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

 

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.

 

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

 

Switching diodes LL4148

 

Switching diodes LL4148

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 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

 

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

So do Shottky barrier types.

 

Yeah I meant to put that, I edited my post straight after but obviously too slow lol.

 

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.