Help me identifying a diode marking

Hello,

If you could re-post, mentioning the make and model of the item the diode belongs to, and the circuit location, it would help all identify the part for you, as someone might have the schematic, and be able to look it up.

Regards, Tim Bristol Electronics

Can't you check a zener diode with a bench power supply and a couple resistors?

Not if it is "short circuited". (ducking)

John ;-#)#

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Diode #ZD1

It's a Zener for sure, since it's labelede ZD1.

** Sure - but the OP has a **short circuit** one !!!

For replacement purposes, you must know what the original part is OR what it should have been - cos one occasionally comes up against incorrect parts fitted in PCBs and even incorrectly marked parts.

.... Phil

OP said it was shorted...

Hello all,

I came across this link:

While it does not give a schematic, it does talk about the power supply, and from what I see there is no 18V supply. (See the "Details about main board" section)

Regards, Tim Bristol Electronics

looks as though you will have to trace out enough of the circuit to decide whether a zener of too low a voltage or too high a voltage will cause colateral damage. Then suck it and see with some values starting from the safe end and note what happens

Well, it could be a "suicide diode", if the power supply regulation fails, the diode quickly fails to short and either the PS goes into overcurrent limit or blows up as it isn't limiting anymore. Many cheap Chinese PS modules now use these suicide diodes instead of a more traditional SCR crowbar circuit.

They actually have a special diode made for this purpose.

Jon

Would "suicide diode" be a valid term for searching within a supplier site?. I've come across guaranteed VTS/diode failure to short and never open, before , but not able to nail it down. Would you know of any VTS/diode type number that is definitely made for this purpose?

TVS not VTS. I just tried an assorted set of search terms and yet again have not found a datasheet for a named TVS, guaranteed (even if only to some probability) to fail short circuit

No, that's my own description. Digi-Key part SA5.0ALFCT-ND is a representative part, although not the one I've seen in a particular power supply. (I can't find the part number of that unit right now.)

Jon

I still cannot find specifically stated , designed to fail short circuit in any associated datasheet for that TVS. Of course any device might fail short or even frequently fail short but I've still never found any TVS designed and stated as such to fail short for safety purposes, ie as you succinctly term such a theoretical device , a suicide device

** Semis like zener diodes and power transistors fail short when over voltaged or overheated and the melted silicon chip does not repair itself. So, the only way it can go open is if a feed wire fuses after the event.

Zeners ( even 400mW ones) have the chip fitted between a pair of copper slugs so there is no feed wire.

Notably, Hitachi lateral mosfets in TO3 pack have aluminium gate and drain feed wires designed to fuse if the chip ever went short - creating the unique situation where one Hitachi mosfet in a parallel group could become short ( drain to source) and yet the power amp still functions.

One then picks out the dud by noting it is much cooler than the rest.

... Phil

Panasonic vcr's from the '90s had ordinary 18v 1-watt zeners across the output of the 15 volt line. When the supply lost regulation, usually due to bad caps, the zener would short quite reliably. This was supposed to blow the fuse but generally took out the switcher transistor as well. Sony's were better designed - the zener would short, and the switcher was so far out of it's resonant range it would simply stop running with no other damage.

Mark Z.

Yes, often such a device will fail short and even anecdotally a lot of recorded times. But can a designer go to some supplier and get a TVS or similar that is stated in their datasheet for that device, that it will fail short circuit with 99.99 percent guarantee or whatever of overload tests. Then the designer ,allowing for such peak current in pcb traces etc will know that his appropriately rated fuse elsewhere will go open circuit?