I want to build some sort of motor run-in station for breaking in motors w/out using a charger to do it. I want to use a rheostat or something similar to drop the 12v from my power supply to a variable 2-4V for motor break-in. Any help would be greatly appreciated.

In the following link, there is a schematic to build a variable PS.
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#317.gif
I believe that you should get what you want if you ignore the part with the AC transformer and the diode bridge. You will have to find a suitable potentiometer to replace R2, as this is the resistor that will control your voltage.
The whole circuit should cost you no more than 10$ in parts.

Hope it helps,

Martin Paradis

The problem with trying to build one with a 12 v source is to keep your 2-4 volts regulated under different amp loads.
No easy to hold 2 volts at 6 amps but also expect that at 10 amps.

I did some extensive research on variable voltage power supplys and came up with some military ones that were outrageously priced.

There not cheap -- but the best bang for the buck is the Competition Electronics GFX that you can adjust the voltage from 1-8 regulated up to 20 amps drawn.-------A great tool to ad to your test bench !

If your budget doesn't allow ---- use 2 cells with an automotive amp meter in line, roughly 2.4 volts.

R.S.

Here is what you need.... http://www.uoguelph.ca/~antoon/circ/pwm555.html
I made one myself and it work really well. it's variable from 0v at 12v. You won't have any problem to drive some motors because the Q1 is rated at 35amp.

Robert Lefebvre

The DuraTrax ICE charger has adjustable volts at up to 10A. Priced at $130 or so.

Not variable but a $25 PC power supply will put out about 30 amps at 3.3 V and 25 amps at 5 V.