From: Watson A.Name
Subject: Re: Parallel LEDs calculation
References: <3D7B8B88.4ED5A338@bellatlantic.net> <3D7CA990.F0432D5C@rsccd.org> <3D7D6315.CE90B627@bellatlantic.net>
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NNTP-Posting-Date: Tue, 10 Sep 2002 02:58:40 EDT
Organization: Prodigy Internet http://www.prodigy.com
Date: Tue, 10 Sep 2002 06:58:40 GMT
In article <3D7D6315.CE90B627@bellatlantic.net>, firstname.lastname@example.org
> 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.
Exactly, and that's the whole point. Suppose the topology of the circuit
is that the LEds are located at the tail light and the resistors are
located some distance from them, upstream in the wiring. If the wiring is
subject to an automobile environment, there could be a short in the wiring
to the frame and the resistors would then be exposed to the full battery
And as you say below, the probability of the LEDs shorting is very low, so
if the resistors and LEDs are on the same circuit board, the probability of
the resistors being exposed to the full battery voltage would be much
In either case, it would be prudent to use a fuse of the right amperage
upstream to protect the circuit. We've all seen people driving around
with a broken taillight, probably caused by backing up too close to the
barriers around the parking space, so these kinds of things aren't all that
As for maximizing reliability in your last paragraph below, I would guess
that the regular incandescent lamp is not all that reliable, lasting only a
thousand hours or so. But if the light, whatever it's made of, causes a
fire, then I'd say that the whole product would be recalled, and that could
cost a whole lot of money!
> 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,
> cost, whatever.
> > --
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
Subject: line. alondra101 hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html