Date: Fri, 26 Jul 1996 05:26:38 -0400
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Cetin Seren <cseren@fore.com>
Subject: Re: basic electronics question
I replied Dan personally, but I'll add it here, hoping it will help people.
One has to be careful about a few points, or it could be sad. I have
two comments on malcolm's solution. And then my reply to Dan is
included.
Cetin
>
>You actually are running about 14v in the car, and normal LED's have
^^^^ quite true....
>about that 2.5v drop across them, so you want a resistor that will pass
>50ma at 11.5v. If you round to 12v (after all, resistors are commonly
>only 10% accurate), this works out to 240 ohms -- 12 = (.050)x(240).
Nononononono.... 14 = (.050) x (280) Soooo, 280 ohms is the min. you need.
(quite possibly more). AND, this is the TOTAL resistance (see calculations
below).
>Notice that by doing this though, you mostly heat up the resistor rather
>that produce light. You might want to series a bunch of these (like 5)
If you choose this route, use at least 6, more like 7, or the instant
you get a glitch in your regulator you'll fry one of these 5 (5x2.5 = 12.5,
and 12.5 < 14 ).
and now, the calculations:
======================================================================
>2.6 V @ 50 mA is the max. it will take. I'll base the calculations
>on 45mA just so that you do not burn the thing.
>
>I'll assume you're getting 14V out of the regulator during normal
>operation.
>
>total resistance ~ 14/.045 = 311 ohms.
>
>now how much of this is coming from the led? ( 2.6 / .050 = 52 ohms ).
>
>soooo, you need a 311-52 = 259 ohms. (this is minimum; I would go
>with a 300 ohm resistor to be on the safe side -- the led will be
>sligthly dimmer, but it'll last longer )
>
>one more thing: what should the power rating of the resistor be?
> I = 14/(300+52) = 0.038 , R= 300,
> power rating = I**2 * R = .038 * .038 * 300 = .475 watts
>
>In other words, you cannot use the cheapest resistors, which come
>at .25 watt rating. Use .5 watt or 1.0 watt resistors.
>
Cetin Seren Direct: 412-635-3479
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