piclist 2001\01\25\102116a >
Thread: Shunt regulator? 317/337 solution
picon face BY : Russell McMahon email (remove spam text)

> > Hi Vasile, that is a series regulator? I think the
> > person wanted a shunt regulator, and I don't think you
> > can do it with a 317 as it uses negative feedback,
> > not positive. :o)
> > -Roman
> >
> It's actually a constant current source in that configuration.  Been
> thinking about this and I can't see a simple solution.  I reckon you could
> make it work with the addition of an opamp (to invert the sense of the
> feedback), but then you might as well stick with a 723 etc.

If you MUST make a shunt regukator with an LM317 try this.
The 317 acts as the controller and the transistor as the dissipative shunt
I haven't tried this circuit (but it will work :-) )

R1 feeds supply voltage to Vout line.
NPN TO220 (or whatever) transistor
   Collector to Vout
   Emitter to ground
   Base via R1 to LM317 input.

   Input to Vout via R3
   Input to transistor base via R2 as above
   Gnd to gnd
   Output to gnd via R4
   Output to supply via R5

R4/R3 form divide.
LM317 attempts to keep V across R4 at its standard value (1.25V I think it

If Vout is too high LM317 shuts down and draws little.current
Transistor is driven on via R2/R3 and shunts Vout to lower it.

If Vout is too low LM317 turns on.
This draws current through R3 thereby lowereing R2/R3 centre point and
turning off transistor which deshunts supply as required.

The advantage of this over a zener is that it would have a MUCH sharper
control knee.
A zener is a truly pathetic regulator by almost any standard.

R2/R3 would need to be fiddled slightly to have transistor biased about
right with the current drawn by R4/R5 but this should not be hard to

I would try and use the LM317 as a series pass regulator as it was designed
to be unless there was some really good reason to use a shunt regulator.


       Russell McMahon

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