Re: Current-driving a powerful IR-illuminator array

On Sat, 01 Apr 2006 13:27:36 GMT, Fred Bloggs <nospam@xxxxxxxxxx>
wrote:



John Larkin wrote:
On Fri, 31 Mar 2006 13:56:34 GMT, Fred Bloggs <nospam@xxxxxxxxxx>
wrote:


Hi! I want to drive a fairly powerful IR-illuminator array using
high-efficiency IR-LED's (Agilent HSDL-4230 to be specific), which can
support continous currents of 100 mA and peak currents of up to 500 mA.
I want perhaps 40 of these.. and essentially I want to flash them all
in sync to an electronic camera shutter of around 1 ms width, with a
duty-cycle of perhaps 1-to-30. Now when googling around for suitable
circuits, most refer to relatively small power demands, with LEDs that
use a current of only a tenth of this.. both with resistors and with
MAX-circuits etc.

Would it be crazy to try to get the right current by the old
resistor-in-series trick ?

Not at all, and this also lets you avoid unequal LED voltage drops while
driving all the LEDs in parallel. You would have 40 resistor + LED
branches all tied in parallel with a medium power MOSFET switching the
whole thing ON/OFF, and a common voltage regulator with large output
filter capacitor supplying the power.



Obviously running 20 amps continously
through some resistors would be crazy but here the duty-cycle is so
high that on average the current is only 20/30 amps..

You will be running only 500mA through the resistors at 3% duty cycle.



20 amps at 3% duty cycle is indeed 600 mA average current. But the RMS
current, the thing wot fries resistors, is 3.5 amps.

John



He's not going to do 20Amps.

Well, he said he was, about 20 lines up. I just wanted to make sure he
didn't size the series resistors based on average current.

John


.

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