I was thinking that if I kept it under 4 amps for the test, the 25W would be O.K., but I can't count on the students to not crank the resistance down all the way and have it overheat. For student labs I need to make it fool proof.

If you use a 1.2 Ohm fixed resistor in series with the rheostat then the maximum current through the rheostat will be 5 Amps. If you use the rheostat you found on ebay it will be fine a fraction of the 6V will drop across the fixed resistor and thus the power dissipated in the rheostast will decrease. The 1.2 Ohms fixed resistor has to be at least 30W in case students set the resistance of the rheostat to zero.

 

I have simulated the power dissipation in the rheostat and the results are shown in the attached graph. As it seen the maximum power dissipation in the rheostat if a 1.2 Ohm resistor is used in series with it is 7.5W. Thus, you can buy a cheaper and smaller rheostat.

 

Thus, you can buy a cheaper and smaller rheostat.

The power rating is down, but not the current.

The size of the wiper & smaller diameter resistance wire of the smaller rheostat might not be able to sustain a continue 5A current at the very end of the rheostat.

 

Thanks for the simulation! It's really nice that you would go through so much trouble for a stranger on an internet board.

I may just purchase the rheostat from Ebay and the proper resistors and experiment. It's worth the money for me just to see if I can make it work. I'll chalk it up to continuing education. The challenge for me is that I can run the equations and make it work that way, but practical application and limitations of the actual devices is where I don't have the experience. It helps to have that experience when you are pushing the limits of components.

I wish I could pack you up in my briefcase and take you all with me to the lab.

 

I wish I could pack you up in my briefcase and take you all with me to the lab.

You can. Get one of those smart-phones.

 

This is what you need. Add a voltage regulation prior to this and you basically have a bench supply like th $280 one, only not so fancy and probably not so stable/precise... Exactly what a school needs to start, I'm sure you don't need accuracy to 1uA.. http://freecircuitdiagram.com/2008/08/27/variable-adjustable-current-limiter-circuit/

If you do need a super stable supply, find a 28-32V/5A transformer and check here: http://www.electronics-lab.com/projects/power/001/index.html

This is nice, I just built an lm317 this morning to get me by, but I am working on this and getting close to finishing. I am going to build a carved oak case for it.