We have a 2 wire (+/-) dimmer input to a ballast that presents about 11V unloaded. Onto these terminals we hook up a simple circuit that basically consists of a NPN transistor (+ to collector, - to emitter) and a manual rheostat (one end to collector, the other to the base) which is used very successfully to vary the voltage across these terminals between 1 and 10V. I am aware that the NPN is operating in the active region as a variable resistor where full resistance of 470K between C and B puts about 0.35V on the base (for 10V across CE) and least resistance puts 0.5V on the base (for 0.5V acorss CE). The idea now is to replace the manual pot with a digital pot controlled by a microcontroller and I have done that. But the digital pot I need to use is half the ohmage of the original manual pot (200K) and the best I can get out of the circuit for V range is 0.5V to 7.5V. Adding a second digital pot in series (to get the full original pot resistance) makes it worse as, I assume, that the base current through the NPN is affected by these DP ICs. So this is where the fun starts. I've replaced the NPN with a darlington pair to try to enhance the current gain and thereby get the full V range back but this has not worked. I even tried two darlington pairs and the best I can get is 0.5V to 9.3V with the original high R manual pot. I've tried all kinds of configurations and biasing as well but no success on the high end of the V range. I'm not sure why this circuit will not go far enough into cutoff to be fully turned off and present the full V range at the terminals?? So two requests here: - can anyone offer any guidance/advice on how to make this circuit work using a digital pot (of lesser ohmage than the original rheostat)? - can anyone suggest alternatives (like how do I use a JFET or MOSFET for this particular circuit)? I appreciate any help. Thanks.