The BSA Workshop > Twins

Zener Diode

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A10 JWO:
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:
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:
Thanks Crofty, what about testing it ? I was going to bolt it near the battery and to the frame. ?

JulianS:
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:
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.

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