Calvin Sambrook wrote:
"Donwill" wrote in message
...
Calvin Sambrook wrote:
"Donwill" wrote in message
...
I purchased a string of 10 Leds powered by 2 AA batteries connected
in series giving 3 volts or thereabouts presumably.
I was a bit sceptical about the life of the batteries so I left
them on 24 hrs to check, I was surprised that they lasted over a
month which makes them very useful as party lights in a damp
conservatory, let's say running them for 4hrs a day which should
make them last for 6 months perhaps.

Hang on a minute, many LEDs won't turn on at 3v as that's quite a
low Vf. Not only that but the normal arrangement for multiple LEDs
is to put them in series chains. You could put them in parallel but
there are issues. I suspect then that the 3V from your batteries is
steped up to something rather higher.

LEDs require a current source rather than a voltage and there are
some nice little ICs available which do all of the step up and
current control in one package, you may well have one of these in
the lights you've bought. Have a look near the batteries, if
they're cheapo it may be a small black blob rather than a chip as such.
The leds are wired in parallell and I have looked, and checked again
after your email for a black blob or any indication of some kind of
electronic gizmo that might control current or whatever, I am pretty
sure there is nothing there. the leds have transparent plastic
housings and I can see nothing in the housings that might indicate
that they have anything significant in there either.

I'm still struggling to imagine how these are arranged. If they are
in parallel then each LED will have a wire going to it so that's 10
wires running along the "string" plus the return which can be common
which makes 11 in all. Each LED will require a current limiting
resistance of some sort. They will not share a limiting resistor as a
mis-match between Vf of individual LEDs leads to runaway conditions
and dead LEDs so there will be 10 resistors. This is however a poor
way to design battery powered LED lights as If will vary significantly
with battery terminal voltage, bear in mind that a fully charged AA
cell may be anywhere between 1.2V and 1.5V depending on its chemistry.

It's just never done this way, in fact the only case where I've seen
LEDs controlled with a resistance commercially is in keyring lights
where the relatively high internal resistance of two button cells is
relied on to limit the current through a single LED. Your chain
simply can't be using the internal resistance of AA batteries as that
can be very low in some cases. In fact as a fun experiment try
fitting NiCd or NiMh cells, either it won't light because they have a
lower V or the LEDs will pop because they have a very low R.
The leds have a thin cable, transparent insulation, and having two
conductors in it; each conductor is connected to a leg of the 1st led
and then carries on to the next led, so in effect they are in parallel.
I found the invoice, and have a ref to the advert on ebay, you can see
on the pic how they are wired.

http://cgi.ebay.co.uk/10-LED-BATTERY...em3ca7 9c497d
I tried using my own camera but it wouldn't focus closely enough to
show the detail of the led connection.

I got to thinking about dimming them, a series resistor would
surely do that, but it would reduce the battery life by warming up
the resistor.

If the lights use an IC to do step up and control then no, a
resistor won't work, or at least not in the way you think.
Even for simple resistor in series with LED circuits you wouldn't
reduce the battery life by increasing the resistor value. You would
waste power of course by heating the resistor but the overall
current draw would be less so the battery would last longer.

Now you can dim filament bulbs by using an astable multivibrator
and varying the mark to space ratio, relying on the chopping
frequency and the time constant of the filament to provide a non
flickering and dimmable light.

Again anything like this will screw up an IC controller, if that's
what you've got.
I don't think there is one, it's just a simple translucent battery
holder which includes a switch, no sign of a "black blob" there
either. The possible explanation that occured to me is that they
have a constant current diode built into the leds during the
manufacturing process and is visually quite undetectable.

Doing that to an led would cause flickering of the light but would
the relativly slow response of the eye smooth out the flicker? as
in film movies which I think have to be in excess of 25 fps in
order to fool the eye.

Movies are 24Hz frame rate *but* they are double or treble shuttered
to avoid flicker. 24Hz is objectionable and unwatchable, 48/50Hz is
right on the limit and anything you'll see in a UK cinema will reach
your eye at 72Hz.
OK, thanks, I hadn't realised that, I was thinking to set the
multivib to 1KHz at least.

In the interests of battery life, are there any dimmers of this
type available for leds?

I purchased them on ebay and also bought a string of 20 and the only
discernable difference between them is that the 20 string has 3 AA
batteries in series.
I eventually cut off the battery housings and connected the two
strings in parallell powered by a Nokia mains charger ACP-7X which
were being sold off cheaply, if I remember correctly for 99p, the
charger got a little warm after 10mins use so I inserted a 2ohm
power resistor in series which reduced the current sufficiently to
keep the charger temperature within bounds. The subsequent light
output was reduced obviously but still adequate for my purposes.
This was just an experiment as I did not want to introduce mains
voltages to the still drying out conservatory (damp).

What voltage is the mains charger? If you have access to a multimeter
why not measure the current.
I think I did at the time, but they are disconnected and back on the
battery packs now. I think it was (from memory) approx 350 mA (30 leds)
and the voltage was just over 3volts. Power supply rated at 3.7 Volts,
355mA



Cheers
Don