I'll order a Tl431 and see if I can figure it out.You can add the PNP transistor to a TL431 programmable-Zener in the manner of a Szicklai pair.
You set the shunt voltage with a pair of resistors, make sure the added emitter follower PNP is enclosed in the nfb loop and the whole thing is as precise and sharp knee as the TL431.
I suspect the voltage that is causing problems it the DC input to the regulator, after the rectifier. It must be exceeding the input limit on the regulator when the charger isn't pulling current.If you have a 5V regulator suppling the charger, why do you need to dump the voltage it will still be 5V, the only thing that will happen is the input to the regulator will rise??
What is the output voltage of the motor?
Years ago I fed a 555 via its own boost inductor and chopped Vcc with a MOSFET and flyback diode. When I checked what the Vcc was boosted to - it was around 30V. Various datasheets gave figures from 16 - 18V.Everyone keeps asking that. A 7805 is rated for 24 V, but can withstand more.
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