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Zener Diode Tester

[Assembled zener diode tester]
Assembled zener diode tester.

Zener diodes, although undoubtedly useful components, can be somewhat inconvenient to work with at times. They come in a variety of breakdown voltages, and generally, you will need a specific voltage to work in a particular circuit. And it is often not easy to determine the breakdown voltage of a zener just by looking at it. Not only are the markings on the diodes small and barely legible to start with, they can quite easily wear off. And even if you can read the marking, this is often not the actual breakdown voltage, but one of a number of of different manufacturer-specific codes, which can generally only be converted into a voltage by looking it up in a table in the device's datasheet.

And measuring the breakdown voltage, while in theory not difficult, is not nearly as simple as measuring a resistor or capacitor. Multimeters generally do not have a dedicated zener testing range (the "diode test" function might work for zeners up to about 3V - if you are lucky), so it is necessary to connect the diode up to a power supply with a current limiting resistor, and measure the voltage drop with a multimeter, which is rather fiddly. Furthermore, this method is limited to testing zeners with a breakdown voltage lower than the supply's maximum output, which is typically only 30V or less for the majority of bench power supplies.

I therefore decided that I would make a dedicated tester for zener diodes. The design was inspired by the availability of LED panel meter modules on ebay for only a few dollars. I had bought a few of these a while ago, without any specific application in mind. The ones that I had bought (and the ones most readily available for sale) are rated at 0-30VDC. However, I discovered that it is relatively easy to change the range on these modules. Shorting the pads marked "S1" increases the scaling value by a factor of 4. It is then a simple matter of changing the upper arm of the input voltage divider (R15) to give the correct calibration. In my case, I had to change R15 from 120k to 510k. This then gives a nominal full scale of 120VDC. The module will actually auto-range to make best use of the 3 digits, although after the modification the resolution is rather coarse at around 150mV, so the last digit isn't really very meaningful for voltages below 10V.

[Digital panel meter from ebay]

[Details of modifications to panel meter]

Having found a suitable display device, it was then necessary to provide a source of moderately high voltage for testing the diodes. To this end, I constructed a one-transistor self-oscillating boost converter that could step up the voltage from a 9V battery to around 70-80V. The circuit is fairly lossy, and so will approximate a constant-power output rather than a constant voltage. The bias resistors were chosen to give a current consumption (no load) of around 55mA at 9V, or around 0.5W. By the time circuit losses and the short duration of a test are taken into account, this is about right for testing 0.4W zener diodes without overloading them.

For a circuit this simple, using a PCB was not really justified, so it was build up in a "point-to-point" style. The circuit was built into a "UB5" size moulded jiffy box (available from Wiltronics, Jaycar, etc), which makes for a compact unit. A standard type 216/PP3 9V battery will just fit across the short axis of the box, though it was necessary to trim the insides of the bosses for the lid screws to provide clearance. Rather than bothering with flying leads or sockets for separate test probes, the input terminals are just the heads of a couple of M3 screws that go though the case wall. This allows a diode to be easily held in contact with the terminals for testing.

[Inside the box]

Given that a test can be performed quite rapidly, and that the circuit requires no appreciable stabilisation time, I decided to use a momentary pushbutton for the power switch - simply hold the component in place, and press the switch. This makes it much less likely that the device will be left on and the battery run flat.

[Circuit diagram]
Circuit diagram of the zener tester.

The circuit diagram of the tester is shown above, if you would like to try building one yourself. Most of the components are not too critical, though some experimentation may be necessary depending on what you use for T1. I used a toroidal ferrite core from the junkbox, about 14mm OD x 8mm ID x 5mm high. The primary was 30 turns of 0.25mm enamelled copper wire, and the feedback winding was 10 turns of the same. R3 and/or R1 should be adjusted to give a no load current consumption of around 40-70mA. If the circuit does not oscillate (indicated by the voltage on C2 not exceeding 9V), then reverse the connection to one of the windings, and try reducing R3.

Q1 can be any small-medium power NPN transistor, as long as its voltage rating (Vceo) is greater than the maximum output voltage. The BD139 and BC639 both have a rating of 80V, which should be suitable. D1 can be any fast recovery diode, again, with a voltage rating higher than the output voltage. ZD1 serves to limit the output voltage. I actually omitted it in my unit, but it should probably be included for reliability. Alternatively, its voltage rating could be increased (with due attention to the ratings of the other components) to enable higher voltage zeners to be tested. R2 serves to limit the current when testing lower voltage zeners (less than the battery voltage).

One final note - the output terminals can deliver enough voltage to give a small shock when unloaded, though there isn't really enough current to pose a hazard. Use caution when connecting components for testing.

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