Zener diodes

[RogerWalker]

I have quite a lot of diodes that I suspect may be zeners, but which have indistinct (or no) markings. Can anyone suggest a suitable circuit for a test jig to test "anonymous" diodes to determine a) what they are, and b) if a zener, the zener voltage, please? I have some variable bench power supplies and meters that I can deploy, but I don't particularly want to destroy them by overcooking them. Cheers!

[mickm3for]

Hi put a 1k resistor in series with the zener diode to limit current mesure voltage across diode, supply voltage has to be larger than zener voltage ie 30 v ish Mick

[Sideband]

Further to the above, remember that a zener diode has to be reverse biased. If you forward bias it, you'll only measure a low voltage (about 0.6V) like a standard diode.

[factory]

Here is one method I use, both at home & work, a low power Megger type tester is the voltage source and a DMM on DC volts to measure the voltage across the Zener diode, if it's the connected wrong way round the normal diode drop will be measured.



The max current available from the small Megger is approx 0.7mA.



Also useful for LED testing, as lots of modern ones have multiple LEDs in one package, diode check on a DMM usually only gives an output around 3V.



If using a bench PSU, a series resistor is a must, I found out the TTI 3022S with current limiting set to 5mA would destroy Zeners when switched on, even the big stud mount types, this is due to the small capacitor on the output zapping them.

David

[David G4EBT]

Originally Posted by RogerWalker

I have quite a lot of diodes that I suspect may be zeners, but which have indistinct (or no) markings. Can anyone suggest a suitable circuit for a test jig to test "anonymous" diodes to determine a) what they are, and b) if a zener, the zener voltage, please? I have some variable bench power supplies and meters that I can deploy, but I don't particularly want to destroy them by overcooking them. Cheers!

Some time ago I found a circuit on internet which I built and which works a treat and will test Zeners up to 50V or so.

I've attached the circuit with the description on how it functions, and some pics of the little unit I built. Ordinarily, I'm not a fan of stripboard and usually design and make PCBs for homebrew projects, but it was so simple, with a low component count, that a PCB was hardly warranted.

The circuit suggests using a multi-meter on the DC Volts range, which makes sense for something that will be used only rarely. Nevertheless, I splashed out a fiver or so on a digital panel meter movement. (As is usually the case, the power to run panel meter can't shared with the Voltage being measured, so needed as separate PP3 battery).

Pic 1: Circuit and description.
Pic 2: Shows the off-load voltage.
Pic 3: A 6V2 Zener on test.
Pic 4: A side view of the box.
Pic 5: An internal view. (A bit manky, it has to be admitted, so apologies for that).

Hope that's of interest and use.

[Silvered_Mica]

I built this one a few years ago, it's the same sort of thing David is showing but if you don't have an audio transformer this uses a mains transformer. The pot controls the output voltage this can be useful for testing other things.

That's a really nice case you have built for it David. mine is just a floating board, maybe one day it will get a case.

[MIKEG4IIA]

That circuit works well. Stripboard pi**es me off these days although have been using it for about 50 years. Just using now and then for prototyping but breadboarding better ...except for RF.

[Radio Wrangler]

If using zeners, look carefully at the current their voltage is specified at. Below this, their voltage can drop quite quickly, leading to poor accuracy and poor regulation. There are low current zeners and high current ones. If you just want a voltage reference, then one of the higher current ones forces you into either getting poor accuracy or into wasting a lot of current. The high current ones are really for clamping things, or for simole resistor-zener power supplies.

The consequence for testing/sorting unknown zeners is that you have to guess the current. A good zener tester would have variable current. Move the current around to see when you get a likely voltage value (E24 series). A really good zener tester would be a curve tracer, then you can see the knee in the characteristic where the zener comes on song.

David

[G6Tanuki]

A few decades back I bought a bargain-pack of 1W Zeners, Al unlabeled.


I wanted to build a stabilised 300V supply that could both source and sink current, for use as the screen feed to a pair of HF PA valves in Class AB2.


To select the high voltage Zeners from my pile I used a 150V AC supply from a little transformer, a 22K series resistor, and an oscilloscope as the thing to observe both the breakdown voltage and the sharpness of the breakdown.

Quite a number of the Zeners had a rather imprecise breakdown voltage, which could explain why they were being sold I'm bags of 100 for £2.

The scope shows up this slow breakdown with increased voltage in a way a simple meter couldn't.