Date:         Wed, 19 Apr 2000 10:32:49 EDT
Reply-To:     JKrevnov@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Rico Sapolich <JKrevnov@AOL.COM>
Subject:      Re: Third brake lights (tack-free)
Content-Type: text/plain; charset="US-ASCII"

< David Beierl wrote:

> Ok, yes, you can wire them in series.  Shoulda thought of that.  But you
> *absolutely* need to have a current-limiting series resistor, because these
> things basically act like switches, on or off.  If you don't use the
> resistor you'll either see darkness or learn what burning epoxy smells
like... >


Oh yes, the power of the internet!  Sometimes it seems like its true prowess
is its ability to disseminate inaccurate information in terms of absolutes.
A CURRENT-LIMITING SERIES RESISTOR IS NOT NEEDED TO OPERATE AN LED AT A
VOLTAGE NEAR ITS TYPICAL OPERATING VOLTAGE.

Don't believe me blindly.  Spend some time with the physics which attempt to
conceptualize how these things work.  Or, look at an equivalent diagram for a
diode.  Or, at least, go get 3 LED's which will be rated at a TYPICAL forward
voltage of about 2 V.  Connect these in series WITHOUT a current-limiting
resistor to 4 fresh alkalines connected in series which will give you a DC
voltage of about 6.24 V.  Then watch them produce photons until you are
certain that I am not full of bullshit.  Hell, if you feel a little frisky,
connect ONE of them across 2 of the batteries (i.e., about 3.12 V) and watch
it glow brightly.

Although an LED has a nonlinear voltage-current characteristic, it is not  .
. . oh, the hell with it.

Look, knowledge is never free, not even on the Internet.  If you do not want
to pay the piper, remember what the Sicilians say," If you don't have brains,
buy brains."  But, no matter what, don't be a sucker for every ersatz
*absolute* you come across.
Rich