Re: Current-driving a powerful IR-illuminator array
- From: "BW" <bjorn@xxxxxxxxxxxx>
- Date: 11 Apr 2006 10:53:04 -0700
Rich Grise wrote:
You don't want to drive 40 LED/R strings in parallel - it's terribly
wasteful. And, as you know, LEDs are current operated, and you can't
depend much of Vf being matched. Do as others have suggested - use
series strings, maybe 4 strings of 10 or 5 strings of 8, then drive
each string with a current driver of some kind - for this, I prefer
an ordinary NPN with a calibrated voltage on the base and an emitter
resistor such that when (Vb - .7)/R = Iled. I've been known to use
two series silicon diodes go get about 1.4V on the base, so then
there will be about .7V across the emitter resistor - just use
Ohm's law to give you the proper LED current (R = .7V/If). The
NPN will regulate the current and the collector gives you the
voltage compliance you need.
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...
Regards,
Bjorn
.
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