## Re: Headlight operates on AC

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• ... it pulses bright/dim at idle speed? It only gets current half the time? Must be. ... Mike, following your letter, I just came back to the group to see
Message 1 of 10 , Apr 1, 2006
--- In royalenfield@yahoogroups.com, michaelszoke@... wrote:
>
> Interesting that the headlamp operates on AC current. Is that why
it pulses bright/dim at idle speed? It only gets current half the
time? Must be.
>

Mike, following your letter, I just came back to the group to see what
As far as a lamp bulb (standard, quartz, halogen, whatever) is
concerned, ac/dc makes no difference, it's power (volts x amps =
watts) which matters.

AC supply (mains or motorbike) is delivered as a sinewave, where (for
a given power requirement) the actual peak voltage and current
delivered is slightly higher that the DC current and voltage which
would be supplied. *BUT* the *effective* AC voltage and current is
0.707 of the peak value. This is known as the r.m.s. (root mean
square) value of an AC supply and is the direct equivalent of the DC
which would be supplied. RMS is defined as the value of an AC supply
which will produce the same amount of heat in an electrical load as
that of a DC supply.

The bright/dim pulsing is nothing to do with it only getting current
for half the time (it doesn't). It is because the AC sinewave is only
being produced very slowly, which gives the lampbulb time to cool down
(dim) before the next sinewave peak comes along.
The lampbulb will produce light on both positive and negative parts of
the AC waveform, but it takes a finite time for the AC current &
voltage to reach a sufficiently high level to heat (light) the lampbulb.
Also, the fact that the alternator is turning slowly means that the
power output from the alternator is reduced from that produced when it
is turning quickly, so the lampbulb never reaches quite the brightness
it does at higher revs.

Hope that clears up any confusion

KeithS
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