SMT Diode Identification

The stripes look grey and red, so I guess that might make it 8V2, which is a standard zener value.

Arfa

Well there is a black stripe in the middle does that matter? I enlarged the picture.

Is this color chart I found correct?

Thanks,

-Mike

Is that black stripe a genuine paint one on the outside of the diode, or a "ring of fire" where it's failed ? It's right over where the actual diode inside is. I'm assuming that it has failed short-circuit or whatever, otherwise, you'd be able to just measure the voltage across it ?? I'd still have money on it being 8V2 as this is such a common value. Are there no other indicators as to what voltage it is ? Voltage that it's zenering down from, value of series reistor, working voltage of any following electrolytics, markings on the board of power supply voltages etc ??

The chart appears to be correct.

Arfa

Yes I believe it is genuine paint. It is on the other diode which has not failed.. I guess I could find out what kind of zener diode it is by testing it as you said, but I have never done this before.. I guess it is time to learn.

Here is a picture of the working diode (ZD2 not ZD1) up close and in the sun so you can really see the colors..

Close-Up

Thanks,

- Mike

OK. Looks like a genuine paint band. I'm still up for it being 8V2 though, if for no other reason than it's a very common value, and corresponds to the other two stripes. If what ever the unit is, is working at all, then just check the voltage across the non-blown diode. If the unit is not working, you can test the other diode for voltage rating easily enough, by removing it from the board and hooking a resistor - say 4k7 - to its cathode. Then put 12v across the combination, "+" to the free end of the resistor, "-" to the diode's anode, then measure the voltage across the diode. Any reading up to about 11v, will be the zener voltage. If you get a reading of 11v or more, then the diode hasn't 'zenered' and you need to up the voltage across the combination a couple of volts at a time, until you do get a steady voltage, that doesn't increase any more, as you continue to up the voltage. You've then found the diode's zener voltage.

Arfa

I was at the local surplus dealer and they had the zener diodes organized by zener voltage. Would a 8v2 be a 8.2v zener? Thats what I got and it seems to ohm very close to what the good zener ohms.

The actual part number for the zener that I got is 1N5237b738.

Thanks for all the help Arfa

- Mike

Zener would have failed for a reason. If you replace it most likely the new one will go that way too.

Peter

Yes, 8V2 is eight point two volts. It is the de facto way of writing it these days - at least this side of the pond - and saves any confusion with flies crapping on the page, or your glasses needing replacement. It is very easy for a decimal point to disappear as a result of poor printing, or one to appear because of that fly ...

It is also the standard way of writing resistor values - 4k7, 1M2 for 4.7k or 1.2M ( see how easily that 4k7 could be read as 47k ). Capacitor values appear like this too. 6u8 for 6.8uF, 2n7 for 2.7nF, 0u1 for .1uF, 3p3 for

3.3pF etc. On some schematics, you will find voltage checking points marked in the same way.

The resistance of a zener diode, is no indication of its zener voltage, however. There are some exceptions, but in general, any small low voltage zener will read the same, ohms-wise, as any other, and indeed, any other ordinary diode, because this is all that they actually are. All diodes exhibit zener effect when reverse biased. It's just that with zeners, the doping of the semiconductor material has been calculated to produce predictable and accurate reverse breakdown voltages.

Arfa

In 35 years of being in this business, I don't believe I can think of more than a very few instances where a small zener diode has failed for any reason other than poor design of the circuit that it's in, or just chance failure. Very occasionally, a zener is placed across a supply rail as an overvolt protection mechanism, and will fail if that rail rises much beyond its nominal value - because of a failed series pass transistor, perhaps - but normally, these are large 1 watt types, that will stand going short circuit without exploding first, not little glass surface mount types, like the OP's example. Also, rarely, a zener in a conventional regulator driver stage, will fail if the transistor that it's connected to, goes collector-base short, but this is a comparitively rare failure mode for transistors.

I think that it is highly unlikely in this particular case, that there will be any problem, other than the diode itself.

Arfa

I tested the good diode on a 12vdc transformer. The transformer had an output of 16.7v and the diode "zenered" at 11.75v... so does that mean it is a 12v zener?

- Mike

That would certainly indicate a 12v zener. This is another very commonly found value.

If that is the case, then the coloured bands on this particular diode, would seem to be meaningless. unless they are signifying a diode type that contains the numbers " 802 " or " 208 " or it's some kind of house code. If the voltage definitely settled at

11.75v, with a 4k7 series resistor, then the only other check that I might do, if it was me, would be to drop the value of the series resistor to perhaps 2k2, to increase the zener current. Sometimes, a zener will not stabilize to its nominal rated voltage, if the zener current is too low. If you still get around the same voltage, then you can assume that 12v is the correct value.

Arfa