Originally Posted by
wphamilton
I think it's just difficult to visualize the forces compared to a mechanical dynamo, and that makes it harder for people to accept that it's more efficient.
There's another thing, which I haven't mentioned because I don't know, but is implied by the design and some hints on their site. This is an expensive headlight, and there are two LED's on it, not even close to the total potential output. It makes me wonder if they're doing something with the rectifier to increase efficiency, or exploiting the efficiency droop somehow, with the cost of the extra LED and circuitry involved not as significant in such an expensive light.
If there's a rectifier in the circuit, a passive one, it will have an on-state voltage drop and hence will dissippate power (rough estimate 0.5 amp * 1 volt *50% duty cycle = 0.25 watts). If the circuit contained an active rectifier (synchronous MOSFET) with a large die geometry, the on-resistance could be as low as a few dozen milliohms, and the power dissipation will be a lot less (similarly rough estimate 0.5A ^2 *.024 ohm *50% = 3 milliwatts discounting MOSFET drive losses and the cost of a drive circuit). But a quarter-watt is really hard to argue about, in terms of an effective change in efficiency.
If possible, the best thing to do with a rectifier is eliminate it, all things being equal.
I have trouble visualizing the Magnic's function, but it's the magnetic field interactions I'm having trouble with, not the forces and torques. Having designed a few handfuls of magnetic and electromagnetic devices, I'm usually pretty good about visualizing fields and fluxes, but not in this case.