IDing transient voltage supressor

[bbjunkie]

Hi Folks.

Have a dehumidifier here that packed in a few years ago.. decided to see if I can get it going again. (totally dead)

Tracing through the power supply from the input I found a halfwave rectifier built with two diodes and another item which, after googling I believe to be a transient voltage supression diode. The symbol on the PCB below the device is that for a diode, but unlike the other diodes on the board it doesn't have a circuit reference like the others, i.e. D1 and D2

The markings on the strange device are:

EIC 5L
P18CA

My question is, how do I test this device? Should it test like a regular diode? (It is reading open circuit both ways on resistance and on diode test mode is also showing open circuit)

If it's faulty, then I will need to source a replacement so any suggestions there would be appreciated too.

Cheers for now & thanks again for a great forum

Jim

[caryg]

It should test like a regular diode. These sit backwards in the circuit and most of the time do nothing, bar absorbing any minor voltage spikes. But if a too high voltage comes through they commit suicide and die as a short circuit. Current then passes straight through that rather than the more sensitive circuitry it is preventing. It's a bit like a reverse fuse in that it conducts when it has blown, so if it's showing open circuit then it needs replacing.

Well actually, if you are feeling brave and you trust your power supply then you don't need to replace it. Just lift one end, you break the short that it creates, and your dehumidifier should work again. Though I've never dared to myself.

If you're in the UK then RSS is the best place I've found to get them. Their minimum order (sometimes 25 or 50) is usually cheaper than one or two via ebay, and they are more likely to be genuine too. http://uk.rs-online.com/web/c/semico...archTerm=diode

[bbjunkie]

Originally Posted by caryg

It's a bit like a reverse fuse in that it conducts when it has blown, so if it's showing open circuit then it needs replacing.

Now im confused, this statement doesn't make sense.

[Krankshaft]

I'm sure the poster meant if it reads as a short circuit.

A working diode reverse biased won't conduct. But if a large enough spike hits it the PN junction can be destroyed allowing current to flow through it in the reverse direction. In other words the diode becomes a jumper wire.

[Toasty]

A transient suppressor diode will read open. You can't generate a high enough voltage with a DMM to cause it to conduct.

Usually placed after a fuse, they may not "die" as a short, but a high enough voltage surge and power (in joules) may cause them to pop the fuse. They "reset" after the surge goes away. Much faster reaction time than an MOV.

Functionality is similar to an avalanche diode.

If it reads open, it's not the reason the dehumidifier is not working.

Toast

PS: Although the board may indicate a diode symbol, that's usually a factory thing so the placement matches the packaging. The actual electronic symbol for these looks like 2 diodes with a single cathode plate between them: ->|<-

.

[bbjunkie]

Ok, all clear now, thanks.

Discovered a short circuit zener diode, think that's whats causing the problem

[bbjunkie]

Replaced the 3.9v zener and all is well.

Thanks guys for the education.

Jim