Motor regeneration Circuit Issues

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?

Attached are the LTSpice simulation files.

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. simulate this circuit – Schematic created using CircuitLab

• thanks for getting back to me! I was unclear as to what I meant by load, which in this case would be R1. The circuit I want will dump energy provided by Vout (a motor being backdriven) through R1 when Vout exceeds a particular threshold above Vin. If I'm right in understanding this circuit, it will dump load when Vout is less than Vin and not the other way around, correct? – Fratink Apr 5 '18 at 14:47
• Then you need to use MOV's or Sidacs to clamp the over voltage. If you know what the exact 'over voltage' point is then a zener diode can be used to turn ON M1. – Sparky256 Apr 5 '18 at 15:02
• D4 would be equal to the expected minimum back-EMF voltage minus 2.5 volts. – Sparky256 Apr 5 '18 at 15:05
• That's pretty much perfect. I liked the idea (at least what I understood to be the idea) from the original circuit by @Trevor_G in the other post in that it would always trigger based on a differential voltage and thus work for any supply voltage. But, this circuit is very stable, showing no oscillation compared to the original or to a similar comparator based circuit. I like it. – Fratink Apr 5 '18 at 15:19
• I suppose the only problem might be transistor heating when in the linear region. Either bigger mostfet or a schmidt trigger should deal with that though. – Fratink Apr 5 '18 at 15:24