# Overly Responsive Pancake Cars



## Grandcheapskate

As I have gotten better at cleaning and tuning pancake chassis, I now seem to be running into an issue. The cars are getting too responsive. Let me s'plain.

I am running on Mattel track using old Aurora 20-22v power packs. The controllers are 60 Ohm Russkits.

As I work on a (Aurora stock) pancake chassis, I can get them better and better. Eventually some of them get so responsive that the slightest touch on the controller gets them going near full speed. There is no slow speed, so negotiating 6" (and 9") curves is often a matter of letting off the trigger completely and then "jerking" the car around the turn.

So I am starting to wonder whether 20 volts is too high to run cars once you get them tuned up, or maybe the 60 ohm controller is not high enough at the 20 volt level. I know when the JL cars came out, the general advice was to get a 90 to 120 ohm controller, but these are not "hot" armatures or high performance parts. In almost all cases, it's the original 40+ year old parts with the standard gearing.

Opinions?

Thanks...Joe


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## ParkRNDL

As I understand it...

You probably DO want a 90 or 120 ohm controller. Cars with "hot" armatures don't like high-ohm controllers such as 90s or 120s. My original Aurora Tuff Ones with Mean Green armatures, which at around 6 ohms are considered "hot" compared to old Thunderjet arms at around 15 ohms, don't work well with a 90-ohm controller. Using that car with a high-ohm controller, I have to pull the trigger 2/3 to 3/4 of the way just to get the car rolling. At anything below half throttle, the car stalls and dies. So I have to do all my throttle modulation in that last 1/4 of the throttle travel. It's kinda like the problem you're having in reverse... pull the trigger slowly and get nothing, nothing, nothing... oh crap that's like full throttle. My cars with low ohm (read "hot") armatures do well with 45 or 60 ohm controllers.

I haven't played with my ohm meter in quite a while, but if I recall correctly, JL/AW arms ohm out significantly higher... is it 15? 18? Something like that... My smoothest AW/JL cars really benefit from the 90 or 120 ohm controllers. Lots of throttle modulation available starting at a steady crawl. I think the rule of thumb is that cars with higher ohm arms are smoother with higher ohm controllers...

Feel free to correct me if I'm wrong. Just throwing that out from what I think I know.

--rick


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## cwbam

Great job Rick
Lower Ohm cars need Lower Ohm controllers
and maybe try lower voltage (tri power pack)
http://www.scaleauto.com/parma/recommnd.htm
and test hookups so you are not connecting (+- poles ) wrong and getting an ON OFF feel (I wonder how I know that)

I know a sweet spot on my track is 45 ohm controllers @ 14 volts using Lifelike T chassis.


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## beast1624

Rick/'Bam: I think ya'll hit the nail n the head. If I try to use anything less than 90-120 ohm on either J/L or Aurora T-Jets I have the same problems Joe mentions...either 'on' or 'off'.


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## brownie374

Running at 18 volts helps alot


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## smalltime

Grandcheapskate said:


> As I have gotten better at cleaning and tuning pancake chassis, I now seem to be running into an issue. The cars are getting too responsive. Let me s'plain.
> 
> I am running on Mattel track using old Aurora 20-22v power packs. The controllers are 60 Ohm Russkits.
> 
> As I work on a (Aurora stock) pancake chassis, I can get them better and better. Eventually some of them get so responsive that the slightest touch on the controller gets them going near full speed. There is no slow speed, so negotiating 6" (and 9") curves is often a matter of letting off the trigger completely and then "jerking" the car around the turn.
> 
> So I am starting to wonder whether 20 volts is too high to run cars once you get them tuned up, or maybe the 60 ohm controller is not high enough at the 20 volt level. I know when the JL cars came out, the general advice was to get a 90 to 120 ohm controller, but these are not "hot" armatures or high performance parts. In almost all cases, it's the original 40+ year old parts with the standard gearing.
> 
> Opinions?
> 
> Thanks...Joe


+1 on the controller advice. Professor motor controller are also a good bet. But the real deal is the armature choice.

When I was racing Magna tractions, the mean greens were for the ovals, the road courses is where I broke out the stock magna traction arms or even a T-Jet arm. Stayin' in was paramount. That was allot easier with a detuned chassis.


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## Grandcheapskate

smalltime said:


> When I was racing Magna tractions, the mean greens were for the ovals, the road courses is where I broke out the stock magna traction arms or even a T-Jet arm. Stayin' in was paramount. That was allot easier with a detuned chassis.


Hey guys, thanks for the input. I had figured my problem was the combination of a 60 ohm controller, 20 volt power source and high ohm armature, and you guys pretty much confirmed that. I may try to lower the power using the Harbor Freight Router Speed Control and see how the cars react. And maybe try to find replacement resistors.

Smalltime - are you saying above that you purposely "detune" some pancake cars so you can control them better, or just a general statement that a detuned car is easier to control?

Here's a question I always wanted to ask. I know there is a relationship between the resistance of the controller and the ohmage of the armature, but isn't there also a relationship between the voltage output of your power source and the resistance of your controller for a given ohm armature? For example, would my 60 ohm controllers be a better fit if I were running at say 16-18 volts? It seems to me that the resistance required in your controller would be somewhat dependant on the voltage output of your power source. Right?

Thanks...Joe


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## dlw

Yes, 14-16 volts will allow you good control with a 60ohm controller.


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## smalltime

> Smalltime - are you saying above that you purposely "detune" some pancake cars so you can control them better, or just a general statement that a detuned car is easier to control?


Yes, I detuned regularly. 

I tuned for throttle response. I hate an all or nothing feel, and worked hard to get away from it. 

There are numerous things you can do: (in order)

#1 get the shoes right first. If you have too little tension, you get blackening, too much and you walk out.

#2 If your controller is set in stone, it's time to change arms. If you have a 60 ohm controller, you will probably need a t-jet arm in the thing to get thru the tight stuff.

#3 set your brush spring tension relative to the coast in your chassis. If you have a ton of coast, then tighten up the spring pressure, if there's not enough coast, lighten up a little.

Hope this helps.


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## slotking

this is why the 90 to 120ohm controllers are selling!

best bet, is the BRP stage III (compared to other electronic controllers)
low cost but lets you run all the cars.


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## Bill Hall

*Take a baby step*

Joe, 

60 ohms just aint gonna cut it for t-jet and t-jet modified. 

Dump the toy/set controllers. You wont regret it! Get yourself a Parma econo with a 90 resistor fer cheap so you can actually appreciate what a properly rated controller has to offer. It's night and day. 

Once yer convinced and wanna upgrade to an electronic cuisine art the econos are easy to get rid of. I'll take it it fer the kids. :thumbsup:


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## slotking

I went to the 90ohm parma and my cars are still a little to touchy with it! 

That's why I suggested the BRP stage III, it will fit any pancake car you are driving as well as some inlines.

I do own 1, but had to borrow them a few times!, Will get 1 as soon as i can


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## mikeponiatowski

*BRP 120 Ohm Resistors*



slotking said:


> I went to the 90ohm parma and my cars are still a little to touchy with it!
> 
> That's why I suggested the BRP stage III, it will fit any pancake car you are driving as well as some inlines.
> 
> I do own 1, but had to borrow them a few times!, Will get 1 as soon as i can


BRP also has 120 ohm resistors for Parma Controllers. They are called "Nitro" Resistors, apparently they are wound differenctly from stock resistors for smoother action. They work great for the touchiest cars and cost $20.00 each.


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## slotking

do they fit the parma 90ohm?

I have to debate the fix ohm controller vs the I can fine tune the ohms for almost anything BRP III??


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## beast1624

slotking said:


> do they fit the parma 90ohm?
> 
> I have to debate the fix ohm controller vs the I can fine tune the ohms for almost anything BRP III??


I can verify that they do. Had an old Parma Econo 45 with a busted resistor and bought one of Jerry's 120's. Been using it for the last year and love it. Just bolted right in. Exact same size as the Econo and Pluss resistors that Parma uses.


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## beast1624

...then again that has gotten me to thinking if a $20 resistor can transform a Parma Econo, how much better would one of his Stage II or Stage III controlers be?


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## AfxToo

> Here's a question I always wanted to ask. I know there is a relationship between the resistance of the controller and the ohmage of the armature, but isn't there also a relationship between the voltage output of your power source and the resistance of your controller for a given ohm armature? For example, would my 60 ohm controllers be a better fit if I were running at say 16-18 volts? It seems to me that the resistance required in your controller would be somewhat dependant on the voltage output of your power source. Right?


<edited: try to answer the question more directly>

The question is about the relationship between 1) power supply voltage, 2) controller resistor size, and 3) armature winding resistance.

1) First, the power supply voltage directly effects the maximum RPM the motor can run at. A 24V power supply will allow you to achieve higher RPM than an 18V power supply regardless of the controller or the armature winding resistance. Motor speed is directly proportional to the applied voltage and motor torque is directly proportional to motor current.

2) The controller resistor (rheostat) is simply a voltage divider. At one extreme it provides 0V to the motor and at the other extreme it provides the full power supply voltage to the motor. As you move the controller wiper the voltage applied to the motor varies over this zero to maximum voltage range. If all the controller was doing is supplying a variable voltage across the arc of the resistor wiper arm then all resistors would have the same exact behavior. The voltage at the 50% point on a 45 ohm resistor would be the same as the voltage at the 50% point on a 90 ohm resistor. With zero load on the controller this is the case.

3) Where the motor winding resistance and all other things like traction magnets come into play is on the load side. With a load attached to the controller the differences between the 45 ohm resistor and the 90 ohm resistor come into play. The resistor/rheostat is still a voltage divider but the difference is that the 45 ohm resistor will be able to supply twice as much current to the motor at any given point in the wiper arc of the resistor other than at minimum and maximum throttle where they are both equal. Since motor current determines motor torque and a motor with lower winding resistance requires more current to start the point in the controller arc where the motor starts spinning will be different between the 45 ohm and 90 ohm resistors. 

The end result is that the usable arc of control for a resistor based controller is dependent on the point in the controller wiper movement that the motor starts turning, which in turn depends on the current requirements of the car. While DC motors are speed controlled through voltage the variable factor between resistor controllers of different sizes, say 45 ohm versus 90 ohm, is one of controlling the current availability at a given point in the controller's arc of motion.

Finally, this all assumes the power supply can supply enough current to start the motor. The controller can only make available the current provided by the power supply.


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## Hornet

Joe,what kind of controller hook-ups do you use.
If you've converted to the standard 3 wire hook-up
You can experiment with an old school homebuilt choke.
The real old school chokes were nothing more then a length of solid core light duty wire (18 to 20G usually) wrapped around a piece of pipe,or even a soda can.
This you just hook in line with the power out (black wire on most 3 wire tracks) from your controller.
Where you hook up on the coiled wire determines the amount of extra resistance.
My old track used kit bashed Tyco resistors as chokes,and you could just alligator clip onto the resistor,where you wanted.
It's not perfect,but it's cheap,and usually something you can build with parts on hand.
Rick


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## alpink

AFXToo, what category does that Jerry Shmoyer stage II and stage III controllers fall into, please.
thank you, al


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## slotcarman12078

One other suggestion Joe. Check the ohms of those super touchy armed chassis. It sounds like a low ohm arm to me. I have a few in the 6 ohm range that are plenty fast (read too fast) with barely a click of a 90 ohm Parma controller.


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## Bill Hall

Gas is on the right grampa!


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## Grandcheapskate

AfxToo said:


> Perhaps I can provide some insight on controllers that will clear up some confusion.
> 
> First of all there are really just two types of controllers: 1) controllers that control current, and 2) controllers that control voltage.
> 
> The traditional Parma, pretty much all set controllers, and transistor amplifier controllers like Difalco are all current controlled controllers. These controllers provide control to the car by varying the amount of current delivered to the car.


Now I am really confused. I thought resistor controllers (like the Russkits and Parmas) varied the voltage going to the car, not the current. I was under the impression that all the amps (current) capable of being produced by a power source were available to the car, but it was the voltage that was being "controlled".

If I increase the voltage output of the power source but leave the amperage capability the same, won't the car go faster at 20 volts as opposed to 16? If a resistor controller is varying the amount of current going to the motor, does that mean the motor is always seeing the full voltage output throughout the entire range of the resistor?

Thanks...Joe


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## AfxToo

From what I can tell the Stage II and Stage III-R are resistor based controllers just like a conventional Parma single fixed resistor controller. The difference here is that instead of having a single fixed resistor it uses additional potentiomters wired in series and in parallel with the main resistor to allow for other total values of resistance (and hence current) to be controlled. These variable resistor networks are provided in various resistance range combinations on separate control boards. 

The Stage III is a transistor amplifier (current controlled) controller similar to a Defalco but with an added twist or two. One twist is that the the transistor amplifier part of the controller is mounted on a separate detachable control board and the base is designed such that it can be adapted to work with either the transistor amplifier control board or a resistor network control board. The other twist is that the Stage III uses the resistors in the base to provide linear control of the transistor amplifier output while a Difalco uses steps provided by fixed resistors in the older base models, variable resistors in older Fanatic models, and a replaceable resistor DIP array in the newer versions. The Stage III also has a separate coast transistor rather than simply using a pot to change the transistor amplifier bias to keep the transistor ON all of the time.

These are all current control based controllers but they do have a very unique and clever modular electrical design and packaging.


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## slotking

amp only matter on how much the car/system requires
ie.. a million amp power and 20amp power supply will not change how the car drives if the car and track only uses 1amp.

I understand and to a degree agree with with a 90ohm resistor and 20ohm resister starts. 

I am not really sure but I think the 20ohm unit has thicker wire.

I have tried using a 45 where a 20 or 25 ohm unit would work better.
what found is that the car feels mushy(with the 90ohm) even at the 15ohm point where both controllers provide enough votage


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## Hornet

I'm gonna take a stab at trying to get this one into layman terms for you Joe,this ones tougher though.
Dave i hope you correct me if i'm wrong.
Years ago when i was a kid right out of high school,i ran into an old electrical engineer,and he told me the easiest way to think of electricity is like water passing through a pipe,or air passing through a tube.

Joe if you think of the resistor as a variable venturi,kinda like an old school carburator.
The air pressure at the entrance is always a constant (voltage),but the flow volume of air (amps) is variable.
To maintain the same amount of flow through the venturi,you either have to up the pressure (volts) or up the flow (amps) to maintain that same volume of air at the exist of the pipe,but the pressure (input volts from the power supply)is a constant non-variable,so the only way to vary the volume of air through the venturi is to vary the input volume (amps) of air.
Let me know if that makes sense Dave,this one took me a minute or two of thinking on how to get it into something everybody can understand:thumbsup:
Rick


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## AfxToo

> Now I am really confused. I thought resistor controllers (like the Russkits and Parmas) varied the voltage going to the car, not the current. I was under the impression that all the amps (current) capable of being produced by a power source were available to the car, but it was the voltage that was being "controlled".


Sorry for the confusion. I updated my original post to better answer your question. You are absolutely correct in assuming the speed is controlled by voltage. However, when comparing differences between different controllers like a 45 ohm versus a 90 ohm the differences in the range of control - the arc from motor start to motor top speed in the trigger pull, this is determined by the current availability at the trigger position. This is what causes the variability in performance and behavior that you see with different motors and cars with a given controller.


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## alpink

these are the kind of posts that could be compiled into an "all you wanted to know about toy slot cars but were afraid to ask" book, CD, DVD, Blu-Ray, online site, all of the above!
there are, of course, many other posts on various threads that would also need including and I am sure there are some topics not yet ventured into, so I think someone could start gleaning the site here for all that can be obtained and saving it to a source like a 'Cliff Notes' sorta thing.
remember when ever volunteers are asked for I take two steps BACK!


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## Hornet

Actually now i'm a bit confused to,.

I always thought the input voltage from the power supply to the resistor was a theoritical constant,and it was the current that was being varied?
Is it the other way around,and the input current is the constant,and the voltage is a variable?


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## slotcarman12078

The way I learned it is if you take a constant voltage, and feed it through a resistor, the voltage drops. That's how I am able to use LEDs with track power. Without resistors, the track voltage would kill the LEDs pretty darn quick. The controller being a variable resistor will adjust the resistance depending on the position of the wiper. That tells me the voltage is dependent on the controller, with there being no resistance and full voltage applied when the trigger is mashed, and corresponding lower levels of voltage through the sweep of the controller.


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## Hornet

So in other words,using the air and pipe anology,the resistor acts like a smaller pipe,instead of a venturi,okay that makes sense


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## slotcarman12078

If I'm wrong, someone please correct me!! I think of it like a faucet which controls the flow of water. When the faucet is off, there is no water flowing ( 0 volts) and as you turn the faucet (pull the controller trigger) the water flow starts and increases (voltage incrementally increases) until the faucet is wide open (full voltage, 0 resistance).


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## Hornet

After some more thinking,the smaller pipe anology is wrong.
I had to think about it in terms of fluid movement using water before i think i re-wrapped my brain around things.

The pipe diameter stays basically the same,but the bends in the pipe determine the output pressure.
The way i've got it figured is,when you bend water around a corner it has a corresponding pressure drop,because of it's inherent hate of changing directions,electricity has the same principles,it hates changing directions.
The resistor is a basically a piece of pipe that is one diameter it's whole length,with a multitude of bends in it,so it actually has the capability of flowing the volume of water (amps),but because of the multitude of directional changes presented,it has corresponding pressure drops at each bend(loss of volts).
The number of bends in the pipe and length of that pipe,is then determined by the position of the wiper on the resistor.
This theory makes more sense to me then the smaller pipe theory,how far out in left field am i


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## slotking

rick

On the water pip thing, the way I read it a while back

say you have 1 pound of pressure pushing water through a 1/2" inch pipe.
that 1 pound of pressure is the voltage

you have a good stream shooting out

now up the voltage to 100 pounds, you that 1/2" pipe is restricting the water flow

so we up the amps (10" pipe) you getting a lot more water (current)

let find it now that i posted it!


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## slotking

not the link I was looking for, but you all may enjoy the site for some strange reason!

http://rikravado.hubpages.com/hub/Watt-are-Amps-and-Volts


ok

found another link that seem helpful

http://wiki.answers.com/Q/What_is_the_difference_between_amperage_and_voltage


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## Hornet

Good reading Mike.
I always thought a faucet /spigot was a current limitator,not a pressure limitator.
It's a variable orifice with a fixed pressure behind it.
If you fill a bucket with water,through the orifice ,it takes quite a bit longer to fill the bucket because the volume of flow (amps) is restricted,not the pressure(volts).
That was the way i used to think a resistor worked,but that's gotta be wrong.
Because in that scenario,we're restricting the flow volume (amps),not the pressure (volts),so i'm leaning towards it being a piece of pipe with a fixed diameter,and a multitude of pressure reducing bends.
The pipe is the same diameter it's whole length,so it has the capability of still flowing the same volume of water(amps),but the bends reduce the pressure(volts)by causing pressure drops at each bend,but not volume drops,while a flow restriction like a faucet restricts the volume(amps),but not the pressure


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## slotking

whatever! I'm still thinking about Naomi Watts:lol::lol:


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## Hornet

If you read the sites close,they're limiting current flow with the faucet,not voltage.
Aren't we actually limiting voltage levels to the car,not amp flow levels.


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## ajd350

The layman is getting cornfused by all this. We know that the water flow is controlled by the valve. That much can be related to controlling the power to the pancake mobile. 

If you had a hose sprayer nozzle that went from closed to a high water volume with very little pull on the handle it would be similar to a low ohm controller. Hard to regulate the power/speed smoothly.

If a different handle was calibrated to control the flow of water more gradually as you squeezed it, it would be like a high ohm controller. Much easier to regulate the power/speed when you need to.

As pointed out earlier, dial back the power to around 18 volts, get some 90 or 120 ohm controllers and tame those flying pancakes. Makes 'em a lot more fun to drive.:wave:


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## Grandcheapskate

My basic electric knowledge left me with one fact - the amount of current into a circuit must equal the current out of the circuit. In other words, there is no loss of current from start to finish.

The analogy to water is simple - the amount of water put into a hose at the start must equal the amount of water that exits the hose (assuming we get all the water out of the hose).

Voltage on the other hand does drop off from start to finish and when you add up all the components of the circuit, you get to zero. What knocks it down to zero? Answer - resistance.

I can visualize it this way. We fill a huge bucket with water and place it 10 feet high . The bucket has a hole in the bottom with one end of a hose attached. We have the open end of the hose also at 10 feet. Let us also assume that if water flows from the bucket through the hose, it is immediatly replaced. There is no pump pushing water through the hose.

Let us also assume there is no inherent resistance in the hose.

The difference in height between the bucket and the open end of the hose is the voltage difference. With the bucket and open end of the hose at the same height, nothing happens - voltage difference (height) is zero and there is no flow of water. As we lower the open end of the hose, water starts to flow. The water pressure in the hose is directly proportional to the height difference of the bucket and hose opening - Voltage! The amount of water that comes out of the hose is the amperage - it must equal the amount of water coming out of the bucket. The more we lower the hose opening (increase the voltage), the more water (amps) flows out the hose. The flow of water continues to increase as we lower the hose opening until we reach the limit of the hose's ability to carry water.

Now that I've said all that, let's see what still bugs me.

Now that we can visualize this, why does no water flow when both the bucket and hose opening are at the same height? Clearly the hose will fill with water, but nothing will flow out. Why? Because there is only enough pressure being supplied (from the bucket) to fill the hose equal to the level of the bucket (water seeks its own level). So the hose fills with water, yet the pressure (voltage) and water flow (amps) at the hose opening is zero.

Now comes the fun part. How does our controller fit into this "circuit"? If we put a valve (controller) between the bucket and hose opening, we can control the amount of water that passes through the hose once we start to lower the hose below the level of the bucket. If we close the valve, nothing will pass no matter how low we put the hose. Open the valve all the way and its like it's not even there.

But what happens when we partially open the valve?

The water pressure (voltage) on the input side of the valve is equal to the height difference of the bucket and hose opening. The amount of water flowing through the valve is restricted based upon the valve opening - this is our variable resistance. So, I can see how the valve (controller) restricts the flow of water (amps) - basically the valve/resistor limits the amount of water flowing through the hose (and because of that, limits the amount of water leaving the bucket) while still maintaining the rule of whatever water leaves the bucket must flow out the hose. Is the system capable of allowing more water to flow? Yes, but the valve limits it. So more water (amps) are available, just not allowed to flow.

What I do not know is the water pressure on the output side of the valve - is it the same or less than the pressure on the input side?

Now that I've written all this, I'll take two aspirin and check back in the morning.

Joe


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## slotking

lets make it real simple

Track power is on
I pull th trigger and the car goes faster

TADA:wave:


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## Hornet

Great post Joe.
The thing that bothers me about the valve anology,is the fact you have to open the valve farther to allow more flow through it.
So the valve is a current limiter,not a pressure or volts limiter.
I used to think that was how a resistor worked in our case,but that doesn't give any creedance to the fact the car can draw more or less amps through the same size valve opening depending on what it needs current wise,with-out adding more initial pressure (volts) to the system.
I'm sorta still sticking with the bends in the pipe are the limiters to volts,while still maintaining the capability of moving the same levels of current.
The valve idea just ain't cutting it anymore in my opinion,and i thought that way for years


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## AfxToo

Joe, your original question asked whether the controller resistor value mattered based on the power supply voltage, i.e., should you use a different controller based on the power supply voltage. Within the scope of slot car applications the answer is no. No matter what resistor you use it still does the same exact thing, it provides a way to vary the voltage from zero to the maximum power supply voltage. The only difference then becomes the amount of current available at each position in the controller trigger movement. If your car needs more current available for a given trigger position, use a lower value resistor.


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## Grandcheapskate

Hornet said:


> Great post Joe.
> The thing that bothers me about the valve anology,is the fact you have to open the valve farther to allow more flow through it.
> So the valve is a current limiter,not a pressure or volts limiter.


Rick,
I'm thinking the valve regulates both.

Here's the question - is the water pressure the same on both sides of the valve once the valve is open even a bit? Can't be.

Obviously when the valve is closed, the pressure on the output side is zero. Once you open the valve, does the pressure go directly from zero to the input pressure? I don't think so, I believe the pressure on the output side rises the more the valve is open, letting out more water (current) under more pressure(voltage) limited only by the input pressure (volts), water quantity (amps) and diameter of the hose (wire gauge).

Now, does this analogy follow through to electicity?

Joe


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## Grandcheapskate

AfxToo said:


> No matter what resistor you use it still does the same exact thing, it provides a way to vary the voltage from zero to the maximum power supply voltage. The only difference then becomes the amount of current available at each position in the controller trigger movement.


So, the resistor does vary the voltage (which is what I thought). But it also sounds like it limits the current.

Let's use the basic V=IR equation and some nice round numbers. The power supply can provide 20 volts and 10 amps. The maximum resistor value is 100 ohms, so our controller can vary the R value from 0 through 100.

If I barely touch the controller so I am getting all 100 ohms of resistance, what voltage and amperage is passed to the car (assuming no other resistance)?

Thanks...Joe


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## slotcarman12078

Easiest way to find out is to set a chassis on the track with the back elevated so it don't take off on you, get your test meter set to volts and touch the leads to the rails while having someone gently pull the trigger. Different motors will have different amp demands, and all have another resistance built in to the arms, so different cars can have very different results. I am curious, Joe... What color wire and tips are on this hot chassis? It wouldn't happen to be dark green wire with grey tips, would it? :lol:


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## Hornet

Good idea.

Dug out my load tester,from post 46,figured it's better then a car,and it's since been modified with external alligator clips so i can hook a meter into it hands free.

http://www.hobbytalk.com/bbs1/showthread.php?t=340565&page=4

Dug out a brand new 25 ohm Parma Econo,and used a set of pliar handles to jam the wiper against the resistor,so it couldn't move,towards the top end of it's travel.
I don't reconmend this test,as it's very hard on resistors,my brand new Parma has a very definite heat stain from the wiper now
Also grabbed another meter,set this one on amp draw,plumbed it inline (series) with my yellow lane power supply.
Set the power supply on 20 volts with the current knobs cranked up.

Then started playing,and the resistor drops voltage,while drawing more amps.
I could take the voltage through the resistor,right down under 1 volt,while at the same time drawing close to 3.4 amps, the amp draw varied from .54 amps all the way up to 3.4 amps,depending on how many lights i turned on.

By that test,it looks like the same controller setting ,will pass the amount of amps needed for the circuit in use,but it varies the volts by load.

Okay you guys draw your own conclusions,i stand by my theory it's the bends in a piece of pipe,not a valve /faucet opening.
My trackmate power supply never varied voltage output during the test

As a side note,i spent way to many years building and troubleshooting giant Natural Gas pumping stations,then i care to remember,and one thing i learned about valves,is you drop the pressure (volts) through a certain sized valve opening,you're also gonna drop the flow (amps) through that opening.
And this test proves that the resistor can't act like a valve,it has to be acting like a single diameter piece of pipe with a pile of bends in it,which if you look at the average resistor that we use,that's exactly what it is,a piece of pipe bent several times around a insulated ceramic barrel
The only way to drop pressure in a pipe while maintaining flow,is to make it have several pressure reducing bends in the pipe,but keep the same diameter it's whole length,so it still has the flow capability


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## Grandcheapskate

slotcarman12078 said:


> I am curious, Joe... What color wire and tips are on this hot chassis? It wouldn't happen to be dark green wire with grey tips, would it? :lol:


None of the armatures are "hot", they are all stock, either in the 15-20 ohm range, or in the AFX range of around 6. The only way they become overly responsive is after they have been cleaned and are apparently getting a real good flow of electricity from the rails to the armature.

Joe


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## slotcarman12078

The 6 ohms are hot!! I have a few NOS chassis with them, and they fly with the first click of the controller. The NOS lighted T Jet chassis brownie has listed in S&S have those 6 ohm arms. They are a stock arm, but they are overly frisky. Based on the ones I got, they are exceptionally smooth runners, but way too fast for my lighting projects.


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## brownie374

slotcarman12078 said:


> The 6 ohms are hot!! I have a few NOS chassis with them, and they fly with the first click of the controller. The NOS lighted T Jet chassis brownie has listed in S&S have those 6 ohm arms. They are a stock arm, but they are overly frisky. Based on the ones I got, they are exceptionally smooth runners, but way too fast for my lighting projects.


Those sold fast,the ones I have now are in the 16 ohm range they have a smoother powerband.The guys like the 6 ohm ones for drag racing


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## slotcarman12078

Cool!! I'll remember that!! I think I can get away with 16 ohms. Please don't think I was bashing the 6's, I just couldn't light them without dropping resistor values for my LEDs, which would leave them unprotected if someone wound up the chassis to full speed.


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## NTxSlotCars

Tycos run smooth as silk with a wide range of controllers.


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## beast1624

NTxSlotCars said:


> Tycos run smooth as silk with a wide range of controllers.


I used to not care for them too much but after racing them in last year's series I gotta' say...they are pretty fun. You are right abou the controlers, I can use a 45, 60 or in some cases a 90 depending on car and track and they respond well.


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## NTxSlotCars

Tyco....... the best ever made.


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## Grandcheapskate

Hornet said:


> The only way to drop pressure in a pipe while maintaining flow,is to make it have several pressure reducing bends in the pipe,but keep the same diameter it's whole length,so it still has the flow capability


Rick, I believe I am a convert to your way of thinking.

Let's say we take a pipe and twist it, so it is coiled like a spring. It now looks somewhat like a resistor. Just for ease of discussion, lets say the pipe has 100 loops in it, each corresponding to one ohm of resistance. On each loop is a cap that can be opened, but only one is open at any one time. The pipe is sealed at the far end, so the only way for water to escape is via one of the outlets (caps) on a coil.

Now, the amount of water pressure you will get at any one outlet depends on which coil it is on. The farther you get away from the source, the lower the pressure. In other words, the pressure at the cap on coil 1 (closest to the source) will be much greater than the pressure at coil 100.

That takes care of pressure (voltage), but not volume (amps).

So, what if the caps were valves that could have a variable opening? Would the valves then act like the resistance of the armature - depending on how open they are will depend upon how much water (amps) they let through?

Joe


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## AfxToo

> So, the resistor does vary the voltage (which is what I thought). But it also sounds like it limits the current.


Absolutely. This is why different resistor values have different behaviors. So the controller is really controlling both voltage and current availability.

1) Keep in mind we are talking about current availability. But the maximum current availability may also be limited by the current capacity of the power supply. 

2) Also keep in mind that the car may not demand all of the available current. Once the motor starts turning the amount of current demanded will typically go down significantly. Cars that present a heavy load to the power supply and controller (think low ohm arm and heavy traction magnets) will also demand more current even after start-up because they need more torque to move the car, and torque comes from current.


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## Hornet

Do some testing Dave,the resistor doesn't drop current,i proved that in the above test,or if it does it's very little,and the current drop,that is occurring is converted to heat in the resistor itself,basically from the friction of the electrons flowing against the sidewalls of the pipe,that's where i've pegged the current drop,as friction in the pipe..
The resistor will flow whatevers needed current wise till it hits the input current limit,figured that one out off the above test.
I'm great for real world numbers ,that's why i like tests,not theories,and one thing i learned,is a Parma resistor doesn't much care for high current draws,lol.
Most of the tests i did involved letting the resistor sit under load for upwards of a minute,and anything above 3 amps for a minute,will basically cook a Parma resistor
Joe,i think it's more where the wiper sits on the barrel of the resistor,i think that detemines the length of the pipe and the number of bends being used.:thumbsup:
Rick


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## slotcarman12078

I'm still leaning towards the faucet concept. The difference in controller ohm values can be looked at as different sizes of faucets. I lower ohm controller being a larger valve which lets the flow start faster, and arrives at full flow quicker, and a higher ohm controller being slower. Both allow full flow when wide open, but the flow greatly differs between the two from closed through the stages of being opened.


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## Hornet

Dan,there's a pressure differantial,as soon as you open a valve in a pipe,and unless you compensation with a higher imput pressure,the flow and pressure through a valve drops off instantly,and we can't compensate as the input pressure or volts is a theoritical stable unit of measurement ,if the power supply is up to it.
For a valve to be the principle,we'd have to have the ability to instantly adjust the input volts,and not the current flow.
But the input volts in an ideal situation,should never vary,assuming the power supply can handle the current loads.
I'm probably making it more confusing
Rick


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## AfxToo

The plumbing analogies are okay I guess, but how do you model an electric motor as a plumbing fixture?

Back to the original question. Rick's experiment is interesting in showing the effect of voltage drop in the controller over various loads. This is exactly what we should expect and is one of the reasons why traditional resistor/rheostat controllers are so inefficient. This is only half of the answer. But it is not answering these two questions pertaining to the controller and motor relationship, which is what we are really interested in:

1) What effect does changing the controller to one with a different resistor value have on a particular motor's performance? In other words, why does a car with a certain motor operate sluggishly or too jumpy depending on the controller's resistor value?

2) What effect does changing the motor have on determining the controller resistor value that I should use?

Let's look at both of these with the following data:

- Power Supply Voltage = 18V
- The controller trigger is set to the halfway point, i.e., half the resistance 

Scenario: I have a car with a 6 ohm motor (M6) and a 45 ohm controller (C45). I am happy with the performance but when I switch to a 3 ohm motor my controller is mushy and the car doesn't start until I move the trigger much further than I had to with the 6 ohm motor. 

Part 1: Why does my 45 ohm controller feel mushy with my hot new arm?

With the original motor and 45 ohm controller the current at the motor is 18V divided by the sum of the controller resistor and the arm resistance. If we assume the trigger is at the halfway point the motor current is about 0.63A. Since the controller and motor are in the same circuit the voltage available to the motor is about 3.9V.

When I change to the hot 3 ohm arm the current at the motor is 18V divided by the sum of the controller resistor and the arm resistance. If we assume the trigger is at the halfway point the motor current is now about 0.71A. Since the controller and motor are in the same circuit the voltage available to the motor is now about 2.1V. That's a lot less voltage at the motor where we were before.

This is where Rick identified half of the behavior leading to the answer. Yes the motor current is increased but it is not increased enough to compensate for the bigger load that the hot 3 ohm arm presents. The lower ohm arm needs more current to develop the same voltage that it had before. What is limiting the current? The 45 ohm controller is limiting the current.

Part 2: Would changing to a 25 ohm controller help?

So lets change out the controller.

I change to a 25 ohm controller. Now the current at the motor is 18V divided by the sum of the controller resistor and the arm resistance. If we assume the trigger is at the halfway point the motor current is now about 1.16A. Since the controller and motor are in the same circuit the voltage available to the motor is now about 3.48V.Now we're somewhere close to where we were in terms of the controller throttle position that we had with the 45 ohm controller and the 6 ohm arm.

What we see here is that at the same throttle position, the 25 ohm controller provided the additional current at about the same throttle position as the 45 ohm controller did to provide enough current for the motor to have the same motor voltage which we can roughly equate to speed reference. In all cases I've assumed that the motor current is sufficient to start the motor. This is really addressing the sluggish case. 

Part 3: What about the twitchy case?

The twitchy case is where the car takes off too fast too soon. This can be calculated by flipping things around a bit and assuming the car ran great with a 45 ohm controller and 6 ohm arm and we're trying the same car with a 25 ohm controller. In this case the motor current is 0.97A and the motor voltage is 5.8V at the same throttle position as before. So simply lowering the controller resistance increased both motor current and motor voltage. No surprise the car is now twitchy, we are giving it a much higher speed reference and more current to generate torque at the same trigger position. 

This is not theory, this is basic math and electricity. But again this is only part of the complete answer to what makes slot cars interesting and very unlike plumbing. The rest of the story is all about understanding the highly dynamic nature of the electric motor and how it interacts with the closed electrical circuit that it is always a part of, and of course, all those electromechanical and electromagnetic behaviors. An electric motor is not a resistor.


----------



## Hornet

Thanks Dave,that helps explain things more.
But there is a set-up in water that is pretty close to an electric motor,when you're driving a fluid turbine,it acts virtually like an electric motor:thumbsup:


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## AfxToo

This is kind of a unique case because we're talking about Parma style controllers which are rheostats. If you had to design a motor controller a rheostat would be your last choice because the control device is in the same circuit as the thing being controlled. This creates a weird mutual dependency between the controller and the motor. A diode controller like the Omni and older PMs don't have this same behavior because they control the motor voltage independently of current. This allows the motor current to be determined solely on motor demand where the Parma couples the controller current to the motor current.


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## Grandcheapskate

Using the water and pipe analogy, I can actually picture this as a pipe with two valves. The first valve is the controller and is closest to the source. The second valve is at the end of the pipe and it is our slot car.

If either valve is closed, nothing flows. Opening one without opening the other does nothing.

So let's say we open the first (controller) valve all the way and don't open the second valve. This is like pulling the trigger full throttle with no car on the track. The pipe fills with water, but it has no where to go. (Let us assume that when the first valve is subsequently closed, all the water between the first and second valve magically disappears).

Let's say we only open the second valve. Now we have a car on the track ready for power, but the first valve (controller) is closed. There is no water available for the second valve.

Now let's put a car on the track (we open the second valve). The amount of amps a car demands depends upon its resistance; the amount of water that can flow out of the second valve depends on how far it is opened - in other words, how much water it can handle. Let us say the second valve is set to let 5 gallons per minute flow.

The second valve is now open part way. We now open the first valve slowly at first, allowing 1 gallon per minute through. Even though valve 2 wants 5 gallons per minute, it will only get as many as valve 1 allows through. However, if we open valve 1 to the point where it can let 6 gallons per minute pass, the most that will pass are 5 gallons per inute because that is all valve 2 needs (and will let pass).

So what have we got? When valve 1 lets in less water than valve 2 can handle, this is the equivilent of starving our slot car of the amps it needs. When valve 1 lets in more water than valve 2 wants, only the amount valve 2 wants is allowed to pass.

Of cource, if we change the diameter of the pipe between some of our key points (source to valve 1, valve 1 to valve 2 and valve 2 to exit), then that introduces more variables into this scenerio.

Joe


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## NTxSlotCars

AfxToo said:


> This is kind of a unique case because we're talking about Parma style controllers which are rheostats. If you had to design a motor controller a rheostat would be your last choice because the control device is in the same circuit as the thing being controlled. This creates a weird mutual dependency between the controller and the motor.


This design was done away with almost fifty years ago in electric forklifts, with the invent of the SCR controller.

I assume these fancy diode controllers operate in much the same way?


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## Hornet

Dave give it to us in layman terms,is the resistor a flow controlling valve or a pressure reducing pipe
A couple of your posts seem to contradict themselves,and you're really starting to confuse me.
I'd like to get my brain wrapped around this resistor idea first.
Rick


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## AfxToo

Rheostat controllers are only viable because of the way we use them, constantly moving the throttle around to change the car's speed. If you leave them at one setting for very long at all they will get very hot very quickly if there is a lot of current involved. Parma includes a specific warning in their packaging telling you not to use them this way. If you wanted to use a rheostat that you could safely use as a fixed speed controller on a higher end HO slot car the rheostat would probably be around the size of a hockey puck in a rotary design or a hot dog in a linear design. 

Diode controllers or more accurately "semiconductor junction" controllers operate around the characteristic that a semiconductor junction operating in the conducting mode drops around 0.5-0.7 volts per junction while passing the full amount of current up to the junction's current capacity. This allows you to divide the voltage into discrete steps based on these voltage drops while passing all of the current to the load. The thing to watch for with these controllers is the current capacity of the diodes or semiconductors cannot be exceeded or the junction will be destroyed. To manage this the higher end controllers use physically large semiconductors with very high current ratings.

Diode controllers are still not ideal however. The control device is still in the same circuit as the load and the voltage steps are tied to the wiper block segments. This can create arcing as the wiper button moves from segment to segment. This tends to foul the wiper block and create a buildup of carbon.


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## Hornet

But we still don't know the basic principle.
Flat out either one or the other,tell us which one,then we can work from there.
I've trained a pile of guys in my time,and they usually have to understand the basic principles,before they can grasp the indepth stuff,and man you're confusing the heck outta me.
So can we have a definite answer on the basic principle,be much appreciated.
Then lets go from there,thanks
Rick


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## AfxToo

> is the resistor a flow controlling valve or a pressure reducing pipe


The plumbing analogies throw me because they do not equate to an electrical circuit, which is a closed system. Is pressure matched to voltage and flow matched to current? You have to look at voltage, current, and resistance all together in the context of the whole circuit.

The closed loop (the circuit) that we are talking about is the positive terminal of the power supply connected to one side of the controller resistor. The other side of the controller resistor is the wiper button which connects to one side of the motor armature windings. The other side of the motor armature windings is connected to the same ground as the negative terminal on the power supply. That is your closed loop or circuit.

How much current flows in this circuit? The amount of current that flows in this circuit is equal to the power supply voltage divided by the total resistance in the circuit. What is the total resistance in the circuit? It is the sum of the resistance in the controller up to the wiper button position plus the resistance of the armature. 

The armature resistance is fixed. 
The controller resistance is variable.

Therefore the total current in the circuit is variable based on the setting of the controller. The controller is therefore determining the current in the circuit, and the same current flows through the controller and the armature. It's a closed loop. 

So what about the voltage? 

The rule about voltage is that the sum of the voltage drops in a circuit must equal the voltage applied by the power supply. This means that if you had only the power supply and only a resistor connected across the power supply the voltage drop in the resistor would equal the power supply voltage. It doesn't matter what the resistor value is, it will drop the entire voltage. What will change with different resistance values is the current. What does this mean in water pressure/flow terms?

So how does the voltage play in our slot car circuit? The same rule applies. The sum of the voltage drops in the circuit must equal the applied voltage. Some of the voltage gets dropped across the controller resistor and some gets dropped across the armature resistance. The voltage proportioned between the controller and the armature is easily determined. All you have to do is calculate the current in the circuit as previously described (power supply voltage divided by total resistance at the controller wiper position and the armature resistance) and multiply the current by the resistance of each resistor.

Since the armature resistance is fixed and the controller resistance is variable, the armature will only get whatever voltage is not dropped by the controller. If the total resistance in the circuit goes down by swapping in a lower armature resistance the circuit current will rise and the controller will drop even more voltage and deprive the motor of voltage. To get motor voltage back up we have to reduce the resistance of the controller so it drops less voltage at the higher current level. In the hot arm case this means we have to pull the throttle further to get the car to move or swap out the controller with a lower resistance model. Yeah, this is a better way to explain the relationship than my previous one, but the relationship between arm resistance, controller resistance, and the mutual dependency to the circuit current is what it is. The bottom line is that with a Parma style controller you have to always be aware of both current and voltage when picking a controller and matching a controller to a car. You cannot treat voltage and current independently.

But there is still the current side. What if you keep the arm the same but tack on a bunch of load like heavy traction magnets. Now we have a heavy load and the motor current needs go up because we need torque and current provides torque. Since current depends on the total resistance in the circuit we are back to having to change our controller to provide more current over the same range of control from the controller. This is why your SS car even with the same arm as a box stock benefits from a lower resistance controller. 

What does this mean in water pressure and flow? I'm not sure. Plus, if we carry the plumbing analogy we have to look at the power supply as something like a pump. How do we look at the motor? At one level the motor is just a resistor. But once the motor is spinning it is also generating voltage - so it is also a pump! It is applying pressure in the opposite direction as the main pump. What does this do to the flow? Well because it is a closed system the flow goes down. That's exactly what happens electrically, but is that even a valid behavior in the plumbing analogy? What about acceleration? When we pull the trigger on the controller the motor must speed up. It does this by asking for more current. But once it reaches the new speed the current demand drops again because the motor is spinning faster and generating more voltage. Maybe it's time for me to look through some fluid dynamics textbooks to get a better handle on the non electrical equivalents to electrical circuits and motors.


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## Boosted-Z71

You guys have a greater knowledge & understanding of this topic than I do, However I think I found the solution

http://www.ebay.com/itm/Vintage-196...t=Slot_Cars&hash=item2c65bc95aa#ht_1559wt_698

Interesting reading, keep up the good info

Boosted


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## Hornet

Jeff,:thumbsup:.

Dave,I can relate better if it is put into a fluid dynamic principle,as that's where i have some experience.
This explanation is alot better though

I'm trying to compare it to something like a closed loop pumping system,that i can relate to.

I even tried thinking about from a turbocharger prespective,where you have exhaust flow and expanding hot gases in the compressor housing pressurizing the flow,but that didn't make sense either,lol

I'm a southpaw,so you might have to hit me over the head before i get it,lol:thumbsup:


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## Hornet

I think it's easier for most people if we can use water.
As water is a tangible thing that we can see and relate to,while to most of us electricity is invisible.
For years i thought of the resistor as a valve,but then you and Joe got me thinking about it,and that's when things didn't start adding up.
That's when i started thinking the resistor must act more like a pipe,things add up,and make more sense if i think of it that way,while the valve idea doesn't make sense now that i've put some thought into it,and that's where most of my confusion lies.
I'm only educated through the school of hard knocks,lol:thumbsup:
Rick


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## slotcarman12078

http://science.howstuffworks.com/electricity4.htm

This gets interesting as you go through it all. It breaks it down to layman's terms. AFXtoo is right on the money as per usual, this link simplifies it even more.. 

I do remember reading that the water analogy really isn't correct when trying to explain electric principals. Pipes are empty until the water flows into them. Electrical circuits are solid wires and the electrons are always present in wire. The current gets them excited and moving, or something to that effect. I wish I saved that link as it was also an interesting read (though way over my head) as far as electrical equations and calculations go.


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## Hornet

Hey that's a good site Dan,but i read this,and it's back to not being a valve again.
Everything i read is using the pressure as a variable to increase flow,and we're not,our input pressure (volts from power supply) is always a theoritical stable number.

Lets say you have a rectangular shaped piping system,with a valve and a variable pitch 3 vane rotator(where the vane is the motor).The variable pitch of the vanes determines the load requirements of the driven vane:ie new style turbochargers,switch pitch torque convertors etc.
On the corner of this rectangle you have a pump,and just downstream of this pump is a valve.Water is returned back to the pump as it's a closed system,a basic hydraulic motor circuit
The pump is set to supply 18 psi of deadhead pressure when the valve is closed,as soon as you open that valve,you have an instant flow through the valve opening.
This flow is totally controlled though by the pressure,and you can't decrease or increase the flow through that valve opening unless you increase or decrease the initial pressure,and that's where i start getting confused,lol
To me the pipe can't always be full,otherwise the motor would be working at it's maximium,so an un-excited electron,to me means it's basically an empty pipe,that's how i'm sorta looking at it
Even increasing the pipe diameter won't do anything to the amount of flow through the valve opening.
A 6 inch pipe won't flow anymore water through a 1/2" diameter opening then a 3 inch pipe will,if they both have the capability of maintaining our deadhead pressure of 18 psi/volts.
So increasing pipe diameter isn't it,and i'm back to it's gotta be acting more like a pipe with multiple bends in it,and a big ole gate that you throw open when you hit the first band of the resistor



This is what i copied off that site,going back for more learning


Let's say you have a tank of pressurized water connected to a hose that you're using to water the garden. If you increase the pressure in the tank, more water comes out of the hose, right? The same is true of an electrical system: Increasing the voltage will result in greater current flow.

Now say you increase the diameter of the hose and all of the tank's fittings. This adjustment would also make more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.

When you look at a normal incandescent light bulb, you can see this water analogy in action. The filament of a light bulb is an extremely thin wire. This thin wire resists the flow of electrons. You can calculate the resistance of the wire with the resistance equation.


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## slotking

the real question is how handle Overly Responsive hot babes??


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## alpink

send the hot babes here and I will demonstrate!


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## slotking

LOL:thumbsup:


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## Hornet

LOL,that made me laugh:thumbsup:


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## NTxSlotCars

All this high tech talk has got me thinking...
I bought a new controller today.


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## Boosted-Z71

NTX, that fits perfectly into the electricity / water analogy, I am going to assume this is the non-brake version, as there is no 3rd connection. 

This type of controller is also quite useful for heat dissipation considering the weather were having here in the Midwest, I need to look into one of these

Boosted


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## Hornet

Prone to heat soak if you leave it laying out in the sun,lol:wave:


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## Grandcheapskate

AfxToo said:


> The closed loop (the circuit) that we are talking about is the positive terminal of the power supply connected to one side of the controller resistor. The other side of the controller resistor is the wiper button which connects to one side of the motor armature windings. The other side of the motor armature windings is connected to the same ground as the negative terminal on the power supply. That is your closed loop or circuit.
> 
> How much current flows in this circuit? The amount of current that flows in this circuit is equal to the power supply voltage divided by the total resistance in the circuit. What is the total resistance in the circuit? It is the sum of the resistance in the controller up to the wiper button position plus the resistance of the armature.
> 
> The armature resistance is fixed.
> The controller resistance is variable.
> 
> Therefore the total current in the circuit is variable based on the setting of the controller. The controller is therefore determining the current in the circuit, and the same current flows through the controller and the armature. It's a closed loop.
> 
> So what about the voltage?
> 
> The rule about voltage is that the sum of the voltage drops in a circuit must equal the voltage applied by the power supply. This means that if you had only the power supply and only a resistor connected across the power supply the voltage drop in the resistor would equal the power supply voltage. It doesn't matter what the resistor value is, it will drop the entire voltage. What will change with different resistance values is the current. What does this mean in water pressure/flow terms?
> 
> So how does the voltage play in our slot car circuit? The same rule applies. The sum of the voltage drops in the circuit must equal the applied voltage. Some of the voltage gets dropped across the controller resistor and some gets dropped across the armature resistance. The voltage proportioned between the controller and the armature is easily determined. All you have to do is calculate the current in the circuit as previously described (power supply voltage divided by total resistance at the controller wiper position and the armature resistance) and multiply the current by the resistance of each resistor.
> 
> Since the armature resistance is fixed and the controller resistance is variable, the armature will only get whatever voltage is not dropped by the controller. If the total resistance in the circuit goes down by swapping in a lower armature resistance the circuit current will rise and the controller will drop even more voltage and deprive the motor of voltage. To get motor voltage back up we have to reduce the resistance of the controller so it drops less voltage at the higher current level. In the hot arm case this means we have to pull the throttle further to get the car to move or swap out the controller with a lower resistance model. Yeah, this is a better way to explain the relationship than my previous one, but the relationship between arm resistance, controller resistance, and the mutual dependency to the circuit current is what it is. The bottom line is that with a Parma style controller you have to always be aware of both current and voltage when picking a controller and matching a controller to a car. You cannot treat voltage and current independently.



Excellent explaination.

So getting back to the original question - does the voltage of the power supply have an impact on what ohm controller to use? Let's take some real numbers.

Let's say we have a 15 ohm armature. Except for the power supply and controller, let's ignore all other components of the circuit (such as rail resistance, traction magnet load, etc.) as I'll have my hands full with this simple example.

When any controller is fully opened (R=1), the voltage getting to the armature is the power supply voltage. Why? Becasue it is like the controller isn't even there.

The basic equation is V=IR ... volts=current x resistance.

Example 1: A 20 volt power source and a 90 ohm controller.

When the controller is at full resistance (90), the R value of the circuit is 105 (90+15). The current through the system is 20/105 or .19 amps. The voltage getting to the armature is .19 * 15 or 3.8 volts.

When the controller is at half trigger (45), the R value of the circuit is 60 (45+15). The current through the system is 20/60 or .33 amps. The voltage getting to the armature is .33 * 15 or 5 volts.

Example 2: Let's crank up the power supply to 30 volts with the same 90 ohm controller.

When the controller is at full resistance (90), the R value of the circuit is 105 (90+15). The current through the system is 30/105 or .28 amps. The voltage getting to the armature is .28 * 15 or 4.2 volts.

When the controller is at half trigger (45), the R value of the circuit is 60 (45+15). The current through the system is 30/60 or .5 amps. The voltage getting to the armature is .5 * 15 or 7.5 volts.

Example 3: A 20 volt power source and a 60 ohm controller.

When the controller is at full resistance (60), the R value of the circuit is 75 (60+15). The current through the system is 20/75 or .27 amps. The voltage getting to the armature is .27 * 15 or 4 volts.

When the controller is at half trigger (30), the R value of the circuit is 45 (30+15). The current through the system is 20/45 or .44 amps. The voltage getting to the armature is .44 * 15 or 6.66 volts.

Example 4: Let's crank up the power supply to 30 volts with the same 60 ohm controller.

When the controller is at full resistance (60), the R value of the circuit is 75 (60+15). The current through the system is 30/75 or .4 amps. The voltage getting to the armature is .4 * 15 or 6 volts.

When the controller is at half trigger (30), the R value of the circuit is 45 (30+15). The current through the system is 30/45 or .66 amps. The voltage getting to the armature is .66 * 15 or 10 volts.

Summerizing the 90 Ohm controller...
At full (90) Ohms 20 volts, voltage at the armature is 3.8
At full (90) Ohms 30 volts, voltage at the armature is 4.2
At half (45) Ohms 20 volts, voltage at the armature is 5
At half (45) Ohms 30 volts, voltage at the armature is 7.5

Summerizing the 60 Ohm controller...
At full (60) Ohms 20 volts, voltage at the armature is 4
At full (60) Ohms 30 volts, voltage at the armature is 6
At half (30) Ohms 20 volts, voltage at the armature is 6.66
At half (30) Ohms 30 volts, voltage at the armature is 10


From what I can see, as the voltage of the power supply increases, the amount of voltage which gets to the armature increases for each relative position on a controller. In other words, at the halfway point on either controller resistor above, more voltage gets to the armature with the 30 volt power supply than with the 20 volt supply - 50% more. And more volts get through the 60 ohm controller (for each relative position) than the 90 ohm controller.

So, if my math and logic are correct, doesn't the voltage of the power supply play some role in determining the correct size resistor to use?

I also have another question. Assume our 20 volt power supply can produce 20 amps. If the power supply can provide 20 amps, do the above calculations still apply? In other words, in example 1, are there still only .33 amps getting to the armature? 

Thanks...Joe


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## Grandcheapskate

Just wanted to bump this up to see if I get any feedback on my last post.


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## demether

> So, if my math and logic are correct, doesn't the voltage of the power supply play some role in determining the correct size resistor to use?



I'm not into math things, but it's also my feeling. Changing my powersupply to a "pro" 8amperes 13volt powersupply changed also the feeling I had driving my cars. 

On lower amperes (around 0.5a by lane), I thought that 45ohm was not enough to control pancake cars. On 8amperes (4 lanes), 45ohm became useable.


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## Grandcheapskate

Just wanted to bump this thread again to see if anyone has any comments/insight on post #80.

Thanks...Joe


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## alpink

:beatdeadhorse:


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## Hornet

I'm not sure what you want either Joe.
Are you wanting somebody to double check your math,if so,it looks good to me:thumbsup:

And yes changing the input voltage to a given controller will change it's operating charastic's.
A controller that works for a certain car, say at 12 volts,probably is gonna be way too touchy to use on the same car if you up the input voltage to say 20 volts


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## Grandcheapskate

Hornet said:


> I'm not sure what you want either Joe.
> Are you wanting somebody to double check your math,if so,it looks good to me:thumbsup:
> 
> And yes changing the input voltage to a given controller will change it's operating charastic's.
> A controller that works for a certain car, say at 12 volts,probably is gonna be way too touchy to use on the same car if you up the input voltage to say 20 volts


Hi Rick,

Way back in my initial post, I had asked whether the voltage of the power supply would have an effect on which controller to choose. It seemed during this thread that the answer was "No, the voltage of the power source doesn't matter". At least, that's the impression I came away with.

In doing the math above, it certainly seems like it would be a contributing factor in determing which controller would be "right" for a given chassis. Yet every time I see the question asked, the replies never consider the maximum voltage of the power source.

So what I am asking (and you answered it) - if my math above is correct, how can the voltage capability of the power source NOT be considered when determining which controler to use?

Thanks...Joe


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## Hornet

LOL,okay i got where you're going,finally.
Hokay i'm not the brightest bulb around:wave:

But yes input voltage changes a controller's charastics,and should be included in your controller choices.

A track with 22 volts acts alot differant then a track with 18 volts and so on down,when it comes to resistor choices:thumbsup:
Rick


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## NTxSlotCars

I think it depends on the track, and the car you are running and the power source.


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## Hornet

Don't forget the most important charastic to consider,"Your Skill Level".:wave:


Don't over-estimate your slotcar driving abilities:thumbsup:


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## Franko

Sooo,get a variable v power supply and run what feels good.


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## beast1624

Franko said:


> Sooo,get a variable v power supply and run what feels good.


On ours we have 'Sober', 'Tipsy' and 'Falling Down' settings and everywhere in between.


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