From: Tony Williams
Subject: Re: current source for array of infrared leds
Date: Wed, 04 Dec 2002 20:35:11 +0000 (GMT)
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NNTP-Posting-Date: Wed, 4 Dec 2002 20:36:46 +0000 (UTC)
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In article ,
Harry Dellamano wrote:
[lots of snippage Harry, to the bits I can remark on.]
> Frank did not tell us how many in his array.
I think he has 7x IR LEDs in series.
Flipping through a few Siemens data sheets, their
GaAS IREDs have the following typical characteristics;
Light output Tempco= -0.55 %/K.
Vforward Tempco= -1.5 mV/K.
Theta(junct-air)= 500 K/W.
Vfwd (Vf/IfmA pairs, picked off a tiny log-lin graph).
1.0/1, 1.1/19, 1.2/28, 1.25/60.
Radiant Flux versus Ifwd, mW/mA pairs off a lin-lin graph.
0.25/12.5, 0.4/25, 0.7/37.5, 0.9/50. (very approx.)
> Bottom line, know your LED's I/V curve over it's temp range
> and a little math will result in a good nights sleep.
Frank wants a constant light output, which is probably
going to be obtained from some optimum voltage value
and optimum series-R. He is operating at around 20mA.
So how do I thrash around some sums here.... like this?
Vf/20mA= about 1.11V, so P(20mA) = 22.2mW.
At 22.2mW and 500K/W, Temp rise= 11 degrees C.
Warmup change in light output= -6.05%.
From the radiant flux versus If, at 20mA, that -6.5%
can be recovered by increasing the current by about 1.1mA.
For an 11C temp rise, Vf falls by 16.5mV, and for const
light output that should cause an increase in If of 1.1mA.
ie Rsource= 16.5/1.1 = 15 ohms.
So the final circuit for 1 LED at 20mA is a voltage
source of 1.41V, and a 15 ohm series-R.
For 7 LEDs it would be 8.9V and 105 ohms.
Does that look a reasonable way to do it, do the final
results look reasonable?