Shorted diode in multimeter at battery supply

[M.Joshi]

Hello,

I have a fairly old multimeter that has a shorted diode near the battery input? Attached is a photo of the diode on the PCB.

What is the purpose of this diode (back EMF?) and what would have caused it to short out?

There are a couple of other similar looking orange diodes on the PCB that measure a forward voltage of 0.56V.

I can't see any markings on the diode, what is it likely to be?

Thanks.

 

[tronitech]

Probably a protection diode to prevent damage if the battery is connected incorrectly.

 

[M.Joshi]

Probably a protection diode to prevent damage if the battery is connected incorrectly.

Thanks, though I thought reverse polarity protection diodes are usually in series not parallel?

How would I find a suitable replacement?

 

[Nigel Goodwin]

Assuming it is a protection diode in parallel with the battery? (protection diodes can be either series or parallel - and series reduces battery life), then it's just a simple rectifier diode, any small rectifier (1A or so?) would do as a replacement. Also, simply removing the S/C diode would make the meter work again, but would give no reverse battery protection.

As for why it's failed, it's because the battery has been connected the wrong way round, even if only very briefly.

 

[M.Joshi]

Assuming it is a protection diode in parallel with the battery? (protection diodes can be either series or parallel - and series reduces battery life), then it's just a simple rectifier diode, any small rectifier (1A or so?) would do as a replacement. Also, simply removing the S/C diode would make the meter work again, but would give no reverse battery protection.

As for why it's failed, it's because the battery has been connected the wrong way round, even if only very briefly.

Thanks Nigel, that makes sense. Yes removing the shorted diode has made the meter work again

I was thinking of replacing it with an LL4148 which looks like the same form factor. How does it defer from a standard SOD-523F-2 package diode?

 

[Nigel Goodwin]

I presume an LL4148 is just an SM version of a 1N4148?, which is a small signal diode. The diode is there to fail S/C and protect the meter if the battery is connected the wrong way round - so it has to be small enough to blow with the power from the battery, but large enough not to blow to pieces. So it depends if the LL4148 would survive without blowing to pieces?, and to be fair it should never have to, just be careful how you connect the battery.

 

[M.Joshi]

Yes, the LL4148 is the surface mount version of the 1N4148. I selected it as it looked similar to the shorted one.

Should I be using a general purpose diode such as a BAV100 instead?

 

[Nigel Goodwin]

I don't know, I've never done any tests - stick it in and see what happens, you'll only find out if the battery is connected the wrong way round again.

 

[sekemiat ke]

from my thought, it is a protection diode against shorting anytime you connected the battery wrongly. The diode might have been shorted hence the blowout.