Testing Zener - With a Multimeter

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brunofunchas

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Hi,

I came across a faulty Zener diode in a SMPS. On the same board I saw one Zener, with the same colour code. A reverse bias it, with my Fluke 289, I thought that with was a 2Volt Zener since I saw on the meter 2V in diode mode.

Wrong! The multimeter does not tell us the reverse voltage of the zener!

I have made this mistake an hope that you do not make the same mistake that I did!

Regards,
Bruno
 

A zener diode can not be tested with simple multimeter.You need to reverse bias it limit the current with appropriate resistor.then you can measure the voltage across it and it will be the break down voltage of that zener diode as printed on the diode.
 
Hi,

This Zener is from a SMPS Matra ADS850. Can anyone help me in identify it's value throw the colour code?

Thanks in advance,
Bruno

 

Hi,

This Zener is from a SMPS Matra ADS850. Can anyone help me in identify it's value throw the colour code?
Click to expand...

These are MELF/ Mini MELF package.There are color coded according to standard by few manufacturers and not by all.Many manufacturers use custom color coding system.Chinese made are more difficult to identify because they don't follow standards most of the time.

The picture you posted is of very low quality and it is not possible to know the color band on those diodes.To me the color band looks like golden.is it?
To identify the sequence of color bands one should start with the color band which is closest to either side of diode.This band will be there to indicate the polarity too of the diode.
 

The colours bands are Yellow (bigger one) and Black. Perhaps it is Golden/black. Does is it identify any diode?
 

Never saw the yellow or gold band on such diodes.May be the gold band is for polarity only.If so then it may be either a zener or just fast switching diode (only black band).But i can't tell for sure.What is the name of this component on the PCB.It say ZD.... or D... or some other word?
one more thing the gold color band may be for tolerance 5%.If so then these are definitely zener diodes.
 

The upper one diode has just one Yellow band and the Black seems to be the GAP between the two contact and not the Color Band. Here is a link giving very nice detail. May help you:
SOD-80 diodes
 

It has no marking on the PCB. I put an normal zener 2V and the SMPS works but the output of the 5V is at 6,2V.

I think I will have to pass on this one.
 

Then I think it is not a Zener diode but a switching diode. Try connecting one 1N4148, may be get the right output volatge.
 
Yes I think thats a right choice. I will be waiting for your reply whether it worked or not for proper output volts.
 

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