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2.2-100V Zener Diode Tester

Got a bunch of unknown diodes and zener diodes? Check 'em all with this . . .

By John Clarke

While most digital multimeters (DMMs) do include a diode test function, they do not test zener diodes. So how many zener diodes do you have stashed away which are not used because their value is unknown? In many cases, the type number will be missing or partially rubbed off or it is difficult to read because the print is so small. And even if it can be read, the type number will not directly give you the voltage rating. So unless you can look up the data for that type number, you are still “in the dark”.

This Zener Tester is the answer. It plugs directly into your DMM, so that you can easily read the breakdown voltage of the zener being tested. The unit can measure all the common types, from very low values of around 2.2V right up to 100V. It’s best for 400mW and 1W power devices, although it will also provide reasonably accurate measurements for 3W zener diodes.

The Zener Tester can also measure the breakdown voltage of other diode types such as transient voltage suppression (TVS) diodes, as well as standard and Schottky diodes with PIV (peak inverse voltage) ratings below 100V. That makes it suitable for testing many Schottky diodes that break down at 20, 30 or 40V depending on the type (eg, 1N5819 or 1N5822).

As with a standard diode tester, you can also measure the forward voltage, which is typically in the range of 0.2-0.8V.

How zener diodes work

Zener diodes are manufactured to provide a specified breakdown voltage where current will flow in the reverse direction. This is known as the “zener” voltage, after Clarence Zener who discovered the effect. The zener diode effect is the predominant operating mechanism for zener diodes with breakdown voltages up to 5.6V.

Click for larger image
Fig.1: the typical zener characteristic. In the reverse direction, there is very little current flow until the "knee" is reached, at which point the zener breaks down and the voltage remains reasonably constant over a wide current range.

Above this voltage, the “avalanche” effect is more predominant. However, avalanche effect diodes continue to be called zener diodes regardless.

Zener diodes (breakdown below 5.6V) have a negative temperature coefficient and avalanche diodes (break-

­­down above 5.6V) have a positive temperature coefficient for their break­down voltage. Zener diodes with a breakdown of around 5.6V have a zero temperature coefficient and so the breakdown voltage does not vary with temperature.

Fig.1 shows the typical zener characteristic. In the forward direction, the zener behaves as a diode and begins to conduct at about 0.7V. Conversely, in the reverse direction, there is very little current flow until the “knee” is reached. At this point, the zener breaks down and the voltage remains relatively constant over a wide current range.

However, the voltage does increase with increasing current and the slope of voltage against current is the zener impedance (or resistance). This impedance can range from 10Ω for low-value zener diodes to above 350Ω for 100V zener diodes.

Fig.1 highlights three operating con­ditions for a zener diode and the two of particular interest are maximum power and 10% of maximum power. These define the normal operating range of the zener. Note how the current/voltage slope is almost a straight line between these points.

At less than 10% of rated power, the zener voltage is much less than its rated value. On the other hand, operation at or above the maximum power rating will destroy the device (unless it is subjected to brief pulses of current).

In any case, zener diodes are not normally operated at maximum power since they must be de-rated for ambient temperatures above 25°C.

Note: some zener diode types have a very sharp “knee” which enables the diode to operate at very low currents, well below 10% of maximum power, while maintaining their rated breakdown voltage.

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