Re: LED voltage drop

On Sun, 22 Mar 2009 08:55:31 -0500, Frnak McKenney
<frnak@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sun, 22 Mar 2009 02:57:15 -0700 (PDT), GarethAdams <gareth.adams@xxxxxxxxxxxx> wrote:

If you connect a 9v battery across a red LED it blows.

For some LEDs and some 9V batteries.

The voltage drop across an LED is fixed at about 1.4V so what happens
to the other 7.6v??

Word of caution: The voltage drop depends on the LED (frequency).
Some are as low as 1.2V and I think I've seen others as high as
3V. But you're in the right neighborhood.

Funny you mention that - IR leds? I just got some tricolor leds that
are specified at 2V red and 3.4 V green and blue - and they drop 2V
red, ~1.7 green, and 1.1 V blue (!)

I've checked my meter several times and even went so far as to put a
"normal" white LED in series and got 3.5 across the white and 1.1
across the tricolor blue. Different meter same thing.

It isn't "possible," but I'm inclined to believe what the meter is
telling me. (particularly since I can light the silly thing on an old
alkaline cell with a 1.1V drop 3 ma and with a 6V supply and the drop
is still 1.1 and at 20 ma)

In the real world, the extra voltage would be dropped across the
internal resistance of the battery. That resistance might be high
(e.g. for a battery of Cu/Zn strips in lemons) or low (e.g. a 9.6V
NiMH robot-competition power pack).

Hope this helps...


Frank McKenney
--
...[S]tigmas are the corollaries of values. If work, indpendence,
responsibility, respectability are valued, then their converse must
be devalued, seen as disreputable.
-- Gertrude Himmelfarb/The De-Moralization of Society

--
.

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