From: firstname.lastname@example.org (David A. Johnson)
Subject: Re: Pulsing LED/High Current - Simple Circuit Question
Date: 8 Jan 2003 09:23:19 -0800
NNTP-Posting-Date: 8 Jan 2003 17:23:19 GMT
Mark Klinger wrote in message news:<email@example.com>...
> Thanks very much for all the helpful responses regarding pulsing LEDs
> and their apparent brightness. I'd like the run the fundamentals of
> my circuit by everyone to make sure my thinking is correct.
> Currently, at DC, my LED has Vf of 1.7V and max current of 100mA.
> Since my supply is 13.8V, would it be correct that:
> 13.8V-1.7V-0.2V (Sat. of Transistor)/.1A = 120ohm resistor?
> Since I am able to pulse the LED at 2A with a very short pulse
> duration, would I then change to a 6 ohm resistor since:
> 13.8-1.7V-0.2V/2A = 5.95 ohm?
> This 2A is peak current I believe, which means that the short duration
> pulses (<1mS) should be significantly "brighter" than the DC 20mA
> intensity, correct?
> Thanks again everyone for helping me understand this.
If you are planning on using the LED as a visible light flasher, with
a slow flash rate, then you should set the pulse at about 3
milliseconds, instead of 1ms. Anything shorter than 3ms will appear
to be less bright. Anything longer will be wasted power. The human
eye responds well to flash durations around 2 to 5 milliseconds.
I would guess that the LED will have an equivalent series resistance
of one ohm. So at 2 amps, I would expect a LED voltage drop of about
3.7 volts. I would suggest using a power FET, such as the IRFD024
instead of a bipolar transistor to drive the LED. It has an on
resistance of 0.1 ohms. So, with a 13.8 volt DC supply, subtracting
0.2 volts for the transistor, another 3.7 volts for the LED, the
current limiting resistor for 2 amps should be about 4.95 ohms, with a
suggested value of 5.1 ohms. But, I would suggest dropping the LED
current down to 1 amp first with a 11 ohm series resistor and see if
the light is sufficient. 1 amp pulses would put less stress on the
LED. With a flash rate of one per second and a peak LED current of 2
amps, the average current from the supply would be about 6ma. With a
LED current of 1 amp, the average would be about 3ma. You will be
amazed just how bright the flash is from a quality LED when driven
hard. Let us know if you get this flasher working.
David A. Johnson, P.E. --- Consulting Engineer
http://www.imagineeringezine.com Home of the Imagineering on-line
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