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  #1  
Old 05-19-2017, 11:38 AM
A/Aracer A/Aracer is offline
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Default Re-wiring speed controllers-need advice

I have old Atlas HO speed controllers that I want to convert for 3 volt operation and not the 18V they are used to. It can be any current cheap plunger rheostat type for that matter, I just happen to have the Atlas. I'm not an electronics guy but I bet it is all based on the number of turns of the wire around the round ceramic cylinder. Any help would be much appreciated. see photo.
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Old 05-19-2017, 06:58 PM
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RichD RichD is offline
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Actually Atlas controllers were available in several resistance values. I once had a 45 ohm one and I still have a pair of 85 ohm models. There may have been 25 ohm ones as well. What are you trying to run at 3 volts? In your case a 15 ohm resistor might work, Parma sells those, but they are much too long to fit in an Atlas controller.
If you wanted to wind your own resistor nichrome wire would be the thing to use. There is a chart here: https://en.wikipedia.org/wiki/Nichrome
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Old 05-19-2017, 07:40 PM
BIG E BIG E is offline
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Rich -

I believe some of the early ELDON/Ungar sets were 3 volts and then later 6 volts, if memory serves.
I'll ask for your expertise here - the voltage of the power supply shouldn't matter, just matching the resistor to the cars as they perform at that voltage, correct?
Love those old ATLAS plunger controllers, especially the original design like the one pictured. They were my "weapon of choice" back in the 1960s days of the Aurora Thunderjet 500 wars, and you could get them to operate pretty smoothly. Still got a box with a few dozen of them, and can't resist buying more every time I see them for sale at the right price - they were made in so many different colors!

Enjoy -- Ernie :>)
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Old 05-20-2017, 12:01 AM
Al's slotracing Al's slotracing is offline
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Working out the resistance is easy enough (see below) but as has already been said, how much resistance you need depends on what you are driving.
To control the same motor at lower voltage, a lower resistance controller is needed.
The same motor at lower voltage will be slow. To get the performance motors for lower voltage typically have a lower resistance. Generally that means an even lower resistance controller is needed.
The bottom line is you are likely to want a lot lower resistance for the low voltage. Without knowing more about what you want to run on 3 volts, what controller resistance is a wild guess

Quote:
Originally Posted by A/Aracer View Post
I bet it is all based on the number of turns of the wire around the round ceramic cylinder.
The resistance depends on the number of turns, the length of each turn, the thickness of the wire and what the wire is made of.
If you want a formula resistance = number of turns x length of each turn x resistance per unit length. Rich's link gives the resistance per unit length for nichrome wire.

Controllers are usually wired with nichrome wire, although other materials such as constantan are sometimes used, for very low resistance copper is sometimes used.
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Old 05-20-2017, 07:13 AM
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If it was me before I attempted to wind my own resistor I would try out a factory one. If you have a Parma controller hanging around you can buy 4, 7, 15 and 25 ohm resistors for those.
Do you have to use a thumb style controller? I turn trigger style controllers around and use my thumb.
The hitch with doing your own resistors is that the turns of wire should not touch. Once you get the wire on the ceramic core you need to coat it with something to keep it from moving around. I would use high temperature exhaust manifold paint.
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