Fwd: Re: [sdiy] VCA controls

[Don Tillman]

   > Date: Fri, 10 May 2002 15:41:18 -0700 (PDT)
   > From: Tim Ressel <madhun2001 at yahoo.com>
   > 
   > --- Don Tillman <don at till.com> wrote:
   > > Use the LM3914 but put the LEDs in *series*, drive the top of
   > > the LED string from a constant current source, and optionally
   > > add a zener diode to keep the current constant for the case
   > > when all the LEDs are off.
   > 
   > I'm wondering why that is nessessary. The lm391x has internal
   > programmable current drivers. Is the CC source to keep the noise
   > out of the power supply?

It's not absolutely necessary; but without it there's nothing to
guarantee that the sum of the current drivers is constant, especially
over transitions.  And besides, you want to keep the current going for
the case where no LEDs are on.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com