I've been trying to get a motor regen power dissipation circuit working that I pulled from here. The circuit works as provided but when I attempt to modify the load resistor, the mosfet is not longer being switched at 10-12V, it generally falls down to 5V or so. It appears that the BJT isn't being driven fully and thus the mosfet is not being driven fully. I'm at a loss as to why this is the case, can anyone explain this?
You need to swap the emitter and base of Q1 around (including base resistor), so that the Vrop across D1 is enough to enable Q2. Q1 MUST have a base voltage at least .65 volts lower than the emitter for it to work and drive the MOSFET.
The following circuit will dump current based on a voltage higher than D4 + 4 volts. At a lower voltage it has no effect.