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'[EE:] Shunt regulator?'
2001\01\23@190407 by Harold M Hallikainen

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       I have a project that involves a variable shunt regulator (10-15V,
200mA). It SEEMS that it should be possible to use an LM 317 or 337 wired
in some manner to make this work, but I'm not seeing it. Anyone see how
this might be done?

Harold



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2001\01\23@192154 by Dwayne Reid

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At 04:01 PM 1/23/01 -0800, Harold M Hallikainen wrote:
>         I have a project that involves a variable shunt regulator (10-15V,
>200mA). It SEEMS that it should be possible to use an LM 317 or 337 wired
>in some manner to make this work, but I'm not seeing it. Anyone see how
>this might be done?

I don't see any easy way to use a linear pass regulator as a shunt
reg.  But have you considered something like a TL431 with a boost
transistor?  That beastie *is* a shunt reg and is both inexpensive as well
as easy to use.

dwayne



Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
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2001\01\23@194519 by Harold M Hallikainen

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       Thanks! I've looked at that and it appears similar in complexity (not
much) to the existing 723 design.

Harold




On Tue, 23 Jan 2001 17:17:27 -0700 Dwayne Reid <.....dwaynerKILLspamspam@spam@PLANET.EON.NET>
writes:
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2001\01\23@210414 by Dwayne Reid

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At 04:37 PM 1/23/01 -0800, Harold M Hallikainen wrote:
>         Thanks! I've looked at that and it appears similar in complexity (not
>much) to the existing 723 design.
>
> >
> > I don't see any easy way to use a linear pass regulator as a shunt
> > reg.  But have you considered something like a TL431 with a boost
> > transistor?  That beastie *is* a shunt reg and is both inexpensive
> > as well as easy to use.

Gosh - my memory could be failing me (yet again), but I thought that a
TL431 shunt reg with boost transistor was: PNP boost transistor, C to gnd,
E to +V to be shunted, B to cathode of TL431.  Use a swamping resistor of
100R or so between E & B of the transistor.  Shunt voltage is set as per
normal: resistor divider from +V to be shunted to control pin on
TL431.  The boost circuit adds only the transistor and swamping resistor to
the standard TL431 circuit.

Just out of curiosity - what is wrong with the existing 723 circuit?  Are
723 regulators getting hard to source?  That would be a shame - it is one
of my favorite chips from years gone by.

dwayne



Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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2001\01\24@073955 by Roman Black

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Harold M Hallikainen wrote:
>
>         I have a project that involves a variable shunt regulator (10-15V,
> 200mA). It SEEMS that it should be possible to use an LM 317 or 337 wired
> in some manner to make this work, but I'm not seeing it. Anyone see how
> this might be done?


You can use one transistor and a zener, this works
very well as a shunt regulator as the zener is
driven from the regulated output and gives a
good precise regulation.
-Roman

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2001\01\24@123159 by Harold M Hallikainen

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       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


On Wed, 24 Jan 2001 23:37:05 +1100 Roman Black <EraseMEfastvidspam_OUTspamTakeThisOuTEZY.NET.AU>
writes:
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2001\01\24@123410 by Harold M Hallikainen

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       Nothing wrong with the existing 723 design. It's just that I believe the
ideal design has zero parts, so if I could get from the chip and a power
fet (with a few other parts) down to a power chip (like a 317) with a few
resistors, I thought we'd be closer to ideal. Doesn't look like we're
gonna get there.
       The 723 DOES do a great job. In the 1970s I worked with an FM station
that couldn't meet the FCC noise requirements due to power supply hum in
the stereo generator. Single chip regulators did not have enough ripple
rejection. The 723 did it!

Harold


On Tue, 23 Jan 2001 18:59:37 -0700 Dwayne Reid <dwaynerspamspam_OUTPLANET.EON.NET>
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2001\01\25@050557 by Roman Black

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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?
:o)
-Roman

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2001\01\25@123603 by Harold M Hallikainen

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Thanks! I'll look at that a little more closely...

Harold


On Thu, 25 Jan 2001 21:02:15 +1100 Roman Black <RemoveMEfastvidTakeThisOuTspamEZY.NET.AU>
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2001\01\25@140653 by Peter L. Peres

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The choice part for shunt regulators is the TL431 (or xx431) as it is
second sourced by many.

A very simple shunt regulator with acceptable performance can be made of a
darlington and a zener diode. By choosing them right one can have
temperature compensation. By replacing the zener with a
transistor-and-potentiometer zener substitute one can vary the shunt
voltage.

Peter

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