View Single Post
Old 27th Jul 2018, 8:24 pm   #9
mole42uk
Nonode
 
mole42uk's Avatar
 
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
Default Re: Gould 4072 bad PSU

I agree Dave, I can't make sense of it either.

The only thing pulling pin 10 down is R41, a 5k6 resistor but if D16 has 12V on the cathode, then the 5k6 won't do much. there is a transistor which, if turned on, will shunt the D16 voltage towards ground but I can't see why it would be turned on.

I've looked at several datasheets for the 3524 chips, and I used them in a commercial item I designed years ago, all seem to indicate the pin 10 is active low, in other words, if it is low the chip shuts down. Perhaps I've misread the data and it's actually active high - in other words, if pin 10 goes above 0.5V the chip shuts down. That makes more sense, the internal circuits I've seen for the 3524 show an NPN transistor with it's base connected to pin 10, collector to the output of the error and current sense ampolifiers and the emitter to ground.

If the pin 10 sense is active high, then the circuit around Q10 makes more sense, if the line voltage, nominally 340V, goes too high, then the 75V zener-controlled base of Q10 will allow the voltage across R40 to rise. D16 is still a fly in the ointment, though, since it would seem to deliver 12V to pin 10, high enough to shut down the 3524, at all times.

I think I'll have to do some more testing and probing although I don't want to use 340V until I'm reasonably certain how the circuit is supposed to behave.
__________________
Richard

Index:
recursive loop: see recursive loop
mole42uk is offline