On Wed, 23 Jun 2004 13:09:31 -0700, james wrote:

I have new shimano nexus dynahub with the switch unit that comes with
it and a B&M lumotec + with standlight

I have the version of the light without a standlight.

(http://www.sjscycles.com/store/item544.htm)and d toplight + rear
(http://www.sjscycles.com/store/item554.htm). The lights are wired in
parallel using heavier-than-usual-for-dynamos speaker wire with proper
soldered ends and spades etc. The light cuts out after anything from
a few seconds to a few minutes.

Could the effect be related to heating? For example, if the light went out
after you were going fast and you only stop for a short time, it may only
work for 2 seconds. But if the light goes out whilst you are going slowly,
then you stop for several minutes - it will work for several more minutes
before going out again?

The standlights remain on (if they had time to charge up).

They are probably a separate circuit (powered by the dynamo, of course) so
I doubt they can tell us that much about how the rest of the light works.

If I stop the front wheel for a few seconds
and then set off again the lights work again. I cannot determine if
the length of time they work for correlates with anything (such as
length of time that the dynamo stops for) or is random. The dynamo
needs to stop for several seconds (eg braking and lifting the front
wheel whilst in motion doesn't do it) It seems as though slowing to
below the speed required to fire the light up (ie 2mph) does the same
thing but I cannot be certain of this. Switching the switch off and
on (without stopping the front wheel doesn't seem to do anything. I
*think* the key is stopping the elctrical output from the dynamo (as
opposed to some sort of mehanical effect from stoppingthe wheel) which
leads me to suspect something to do with the zener diode protection or
somesuch.

Could be...

IRRC both the switch unit and headlamp have zener diodes.

I have never knowingly shorted any terminals.

If you had, this would probably damage the dynamo rather than the lamp.

I have had the unit up to 40mph (although not with lights on) but dont
often go much above 25 mph (peak speeds).

At least one dynamo - the expensive and sophisticated Lightspin - is
destroyed whenever the rider exceeds a certain, rather low speed (3X mph
IIRAC). Those continentals don't have steep hills like we do, so their
bike lights aren't always designed to withstand the speeds that we can attain.

Open-circuit dynamos can produce quite high voltages at these speeds, and I
wouldn't be surprised if they could roast a small varistor or zener diode
placed accross its output. You might put such a component in front of the
switch (i.e. not turned off when the light is turned off) in order to
protect it from the sparks the dynamo would otherwise make if you tried to switch
off the lamp whilst you were moving.

Can anyone suggest a likely hypothesis for these
syptoms or has anyone had similar eperiences.

If I short out my Schmidt dynohub, it starts to produce a moderate amount
of resistance as I try to turn the wheel - it's a small recumbent wheel so
this may be easier to detect. On the other hand, if it's open circuit then
it just keeps spinning.

I would suggest you put your bike upside down, and spin the wheel until
the lamp goes out. If the wheel suddenly starts to slow down rapidly,
this would suggest that the problem was a closed-circuit fault - perhaps a
failed Zener diode. On the other hand, if the wheel just keeps spinning
then you probably have a good old fashioned loose connection.

AC