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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.

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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ 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? \$\endgroup\$ – Fratink Apr 5 '18 at 14:47
  • \$\begingroup\$ 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. \$\endgroup\$ – Sparky256 Apr 5 '18 at 15:02
  • \$\begingroup\$ D4 would be equal to the expected minimum back-EMF voltage minus 2.5 volts. \$\endgroup\$ – Sparky256 Apr 5 '18 at 15:05
  • \$\begingroup\$ 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. \$\endgroup\$ – Fratink Apr 5 '18 at 15:19
  • \$\begingroup\$ 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. \$\endgroup\$ – Fratink Apr 5 '18 at 15:24

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