fechter wrote:Good work! I wish we could get a schematic of that.
Thanks! I have a hand-drawn pencil and paper one here... Anyone wants to feed it into a computer for an electronic version? Otherwise I can scan it but it might not be too easy readable.
Most components I noted the value, some include the reference number so you can find them easier on the board.
Almost all of the schematic is straightforward what you would expect, except that some choices are made to use specific inputs and not others - the VFB input of the PWM controller is not used, all feedback is tied to the COMP input.
The voltage and current limit are combined (using U100 and two diodes) into one feedback opto (U2) and the over-voltage protection as well as the over-temp protection share the other opto U3.
The only quite tricky part is the use of a single thermal sensor RTH2 to both turn on the fan (when the voltage on the OTP input to SHR2 has risen to 2.5V) as well as detect overheating (when the OTP voltage has risen high enough to turn on opto U3 through R153 and D121, which triggers SCR1 so the voltage on the COMP input is shorted and the controller shuts down. One of the other smart things is that the over voltage/temp protection will cause a hiccup operation, because SCR1 is tied to a supply voltage from the transformer. Once the supply cap C36 that also feeds Q5 has been drawn down far enough for SCR1 to let go, the controller will restart and the protection cannot engage before the transformer has put some charge in the caps again.
fechter wrote:It would be important to find out if the current limit can be lowered with R134 and maintain stability.
I see no problem with that, because R134 is essentially a current source (or voltage divider if you will) to lift the reference voltage from the shunt resistor by 59mV to keep it above negative output reference, which is the level at the positive opamp U100 input pin 5. As soon as current gets too large, the shunt drops more than 59mV and since it is in the negative output, the upstream side goes more negative than 59mV so the +59mV voltage shift by R127 & R128 no longer is positive and the negative input pin 6 being lower than pin 5 means that the output becomes positive which fires the feedback opto U2 through D122. Reducing the current through R134 will reduce the lift by R127 and R128, so the voltage goes negative at a lower current through the shunt. In theory you can turn the current limit down to zero. I need to fix this NES before I can try that experiment.
To avoid blowing it right away, I want to test as much of it first, so I will feed voltage to the output of the secondary rectifiers to see if the circuits after them work as expected (2.5V references and such) then check the PWM controller by supplying power via Q5 (which is an emitter follower with a zener on its base)
then measure all semiconductors for dead shorts and finally apply AC and see what happens...