Re: Driving LEDs with a battery pack
- From: default <default@xxxxxxxxxxxxx>
- Date: Wed, 08 Jul 2009 08:13:40 -0400
On Sun, 5 Jul 2009 11:22:29 -0700 (PDT), fungus
<openglMYSOCKS@xxxxxxxxxx> wrote:
Hello,
I'm trying to drive some LEDs with standard batteries (eg. from a
battery holder with 3 AAAs in it). Should be easy ... right?
There's various colors of LED which need from 2.2 to 3.3 volts at
around 25mA and I want to drive maybe half a dozen from the battery
pack.
I can calculate the right resistor for any give voltage, no problem,
but how do I deal with the wide range of voltage over the lifetime of
the battery. With a brand new battery the voltage is around 4.6V but
as it discharges it goes down to about 3.3V (with about 10% battery
left). If I pick resistors which work at 3.3V then there's far too
much current when the batteries are new (I measured 60mA on some of
them and they get warm to the touch so I'm guessing that's bad)
So:
a) How delicate are LEDs? Is 60mA going to burn them out?
b) If it is, what's the simplest/smallest circuit which will give me
(eg.) 3.3V @ 150mA from a set of AAA batteries? Size is important as I
want to pack it into a small space.
I did some Googling and tried a 3.3V Zener diode to drop the voltage
but it only dropped the voltage by about 0.2V. I'm guessing the reason
for that is something to do with the the load current being quite high
which makes the Zener resistor very small (two or three ohms).
I also looked at voltage regulators but is seems a 3.3V regulator
needs a higher starting voltage than the batteries can provide.
I have a couple of cheap LED flashlights that just wire the 14 Leds in
parallel and power them from 3AAA cells - apparently depending on the
internal resistance of the battery to keep the 14 Leds alive . . .
Bob Pease published a constant current circuit for leds that only
drops <100 millivolts in his Electronic Design column awhile ago.
http://www.worldtorch.com/LDO-fixed-current.php
the circuit uses a commonly available LM334 three terminal constant
current source to control a PNP pass transistor
--
.
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