testing zener diodes in circuit

using a multimeter, you can check the zener diodes. first get the true values from a good one then compare the values. while measuring, first make the forward-biased the diot then reverse- biased.

You can check for only very basic diode functionality, in circuit, and without the unit powered. You cannot check for zener voltage, and any which were open circuit or leaky, may be difficult to spot. Short circuit ones should show up ok, but bear in mind that you will probably have a decoupling cap directly across the zener, and the zener may be strung directly across a transistor junction if it's part of a 'conventional' linear regulator. If either of those additional components was leaky or short, the reading across the zener will be directly influenced. On the other hand, zeners that are perfectly ok may show what appears to be reverse leakage, but is actually the influence of attached circuitry, and if any are very low voltage zeners - say 3v3 - depending on the meter's test voltage, they may *just* start to conduct in the reverse direction, giving the impression that they are reverse leaky, when they're not. Is there a big problem with powering the unit ? Do you suspect that it has failed zeners ?

Arfa

Yes, that is right. But you can only measure the typical Silicon-Voltage (approx .5 to .8 Volts). Switch your Multimeter to Diode-Check (that supllies a higher Voltage than normal Resistor-check). In one Direction you should get overflow "1" and in the other direction the Voltage named above. That means: The Diode is OK. Sometimes, you will get in both directions any other Voltage or even a shortcut. That is mostly caused by other parts connected to your Zener.

Best Way to solve this Problem: Dissolder one of the Zeners Pins and get the right Voltage...

What you can NOT measure that way, is the real Zener-Voltage. You would need a little Test-Circuit (and dissolder the Diode completely). Connect The Zeners cathode via a 10K Resistor with e.g.15V. Ground the Anode. A Zener is only used in the "wrong direction" The get the Voltage above the Zener. That is the real Zener Voltage.

Many Greetings from Germany, Felix

You can find a shorted circuit node with an ohm-meter. If you've got stuff in parallel that could be shorted, you have to determine where the current is going. Easiest thing is to unsolder something. If you put a bunch of current into a shorted node, the offending surface mount will desolder itself...sometimes explosively. Or use a high-tech infrared thermal imager. Used that back in the day to find internal circuit board shorts. Low-tech non-contact thermometer works if you can find one with a very narrow beam width.

Another approach is to use a HP current tracer and HP current pulser to determine which way the current is going in a trace/wire.

Open zeners are a bit more difficult. Easiest thing there is to power the circuit and measure the voltage across the zener. If you can't put the circuit in a mode that turns on the zener, it's not likely the cause of your symptom...or you didn't need one in the first place. A curve tracer can often tell you a lot about the condition of your zener, but it depends heavily on whether you can reach zener voltage without blowing up something else. YOu can make one from an AC wall wart, a resistor and scope. Modems often have AC wall warts.

Would be interesting to know why you can't power it on??

If you built the circuit, you should be able to get close to the problem by analysis of the symptoms.

And zeners do fail in modes whereby the small signal behavior is more like a resistor. mike