piclist 2001\01\25\050557a >
Thread: Shunt regulator?
face BY : Roman Black email (remove spam text)

Harold M Hallikainen wrote:
>         But, it's not adjustable, unless you use an adjustable reference instead
> of the zener. I assume you have in mind a PNP with the collector to
> ground, the zener between the base and ground, a base pull-up resistor
> base to emitter, and the emitter being the top of our shunt regulator. I
> also wonder how "tight" the regulator would be. Seems like the inpedance
> would be the r'e of the transistor (25mV/IE) plus the zener dynamic
> resistance/beta.
>         Thanks for the comments!
> Harold

Harold, it's as adjustable as the LM317! Substitute a
pot for the base resistor, and use a lower voltage zener,
and you can get a fairly wide range of voltages.

I see one transistor, one zener, one resistor and one
pot. How simple do you need it? This circuit will give
reasonable regulation as the zener is driven from the
output voltage which is regulated, keeping zener current
fairly constant. The only thing that changes with load
is transistor current, so using a higher gain transistor
or darlington will minimise this as will the old rule
of 10:1 for the zener through current vs draw off
current. With the right four parts you will get as good
a regulation as you would expect from a low cost chip
like a 317. And only one zener more cost than a 317?

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