From: "Mario C."
Reply-To: Remove, all, x'es
X-Mailer: Mozilla 4.7 [en] (Win98; I)
Subject: Re: Pulsing LEDs - need more current, how?
Date: Thu, 02 Jan 2003 02:25:03 -0500
NNTP-Posting-Date: Thu, 02 Jan 2003 02:23:02 EST
Organization: Bell Sympatico
Frank, I may be reading this wrong, but it would seem that you have no
problem. Your pulse width is 2 ms and the pulse repetition frequency is
(average) 60 Hz. The duty cycle is therefore 12 %. If the measured current
(with a current averaging multimeter) is 50 mA, then the instantaneous
current (pulse currrent) is roughly 417 mA, which is not that far off the
wanted 500 mA current.
Of course, there is only small energy per bit with this scheme which may
explain the restricted range.
Frank Petrof wrote:
> I'm stuck with this problem, and can use some good input. I am
> trying to use an array of 4 high power IR LEDs for communications.
> Each LED drops 2.84V, and I'm driving them with a 13.8V supply
> controlled by a ZTX605 NPN Darlington transistor from a PIC.
> I can run them up to 500 mA pulsed current, but at present I can't get
> much more than 50 mA out of them when I pulse them, and their
> brightness and ultimately range is severly decreased. At DC, with an
> appropriate resistor, I can obviously get them to work just fine.
> My pulse width is 2mS, and in this application it needs to be that
> short, frequency is between 50 and 85 Hz. If I go so far as to put a
> pot inline and vary it down to almost 0, still about 50mA max is all I
> can get out of them with this pulse width.
> Can someone help me understand pulsed LED operation and its resultant
> effect on brightness? In order to put 500 mA through the array with
> such a short pulse width, do I need to substantially increase the
> voltage feeding them?
> Any pointers or suggestions would be greatly appreciated.