Zener Diode Advice

[Stylo N M]

Hi,

I'm still battling on with my Mettoy music master stereo battery record player, but I need a bit of advice on zener diodes, there's two in the entire circuit only tiny in size, one is in conjuntion with a 1uF capacitor that feeds the magnetic reed switch.

Can I read them on a diode test and can I replace it with similar or similar reading? I do have a multi bag full of these things in the attic, are any of them likely to be any good or do these diodes have to be a specific replacement? It's only for battery use, both diodes still have life in them but one of them may be a bit intermittent, and I would like see if it makes a difference on changing one of them, as I've still got this slowing down issue haven't quite tested all the transistors yet.

Thanks, Paul.

[paulsherwin]

Zeners have a specific breakdown voltage which enables them to be used as voltage regulators or references. If you test them on a simple diode tester they will test like an ordinary silicon diode.

https://en.wikipedia.org/wiki/Zener_diode

[Stylo N M]

Hi Paul.

Thanks for the link, I will have a look through this bag of diodes, and see if I can find something that reads similar on the multimeter and try it.

Paul.

[Station X]

All a DMM will tell you is forward volt drop and backward resistance.

What you need to know where a zener diode is concerned is breakdown voltage. You can easily check this in circuit by measuring the voltage across the zener diode.

[Stylo N M]

Hi graham,

i have a voltage reading across the diode of 3.6 volts DC and 4.7 volts AC but of course it runs on batteries so it's powered by a DC supply, the batteries were in and on when i took the readings.
But i don't know how to select a replacement unless it's something to do with a number on the diode, i don't know how to work it out.

paul.

[paulsherwin]

Your reading suggests it's a 3.6V zener diode and is working normally.

I've never know a zener fail in an odd manner. They fail like any other diode, either open or short circuit, and they don't do that very often.

[Bobdger]

Hi Paul. You can find the voltage of your odd zeners by connecting a 1k resistor in series with each diode. if you have a bench psu set this for say 12 volts, You then connect your meter across the Zener. The voltage that is displayed will be the breakdown voltage. If you get a reading of about 12 volt, up the psu voltage slowly to a point where the diode conducts.

[evingar]

Originally Posted by paulsherwin

I've never know a zener fail in an odd manner. They fail like any other diode, either open or short circuit, and they don't do that very often.

I generally agree, but a while back I had one that had become extremely leaky. Although it measured out OK with a meter, the voltage across it was low and variable.

[Stylo N M]

Hi,

thanks paul, i think i will just carry on with the transistor checks then for now, thank you bobdger for your kind input and help to.

paul.

[Stylo N M]

Hi Chris,

Ah yes my reading was fluctuating too, but I put that down to the switching of the magnetic reed switch ?

paul.

[AC/HL]

The whole point of the Zener is that the voltage across it should be stable. That's what it's there for.
Does it have any markings on it?

[TonyDuell]

Yes, the voltage across a zener diode should be stable, if there is nothing in parallel with it.

I don't know the circuit here, but I can imagine a circuit as follows : Resistor and zener in series across the battery supply and a reed swtich in parallel with the zener only. Then, (assuming the contact resistance of the reed switch is low compared to the resistor) when the reed switch is open you get the zener voltage, when it is closed you get essentially 0V. So as the switch opens and closes you get a rectangular wave with an amplitude determined by the zener voltage.

[Stylo N M]

Hi,

it looks very much like 1N4148 i have that as being a fast switching diode, i think that makes sense in a magneticly driven circuit and i've found some in a bag, but i haven't taken the plundge yet, it definatly starts with 1N anyway and you can just make the rest of it out.

Do you think that makes sense?

paul.

[paulsherwin]

Practically all American diodes have codes starting with 1N. The 1N4148 is a general purpose silicon diode which is used for a wide range of purposes inluding transient suppression, so it may well be present on a motor control board.

However, I strongly advise against arbitrarily changing components, especially when you're not certain what they actually are.

[Stylo N M]

Hi Paul,

Thanks I shall proceed with caution.

Paul.

[Diabolical Artificer]

Look for something- a number V then another number, EG 3V6 or 1V5 at the end of the diodes body. These two examples would be 3.6v and 1.5v.

Re testing and ID, your best taking them off the PCB to do this as the tiny number you need is always on the back : ) use a magnifying glass for ID. It's easier to be honest to replace them than test them properly if they're unstable. I had some go in a guitar amp, they were causing intermitent distortion on a boost/gain stage but measured ok as regards voltage regulation every time I tested them.

If you can post the schematic here it may help ID.

Andy.

Zeners near heat sources such as big resistors can be a problem.

[Station X]

Stylo N M.

How do you know that these components are zener diodes?

If you have a circuit it would be useful if you could post it.

[Station X]

Mettoy patented the idea and a circuit is included in the patent.

[David G4EBT]

It's not a lot of effort to knock up a simple zener tester. Some time ago I made a little Zener diode tester which uses a single 555 Timer IC and a small audio transformer such as the LT700 to generate a high(ish) voltage from a 9V PP3 - in my case, just under 60V. This circuit will test zener diodes of voltage ratings up to 50VDC. The circuit can be found here:

http://www.zen22142.zen.co.uk/Circui.../zenertest.htm

I build quite a few bits of simple test gear and usually design and make a little PCB, but this circuit is so simple that I just used a small piece of stripboard. As designed, it has two sockets to which to connect a multimeter for Voltage readings, but digital panel meters are now so cheap, (typically £3.45 post free) that I popped one of those in and made a little box in which to house the unit. All in all, well under a tenner and not a lot of effort.

The little meter module I used has a stated measurement accuracy: 0.1%
The display digits are 0.56 " high and the measuring range is 0V-99.9V

The power supply range of the meter module is DC 4.5V-30V so a PP3 is fine, and as the operating current is less than 8mA and the tester is only used briefly, a battery lasts for ages. I made the tester in May 2010 and it still has the same Poundland PP3 in it.

Some of these meters (including the one that I used) must not share a common ground with the equipment power supply, which is why mine has two PP3s - one for the meter, the other for the tester circuitry. However, some are three-wire types which can share a ground so one PP3 would be fine. Typically, they have a choice of red, blue or green displays and can be found on e-bay for about £3.50, post free. (Just google: ‘ DC 0-99V 3 Wire LED Digital Display Panel Volt Meter’). Or of course, the meter could be left out as in the original design, and a multimeter on the DC voltage range plugged into a couple of socket.

The first picture is of it switched on with no diode connected to it indicates the off-load voltage, which on my tester is 57.5V.

The second pic is of a 6V2 zener under test, which confirms that the diode isn't faulty.

No adjustments to make - just connect any zener within the maximum usable voltage to it and it directly reads off the zener Voltage, assuming that the zener isn’t faulty.

The third pic is of the inside showing the little stripboard I knocked up.

The last pic of the project built into a small oak box 5.5" x 4" x 2".

Hope that’s of interest.

[Station X]

Having looked at the circuit I can't see any zener diode symbols. I wonder if the fact that one of the four diodes is unshaded has any significance?

Studying the circuit description may give a clue as to whether any of the diodes are zeners.