Zener diode test with dvom

[eyoung]

Hola Y'all,

My question is there a quick test to check a zener diode to see if its good or bad?? I checked a new one and it measured open in one direction and several meg ohm in the other direction and the zener in question read ~75 ohm in both directions which I suspect is bad. Your comments are welcome.

Regards, Elwood

[Steerpike]

Most 'Zener' diodes are in practice several avalanche diodes in series - how many depends on the voltage rating.
So both the forward and reverse voltage drop may be larger than an ordinary digital avo-meter is designed to cope with.
Some old moving coil meters are better at this since they use a rather high voltage battery (some up to 15 or 22V)

[Mooly]

Should read as a normal diode when forward biased... ie on the diode range on a DVM it will read 0.7 ish meaning 700 mv dropped. 'tother way about and it shouldn't read as general rule for anything over a couple of volts or more but that's useless for testing accurately.

All you need is a battery of higher voltage than the zener and a restor to limit the current. Choose resistor as follows (or even better use a FET as a constant current source)

R= (Vbatt-VZener)/0.010

and measure the voltage across the zener and see if it agrees with the device in question.

Oh yes 75 ohms each way... "Oh my" so it is faulty

[Steerpike]

Originally Posted by Mooly

Should read as a normal diode when forward biased... ie on the diode range on a DVM it will read 0.7 ish

Only some 'zeners'. Not if it is a compound device, which many are.

[eyoung]

Thanks a heap you guys I guess I'll get the soldering iron out and solder and test.

Thanks again, Elwood

[eyoung]

Just a quick update... replaced zener along with several trannies and my amp turns on, somewhere in this conglomeration of threads there is my B2 thread meet me there...

Thanks again, Elwood