# Light dimmer for variable voltage



## jlong (Feb 20, 2010)

Anyone elese do this? Today I put my AFX wall warts on an outlet controlled by a light dimmer and now have variable voltage to my controllers for the sake of trigger happy kids. 

I have to wonder if this is hard on the wall warts, controllers, or cars for that matter. It has worked all day quite beautifully. Nothing has acted abnormal or gotten unusually hot.


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## roadrner (Jul 21, 1999)

A resistor is a resistor, which is all a light dimmer is. Since it's designed for use on a 110 volt AC circuit, using it to feed a DC power pack is interesting. Have never thought about doing it this way. I have always thought about putting a variable resistor between the DC power packs and the track/controllers. Keep us updated. :thumbsup: rr


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## jlong (Feb 20, 2010)

A light dimmer is actually a type of rectifier (SCR) that manipulates the AC sine wave somehow. I'm no electrical expert and don't know exactly how. It makes me wonder if the DC current coming out of the wall pack is pulsed to a point it's hard on the motors.


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## AfxToo (Aug 29, 2003)

Light dimmers can be variable resistors (potentiometers and rheostats) or electronic devices. The variable resistor types work on exactly the same principle as Parma controllers, they act as a voltage divider. The downside of a voltage divider is that as the current draw of the motor increases the power dissipated in the variable resistor goes up and the unit gets hot. As long as you do not exceed to power rating of a variable resistor based dimmer you will be okay. If you exceed the power rating the windings in the dimmer will melt and everything will come to a screeching halt. 

Semiconductor based dimmers work by converting the AC sine wave into a series of pulses or various duration (or pulse widths) and/or various amplitudes. The net effect is that the average voltage and current delivered to the load is effectively reduced by rapidly turning on and off voltage and current going to the load. Loads like incandescent light bulbs and motors are actually quite tolerant of being driven in this manner. Fluorescent lights, not so much. However, even for loads that are compatible with being driven by pulsed or "chopped" input instead of the expected sine wave, and especially as the ratio of off time starts to get high you'll start to see an effect, like flickering lights or a noisy motor. A secondary effect of chopping up a sine wave into pulses is that it generates many high frequency harmonics. These can be reduced through filtering, either explicitly with filter circuitry, or implicitly through the characteristics of the load. Motors are naturally low pass filters which is one of the reasons they are tolerant to pulsed control. Keep in mind that this is all based on having an AC load that is attached directly to the dimmer.

If you use a pulsed dimmer to drive a transformer, or AC-to-DC power supply, the power supply transformer helps reduce the negative effects of the pulsed drive signal as does the DC motor itself. Having some good filter capacitors on the power supply output will help even more. While not as clean as a pure DC source like a battery it is still a pretty good source for a slot car motor. I wouldn't be concerned about the residual ripple for run of the mill stock type motors. A low inductance $40 custom wind, maybe then I would have more concern and demand a cleaner power source.

If your goal is simply to have a variable output voltage at minimal cost, using a dimmer to power wall warts or a fixed voltage power supply is a viable option. What you give up over a higher priced conventional variable voltage bench or lab style power supply is what you'd be giving up anyway, like better voltage and current regulation, over current protection, better filtering, higher current capacity, etc.


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## shocker36 (Jul 5, 2008)

Before I bought my variable power supply I would just take a scrap tire and slid it over the trigger on the controller. Cheap and simple:thumbsup:


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## Pete McKay (Dec 20, 2006)

shocker36 said:


> Before I bought my variable power supply I would just take a scrap tire and slid it over the trigger on the controller. Cheap and simple:thumbsup:


Sounds like a restrictor plate.


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## jlong (Feb 20, 2010)

Thanks guys. I was speaking of the home center variety of light dimmers for incadescent lamps which are in most cases SCR's. I didn't think the resistor type was used anymore due to wasted power and heat generation.


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## NTxSlotCars (May 27, 2008)

Hey Too,

I used a large rheostat to to reduce the overall track voltage from a 24v battery pack.
It worked great when running only one lane, but with two or more lanes, it seemed to
amplify the resistance in some way. This was while running stock cars, nothing fancy.
The surge effect was also amplified, when any car came off, the other cars seemed to get twice the power.

Any thoughts?


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## jlong (Feb 20, 2010)

NTxSlotCars said:


> Hey Too,
> 
> I used a large rheostat to to reduce the overall track voltage from a 24v battery pack.
> It worked great when running only one lane, but with two or more lanes, it seemed to
> ...


What you are doing is running two cars branched in parallel but in series with one resistor (rheostat). You want to have one rheostat for each lane.

This is why. Ohms law states: amps x resistance = volts. 

In basic terms, when you put two cars on the track, you double the current draw (amps). Using the equation, the cars want to see the total voltage roughly doubled verses one car which is what you are doing with two cars. When one car leaves the track, the remaining car sees roughly twice the voltage. This is also why two cars on the same lane slow down considerably with one resistive controller. I say "roughly" because current (amps) is being drawn by the rheostat too which compounds the equation.

SCR light dimmers work on a different principle than resistors so it hasn't been a problem. There is some voltage drop with multi cars but that has more to do with an underpowered wall wart as it is no different than without the SCR dimmer. Otherwise, they would cause the same problem with a bank of lights when one burns out. They must be on the 120VAC side of the wall pack to work with slot cars.

I ran all day with the kids. They chewed up one crown gear and tore a bumper half off. No motor damage to report yet.


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## Grandcheapskate (Jan 5, 2006)

I use a Router Speed Control from Harbor Freight. Usually $20, sometimes they sell for $15. I have one 20v Aurora power pack per lane plugged into a power strip. The power strip is plugged into the RSC, which is then plugged in the wall. I have found it an effective tool for lowering the voltage when I run some overpowered/twitchy JL cars.

This must be a pulse device as AFXTOO explained. I connected a volt meter to the output side of the RSC and although I moved the dial up and down, the output voltage never changed. Yet the results speak for themselves. Plus it works on all your non-variable power tools. So it must not actually lower the voltage, but mimic doing so. 

The ideal solution would be one of these RSC units on each lane so you could tune the "voltage" to your personal preference.

Joe


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## AfxToo (Aug 29, 2003)

> I used a large rheostat to to reduce the overall track voltage from a 24v battery pack. It worked great when running only one lane, but with two or more lanes, it seemed to amplify the resistance in some way.


As already mentioned, using a single rheostat to control a variable load does not work well because the output voltage varies according to the load. In slot car terms, the effect you are seeing is exactly the same as you see when using a standard Parma controller with various resistance motors. For a low resistance motor you want a low resistance controller and for a high resistance motor you want a high resistance controller. In your setup, you are setting the rheostat (which is acting like a single "master controller") based on the combined resistance of 4 cars in parallel. This looks like a single very low resistance motor to the master controller. As soon as the load changes significantly, by a car coming off, the master controller is no longer set correctly for the load.

Imagine what would happen if you used a 45 ohm Parma controller and locked it at halfway with a 6 ohm superstock with fairly heavy downforce magnets. The car would go around the track at a certain speed, probably not super fast because you've noticed that you really have to move the controller a bit just to get the car to move. If you left the controller at the same setting and replaced the superstock with a JL TJet the TJet would probably fly off the track. That's exactly what you are seeing with your rheostat. In fact, even if you wired up one rheostat per lane (and with a battery power supply you have plenty of amps to do so) you would still have the same problem with different types of cars behaving differently on the same lane, though you would be able to compensate with your hand controller.

Note that this type of surge effect is uniquely different than what you experience with underpowered and unregulated power supplies.


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## NTxSlotCars (May 27, 2008)

Thanks for the answers guys. I knew it was something like that, but couldn't visualize it.
I have now replaced that system with four 6v batteries and a switch. 
I can now do 12v, 18v and 24v as desired. 18v is the favorite.

Thanks again.


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