# What are OHMS for in controllers??



## guinnesspeanut

I keep seeing different Ohms for controllers in slotcars.. I think ohms are like resistance. The higher the resistance, the hotter the wire, but don't know if higher ohms is more or less resistance, or why any of it is significant. Can someone please enlighten me? I'd like to use what I'm supposed to, so I think I should know this stuff... Baby steps please. I want to make sure I grasp this.


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## alpink

ohms are steps of resistance. low ohms have low resistance, so low ohm armature should spin faster than a high ohm armature given everything else is equal.
a high ohm controller will have more range and allow better handling.
most controllers have a wire wrapped around a piece of ceramic. when one begins to pull the trigger the button touches the wire at it's longest point and the resistance is highest.
as the trigger is pulled more, the button touches the wire closer to the point of origin decreasing the resistance and allowing more "juice" to the motor and the car should go faster.
when the trigger is pulled all the way and the button is at the beginning of the wire, the resistance is very low allowing a lot of "juice" to get to the motor.
so, the resistance started high, let's say 100 OHM and decreased a little at a time as the trigger was pulled allowing the car to go faster.
that is a basic controller and there are many more that control the power different ways.
it is said that a high OHM controller, 90+ OHMs is good for t-jets.
each person learns to drive a little differently, so their choice of controller might be different.
other, easier explanations due real soon!


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## slotking

I agree with Alpink on almost everything he said.

the only exception is


> "a high ohm controller will have more range and allow better handling"


if I use a 100ohm controller on a t-jet, he is correct.
but if i try to use on inline unlimited car, the car will not go till 100ohm controller is almost full open. thus only allowing an off/on control.
but a 4ohm controller would work much better.

a sg+ or other normal inline would be sluggish at the start of the trigger pull so a 35 to 60ohm controller would work better.

JMHO


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## cwbam

the Faster the car (usually a lower ohm car) the lower the ohm Controller.

Think of the wire bands on the controller as a Dimmer Switch on a light switch.
A 16 ohm stock tjet is more in control with 120-90 ohm controller.
A 2 ohm Dragster tjet with NeoMags can use a 2 ohm controller (basically ON OFF)

http://www.scaleauto.com/parma/recommnd.htm

60 to 90-ohms - Suggested for T-Jets on any type of layout.
45-ohms - Suggested for BSRT G-Jets or Stock Tyco/Tomy/Life-Like cars on small twisty layouts (using wall-pack power).
25 to 35-ohms - Works well with BSRT's G-Jet cars as well as Stock or Modified cars on large, fast layouts(wall-pack or transformer power).
15 to 25-ohms - For Modified/Polymer cars on fast layouts (high-output transformer or battery power).
4 to 15-ohms - For Neo/Unlimited cars with high-strength magnets on layouts with plenty of power.


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## slotcardan

you could write pages on the subject.

to add to things:

you have amp draw, and static friction caused by magnatraction. then there is voltage to consider.

Basically for Older cars you use a 75-120 ohm controller, these are pre- magnatraction or brass wars cars.

these cars have Low amp draw and low static friction. they roll very easy and also usually need braking, braking is a closed loop back to ground the resistance of the brake loop is what allows adjustment the motor basically acts as a feedback generator and slows the car down faster then if you had no brake. 
For these older cars when you have a hot motor low ohm. then you need to move towards 45 ohms. the reason is due to the torque drop off.

There is also the effect of voltage the old cars originally started out at 8 volts, and slowly jumped to 12 volts, then finally to 18 volts. at 8 volts some sets had 35 ohm controllers and at 18 volts they went to 75-90 ohm controllers.

basically it is ohms LAW these old cars had like .2-.4 amp draw, so they setup the sets to work in that range.
at 8 volts and 35 ohms you get 0.22857 amps
at 12 volts 60 ohms you get .2 amps
at 18 volts 90 ohms you get .2 amps again... it was the given range that would make the throttle control manageable.

things get more confusing when you talk about motors. older cars used 15ohm to 30 ohm motors. these motors were high torque low rpms.

Slot cars gauge the motors different then the rest of the motor world. they say a motor is 15 ohms or 25 ohms. the correct way to talk about a motor is how many turns the motor has and the thickness of the wire. is it a 13 turn double wind or a 44 turn single wind.

Basically a motor with high Ohms must have more turns then a low ohm motor with the trade off being the thickness of a wire.
a high ohm motor may have 60 turns on each coil, and a low ohm motor may have 40 turns with a thicker wire. 
the difference is more turns or higher ohms means more Torque less rpms.
more rpms means lower torque with a low ohm motor.

So when people say they swapped out a 6 ohm motor for a 3.3 or 2.8 ohm motor what they have done is increased rpms from 500- 5000 rpms, depending on the motor. but they have dropped off the torque, making the car very lazy if the car must constantly slow down and speed up as opposed to going flat out all the time.

but lets get back on topic.

When magnatraction came out the controllers had to change. a magnatraction car will pull .6-.8 compared to .2-.4 on a non-mag car. this is due to the static friction caused by the magnetic downforce holding the car to the rails.

at that point at 18 volts which is where things started in the mid 1970s you needed a 25-55 ohm controller to maintain the .5-.6 amps. later the voltage increased 20,22,24 volts and the magnatraction got stronger and stronger. eventually you had 24 volts at 25 ohms with some sets giving almost an amp of capacity. 

the reason is as the magnatraction effect is increased, your range of voltage that moves the car gets smaller and smaller. before a car might start moving at 6 volts now with heavy magnatraction the car may require 18 volts before it starts moving. thus you need a very narrow range of control over the voltage requiring a 2-20 ohm controller. if you tried a 20 ohm controller on a old brass wars car the car would be undrivable the fine movements required for mid throttle control would be impossible for a person to control thus you need a much wider range....

So typically stock T-jets work best in a 90-120 ohm range. (15-30 ohm motors)
stock tyco pro 1 and 2 and S series 75-120 ohms. (tycos used a little hotter armature 8-20 ohm)

then when you enter the magnatraction era you see a range like 35-65 ohms

when you get into neo magnets or competition and very low ohm motors you see 1-45 ohm controllers. this is a big range because of all the different configurations you may see. 


So this is why you see a huge range of ohms listed for controllers. it depends on the type of chassis you have. older cars pre magnatraction require a larger ohms range because they have lots of mid range throttle control and low amps.
newer cars have a much lower ohms( resistance) because they have a very on/off feel due to the magantraction, requiring a shorter range of throw for your finger to produce the same throttle controlling effect.

if you take say a 120 ohm resistor and use it on a magnatraction what you will find is the car will not move until 75-85% of finger throw that leaves you 10% of travel left on the resistor to try and maintain mid throttle control because anything after 90% will feel like full power. by using a smaller ohms, your finger range over the trigger is increased making fine throttle adjustments easier on you. 

by comparison a old brass car that will roll by breathing on it will require a huge ohms resistor because just touching the trigger will send it off down the track like a rocket ship. again it gets your finger on the trigger into a comfortable range of motion to maintain mid throttle control and drive the car.

so you take a 120 ohm controller on a magnet car and it will feel like nothing nothing nothing , suddenly rocket ship.
you take a 35 ohm controller on a non magner car and it will feel like, rocket ship, warp drive, lunicous speed. 
both situations make the cars almost undrivable. 

after all this you get into braking the old cars need them the new ones sort of use the traction magnets and down force to adjust braking. braking just allows you to enter a curve at a higher rate of speed and let off power later then you would normally, thus lowering your lap times.


well i could go on, but it will give you an idea of why things are low or high ohm and what i means as far as the controller is concerned.

also when you modify a car and use a hotter motor you drop the ohms resistance of the controller so to your finger and range of motion is the same as before this is due to the torque dropping off.
as the torque of the motor drops you start getting that on/off feeling and you must narrow the range of the controller to maintain your range of motion of your finger and thus have good mid throttle control of the car.

so take a tyco pro with a 15 ohm stock motor using a 75-90 ohm controller, it has low rpms and high torque.
you swap out the 15 ohm with a 3.5 ohm, now the rpms is up and the torque is down your range of motion just shrank on the original controller due to the torque loss so you go to a 35-55 ohm controller to have the same range of motion your fingers are used to. 

(you should also have regeared the car but getting hopup parts is hell now, proper gearing is a whole new topic as well. the mistake people make is swaping in a low ohm motor with stock gearing, the car will scream in going flat out, have no power in the corners and you will overheat the motor and blue the commutator surface wrecking the motor. you must always gear down you will make up the rollout loss with the lower ratio with the higher engine rpms.)


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## jobobvideo

wow! great info guys


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## Bill Hall

That should be a sticky.


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## slotcardan

it always begs the question; 

Well what motor and controller should i get? 

The answer is the track dictates that, and if you change the track then you have to start all over again.

and the voltage you run at can change things up again. that is why you always see posts about adjustable power supplies and i found the perfect voltage of like 10.125. well for that track and that car that is what works.

one thing that can be super annoying for a beginner is how sensitive ho scale is. what works on one chassis may not be reproducible, tolerance, friction, chassis warpage each ho chassis is like a fingerprint and you could spend weeks making one run the way you want just on a home track. you pick up your winning chassis and head over to a friend's house and suddenly it runs HORRIBLE and you are at a loss as to how that is possible. this is very much like real life as with a real racing car you have totally different setups for the chassis, motor, and transmission depending on track, atmospheric conditions what wins for you at daytona will not work at all at walken's glen.

I can tell you having a track in my basement the difference between 50 degrees and 75 degrees of temperature in the room makes for a huge difference in track times.


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## slotking

> So when people say they swapped out a 6 ohm motor for a 3.3 or 2.8 ohm motor what they have done is increased rpms from 500- 5000 rpms, depending on the motor. but they have dropped off the torque, making the car very lazy if the car must constantly slow down and speed up as opposed to going flat out all the time.


just to clarify, this is true if using the same wire gauge.
For example a 6ohm stock arm does not have the torque needed to move my 6mag unlimited car. i need 1/2 an ohm at lest to get it moving and to make drivable.



> The answer is the track dictates that, and if you change the track then you have to start all over again.


not always
for t-jets we can run on big or small tracks just fine with a single ohm controller.

Many racer have purchased more advance controllers that let us use 1 controller for all types of cars. these may range in cost from $100 to $400.
I have a stage III and Theisen (same as lucky bobs') 
The theisen lets me run t-jets to 6mag unimiteds but adjusting voltage and not the ohms


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