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From: "R. Lewis"
Subject: Re: Best way to power array of LEDs?
Date: Sat, 2 Nov 2002 18:22:31 -0000
Organization: Nextra UK
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <7tGLdeAS6fn9EwBv@jmwa.demon.co.uk>
NNTP-Posting-Date: Sat, 2 Nov 2002 18:22:00 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Sofie" wrote in message
> R. Lewis:
> I know I will get shot down here...... but here is a very inexpensive,
> SIMPLE and RELIABLE scheme:
> Depending on the specifications of the LEDs and how hard you want to drive
> them, you can adjust the value of the series dropping resistor. For
> example.....In series with each LED, a 680 ohm half watt resistor will
> the current to each LED to roughly 19 ma.... actually the total current
> swing of about 17 ma to 21 ma (a swing of only + or - 10% total) over
> the nominal automotive voltage operating range, driving, idling, and the
> motor not running. At a total of 36 LEDs, that is a TOTAL current draw
> only 0.75 Amps..... LESS than a side marker or dome lamp..... so WHO CARES
> about "wasted power" in the dropping resistors.
> So, how complicated is this?....... 36 LEDs and 36 resistors..... not
> to go wrong here.
> At that rate, you could hook up 100 LEDs and pull about 2 Amps.....
> the same amount as a brake or tail-light...... and the heat and wasted
> from the resistors will still be negligible..... and would not affect your
> gas mileage..... or I suppose a single drop of gasoline in several
> tank-fulls should take care of the extra power used.
> Best Regards,
> Daniel Sofie
> Electronics Supply & Repair
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> "R. Lewis" wrote in message
> > Amid all the jollity has anyone come up with an economical yet reliable
> > of making a 36 led array with anywhere near a standard (constant)
> > specification?
I now have a side lamp with an MTBF of what?
It also has a unit to unit light output of around +100%, -50% from the
average at 20 degC and which gets dimmer as it gets hotter and dimmer as it
It does not have a catastrophic failure mode which has an advantage -one led
failing does not cause the lamp to cease completely - and a disadvantage in
that a user without a light meter cannot tell if n of the leds have failed
and the lamp is illegal.
I am reasonably sure that a lit incandescent bulb is reasonably close to its
specification provided the supply is OK
With such a proposed led scheme I may be driving around with an illegal tail
lamp without knowing it ---- until some more points appear on the license.
I repeat my question
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