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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?

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    \$\begingroup\$ (I suspect this has something to do with the "minimum zener current" but would appreciate insight into how that interacts with the diode testing method used.) \$\endgroup\$
    – natevw
    Commented Oct 29, 2019 at 4:08
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    \$\begingroup\$ Probably has to do with the current it attempts to use in making the measurement. Different instruments from different families from the same manufacturer, or different instruments from different manufacturers, will use different design choices here. There is, so far as I'm aware, no standardized current to use. And even if there was, it wouldn't be appropriate for measuring the zener voltage, when reversed. You need to instead provide the correct current and then measure the voltage with the usual voltage mode of the meter. Supply it with a largish voltage and a current limiting resistor. \$\endgroup\$
    – jonk
    Commented Oct 29, 2019 at 4:08
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    \$\begingroup\$ The zener voltage might be specified at 10 mA whereas your meter might be injecting 1 mA or less. \$\endgroup\$
    – Andy aka
    Commented Oct 29, 2019 at 8:07
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    \$\begingroup\$ @jonk The test current will always be specified in the datasheet, and JEDEC part numbers have standardized test currents, however those currents tend to decrease with increasing zener voltage. Lower power zeners and zeners originally made by Japanese companies tend to be specified at lower currents, in my experience. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 29, 2019 at 11:33
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    \$\begingroup\$ @Spehro Thanks for the added thoughts on this. I've simply assumed there is no standard and that the OP's differing results with different Fluke units supported my assumptions. Of course, I knew that datasheets for an instrument may (not always) specify test conditions. And I'm well aware that different zener families, as well as different zener voltages in the same family, have different test currents. \$\endgroup\$
    – jonk
    Commented Oct 29, 2019 at 11:38

2 Answers 2

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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.

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The value you read is not wrong, it’s correct for the test current your meter uses. Zeners less than about 5-6V have very rounded knee characteristics- to the point of being fairly useless for many purposes.

For example, the 1N4728 is specified at a zener test current of 76mA! Your meter probably uses a test current in the 1mA range, limited to 5 or 7V.

From this datasheet see how rounded the curves of low voltage zeners at the 1mA range:

enter image description here

Usually it’s better to use some alternate method than use a low voltage zener, for example an TL431 or TLV431, or adjust the circuit to use a higher voltage.

One common situation is when a designer wishes to clamp the input voltage to an ADC without affecting voltages close to the maximum (sharp clamping). For that application, a 3.3V zener is not very useful.

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