so I have this diode that is under a transformer on a battery charger. I was looking for faults on it with a thermal camera and noticed the one next to it was glowing at over 120C . I removed the hot one and it is pretty much shorted as shows 0.03 in both directions with the diode test. Iv also got a peak tester dca 75 and it won't even identify it as a component.

The one next to it is marked with V21 and is just a regular looking brown / orange looking colour and is glass. I would go on what the dca 75 says, but each time it tests the working one, the vf goes up.

It's been so hot that it's burnt 99% of the markings off it, but it looks to be paired with the one next to it. The problem is I can't seem to find a replacement for it. "V21 Zener diode" just brings up an smd diode sot 23 package and not a through hole glass diode like this.

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  • 2
    \$\begingroup\$ Can you draw a schematic and post it? You can generally come up with a replacement if you can figure out the circuit. V21 is probably an in-house number. \$\endgroup\$ Apr 22, 2016 at 2:49
  • \$\begingroup\$ Ya Vince Patraon is right .. its just an in-house number. The diode is a simple zener diode. \$\endgroup\$
    – Honeybee
    Apr 22, 2016 at 6:14
  • \$\begingroup\$ I am guessing the diodes are a two diode rectifier for one of the transformer windings as the low voltage side is not working. \$\endgroup\$ Apr 22, 2016 at 10:01

2 Answers 2


SMD devices typically are marked with random letters/numbers, but through-hole devices usually try to use markings that resemble the actual name.

"V21" sounds like the BAV21, which is a general-purpose diode made by many manufacturers.

  • \$\begingroup\$ Thanks, that makes sense as I think it's 250v coming in, have ordered a few of this's and will see how it goes. \$\endgroup\$ Apr 22, 2016 at 14:19

Looks like a standard IN4148 diode. Google for it and see pictures. If it is hot it means a lot of current is going thru. Check your schematics.


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