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Replacement diode/zener for ITT Metrix Lab Power supply

Lucy Electronic

Mar 11, 2022
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In the process of trying to repair an 0-30V (2.5A max) adjustable power supply. ITT Metrix 321

Output terminals seemed to be shorted to each other, current limit would engage on power on.

There is a diode or zener on the pcb connected between the +ve and -ve output, I unsoldered one side of the component, and the unit now powers on without the current limit engaged. So I'm guessing it's blown and shorting.

The component says M219 (and looks like a Motorola M) and has ring at one end making me think it's a diode or zener. I'd like to replace it evidently.

1) With no schematic available, what would be a reasonable guess as to what specs that component would be?
2) What function would it have? I understand zeners are generally used to stabilize voltage but I don't understand how it would work in the context of a variable supply

There's another M219 elsewhere on the PCB which I could pull and try in a cheapo component tester, with the hopes of determining some specs, but I'd rather not unsolder working components if possible.
 

Harald Kapp

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Nov 17, 2011
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With no schematic available, what would be a reasonable guess as to what specs that component would be?
max. output voltage of the power supply + 10 %: 33 V or 36 V in this case
What function would it have?
Overvoltage protection. In case of a failure of the power supply that results in output voltage rising above the nominal max. value., the zener will limit the voltage, so will the current limiting that sets in with the zener becoming active - as you noticed.
 

Bluejets

Oct 5, 2014
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Not a normal approach to parallel zener diodes.
Associated resistor usually chosen to ensure zener operates in the correct current region.
 
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