Originally Posted by
dweenk
I have an opportunity to buy a bike with the affore mentioned hub. There is no wiring or light. The wheel seems to be a repalcement. So my question is how do I test the hubs output?
Is a VOM set to AC voltage adequate, and what reading should I expect to see with a vigorous spin of the wheel?
I have a digital meter that can measure current, and with that, the most meaningful measurement would be to measure the current out of the dynamo. With a modern dynamo, it should be producing 400mA or more at decent speeds. I know that the SA dynohub produces less power, so maybe 250mA to 300mA?? edit: obviously, this would be a measure of AC current. If the meter is set up to measure DC current, the reading will be zero.
With a literal volt-ohm meter, then you can always check the open-circuit voltage. The problem is that this voltage is mostly proportional to the wheel speed, so the value could be anything. If you can connect a resistor that is close to 12 ohms to the terminals, then an output voltage of 3VAC or more is probably a good sign (but again, I'm not certain exactly what the output of these is).
You could also use the ohm-meter to make sure that there isn't a break in the dynamo's wiring.
The problem with the SA dynohub that I've heard of is demagnetized magnets. This will result in reduced power output. To measure this, you'd need to either measure output current or else connect a known suitable resistor and measure the voltage across the resistor.
The alternative to using a meter would be to short out the dynamo terminals. If it is working properly, it should produce a significant drag when spinning. This is very obvious with the Schmidts that I own. Not sure how obvious it would be with the dynohub, but you should notice a difference between the open-circuit and short-circuit drag.
Steve in Peoria