Re: Current-driving a powerful IR-illuminator array



BW wrote:

Ok I'm back after some actual lab-work ;)

I studied the current-source design with an NPN with an emitter
resistor (Re) to GND and a string of 5 leds between Vcc (12V) and the
collector. I tried to make the dimensions so that Ve is around 500 mV
(to leave space for a Vce of over 1 V and the voltage drops across the
LED's which is around 2.1 V per LED at the currents I'm interested in),
so for a LED current of Ic=250mA, I chose Re=2.2 ohm. Given the drop
over the base-emitter port of the NPN of 0.7V, I'd have to have a Vb of
1.2 V.

Somewhere here I realised that at a beta of say 30-40, I would have to
support an Ib of around 10 mA (I actually measured this), which is
quite much. The trigger source is 5V and I have difficulties of getting
those 5V down to the required 1.2V. A resistor-based voltage-divider is
not good enough since choosing small R forces a too heavy load on the
trigger buffer, and choosing a high R makes the 15 mA create a too big
voltage drop (pulling the NPN out of the "easy" feedback model). So I
switched the transistor to a darlington NPN with beta > 750 giving an
Ib of about 0.2 mA (measured). Still this did not create a sane
configuration with the voltage divider model (the 0.2 mA creates a too
big voltage drop nonetheless).

I also tried various combinations of 1N4148 strings to drop the
voltage, but it does not work as I think it does (I get voltage drops
of only 530mV over each 1N4148 can that be right?). I tried both simply
putting the diodes in a series from the trig buffer (5V) down to the
transistor base. I also tried a resistor from the trig buffer to the
base, and a diode string from the base to GND, but this configuration
didn't work as expected either :)

I'd be happy to have some thoughts on this seemingly simple circuit :)
Perhaps adding a voltage follower (another buffer) at the input port to
support the higher currents is enough ? I'll try that in the lab I
guess...


Your main problem is being sent on a wild-goose chase by a well known newsgroup pest, troll, and ignorant pretentious idiot, "Rich Grease"- the "dreaded" this and that. You will notice the complete fake offered no further suggestions to handling your quandary as is usual with that kind. At 12V you end up with bunches of current source strings, each requiring a bunch of parts overhead and delivering poor accuracy.

.

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