[sdiy] LEDs and noise

John Speth jspeth at avnera.com
Mon May 11 19:17:20 CEST 2015

> > constant current source (transistor,
> > three resistors)
> no diodes? (gnd to base)

I was puzzled about that too.  I think what Neil meant is a constant
current driver using a PNP transistor as described in the transistor
chapter of the Art of Electronics.  Here's a drawing of what me meant (I


R1/R2 are selected to put the transistor into the active mode (not
saturated).  Rset sets the LED current assuming the current gain is high
enough (usually true).  The shorting transistor diverts current into the
load (LED or shorting transistor).

R1/R2 is a simple voltage divider in which the base current can be
considered negligible.  The base voltage but must be constant to deliver
constant current.  A zener diode or two or three forward biased series
diodes are another way to guarantee a constant base voltage.

I'm particularly interested in this circuit because I'm in the middle of
debugging a tremolo circuit using a micro with PWM driving a Vactrol.
Even with the PWM frequency at 30 kHz, the switching transients are
causing all sorts of noise problems.  I'm hoping this CC driver method
will solve that problem.


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