Originally Posted by
noglider
Bottle or hub dynamo, I found neither mystifying to hook up. The concept of polarity on an A/C circuit is a little confusing to me, but there are only so many combinations to try, and I eventually find a working one.
"polarity" of an AC voltage source is, to some degree, a matter of semantics.
As an electrical engineer, we don't really think about polarity for AC. Instead, we worry about phase angle, which is how much the sine waves of two different AC voltages differ. This is similar to how the sine and cosine function have the same shape, but the values are different for a given angle. This is a useful concept for electrical power transmission, which typically uses three sine waves spaced 120 degrees apart. For house wiring in the USA, we usually split the incoming 240VAC into two 120VAC voltages, but one is 180 degrees from the other. You can also think of this as one being an inverted version of the other.
Anyway... the problem isn't so much "polarity", it's that some lights and dynamos have one terminal connected to the bike frame (i.e. "grounded"), and this may not be well marked or documented. The ungrounded terminals of the dynamo and lights need to be connected together. Similarly, if you do want to run wires to the terminals that are connected to the frame, these all need to be connected together.
Failure to do this will mean that something is being shorted out, usually meaning that the lights won't illuminate.
Steve in Peoria