From: "Spehro Pefhany"
Subject: Re: What is maximum current for LED?
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Thu, 03 Oct 2002 15:17:23 GMT
NNTP-Posting-Date: Thu, 03 Oct 2002 11:17:23 EDT
The renowned markp wrote:
> This is true, the eye remembers peak brightness too. By the way I don't
> agree with Spehro here. Let's say you had a 5V source, 330R resistor and LED
> at 1.8V forward voltage. So 3.2V across the resistor gives 9.6mA, power is
> 9.6e-3 * 3.2 = 31mW from resistor and 9.6e-3 * 1.8 = 17mW across LED. Now
> double the current for 50% of the time. Power is now (19.2e-3 * 3.2)/2 =
> 31mW across resistor and (19.2e-3 * 1.8)/2 = 17mW across LED. I.e. the same.
The LED dissipates more power, however, since Vf is not fixed- it
increases with increasing current. If your supply is fixed voltage, there
is no efficiency consequence to this (you're throwing away the majority of
the power anyhow) but the LED runs hotter, and hence less reliably.
> However, the persistence of peak brightness means you can probably get the
> same perceived brightness at lower current, hence less power.
Not significantly *if* the frequency is high enough to eliminate visual
flicker. Try it and see, it's an easy test to do on the bench (use a
visual matching technique). From my tests, any differences are not
significant in the context of the overall design. I used super-bright
(red) and high efficiency (orange-red) ~9 mil dies, IIRC.
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