Date: Fri, 16 Oct 1998 19:45:57 EST
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Alternator warning light
** Reply to message from YauMan Chan <YauMan@CCHEM.BERKELEY.EDU> on
Fri, 16 Oct 1998 08:08:01 -0700
With respect, I have to add a few caveats here...
> When an LED comes out of the LED stamping machine, the longer leg
is
> theanode and the short is the cathode. But when it get installed,
they
> areclipped... so there is no easy way to tell.
Most LEDs have a flat on one side of the rim -- match it with the
others in the panel.
> But fear not. LED cannot bedestroyed if you put in it backward..
it just
> won't light up. Just turnit around and it will work again.
In *theory* this should not work. Most LEDs are rated to withstand
about 3.5 volts in the reverse direction, not 12. In practice,
though, I haven't had a problem -- but I don't leave them plugged in
in reverse for more than an instant.
> If you want to test it first, you canuse an ohmeter. Measure the
resistance
> one way, and then reverse theleads and measure again. The "forward"
> direction is the low resistenceconfiguration. remember which leg
had the
> red lead from the ohmmeter..that's the leg that should be positive.
Couple of points here: First, most modern ohmmeters, particularly
digitals, use a very low voltage to measure resistance and will show
infinite resistance in both directions on a good diode (light-emitting
or not). These meters almost always have a diode-check setting which
uses a higher voltage and will give an open indication in the reverse
direction, and a reading indicating the voltage drop across the diode
in the forward direction. Red LEDs are generally around 1.8 volts.
Orange, yellow, green are higher, around 2.2 volts, and may not be
measurable on some meters. The new blue LEDs are even higher (but the
blue VW high-beam indicator is a light bulb, not an LED).
Second, a few meters (I think only older ones) have the testing
voltage *opposite* to the color of the test leads -- I've owned at
least one Radio Shack meter that was that way.
>
> Caution. If you don't have an ohmmeter, and wants to test which
way
> tolight up, don't just connect it to a battery. It will survive a
> reversedirection, ie no light up direction, but in the corrrect
forward
> direction(light up) there will be too much current and may blow the
led.
*Will* blow the LED. It will smoke almost instantly, probably crack
the epoxy, make a terrible smell, and burn your fingers if you're
unlucky. Typical LEDs have a working current of around 20 milliamps,
but a direct connection to a 12v battery may give it ten amps --
poweee! One flashlight battery won't light it, but two will smoke it
-- you *have* to have a limiting resistor as Chan says below, and 2k
is a safe value. If you want to see normal brightness, you'll have to
use the formula I give below. But in general, LED's are probably the
least likely element to fail in the whole circuit, unless something
accidentally shorts across the limiting resistor (resistors usually
fail open, incidentally).
> LEDcircuits have current limiting resisters so it is much safer to
test
> theLED in circuit rather than putting it across a battery unless
you
> putsomething like a 2K ohm resister in series.
If you want to use a different LED (I put in a super-bright for the
oil pressure so there is NO WAY I can miss it) here's the formula for
figuring the right resistor to use:
R = (Vs - Vd) / Id
Where R is the resistor in kilohms, Vs is the highest possible
battery voltage, Vd is the rated diode forward voltage (off the
package or assume 1.8 for red, 2.2 for others), and Id is the desired
diode current in milliamps (off the package, or assume 20 ma -- this
is a shakier assumption than the voltage).
Be conservative with the working current, don't use the "absolute
max" ratings, and use the next *higher* standard resistor size. With
a red LED in a 14 volt automotive circuit the resistor will be
dissipating .12 watts at 10 ma, .18 at 15 ma and .25 at 20 ma. Best
practice is to use a resistor rated at twice the expected power, hence
quarter-watt for 10 ma, half watt for 15 or 20. There's a good chance
that the existing resistor will be usable -- you can check the value
by meter or by reading the coded stripes on the resistor. The stripe
nearest the end is a number, so is the second, and the third says how
many zeroes to put after the first two. If there's a fourth stripe,
it indicates the accuracy of the value: no stripe = +/- 20%, silver is
10, gold is 5. The code for the first three stripes is:
Black = 0, Brown = 1, Red = 2, Orange = 3, Yellow = 4, Green = 5,
Blue = 6, Purple = 7, Gray = 8, White = 9.
So a Yellow Purple Red Silver would be a 10%, 4700 ohm resistor, Red
Red Brown Gold would be 5%, 220 ohm, Green Brown Black would be 20%,
51 ohm etc. Have fun :) and I'm happy to try to answer questions.
david
David Beierl <dbeierl@ibm.net>
401 274-5827 voice, -6349 fax
OS/2 V4, FP7, JVM 1.1.6, JSM 98.6.3
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