Hi all,

  I tried but I could not resist correcting the info in this post. :)

 The forward biased voltage across a silicon based diode is about 0.7V.
The reverse biased voltage depends on the construction of the diode.
(I have only seen a 2v reverse bias breakdown voltage in a zener diode.)

  LEDs (Light Emitting Diodes) are gallium-arsenide based diodes. They
have a forward biased voltage drop of about 2 volts depending on what
color the LED is. The color depends on what else besides gallium-arsenide
is used to make the LED.  e.g. Phosphorus, Boron, ...

  What does this mean? Well when you connect up a LED, if you read 2 volts
across the leads you have it connected correctly. If you read more than
2 volts across the leads, you have it hooked up backwards, or the LED is
bad. The resistor in series with the LED sets what the current through the
LED will be and how bright it will be. A good starting point for most LEDs
is to plan to drive 20mA of current through the LED.
(This is for the RED LEDs that are about 3/16 in diameter.) Smaller LEDs
will need smaller currents.

  If you are powering one of these LEDs from 12V you will need a 500 ohm
resistor. (12V - 2V)/20mA = 500 ohm. But 500 ohm is not one of the
standard resistor values in the 10% tolerence range. So use a standard
470 ohm resistor instead. This results in a current of
(12V - 2V)/470 ohm = 21.28 mA through the LED.

  I agree with Carl that LEDs are very durable indicators. Like most
semiconductors, they will either fail within the first month or they
will last >30 years. i.e. they do not wear out like a light bulb.
This assumes that they are not abused by driving more current through them
than they are designed to handle.

Bob

> Date:    Sat, 8 Aug 1998 23:45:38 -0500
> From:    Carl Hansen <carl_hansen@IEEE.ORG>
> Subject: Camper Indicator Panel LED's !!!
>
> Hi, I have to appologize for my lame post on this issue.  It was late, I'd
> been up all night the night before, etc., etc.......  So I determined to
> right the poor information I'd given you all, but then later I read more
> posts and lo and behold, there was the answere.  The information below is
> correct.
>
> 1.  The reverse voltge of a standard silicon based diode is about 2 volts,
> this is when voltage polarity is applied in the correct direction so as to
> light it.  See the original post below, Darrell measured 2.0 volts.  Good
> for you Darrell, thanks.  Note that if you check 97.29 in Bently, they use
> a 470 ohm resistor in line with the LED, for an operating current of 21.3
> ma.  That's about right.
>
> 2.  The forward voltage is about 0.7 volts, and that's for any silicon
> diode.  If you apply voltage polarity in the wrong direction, the LED won't
> light, and you'll blow it out.  LED's are polarity sensitive.  Not like
> incandenscents.
>
[cut]
>
> In all my 25 years as an electrical engineer, not necessarily working
> directly with LED's mind you, I have NEVER known an LED to fail, except
> when not biased or powered correctly.  So follow 97.29 to check your panel
> FIRST.  Do it carefully, then double check it again.  If indeed you have a
> faulty LED, then carry on.
>
> Ch
> '89 SyncroBeast Vanagon
>
> '87 16V GTI
> '87 16V GTI w/ hale damage
> '89 16V GTI w/ toasted motor
> '86 Scirocco (8 valve w/ auto)
>
> '80 850 Yamaha Special
>
> Prior Lake, MinneSnowta
> well, ok, it's summer
> Land of Sky Blue Waters
>
>
> *** Original post follow up ***  <<<<<            AND UPDATES TO IT IN CAP'S
> Darrell blows this issue out fast! Decisive, fast...wow.
>
> Here are the part numbers he offers for refrigerator water level LEDs.
>
> >From: "Darrell Boehler" <midwesty@taos.midwest.net>
> >To: "Derek Drew" <drew@interport.net>
> >Subject: LEDs  above reefer
> >Date: Sat, 8 Aug 1998 08:20:45 -0500
> >X-Mailer: Microsoft Outlook Express 4.71.1712.3
> >
> >-------------------------
> >Hi Derek,
> >    All the LEDs that I checked are 2 volt,  T1 3/4   5mm diameter. .  I
> >checked them all  but the snowflake. So Radio shack part numbers follow.
> >
> >red   276-041a           <<<<<< THIS STUFF IS GOOD ****************
> >yellow   276-021a             ** I REMEMBER SEING BLUE ALSO
> >green   276-022a               ** PARTS WERE .99 CENTS EACH.
> >
> >Good luck and  have a good weekend.
> >
> >
> >Darrell
> >
> _____________________________________________________
> Derek Drew                           New York, NY & Washington DC
> ConsumerSearch
> drew@interport.net
> 212-580-6486, 202-966-7907 (W)
> 212-580-4459; 202-966-0938 (H)
>
>  -----------------------------
>
> ------------------------------