[sdiy] LED's on negative rail??

[Trevor Page]

Hi,

I see no reason why you shouldn't drive a LED from the negative rail. This
is done. Of course, it really does depend upon the particular application.
In some circuits you aren't always necessarily using a power supply with
equal current capability on both rails.

Bipolar transistor, field-effect transistor or op-amp - I guess it really
depends upon what sort of control signal you're wanting to drive the LED
with (voltage range, available drive current). I'd usually use a bipolar
transistor, but you could use a FET if you're driving the LED with a
high-impedance control voltage. I'm not sure why to use an opamp - unless
you have one 'spare', or maybe you'd have it configured as a comparator, for
example, so that the LED turns on when an input voltage reaches a specified
threshold.

If you're concerned about noise issues, one solution is to drive the LED
through one side of a differential transistor pair so that current drawn
from the PSU remains fairly constant.

I'm no expert - I guess others will correct me or give you better advice.

Trev



----- Original Message -----
From: <media at mail1.nai.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Friday, January 25, 2002 7:27 PM
Subject: [sdiy] LED's on negative rail??


>
> 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.
>
> 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??
>
>
>