[sdiy] LED's on negative rail??

[Jim Patchell]

    I have used all kinds of ways of driving LED's.  Regular transistors work
as well.  Mosfets are nice in that they require no base resistor, but FETs do
cost more like you said.  I am not sure if trying to ballance the current draw
between you positive and negative rail is going to pay off.  I generally worry
about the amount of current that is going to flow through my ground circuit.
If you are going to have a noise issue, that is probably more important.

    -Jim

media at mail1.nai.net wrote:

> 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??