[sdiy] led driver from negative power rail

[Oren Leavitt]

On 9/7/2010 6:05 PM, Derek Holzer wrote:
> Can someone please explain this to me:
>
> http://www.muffwiggler.com/forum/userpix/541_led_driver_1.gif
>
> I'm having a hell of a time getting it to work!!!! I'm using a 2N3906
> PNP in place of the listed transistor. Output is indeed from the Serge
> SSG, either Smooth or Stepped.
>
> How exactly does one drive an LED from the negative rail? I can't Google
> up a single instance of this.
>
> Incidentally, I would just use an LED driver powered from the positive
> rail, except that the Serge modules don't have positive anywhere on the
> panel, all the pots are wired between ground and negative. Sigh....
>
> Best,
> Derek

Hi Derek,

Looks like the diode (as drawn) across the transistor's base-emitter is 
the culprit.
The intent appears to be to protect the BE junction from reverse 
voltage, but it is drawn backwards. Reverse the direction and it should 
work.
In your case, that diode is probably conducting just before the 
transistor has a chance to turn on.

- Oren