0
\$\begingroup\$

enter image description here

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.

\$\endgroup\$
6
  • 1
    \$\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
    Commented Feb 26, 2018 at 9:37
  • \$\begingroup\$ @Neil_UK VR4 = 1N5234 (500mW) \$\endgroup\$
    – Decapod
    Commented Feb 26, 2018 at 10:01
  • 3
    \$\begingroup\$ I think your schematic reconstruction is fawlty \$\endgroup\$
    – Andy aka
    Commented 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
    Commented 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
    Commented Feb 26, 2018 at 15:11

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.