Need some help with a potentiometer as a rheostat - UPDATE page 2 - Model Railroader Magazine

 Good morning!

Rich, thanks for the circuit.  It's identical to the one I was trying to build (with no success).  More on that in a minute.

Fred, the circuit you referred to is, I believe, Peter Thorne's throttle in the August 1976 MR entitled "The Simplest Transistor Throttle?".  

The circuit calls for a 2N6040 which after some checking, cross referenced to a TIP120.  However, the 2N6040 is a PNP type and the TIP120 is an NPN type transistor.  Does this cause a problem?  I used the TIP120 in the circuit and I got a constant ~11 volts output no matter where the pot was set.

Fred, I have a 4700 uf 35 volt cap leftover from another project.  Is that to large to use as a filter?

Again, I really appreciate your help on this project.

Tom