Circuit tracing - zener, or not zener?
 

Hi folks

More slightly embarrassing basic questions, I'm afraid.

When tracing a circuit and coming across a diode; if it's in a glass package and kind of amber-coloured - how can one tell whether it's a Zener or not? I certainly have Zener devices in that kind of package, and I think (but I'm not sure) 'normal' diodes too. My amazing-Chinese-component-identificator looks at a known Zener and calls it a diode, so this is no help - I wonder if I have to go so far as to set the unknown device on a breadboard with a series resistor and up the reverse-bias it until something happens...or not.

Thanks
Mark

Re: Circuit tracing - zener, or not zener?
 

It would be worth taking a magnifier out and seeing if you can read anything printed on it. Does it make sense for there to be a Zener at that point in the circuit ? Failing that I'm afraid you're right, all you can do is put some volts on it and see. Don't get too carried away of course - it'd be a shame to destroy the device in the act of trying to identify it.

Cheers,

GJ

Re: Circuit tracing - zener, or not zener?
 

My knowledge is OK for comprehension when I read someone's description of circuit operation - when I might appreciate a certain diode must be a zener - but much more flakey when I'm trying to work it out with no description and no diagram. I'd be more forthcoming but the job is off-topic and I'm keen not to try anyone's patience.

> Don't get too carried away of course - it'd be a shame to destroy the device in the act of trying to identify it.

:) You read my mind.

Re: Circuit tracing - zener, or not zener?
 

Well, if you can power up the circuit you can measure what voltage is across it of what polarity, that will be a good clue. Also, just its polarity in the circuit is another clue. Is it shunting something?

A while ago I was breadboarding something and it just would not work and eventually I realised with some embarrassment that I'd put in a couple of zeners instead of 1n4148s.

Re: Circuit tracing - zener, or not zener?
 

Ah - the clouds are parting - I see why shunting something would be a useful thing to do with a zener. Thanks, that's a useful clue.

Re: Circuit tracing - zener, or not zener?
 

Hmm, not sure what happened then, but I found I had a 3" wide by 4" deep only display of the forum??
Any way, zeners. If you suspect one, and that it is not more than say 13v, just test with an avo or similat test meter with the 15v battery for the high ohms range. It will give you an approx idea of value, and confirm it is a zener.
Les.

Re: Circuit tracing - zener, or not zener?
 

A good excuse to keep that high range battery in use on AVOs etc.!- I suspect that they don't get used that often in the era of the ubiquitous DMM. A sensibly high impedance volt-meter connected across the diode in question will give at least an indication of turnover voltage, though the reading obtained is almost certain to be on the low side because of the very low test current. Some DMMs will turn over LEDs and low-voltage Zeners on their diode test range and give a voltage reading, but again the reading will be a little low for the same reason. Not sure if DMM manuals tend to give a "max test voltage out" on this range, must read more deeply (or read at all....),

Colin

Re: Circuit tracing - zener, or not zener?
 

If you find a diode in a circuit and you are wondering if it is zener:

Power the circuit, check if the diode is forward or reverse biased. If its forward biased and dropping around 0.7V , likely it is not a zener or a diode used as a zener.

If it is reverse biased (positive on the cathode(line)) then it may or may not be a zener.

Typically , in most (but not all) zener circiuts, the zener anode is grounded (for a neg earth system) So if you trace out the circuit, and the diode anode is grounded, then it is likely a zener shunting a resistive load (Although other times its possible to see diodes with grounded anodes in flywheel and snubber circuits)

If in doubt, its easy to find out. Disconnect one leg of the diode in question and reverse bias it (positive applied to the cathode) with a power supply, say 15V to 20V with a series 4.7k or similar resistor to limit the current (don't just apply voltage without a current limiting resistor). Measure the voltage across the diode with a DVM. If it is a zener, and not a 1N4148 or signal diode of some sort, it will shunt the voltage down to some lower value close to the zener voltage.

Re: Circuit tracing - zener, or not zener?
 

Most of these suggestions have to carry the qualification "probably"

All diodes break down into an avalanche mode at some voltage. There are some zeners made with quite high breakdown voltages and there are signal and switching diodes made with breakdown voltages as low as a few volts. So there is a lot of overlap between the voltage range of available zeners and that of other diodes. Most commonly used ordinary diodes have higher breakdown voltages than most commonly used zeners. Just be wary that there are enough exceptions to catch you out occasionally.

In a purpose-made zener diode the processing is controlled to set the breakdown voltage - usually close to an E12 or E24 value (at their specified current, and that throws in another unknown!). The processing is also designed to remain stable over a long period spent in breakdown. Ordinary diodes will degrade under this treatment. Intentional zener diodes are designed to show a sharper knee in their characteristic. These aren't hard boundaries that can give certain identification of zeners so there's no certainty here.

Not all zeners are used as reference voltage sources, they get used in signal circuits to offset signal voltages or to clamp them, so they can be found floating around away from ground potential.

Some of the signal diodes with low breakdown voltages can be rather valuable. Tunnel diodes, microwave noise sources and microwave detector diodes. Not commonly met but can be damaged by an ohmmeter test.

The only sure identification comes from either reading a type number, or seeing the schematic.

Measuring the breakdown voltage gives probable identification, but you should at least be aware of exceptions, otherwise when you meet one it will really lead you up the garden path.

David

Re: Circuit tracing - zener, or not zener?
 

"Some of the signal diodes with low breakdown voltages can be rather valuable. Tunnel diodes, microwave noise sources and microwave detector diodes." I have some diacs in the package Marc mentioned too, no idea if they come under that catagory.

Diode type's are often indicated on the PCB by symbol usually hiding under the diode, or by the prefix ZD or similar which you probably know. The very rarely go wrong but on some circuits I've had to replace several.

To check mine I use a little zener checker someone off the forum made and let me have, have the plan's if you want them or you can check them with a V/I component checker that uses your XY facility ont scope. Either way it entails lifting their legs, bit like sexing dogs or newts, but like birds or
spiders, diodes are hard to sex too : ) Easiest ID as Alistair says is if they're upside down.

Andy.

Re: Circuit tracing - zener, or not zener?
 

Thanks for all these ideas.

So on a 15v ohms range Avo, forward bias will read about .7/15 of the way along from 0 ohms, and reverse bias will be Zener V./15 - so a 7.5v hypothetical Zener would give half-scale deflection?

I think I'm confused over the difference between a resistor between the probes (where R is fixed, and the i that flows due to battery and shunts happens to produce 7.5v across the probes) and a zener (where V is fixed...but does that mean i through the thing and therefore it's effective 'R' are the same as for the resistor example, assuming the shunts etc in the meter and the state of the battery are the same between the two tests? Sorry folks, I think this should be trivial but I'm a bit slow at the moment).

Re: Circuit tracing - zener, or not zener?
 

Mark, first your points about the Avo are correct. You effectively have a battery, resistor and meter in series (loads of resistance when on the high ohms range, otherwise the needle would wrap itself around the upper stop. It is only approx, but give a general answer "is this a low voltage zener?" your original question (except you did not mention low voltage). Please note that if you use the Avo on mid ohms range, or worse still on the low ohms range, not only do you not have the voltage to assess any zenering, but you may well have enough current flow to get all destructive. Back in my compact disc repair days, I used an AVO on the low ohms range to test sled motors etc. A very low reading usually meant faulty, whilst a middling reading also had the motor running happily.
The idea of a battery in series with a resistance (4K7 was mentioned) is providing a current limit and chance of meaningful reading. In this case the meter is reading volts across the diode.
Les.

Re: Circuit tracing - zener, or not zener?
 

Thanks Les, I think I rather tied myself up in knots!