Re: Driving LEDs with a battery pack

On Jul 6, 9:42 pm, David Eather <eat...@xxxxxxxxxx> wrote:
fungus wrote:
Isn't it basically the same thing...?

Ouch! you need to start at the very beginning i.e. Ohms law. V=I*R or
I=V/R or R=I/I. (and P = V*I)


Yes, I heard about that one...

I was under the impression that if you fixed one of
the values in part of a circuit (volts or amps) the other
value would sort of figure out what it was supposed to
be (assuming the power supply can provide it)..

So if I figure out a way to fix the current at 20mA
the voltage will sort itself out and I don't need to
worry about it.

...
To check for power requirements of the resistor (so the pixi smoke stays
inside the resistor)

P = (4.5v - 2.2) * .02A
   = .046 watts - any small sized resistor will be fine.


The specs on the LEDs aren't very exact so the way I
did it was to put the LED in series with a variable resistor
(a ten turn wire-wound pot - quite fine tuning), set up my
multimeter to measure current, dial "20mA" using
the pot ... then take it apart and measure it.

Not very scientific but I didn't see any other way to get
exactly 20mA using the loose specs of the LED.

As you have noted the battery voltage drops over time. If you want just
a simple resistor solution you cope for it this way. Find the maximum
current the LED can RELIABLY withstand this is probably 20 milliamp and
calculate R using the new battery voltage.


The batteries give between 3.6 and 3.9 volts for most of their
lifetime so
I was designing around an average value of 3.75V.

See this chart, which seems quite accurate:
http://www.powerstream.com/z/AA-100mA.png

The problem is that brand new batteries give out 4.7V for a while and
this makes the LEDs get warm (not hot, just warm to the touch).
I discharged two sets of batteries through it and got peak currents
up to 45mA but nothing bad happened.

My idea was to use a Zener diode to take away the extra current
at the beginning but I didn't figure out all the resistors before
the pixie smoke escaped. I'll get another one tonight and try
again...

Hope some of that was useful

Yes, of course. I'm a complete newbie at this...all I know
is what I've read on the web over the last few days.

.

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