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I am trying to repair an old SMPS. The first step is to reconstruct the schematic and work from there on. The schematic on the low voltage side is completed for the +12V and the +5V circuits. However what is the functionality of the SCR and Zener in the circuit.

  1. Since VR4 is a zener I have been thinking of the +5 going up over the threshold of 6.2V but that would not trigger the SCR.

  2. If the +5V is going down the zener would act as a diode and the SCR would trigger if the +5V line comes below GND. But in that case the zener functions as a diode.

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    \$\begingroup\$ comment rather than answer. The gate's connected to ground, normally VR4 (unusual ref des for a zener?) will be rev biassed by the 5v. If 5v goes -ve wrt gnd by more than 2 or 3 volts, the SCR will be triggered, either collapsing +12v, or recruiting the +12v to get the +5v back up again? Is VR4 beefy enough to do that? Interesting puzzle. \$\endgroup\$
    – Neil_UK
    Feb 26, 2018 at 9:37
  • \$\begingroup\$ @Neil_UK VR4 = 1N5234 (500mW) \$\endgroup\$
    – Decapod
    Feb 26, 2018 at 10:01
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    \$\begingroup\$ I think your schematic reconstruction is fawlty \$\endgroup\$
    – Andy aka
    Feb 26, 2018 at 12:34
  • \$\begingroup\$ @Andyaka You are right. I verified the symbol in the drawing database with the original datasheet and I found that the Gate and the Cathode are interchanged. \$\endgroup\$
    – Decapod
    Feb 26, 2018 at 15:00
  • \$\begingroup\$ Then it's starting to look like a crowbar circuit but you should update the diagram to be sure. \$\endgroup\$
    – Andy aka
    Feb 26, 2018 at 15:11

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