Author Topic: Zener Diode  (Read 311 times)

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A10 JWO

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Zener Diode
« on: 25 January, 2018, 16:04:12 »
Can someone tell me what the Zener  Diode actually does. I assume it must be bolted to a good earth, this came with my basket case so please don't mock. This is what the forums are for. Can it be tested with my Avo meter.
Many thanks.

CROFTY1984

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Re: Zener Diode
« Reply #1 on: 25 January, 2018, 16:21:30 »
The job of a diode is to let current flow one way through it, but block it completely if installed the other way.
A ZENER diode is a bit special in that it will let current flow one way, but if you install it "backwards" it will let a current through but only at a certain voltage, say 12v.
So if you fed the forward and backward current from the alternator (AC) into a ZENER diode installed backwards, you'll get a DC output of 12v. Sounds pretty handy on a motorcycle!

It's a primitive regulator/rectifier.
Hope that helps.

Actually, now I think about it, I think it may well be completely blocked in one direction and 12v in the other. But my general point remains!

A10 JWO

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Re: Zener Diode
« Reply #2 on: 25 January, 2018, 16:47:50 »
Thanks Crofty, what about testing it ? I was going to bolt it near the battery and to the frame. ?

JulianS

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Re: Zener Diode
« Reply #3 on: 25 January, 2018, 16:52:50 »
Photos 1 and 2 from Lucas service manual describing the zener diode.

It is just a regulator, not a rectifier.

Photo 3 again from Lucas, describes testing the zener.

The zener gets really hot and need to be mounted in airflow with heat sink. BSA chose under the bottom fork yoke as the best position.

CROFTY1984

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Re: Zener Diode
« Reply #4 on: 25 January, 2018, 17:06:43 »
To add to Julian's post, mine is mounted attached to the sorbic case by the oil "tank" on my '71 OIF, but usually they're where he said, it is on my '69 Starfire.

A10 JWO

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Re: Zener Diode
« Reply #5 on: 25 January, 2018, 17:47:32 »
Appreciated lads.

AWJDThumper

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Re: Zener Diode
« Reply #6 on: 25 January, 2018, 18:00:55 »
Just to add that a Zener diode only differs from other diodes in being able to operate safely above its reverse bias breakdown voltage which is chosen to be about 14.5V for a 12V electrical system. With most diodes, operating beyond their breakdown voltage will kill them - they overheat and burn out. With the Zener, they are designed to be able to conduct the heat away efficiently enough so that their so called junction temperature doesn't rise above the normal max of about 100 deg C. If the charging voltage to the battery rises above 14.5V, they simply short the excess charging current to ground, getting very hot in the process. If the heatsink is not properly cooled by the air flow then the junction temperature will rise above the normal limit and this will reduce the diode's reliability and lifetime.

Caulky

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Re: Zener Diode
« Reply #7 on: 27 January, 2018, 12:48:54 »
To test the zener with a multimeter, treat it as a normal diode (with connection wires removed).

I just tested a 1N4148 on my old analogue meter, about 20 Ohms in the forward mode, infinite in the reverse mode.
Use this if you have one, digital meters may give erroneous results.

Be warned, autoranging digital meters may give silly results when testing diodes on the Ohms ranges.

AWJDThumper

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Re: Zener Diode
« Reply #8 on: 27 January, 2018, 14:24:23 »
Most modern multimeters have a diode test mode which should give reliable results.

A10 JWO

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Re: Zener Diode
« Reply #9 on: 27 January, 2018, 15:01:37 »
I have an old analogue meter, will have a look in the morning, many thanks.

AWJDThumper

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Re: Zener Diode
« Reply #10 on: 27 January, 2018, 21:20:31 »
It might be worth clarifying the differences in testing a diode (rectifier or Zener) using either an analogue (AMM) or a digital multimeter (DMM).

Using an AMM's resistance measuring modes is the straightforward. An AMM will measure a relative low resistance with the leads connected one way round and a very high resistance with the leads connected the other way round, assuming the diode is ok. However, the low resistance measured will be much higher than the real value because of the way the measurement is done.

In the case of most DMM's, it will not be possible to test a diode using the resistance settings because of the way the DMM works - it will indicate an open circuit with the leads connected either way round. To overcome this problem, most DMM's have a diode test setting. In this case, in the forward bias case, the DMM will measure the voltage dropped across the diode which will normally be 0.5-0.7V - it doesn't indicate the resistance. In the reverse bias case, it will indicate an open circuit.

Therefore, either type of multimeter can be used but you need to be a bit more careful with the digital version.


A10 JWO

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Re: Zener Diode
« Reply #11 on: 01 February, 2018, 17:24:28 »
Thumper, it's gone straight over my head now. Will get one of my sparky mates to read your entry and let him do it. Many thanks for your time.

JulianS

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Re: Zener Diode
« Reply #12 on: 01 February, 2018, 18:09:46 »
It is quite hard to properly test a zener diode off the bike. It is a matter of producing the current which would be applied by the bike alternator and whether the diode breaks down as it gets hot in use.

A10 JWO

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Re: Zener Diode
« Reply #13 on: 02 February, 2018, 15:26:58 »
Thanks Julian. Thats a few weeks off yet. 's loads of engine parts arrived from Kidderminster M/C's so I have some building to do first, then lifting the beastie engine back in.

AWJDThumper

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Re: Zener Diode
« Reply #14 on: 03 February, 2018, 11:35:36 »
If you perform the simple diode test on the Zener with a digital multimeter set on diode test then it will measure the potential barrier for the diode when it is forward biased (0.5-0.7V) - that will show that it is still working as a semi-conductor. It's very unlikely that its current-voltage characteristics will have changed in this case and it should still be working in its Zener diode reverse bias mode. To test it fully off the bike you would need to use a variable voltage source which would show that at about 14.5V the current it draws in reverse bias mode suddenly goes from ~zero to a very large amount as it displays the Zener breakdown effect.