undervoltage lockout for flyback powersupply input

here is the schematic:

cheers, Jamie

For simplicity reasons, Get your self a DIAC rated around 75 volts connect one end to the cathode of D34 and the other end will be the V+ supply for R5 instead of connecting R5 directly to V+.

Using a comparator circuit would also be fine how ever, you would need to use a voltage divider and protection zener to make sure you don't over do the input. The output could then simply control the base of that transistor switch you have.

Never used Diac's before, from what I read it looks equivalent to back to back zeners except the DIAC stays on until the current drops below a threshold, is this correct? Would back to back zeners, work for this as well?

cheers, Jamie

Oh nevermind, I read a diac is just like a triac but without the gate.

cheers, Jamie

Yes, you have it correct, the DIAC are a cheap way of closing a circuit and opening it how ever, depending on the spec's, You need to tailor the circuit so that the holding current will drop below spec at the desired point to have it open again.