zener diode info and ID ?

If you can post a picture to a photo hosting service, someone might recognise the type.

If the zener is faulty, measuring its voltage won't tell you anything useful.

A home brew zener tester I use for checking salvaged zeners works well enough, but its not for the faint hearted.

Start with 2 identical mains rectifier/reservoir electrolytics, typically

220/330uF and 385V for European supplies. assuming you get the caps from scrap monitors, one of them will also donate a bridge rectifier.

Wire the 2 caps in series, connect the + terminal of the top cap to the + terminal of the rectifier and the - terminal of the bottom cap to the - terminal of the rectifier, then connect the junction between the 2 caps to one of the ac terminals of the rectifier - this makes a voltage doubling rectifier that will give you a maximum of about 600V on a European supply.

On the ac input to the rectifier that you connected to the +/- junction of the caps, connect in series 2 resistors between 33 & 56k rated at 2W - this is the current limited live input. The other ac terminal is the neutral terminal.

To avoid blowing components under test, you will need a shorting switch to completely discharge the caps between tests. If you just use a switch, the charge in the caps will blow the contacts to smithereens first time they're closed. Search a scrap PSU or monitor mains input section for the inrush surge limiting ntc thermistor - put this in series with the shorting switch.

The large capacitors cause the voltage to rise fairly slowly as the current limiting resistors are comparatively large, I have used mine to test not only zeners but the reverse leakage voltage of ordinary diodes like the

1N4148 etc.

Note that *both* output leads are potentially live when its operating, so I'd recommend a 2 pole isolating switch for the mains input so you can safely change the component under test.

That is a DANGEROUS circuit and not required to test a little zener diode.

All you need is a variable power supply, a resistor to limit the current to the zener under test, and a volt meter across the zener. Watch the volt meter, turn up the power supply and watch the volt meter reading as you reach it's operating voltage.

Before you do this check the zener with a VOM to see if it is still 0.6 to

0.7 volt ( out of circuit) on diode test in the forward direction. This way you'll know it's still a diode and not shorted or open.

Shaun

That'll do it, provided your variable supply goes high enough for the zener being tested. Ian mentioned taking parts from an old monitor. I made a zener tester, using parts from an old monitor, that runs from a 9V battery and produces up to

170 volts across the zener at regulated low current. You just connect your meter across the zener with the zener connected to the tester. When you press the test button, your meter reads the zener voltage. No dangerous current, no caps to discharge, no need to adjust a supply, no need for a variable supply for a wide range of voltage.

I'll post the schematic on abse. The circuit is generic - oscillator & xformer make a HV source, which feeds a current regulator. All the parts were scrounged from an old monitor - I designed it around the parts I got from it. The same circuit will work with different parts. You need to be sure that ratings for the parts you use won't be exceeded. Example: output transistors (Q2, Q3) handle the high voltage at the transformer secondary so need to be rated accordingly.

Ed

ehsjr wrote in news:hd07dk$igs$ snipped-for-privacy@news.eternal-september.org:

Neat idea. Any chance there's an easy safe mod of a small disposable camera flash to do this? As they are, they do have a risk of a heavy discharge, but the basic idea is very similar, and most of the parts are there.

Dunno. The transformer in the camera may be too wimpy to provide the current. But it seems worth trying - so what if the transformer fails, you haven't lost anything. :-) Just be sure to remove the stuff (except the diode) from the secondary of the transformer before adding the current regulator.

Ed

you missed out the isolating transformer to run this device off...

I use a flyback converter built round the 5V standby transformer from an on PC powersupply running as a blocking oscilator, I run it from

2.5V (2x AA rechargable cells) and it produces at peak a little over 200V

the 200V charges a capacitor (10uF or so) and then flows through a resistor into the diode I'm testing.

anthing that starts conducting at over 100V is probably not a zener, my 1N914's start at around 120V.

Most cheap camera flash inverters use a blocking oscillator with the capacitor charging circuit in the feedback loop, this is designed to give the most rapid charging possible to the discharged capacitor, as the capacitor charges the feedback current falls away and a point is reached where the secondary winding no longer produces enough voltage to drive sufficient base current to maintain oscillation.

There shouldn't be any problem using such an inverter to provide high voltage for zener testing but don't use the inverters own regulation system - if you're ripping parts from an old monitor, I'd suggest using a salvaged video output transistor to make a current source.

I did say "not for the faint hearted".

PCB

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[trim zener tester]

Thanks Ian, I'll look into the zener tester.

robb

PCB

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Thanks for reply and help Shaun, robb

PCB

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oscillator

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Thanks for the help Ed, robb

If you're not experienced with handling live circuitry you might be better off with the blocking oscillator method being discussed elsewhere on the thread.

The zener tester I described certainly works, but its a risky design - as someone else suggested, an isolating transformer would be a very good idea if you have one.