Indentity of 12V Converter

Phil Mahood

VOC Hon. Computer Officer
VOC Member
I'm working on a Comet that has a mystery 12V converter atop a standard Miller generator. The bike was restored many years ago and never ridden. There was no battery connected and I cannot tell what polarity the device is. Can anyone help me identify the box and improve the 50-50 odds of me getting it right? Or should I just motor the dyno and assume that its polarity matches the converter?

The unit is a Miller regulator look-alike. There are no printed markings on it whatsoever. All the solid sate components are potted and sealed in epoxy. This device was probably fitted new about 15 years ago. It is obviously a JG type unit with Miller appearance.

Any educated guesses or speculation appreciated. Thanks.

Phil Mahood


Well Known and Active Forum User
VOC Member
Electronic Regulator Identity

If you tell what the wire colours are I, or someone else, might be able to assist you. Also does it have a plastic Miller cover or steel? Steel or stainless steel base-plate?

Cheers, Peter
Bristol, UK

Pete Appleton

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The clue to the polarity of most solid state regulator boxes is the fact that they have a diode (one way valve for electricity) in them to perform the tasks of the original mechanical cut out. In this case all you have to do is Disconnect the dynamo, connect a test light between earth and the wire that was previously connected to the 'D' terminal of the dynamo. You can then try connecting a battery to the bike either way around. The way that causes the test light to glow is the INCORRECT way aroundas the blocking diode is not working. Swap the battery the other way around, so that the light does not glow, and everything should be fine. Click on the (dodgy) picture below to see how to set it up.

In the unlikely event that your regulator does not have a blocking diode and relies on the cut out in the miller dynamo then I am afraid that motoring the dynamo will not help as connecting power to the fields re-polarises the dynamo anyway. The direction of motoring is related to brush position and not polarity.

Don't know if this helps or just confuses you more :)


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Phil Mahood

VOC Hon. Computer Officer
VOC Member
Thank you bith for this. When the owner returns from vacation I'll check the details of the unit more closely.

The test suggestion and diagram were terrific. It is always great to learn a new trick.

Cheers, Phil