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Old 01-11-22, 01:38 PM
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Join Date: Jun 2010
Location: Medford MA
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Bikes: Ron Cooper touring, 1959 Jack Taylor 650b ladyback touring tandem, Vitus 979, Joe Bell painted Claud Butler Dalesman, Colin Laing curved tube tandem, heavily-Dilberted 1982 Trek 6xx, René Herse tandem

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The Old Way was to put a screwdriver across the terminals to short them, give the wheel a vigorous spin, and pull the screwdriver blade a minuscule amount away from one terminal. You should see and/or hear a tiny spark jump the gap. Might take a few tries if it's your first time. Can also do it with a wire if you have one handy. Also, when it's shorted, you will notice more drag and vibration via the axle if you don't have it securely clamped in the dropouts.

It is not harmful to short a dynamo like this. Well, it might heat up if you shorted it and then went on a ride for half an hour.. They are naturally current-limited by the gauge of wire used in the windings, so no damage will occur, unlike when you short a battery (which will try to supply all the amps you can draw from it, right up until the battery explodes).

Digital voltmeters tend to be unreliable for this sort of thing. The voltage and frequency will be varying all over the place as the speed of the wheel changes, and it can be hard for a digital meter to follow. I prefer an analog meter if I'm measuring a dynamo, but in practice I rarely have needed to.
Owner & co-founder, Cycles René Hubris. Unfortunately attaching questionable braze-ons to perfectly good frames since about 2015. With style.
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