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I have switch mode power supply (SMPS) rated for 32V 50 watt. I plugged it in main AC, checked its output then unplugged it.

Then to put the SMPS back into its casing, to be on safe side I (tried too) shorted 400V filter cap but accidentally shorted it with something else, the SMPS stopped working after that

Upon diagnostics (randomly probing components) I found a Zener diode completely shorted (aka closed) in both directions and behaves the same way out of the circuit board.

Zener have no other markings other than 24 written on it so can I assume its breakdown voltage is 24V? Also, what other components might have gone bad because of it.

My (blind) guess: it is used for some over voltage protection because I didn't find any other bad component on input or output side, the UC3842 also appears to be fine.

The problem is, I cannot find any 24V Zeners on similar electronic circuits like SMPS or UPS kits, what else can I replace it with?

Update: Circuit trace

Note: The SMD transistor is so tiny that I only managed to guess its marking using optical lens.

The cathode of the Zener is connected to Vcc.

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  • \$\begingroup\$ It could be part of the snubber circuit if it’s a flyback. Can you take a picture of both the diode and the PCB? Can you trace out a sketch a schematic around the failed diode? \$\endgroup\$
    – winny
    Oct 4, 2020 at 17:29
  • \$\begingroup\$ It is so congested that it is almost impossible to trace it @winny \$\endgroup\$
    – asim
    Oct 4, 2020 at 17:36
  • \$\begingroup\$ Please try. It’s impossible to guess otherwise. \$\endgroup\$
    – winny
    Oct 4, 2020 at 17:36
  • \$\begingroup\$ I tried my best and updated question with what i found @winny \$\endgroup\$
    – asim
    Oct 4, 2020 at 18:06
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    \$\begingroup\$ Unfortunately, without a circuit diagram, this is just a guess. Replacing Zener can help you decide if your system has stayed working or not. \$\endgroup\$
    – csabahu
    Oct 5, 2020 at 13:51

1 Answer 1

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It is working as Voltage Regulation for UC3842

You should use 24V zener

So let's understand how it is working When your tapped pin of transformer gets > 24V Zener start conducting and the 1K resistor sees Vtapped-24V on one of its end. The resistor sees 0.7V on its other leg because of its transistor base. The voltage drop on the resistor is (Vtapped-24V - 0.7V). If this voltage is >0v transistor starts conducting and the COMP pin gets 0V which disable the output signal for the Mosfet driver of your SMPS. If the voltage comes below 24 Volts it again starts the signal. This way it regulates the voltage.

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  • \$\begingroup\$ Removing the zener did not help \$\endgroup\$
    – asim
    Oct 5, 2020 at 11:09
  • \$\begingroup\$ who told to remove the Zener? Do you have any Zener diodes with you? \$\endgroup\$
    – Deepak
    Oct 5, 2020 at 11:18
  • \$\begingroup\$ it was faulty and was possibly shutting down uc3842, I do but non within the voltage range of 24v @Deepak \$\endgroup\$
    – asim
    Oct 5, 2020 at 11:19
  • \$\begingroup\$ Do you have any zener diode with you? \$\endgroup\$
    – Deepak
    Oct 5, 2020 at 11:20
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    \$\begingroup\$ We can make series connection to make 24 v or around it \$\endgroup\$
    – Deepak
    Oct 5, 2020 at 11:21

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