Zenor

If you apply 12v to the spade terminal on the zenor should you read any voltage on
the stud? This is off the bike completely just on the bench.

Hope this help ...

Onder said:

If you apply 12v to the spade terminal on the zenor should you read any voltage on
the stud? This is off the bike completely just on the bench.

Click to expand...

On the bench with battery/meter + to the stud and battery/meter - to the tab you should see the battery voltage. If you see smoke you have a shorted Zener. If the Zener is defective open, you've proven nothing as the Zener's job is to be "open" below its cutoff voltage. If you add a charger to the battery, the voltage should be a little higher than the battery voltage and a little lower than the battery voltage with the Zener disconnected.

On the bike, assuming positive ground, meter's + on ground (the stud), - on Zener tab and bike turned on you should see the battery voltage. The Zener is directly connected to the battery - via wire. The Zener's job is to bleed off the higher voltage from the charging system and to do nothing at the battery fully charged voltage.

So if reading Grant's test correctly you will not see any voltage across the Zener until you are over 12.75V as it will not allow current through until the V is 12.75V which makes sense as if it worked below 12.75V you would have difficulty keeping the battery charged.

kommando said:

So if reading Grant's test correctly you will not see any voltage across the Zener until you are over 12.75V as it will not allow current through until the V is 12.75V which makes sense as if it worked below 12.75V you would have difficulty keeping the battery charged.

Click to expand...

Nope! The Zener bleeds the higher voltage to ground. Across the Zener you will see the battery voltage if connect like I said since it is basically an open circuit.

Grant's test: https://granttiller.com/zener is different - he is showing the Zener bleeding off the excess. If the supplied voltage is higher than the Zener cutoff voltage then the bulb will start lighting. Of course, he gives a VERY complete answer. I was answering the question asked.

It looks like my stash of zenors are all NFG. Wishing to keep the stock charging system, is there anyone out there willing to make an offer
on a working unit? There seems to be one on ebay, claimed new, that goes for 194 quid! Im amazed some one has not made a run of them off shore as they were fit to nearly every brit bike at one point.
Anybody...? I live in hope.

Are you sure you are not applying the voltage in the reverse direction. In that direction, the voltage drop should be that of a standard diode, app. .65 volts. Be careful as without a resistance you are nearly shorting the batter since the diode is biased to conduct in a near dead short.

If mine failed I would just convert to an electronic regulator of somewhat greater sophistication. That being said I am still using the gallium rectifier with rather high leakage current on my 20M3.