# Voltage rating of a Zener diode

How do we find the voltage rating of a Zener diode found alone by chance, when the only way to identify it is to inspect, test, and so on?

• Zener diodes are usually cheap enough to throw away. So, recycle it and buy the appropriate component when you need it. A component found by chance is not a usable component most of the time due to uncertainty about its reliability. Jan 23, 2023 at 10:45
• @Andyaka, That might depend on your circumstances. The requirements in manufacturing a commercial product can be different from the requirements in a prototyping lab, different from the requirements of a hobbyist who is trying to cobble something together tonight from parts-on-hand. Jan 23, 2023 at 16:22
• @SolomonSlow the Zener diode was "found alone by chance" so it's hardly relevant some candlelight experiment that might happen later in the day. No, Zeners are quite often used as safety devices and, should not be relied upon when found in some dusty corner. Would anyone eat food from a discarded plate left by someone unknown in a restaurant? I guess someone starving would but, anything that has to be cobbled together "tonight" should still not compromise safety. I am not convinced by what you said. Jan 23, 2023 at 16:37

I use a curve tracer. There are some very inexpensive semiconductor testors that will do this as well. You clip on two leads and the display displays what it found. Not sure about voltage above its vcc.

• This also has the further advantage that a curve tracer can validate that the device actually is a Zener diode, and not just some other 2-terminal device Jan 23, 2023 at 7:28

You can measure a diode's properties with a voltmeter, a DC voltage source, and a resistor.

First, connect the diode in forward-biased mode, with the power on, and measure the voltage drop across the diode:

simulate this circuit – Schematic created using CircuitLab

(As you can see, for this diode, the simulator calculates a forward voltage drop of 537.1 mV.)

Note the use of the 10 kΩ resistor: I chose arbitrarily, but this resistor is big enough to limit the current across the diode to no more than 1.2 mA (and the power to no more than 14 mW), which seems low enough to not overheat. If you need to test the diode at various currents, you can change the resistor's value. Measuring the current through the resistor and the voltage across the diode will let you plot an I-V graph, fully characterizing the diode.

Next, do the same for reverse bias (ammeter shown this time):

simulate this circuit

Again, varying the resistor and measuring the current will let you plot an I-V graph for the diode. If you can't get current to flow even with no resistor, your diode needs a higher voltage. Once you've done testing both directions at whatever values of current you need to test, and characterized the diode in blocking and conducting regions for both directions, you're done.

• Note that this method works for rectifier diodes as well; in which case, choose a much lower current (<1mA), and a higher test voltage (at least the peak voltage you intend to use the diode at!) Mind that older rectifiers may be destroyed by avalanche; modern (glass passivated, etc.) types are robust enough to handle a little current this way. (Zeners are made to handle more current, and be stable and accurate as well; and TVSs, very high (peak) currents indeed.) Jan 23, 2023 at 4:21
• This "even with no resistor" is somewhat risky business. Jan 23, 2023 at 9:49
• @fraxinus Certainly don't start with no resistor, but I'm picturing turning a pot all the way down to zero, reading full power supply voltage across the diode, and still seeing microamps. Jan 23, 2023 at 18:31

Here is a simple test circuit. Vary V1 and measure the voltage and current. When the current is about 20 mA, that is approximately the zener voltage. For low voltage zeners (< 6V) the "knee" will be soft so the current will increase gradually.

The value of R1 is not critical, but it should be a power resistor, or it may get very hot.

If the zener is 12 or greater, you need to watch the power in the zener also. You may want to put a fan on it.

If you only have one meter, measure the voltage across the resistor instead of using an ammeter. Then you only need to move one lead for the second measurement.

simulate this circuit – Schematic created using CircuitLab

You can check it by using 12V or 24V power supply and some resistors in range of 1k~10k ohms. Connect your Zener diode with resistor in serial to the power supply. Measure the voltage of the diode.

• There are common Zener diode voltages like 2.7V, 3.0V, 3.9V, 5.1V, 5.6V, 7.5V, etc. Thus, if your multimeter shows 3.88V for example, you can think it's 3.9V rated zener diode.
• If the voltage is very near to the power supply voltage use used (for example, 11.6V on diode while you're using 12V power line), you can think it has bigger rating than power supply voltage. You'll need to use higher voltage to measure it.

Each Zener diode has proper operating range of Zener current. Thus, simply knowing the voltage may not be enough for your usage. You need to choose the serial resistor for the correct reverse bias current setting.