Need to identify SMC diode for repair project

Thread Starter

Simon Boyd

Joined Feb 9, 2016
10
Hi all,

I am trying to fix a board, looks like an easy fix as a screw driver broke a diode in half.
On my meter set to diode test I get a value of 278 on the good diodes. Picture attached.
I am not sure what diode to order to replace the broken one and the suspect one.

Any advise much appreciated.

Simon
 

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ian field

Joined Oct 27, 2012
6,539
Hi all,

I am trying to fix a board, looks like an easy fix as a screw driver broke a diode in half.
On my meter set to diode test I get a value of 278 on the good diodes. Picture attached.
I am not sure what diode to order to replace the broken one and the suspect one.

Any advise much appreciated.

Simon
If you've destroyed the diode, there isn't anything left to measure - and the diode test function should display a volt drop, not a resistance.

If you're sure the adjacent diodes are identical, you can take one of those off the board and measure it.

A Vf of anything under about 0.4V is probably Shottky-barrier. You have to watch out for reverse voltage rating - they start at only 20V and anything over 60V starts getting expensive.

Zener diodes come in closest to the much mentioned magic figure of 0.7V - if you've destroyed the original; you're probably screwed.

Standard recovery rectifiers are usually somewhere around 0.65V. Faster recovery diodes tend to have lower Vf - in extreme cases they can be hard to distinguish from SB.
 

Thread Starter

Simon Boyd

Joined Feb 9, 2016
10
I have a mirror of that board on the other side of the equipment.
Testing of the 5 remaining good diodes all give a read of between of 278 - 284
 

Thread Starter

Simon Boyd

Joined Feb 9, 2016
10
The TS has repeated what I assume are resistance readings for a meter that's claimed to be set to diode test after I've pointed out that normally produces a Vf reading.

My suspicions have been raised...........................
The reading is consistent with a germanium diode I believe.
As far as I can tell germanium diodes will give a reading of between 200mV and 300mV or a Schottky
 
Germanium diodes would be unusual in digital electronics, but I have to admit that we don't really know what the circuit is. Can you give us a good picture of the whole PCB?

From what I can see, the right-most two diodes (one of which was broken) look like they're doing some kind of rectification, or perhaps a diode-or combiner of two power sources. Or some kind of AM detector.

But my best bet would be on a 1N4148 SMT type like Dave says.
 
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