Electronics > Repair

Identify zener in a SMPS feedback circuit?

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Pinball_Newf:
Hi, I've a blown zener in a plasma cutter with no markings.  Is SMD glass package and is used with a PC817 optoisolator to provide feedback [to a Viper22A PWM controller].  The input voltage to the zener measures 7.4V.  Typical forward voltage of PC817 is 1.2V The thing has zero markings except the blue stripe. 
Is there any way to identify this or any suitable replacement?

magic:
Whatever you install, your new output voltage will be Vz+1.2V or something like that.

If my guess is right, you need to figure out exactly how the feedback circuit works and find what's the proper output voltage from this PSU, then select the right part to achieve it.

edit

--- Quote from: Pinball_Newf on February 29, 2024, 04:40:43 am ---The input voltage to the zener measures 7.4V.
--- End quote ---
What do you mean by this?
A schematic of the PSU would help.

Pinball_Newf:
Hmmm, I can attempt to reverse engineer it. The diode is reverse biased to drive the optoisolator's LED...  So presumably I'd select one that has a Vz of whatever PSU voltage it should be [say 5V] - the 1.2V needed for the optoisolator. Am i thinking this right?   Basic digital electronics are more what i do, so i am a bit out of my element here...

fzabkar:

--- Quote from: Pinball_Newf on February 29, 2024, 07:44:40 pm ---Hmmm, I can attempt to reverse engineer it. The diode is reverse biased to drive the optoisolator's LED...  So presumably I'd select one that has a Vz of whatever PSU voltage it should be [say 5V] - the 1.2V needed for the optoisolator.
--- End quote ---

That would be correct ... if the zener is directly connected to the rail that it is trying to regulate.

Pinball_Newf:
It's connected to a dedicated tap on the secondary side of the transformer.  That tap goes to just over 7V before the PWM controller cycles on undervoltage.  I'll need to determine the ratio of this tap vs. the one for the main rail and go from there.

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