Eric Kort wrote:
> A simple question about transistor switches:
>
> I am using NPN transistors to drive several RGB LEDs (in parallel),
> with the base current being provided by PWM signals from a PIC.
> However, my LEDs are common cathode.  It seems that all the diagrams I
> see around the internet about transistor switches (e.g.
> http://www.kpsec.freeuk.com/trancirc.htm#switching) and driving LEDs
> with transistors show the load, including the LED, on the collector
> side of the transistor.  However, I need to put the LED (anode) on the
> emitter side since the cathode is common.
>
> So, I have put the current limiting resistor on the collector, and the
> LED anode on the emitter.  This works fine on the breadboard, and
> based on my very limited knowledge of transistor function I see no
> reason not to do it this way.  My question, then, is whether there is
> some something about this configuration that is not advisable that I
> am missing?
>  =20
In this case it's IMO quite a reasonable soloution
though a rather unconventional one.

In general the main limitation of this setup is that the emmitter of the
trasistor will sit about 0.7v below the base. So if your the forward voltag=
e
drop of the emmitter side part of the load is too high you won't
be able to turn the load fully on.

The normal way to switch a common cathode load would be to use a PNP
transistor (or a P channel mosfet).
> -Eric
>  =20

--=20
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