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Subject: Re: Parallel LEDs calculation
References: <3D7B8B88.4ED5A338@bellatlantic.net> <3D7CA990.F0432D5C@rsccd.org>
Date: Tue, 10 Sep 2002 03:13:08 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 23:13:08 EDT
Lizard Blizzard wrote:
> Martin Riddle wrote:
> > Actuall you should derate the resistors according to temperature.
> > If this is a 85c enviroment, then the resistors should be derated.
> > Check the ohmite web site, the info is in their catalogs.
> Of course derating the resistors still doesn't take into account the
> possibility that the series string of LEDs might get shorted, and the
> full voltage would be across the resistor(s).
Anything is possible. But do you really think 5 LEDs in a string
are each going to short? Regarding LED failure in general, open
is a more likely failure than shorted. Regarding the string,
if one shorts, the probably of failing open for one of the
remaining good LEDs goes up, and increases with each succeeding
shorted LED, afaik. Please correct me if I am wrong.
> If so, then plan for the
> fault by either using higher power resistors (E^2/R) or else flameproof
> fusible resistors or a fuse.
You could do that, or use the approach I mentioned:
two 1/2 watt resistors. Or use 2 1 watt units if you feel
the need. But at some point there must be an engineering
decision as to how far you want to go. You can maximize
reliability at the expense of parts count, expense, volume,
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