Thanks MrAl. I am having a problem with my circuit; let me give you a background.
I have a LiIon battery, which has discharged very low; the charge chip will not charge it due to it's voltage. I am trying to make a variable current source so that I may bypass the battery's charge circuitry and charge the battery (no lectures on LiIon charging please, I am familiar with their charge curves/dangers).
My circuit is a pwm output --> low pass filter --> darlington pair [control charge current] --> Shunt --> ground.
I am testing the circuit with an automotive light bulb for now. When I have the shunt (toaster) in place the circuit does not work. When I remove the toaster, I can dim the bulb at will using the transistor (the max intensity is about the same as without the transistor). When I remove the transistor from the circuit, the bulb illuminates (albeit at a lower intensity due to the resistance of toaster).
I have tried this so many times to make sure I'm not making a mistake, what gives? Why doesn't the bulb illuminate with the toaster AND transistor in the circuit but does illuminate with one or the other in place?
Stumped.
EDIT: DOH! I just figured it out, so simple but it stumped me. The toaster is acting like a divider, the voltage drop at the toaster must be greater than 5-0.7-0.7. I can't believe I didn't realize this problem. I don't know a simple way to get around it, I could put the toaster before the transistor (which would take out the need for a Ibe current correction). The problem is then there could be modes of operation where the voltage across the shut would be >>5v which will fry my ADC.