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A-Reg rectifiers

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My electrical skills (still more electronic ones) don't extend much beyond checking continuity with a battery and light bulb, so I'd be interested in Al Osborn's comments on the following:

I'm fitting one of these to my Commando. A friend of mine suggests that one might consider leaving the Zener diode in circuit as a safety valve in case the rectifier fails in such way as to subject the battery to excess voltage.

Although this seems superficially plausible, my thought is that the rectifier needs to see the full unmodified signal from the alternator in order to function properly.

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Leave it in place but not connected, mine is there on the Z plate and the connector taped up ready to use if ever needed. A voltage led tells me if the voltage ever gets too high.

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Leaving the Zener working sounds like a good safety valve if the rectifier fries. What does AL O,think about the idea?.

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You will have 2 devices trying to control the voltage and neither will be set exactly the same as the other, they will fight each other. Hence why on the MK3 that had 2 zeners they were matched by Lucas to be at the same voltage. Not advisable.

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I dumped the zener and rectifier on my Commando long ago. I have fitted a Wassell type black box to control stator output, this does away with both of the above. The unit has 4 wires

  • Yellow x2
  • Black-negative 
  • Red positive

The yellow wires connect to the stator, it doesn't matter which way around. Assuming your Commado is positive earth-red wire to a good earth. Connect the back wire to a negative terminal on the ignition switch. I consider it important that the circuit is broken when the machine is switched off hence the black wire to the ignition switch. The unit needs a good heatsink, I made a plate from stainless steel-the black box has two mounting holes and mounted it under the frame top tube.

True the Zener and the A reg will 'go off' at not quite the same voltage, but they won't fight one another, they work quite differently. Provided the Zener was a highish voltage then it could act as a 'back stop'. For your information if the A reg should it fail mostly goes low output as opposed to high volts.

MKIII never had matched Zeners. The MKII cct has the two Zeners in the AC circuit as part of the bridge hence they do NOT need to be matched. From the 1980s there was a high output 3 phase kit using Lucas parts commissioned and sold by Mistral Engineering, (and AO Services) that had a pair of matched zeners but again not matched by Lucas but Mistral engineering. I can still do this job.

But back to the original question, Leaving the Zener in place should not cause any problems BUT if you have a high output alternator AND the A Reg (or similar) fails-high volts, then the Zener will be over-volts and also fail. The only real answer is the BSM or other accurate Voltage monitor on your battery.

Graham Powis-the Wassell box is exactly the same as the A reg but as far as I am aware there is no technical backup for Wassell products. The A Reg has me. Also having the Regulator/Rectifier does not obviate the need for a decent voltage monitor.

Hello Alan,

I know that you are the 'Electrical Oracle' on this Forum and I would not try to contradict your expertise, I don't think my black box is a wassell but I can't remember where it came from. The black box system makes things a lot more simple {in my opinion}. I haven't had any issues since fitting it to a MK1 850 but I also understand there is more than one method solving Julian's problem.

Graham

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Thanks to Al for his reply. As he may recall, I have the original low output alternator, so it looks as if my mate's safety-valve scheme is a runner if wanted.

In the meantime I have had the bike running with A Reg and BSM, and can confirm that the latter provides useful reassurance that all is functioning as it should.

Although it is a '72 at some point in its past it has acquired a headlamp with a rather nice Jaeger ammeter. Without taking too seriously the precise numbers it reports, qualitatively its readings correspond to what the BSM shows.

 

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