# In series resistor help

Discussion in 'General Electronics Chat' started by Deane, Mar 4, 2015.

1. ### Deane Thread Starter New Member

Mar 4, 2015
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0
Morning!

I race slot cars and recently hit a problem with resistors. The cars are controlled by devices with a trigger and wiper that passes over windings to increase speed as you increase pressure on it. This is a typical unit:

You can see the resitor and windings clearly at the top of the yellow one.

Now these resistors and windings are rated at 45 ohms, 60 ohms etc. Different rating suit different cars. Units where the resistance can be altered with one unit or are transistorized and don't use resistors so exist but their cost means many don't have those.

The problem we now have is that a new car has come along that works best on 120 ohms. They come in toy sets with small and cheap controller units not suited to our use. There isn't as yet a 120 ohm Unit of the type above. Even if there were buying another Unit for this one car is a pain.

Reaching back to my school physics I seem to remember that connecting resistors in series only ever increases the resistance, in parallel only ever reduces it. So my thinking is to wire in an external resistor of about 65ohms so that we can all carry on with our current units and still race the new car.

This diagram show typical track wiring:

The black wire runs to the windings and resistor represented by the squiggly horizontal line. So my question is, assuming the unit in the diagram is rated at 45ohms what kind of resistors should I be looking for and where do I wire it in to get the effect I am after?

2. ### #12 Expert

Nov 30, 2010
17,838
9,176
Two problems. 1) We can't calculate the power value because you did not tell us the voltage or current. 2) Adding a resistor in series will limit your top speed because the control can not short it out. (You would put the resistor in series with the wiper.)

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3. ### Deane Thread Starter New Member

Mar 4, 2015
4
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We mostly run on 18V power supplies.

But my understanding (which I realise is limited, hence being here) is that two resistors in series always adds the two together? 45 in the controller, 75 externally would result in the 120 this new car 'needs'.

As for the second resistors limiting top speed, that is over my head, which is why I am here also

4. ### #12 Expert

Nov 30, 2010
17,838
9,176
Trust me.

Your controllers are wired to have a certain maximum resistance and they short that out for top speed. If you put the resistor on the "brake" side, you won't have brakes. Anywhere else, and you won't be able to short out the resistor.

Still, there are other really smart people here. Give them a day to find out if they have a better idea.

5. ### Deane Thread Starter New Member

Mar 4, 2015
4
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I think I understand you. There is an area on the right hand end of the 'windings' that is smooth, which lines up with the wiper when the trigger is at max. That shorts the resistor.

So the 2nd resistor would do the job required most of the time, but you would lose top speed?

6. ### #12 Expert

Nov 30, 2010
17,838
9,176
That's what I said.

7. ### Deane Thread Starter New Member

Mar 4, 2015
4
0
Thank You. Bear in mind like most people I just pick up a controller and use it without understanding how it works.

I do vaguely get that the lower the ohm rating of the armature in the car's motor, the lower the rating of the controller needs to be have the real feel when using it, but until now that was about it.