On 2013-08-13, James (aka Bruce)
was almost, but not quite, entirely unlike tea:
On 12/08/13 16:43, Trent W. Buck wrote:
James writes:

On 06/08/13 18:41, Trent W. Buck wrote:
I suspect the responses are all gonna be "I hate dynamos! Batteries are
not that hard, just HTFU" so I haven't bothered to dig up hub specs &c,
but I'll do so if there's interest. The bulb claims to be a Philips
HPR60, 3.6V 2.4W.

Interesting that the bulb is rated for such low voltage.

What is the rating of the hub dynamo you have?

Most dynamos and the required bulb are rated something like 6V, 3W.

Now you've got me freaked out, because I tried a "HPR53 4V0.85A JAPAN"
in it, and it didn't light up *at all*. (A replacement bulb finally
arrived from LBS, and that's working, hooray.)

Ah, well that's good. Perhaps the light housing has a voltage limiting
zener diode or similar, to protect the 3.6V bulb from the full 6V. Or
perhaps the original lighting circuit had the front and rear bulbs wired
in series, so that the 6V was shared across two bulbs. I can't tell
from here.

Dynamos are current sources (not voltage sources). LEDs are current
sinks (not voltage sinks).

Your 3W LED will typically have about 3 or less volts across it, and
if you wired it up to a non-current limited 12v power supply, it would
blow. If you hooked up an appropriate resistor in series with your 3W
LED, and then the un-current-limited 12V power supply, then you'd end
up with a voltage divider and the appropriate current flowing through
the LED that caused it to have a ~3V voltage drop (with 9V and most of
the power being dissipated by the resistor). But in the ideal case of
having a current source, then the LED will just sink as much current
as the source supplies. Both will match their voltage if you've got
no resistor or anything else in the circuit. But of course, you don't
presumably know the actual current limit of the dynamo - at least I've
never seen it documented before. But given they are ~3W, it should be
more or less right. The LED won't work perfectly at 2.9W and blow
catastrophically at 3.1W.

LEDs of different types *have* to be wired in series (the current
source will just double its voltage output in order to maintain the
same current going to 2 equal LEDs in series). You can get away with
very similar lights being in parallel, until they age. Then one will
start taking more current, until the power supply starts overpowering
it, and there'll be thermal runaway and that LED will completely short
out, rendering you with a Dark Emitting Diode.

--
TimC
My mom says you shouldn't encourage me. -- Theresa Willis