Hello, I Was wondering: I have a variable resistor (well, actually a tamiya mechanical speed controller, but they work the same) and its rated at 8.5vdc max, but I need to drive a motor with it that requires 12vdc (the motor draws 4 amps, and the var. resistor is rated at 200 or something so thats not a problem) But would anyone know how to make the resistor still drive the motor without burning it out? thanks for all help.

 

Well, the design tests probably rated the speed controller at 8.5 volts and no higher because the greater voltage may lead to an arc. I think the design specifications won't allow you to run a 12 VDC motor using an 8.5VDC speed control, simply due to the physical structure of the speed control (Contacts close to each other etc.)

I'm sure that there is a way to ramp the voltage up after the speed controller, but I'm not exactly sure how this woudl be done. Some sort of voltage pump, or step up transformer.

 

so even if I run it higher than 8.5 volts, It probably won't harm anything?

 

If you run higher than 8.5VDC, it may harm something because the contacts may arc. You'll need to ramp the voltage up after the speed control somehow, using a transformer or something.

so you have

8.5 VDC in---speed control----voltage ramp to 12 vdc---motor.

I wouldn't suggest

12vdc---speed control---12vdc motor

Maybe you could look at step up transformers and see if you can find one that goes from 8.5VDC to 12VDC. I'm not sure...

 

The only problem with using a transformer with DC voltage, is that it gives one shot of output power, not continuous output.

 

Why dont you just try it out? with one hand on the 'off' switch you should be able to safe it if there is a problem.
8.5 > 12V is not a big difference if the device can handle the 4 amp current of the motor.

Forget any suggestions made here about a transformer. DC and transformers do NOT mix!! Definitely a NO NO, transformer windings are like a short circuit to DC