Electronic – Why doesn’t the multimeter show the correct Zener reverse voltage


I have a 3.3V Zener diode, brand new from a reputable source.

When I set my Fluke 101 multimeter to diode test mode, and put the black lead next to the dark bar on the diode and red on opposite side, the meter reads a voltage drop of about 0.689V. This would be the forward voltage of a "normal" diode.

When I reverse the leads, however, the reading does not match the expected reverse voltage. Instead my meter reads 1.746V. I tried also with a Fluke 87V which gave a slightly higher reading of about 2.2V, but still not the 3.3V that it should be.

Now obviously for most diodes, a multimeter will just read "OL" rather than finding the actual breakdown voltage (which could be quite high). Why in the case of a low-voltage zener do multimeters give wrong (and different to each other) readings of the reverse voltage?

Best Answer

Because the limited current sourced by the meter is not enough to bias the Zener diode into the reverse avalanche region. Instead you are biased some where on the knee of the Zener reverse characteristic curve.

It is typical for meters to source about 1 mA in the diode test mode. Most Zener diodes will not reach their steep breakdown region until 10 mA or more.

For testing Zener diodes in reverse bias you are much better off biasing the Zener diode via a resistor from a separate power supply and just using your meter to measure the voltage drop across the diode.

Be aware that meters in diode test mode typically limit the maximum voltage across the leads. This may be limited to the meter battery voltage or some lower value for better quality meters. This places a distinct limit on how much voltage the meter would show for reverse biased Zener diodes. It would be extremely uncommon to find a meter that could reverse bias a 27 V or 48 V Zener diode when the meter runs off a nine volt battery.