[sdiy] LED's on negative rail??

[Magnus Danielson]

From: media at mail1.nai.net
Subject: [sdiy] LED's on negative rail??
Date: Fri, 25 Jan 2002 14:27:46 -0500 (EST)

> 
> I'm working on a few things that run almost entirely off the postive rail.
> Since LED's draw a significant amount of current (the light has to come
> from somewhere), I'm thinking that I should run the LED's off the negative
> rail to balance things out.  However, I've never seen that done.  Is there
> any reason why??  Is it a noise issue??  As far as I can tell, the LED's
> would "see" the same difference.

I think we are talking about a matter of habbit. That should explain
it for 99.7 % of the cases (the good engineering 3 sigma's) at least.

I've seen people use the negative rails for LEDs and then only
becauseit came out the easiest way of doing things for that
design. Which is actually a very good argument to use.

> Any suggestions on what is the best way to drive an LED??  I've seen
> circuits that use op-amps or FET, but wouldn't regular transistors work
> just as well for less money??

Driving an LED from a regular transistor is fine. You only have to
bother with tossing the right current and voltage at the LED and that
you are able to acomplish whatever task you where aiming for and doing
it safely. Running it of the negative or positive rail is just a
matter of implementation most of the time.

Cheers,
Magnus