My Zener Tester

[David G4EBT]

I’ve always enjoyed building test gear and servicing aids, from simple projects such as signal injectors and tracers, to waveform generators, transistor testers, frequency counters and a ‘scope – the PW Purbeck in the late 1970s.

My most recent project is a little zener diode tester. I found the circuit on internet – it’s very simple, and as designed, used an external digital voltmeter, but digital meters are now so cheap that I decided to build one into the tester. The rest of the components were out of my spares box.

These meters must have their own separate power source, hence, the two PP3 batteries – one to power the meter, one for the circuitry. (Thought backlit, the meter draws less than 5mA).

I’m not keen on plastic or metal enclosures, and as a woodworker, tend to make wooden ones, which cost me nothing. The one I made for the tester is oak, (abouy 5" x 4" x 2")with comb-jointed corners. (Salvaged from a kitchen cabinet door I fished out of a skip). The front panel is a piece of SRBP single sided PCB, sprayed appliance white, with blue dymo labels to match the bue backlit display.

Description of the circuit:

An NE555 IC is wired as an astable multivibrator and the output of the IC is stepped up to a high voltage AC using the transformer T1. I used a little audio output transformer intended for use in transistor radios – an LT 700, the unloaded AC voltage from which is around 55V. However, if it’s desired to test Zeners higher than this voltage, a small 9V transformer with a 120V primary would provide 120V.

The AC voltage is rectified by diode D1 and filtered by the capacitor C3.This voltage is applied across the Zener diode through the current limiting resistors R2 or R3 which can be selected by switch S2 for testing the diode at 1mA or 2mA respectively. (C3 must be rated at 150V)

The meter – either built in or a digital multimeter set to the DC Voltage range is connected across the Zener diode under test. If the Zener diode is good, the meter will show the correct Zener voltage.

Hope it's of interest.

David,
G4EBT

[0108vesa]

Hi!

David, I have just same kind of plans to build Zener tester!! And I foud same circuit. But at the moment I am working with "Sussex" tube tester (and also hf- linear amplifier ) so Zener tester comes later.

Vesa,
oh7xi

[Guitarist28]

Hi David,
Your tester looks good - yet another project to add to my growing list.

Your case looks very impressive - well done.

Regards

Rob

[daviddeakin]

Nice project! Where do you buy your voltmeters?

[David G4EBT]

Originally Posted by daviddeakin

Nice project! Where do you buy your voltmeters?

eBay.

There are various sellers, I think the last one I got came from 'Asia Engineer' in China. It was $9.99 = £7.21 with free P&P and only took about 10 days to arrive. (I used paypal).

That one measured up to 200V, but I'd had a couple before from 'Virtual Village' for the Capacitor tester/reformer project which featured in Bygones/BVWS Bulletin. I say 'a couple' because I sent one to the skip in the sky by squirting more than 500V into it, and discovered that they don't like it up 'em, so I had to replace it

As I said earlier, it's also certain death if you don't feed them from a power supply isolated from that being measured. Other than that, they seem reliable

You might like to check out 'blue panel meters' at this link:

http://business.shop.ebay.com/Amp-Vo...-/25411/i.html

And this one:

http://cgi.ebay.ca/3-DC-200mV-Blue-L...#ht_2784wt_941

Or you could google 'blue digital panel meters' and see what comes up.

There are lots of firms who do them, and they do them for current as well as volts, and for high current, included an external shunt.

Hope that helps.

David,
G4EBT

[daviddeakin]

Thanks for the info David!

Incidentally, many of those meters you linked claim "Doesn't require power" or similar. Surely they don't draw the power from the voltage being measured?! But how else can they work?

Also, if the meter is ground referenced, are you sure you need an isolated power supply?

[Guitarist28]

Hi,
If these are the same ones that I have ordered (it seems that there a lot of sellers of the same products on ebay from the far east) then they need an independant power supply (i can't recall if this is 6 or 12 volts).

Hope this helps

Rob

[David G4EBT]

Originally Posted by daviddeakin

Thanks for the info David!

Incidentally, many of those meters you linked claim "Doesn't require power" or similar. Surely they don't draw the power from the voltage being measured?! But how else can they work?

Also, if the meter is ground referenced, are you sure you need an isolated power supply?

It may be that there are some digital panel meters that don't need to have the measured supply and power supply separate, but the ones that I've used do, which I guess can be a disadvantage in equipment using more than one meter.

I can best quote verbatim from the instructions which came with the meter:

'Warning: The working power and the measured power can't use one power system together. The Working power and measured power can't [have a] common ground'

It also makes clear that if more than one meter is used - ie, for current and voltage say, they must not share the same power supply. As to the power supply, it states:

'Meter working power must be 9 - 13.5 V, if greater than, then be burn'.

The power supply can be AC or DC, and the polarity doesn't matter.

I've appended below the diagram which came with the meter, which gives the arrangmenets that are permissible (if meter not 'be burn'!):

Hope that helps.

David,
G4EBT