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'[PIC]: Controlling resistance'
2002\11\01@215458 by Jai Dhar

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Hello,

I am looking to make a simple linear psu using lm317/337's adjustable
regulators. Basically, they use a pot from 0 -> 5k to adjust the Vout. I was
wondering if there is a way I can control a resistance using a 16f628, so I
can incorporate a sort of digital feel to my psu. Thank you for your time,

Jai

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2002\11\02@053529 by John De Villiers

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i wonder if you cant use the v-ref out to drive the regulator ?


On Sat, 2002-11-02 at 04:53, Jai Dhar wrote:
{Quote hidden}

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2002\11\02@112123 by Manu Nair

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You can control the resistance by using a digital pot. You just send the
values of the resistance you want via SPI and then the digital pot will
adjust to it. Microchip, among others, makes a variety of them. Their part
number (Microchip's) starts with MCP41 /MCP42-then the value of the pot.
Application note AN746 from Microchip's website shows you how to interface a
PIC16F876 to their digital pots. I am sure you can figure out how to port it
over to the 16F628. Hope this helps.

AN746
http://www.microchip.com/download/appnote/analog/mcp4xxxx/00746a.pdf

\manu
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{Original Message removed}

2002\11\02@130956 by Bob Axtell

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Digital Pots are cute, but be sure to notice that the current capabilities
of those pots is very small. Make sure that over the entire range of use
that the current is not exceeded. The other limit is the fact that they only
work within the GND-VCC range as it is connected.

Have been very happy with their except for these limitations.

This is my first posting to the PICLIST.

--Bob Axtell


----- Original Message -----
From: "Manu Nair" <EraseMEmnair066spam_OUTspamTakeThisOuTUOTTAWA.CA>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Saturday, November 02, 2002 9:20 AM
Subject: Re: [PIC]: Controlling resistance


> You can control the resistance by using a digital pot. You just send the
> values of the resistance you want via SPI and then the digital pot will
> adjust to it. Microchip, among others, makes a variety of them. Their part
> number (Microchip's) starts with MCP41 /MCP42-then the value of the pot.
> Application note AN746 from Microchip's website shows you how to interface
a
> PIC16F876 to their digital pots. I am sure you can figure out how to port
it
{Quote hidden}

> {Original Message removed}

2002\11\02@203912 by Jai Dhar

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Thank you for all your suggestions. But in regards to the current capacity of
the dpot, I do'nt think that will be a problem. Reading the lm317 datasheet,
it appears that only a few uamps pass through the adjustment pin, which should
mean that the range across the dpot will be between 0 and GND. I will measure
it with an analog pot first tho, just to make sure. In relation however, I had
another simple question. I could use a mech. switch to turn on the adjustable
output (ie: from the output pin of the regulator to the jack or whatever), but
I was wondering if it were possible to use a mosfet/bjt triggered by 16f628?
I'm not too familiar with what sort of transistors to use since I have limited
experience in D to A interfacing, but would this suggestion work in place of a
mechanical switch? Considering that I could have 1.5A max running through the
output, would a transistor be able to handle this?

Thank you once again,

JAi

Quoting Bob Axtell <KILLspambobKILLspamspamEDTEC.COM>:

{Quote hidden}

> > {Original Message removed}

2002\11\02@205420 by Jinx

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> I was wondering if it were possible to use a mosfet/bjt triggered
> by 16f628?

An electronic variable resistance can be made from FETs. For
example a 4066 (which has FET gates) or similar quad CMOS
switch can be controlled by PWM to get an effective resistance
between Ron and Roff. Note though that Ron for a 4066 is a few
tens of ohms. The best in a chip is possibly the ADG736 and
that's 4R5. Discrete FETs can have Ron in milliohms

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2002\11\02@212951 by Manu Nair

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I was thinking that if you wanted a simple on/off switch between your output
and your load that you can use a power mosfet like the IRFP260N from
International Rectifier, or something similar (or smaller) than that. These
power mosfets can handle way more than 1.5A that you need (up to 50A on this
one, I think) and has low on resistance. I am not sure if the ADG736 can
handle the current that you need it to handle.



{Original Message removed}

2002\11\02@215950 by Jinx

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> I am not sure if the ADG736 can handle the [adj] current

LM317 adj current is only 50-60uA for 10mA < Iout < Imax

http://www.analog.com/productSelection/pdf/ADG736_0.pdf

shows through-current for ADG736 is 30mA max (4066 is 25mA)

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2002\11\02@223440 by Jai Dhar

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Yes, the adjuster pin for the 317 has only a few uA, but the switch
application is for the actual output from the regulator, which will be going
up to 1.5A. I mentioned the adjustor pin because I needed advice on
controlling the resistance digitally, hence a digital pot. Will the IRFP260N
act as a simple switch with +5V = on and 0V = off?

Jai
Quoting Jinx <EraseMEjoecolquittspamCLEAR.NET.NZ>:

{Quote hidden}

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2002\11\02@225602 by Jinx

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> I am looking to make a simple linear psu using lm317/337's
> adjustable regulators. Basically, they use a pot from 0 -> 5k
> to adjust the Vout. I was wondering if there is a way I can
> control a resistance using a 16f628, so I can incorporate a
> sort of digital feel to my psu. Thank you for your time

This is what I was referring to wrt using a small FET on the adj
pin to alter the voltage. Obviously you do need something bigger
for the 1.5A o/p switching. You could make a low-side switch
with a 5V logic FET, ie your load goes between the o/p of the
317 and the D of the FET. The FET's S goes to 0V and the
micro control goes to G

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2002\11\03@002231 by Peter L. Peres

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On Sat, 2 Nov 2002, Jai Dhar wrote:

*>Yes, the adjuster pin for the 317 has only a few uA, but the switch
*>application is for the actual output from the regulator, which will be going
*>up to 1.5A. I mentioned the adjustor pin because I needed advice on
*>controlling the resistance digitally, hence a digital pot. Will the IRFP260N
*>act as a simple switch with +5V = on and 0V = off?

Why can't you switch the digital resistor from 0 to set value ? Saves you
a switch. Minimum out voltage will be 1.2V with LM317.

Peter

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2002\11\03@002425 by Peter L. Peres

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On Sun, 3 Nov 2002, Jinx wrote:

*>LM317 adj current is only 50-60uA for 10mA < Iout < Imax

Wherever you get that. LM317 adj current is exactly 120uA no matter what
the load. That's how it comes to set the voltage by using just a resistor.
Constant current across a resistor == constant voltage == regulation.
There is only the 1.2V constant voltage between ADJ and Out to be added to
this.

Peter

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2002\11\03@004747 by Jinx

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> *>LM317 adj current is only 50-60uA for 10mA < Iout < Imax
>
> Wherever you get that.

1) NatSemi Linear Databook 1

2) NatSemi LM317 web pdf

http://ontwerpen1.khlim.be/~groepd/LM317.pdf

Both say 50uA typical, 100uA max and adjustment pin current
change of 0.2uA for 10mA < Iout < Imax and 3V < Vout < 40V
plus Iadj vs Temp graph showing ~52uA @ 25dC

> LM317 adj current is exactly 120uA no matter what the load

Wherever you get that ? ;-)

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2002\11\03@012343 by Dale Botkin

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On Sun, 3 Nov 2002, Peter L. Peres wrote:

> On Sun, 3 Nov 2002, Jinx wrote:
>
> *>LM317 adj current is only 50-60uA for 10mA < Iout < Imax
>
> Wherever you get that. LM317 adj current is exactly 120uA no matter what
> the load. That's how it comes to set the voltage by using just a resistor.
> Constant current across a resistor == constant voltage == regulation.
> There is only the 1.2V constant voltage between ADJ and Out to be added to
> this.

Ummm...  he got it straight from National's data sheet, page 4 I would
guess.  I just happened to be looking at the LM317 data sheet when this
arrived.  The adj current is specified as 50 to 100uA, and according to
the data sheet the LM317 develops a reference voltage of 1.2 to 1.3V
between the adj pin and the output.

Dale

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2002\11\03@014755 by Russell McMahon

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> > *>LM317 adj current is only 50-60uA for 10mA < Iout < Imax
> >
> > Wherever you get that. LM317 adj current is exactly 120uA no matter what
> > the load. That's how it comes to set the voltage by using just a
resistor.
> > Constant current across a resistor == constant voltage == regulation.
> > There is only the 1.2V constant voltage between ADJ and Out to be added
to
> > this.
>
> Ummm...  he got it straight from National's data sheet, page 4 I would
> guess.  I just happened to be looking at the LM317 data sheet when this
> arrived.  The adj current is specified as 50 to 100uA, and according to
> the data sheet the LM317 develops a reference voltage of 1.2 to 1.3V
> between the adj pin and the output.

The current does vary somewhat in practice but typically not nearly as much
as the spec sheet worst case figures. See spec sheet graphs.

This is why the divider uses lowish value resistors. The designers TRY to
keep this current as low as possible and across usual temperature range it
varies by about 10% typically. If this current formed a large percentage of
the current through the lower resistor in the divider than it would have a
severe affect on regulation. Ideally this current should be zero and the
divider current would then only be the current in the upper resistor (with
1.2V across it) and the divider would be in the exact ratio of voltages
desired. eg if there was 1k2 from Vout to Vref then you would get 1 extra
volt  per kohm of resistance from Vref to ground. This (of course) is not an
ideal world we have the added current. to contend with.

National typically recommend a 240r resistor from Vout to Vref giving
1.2/240 = 5mA "regulation" current. A change of 5uA in the bias current
mentioned above will lead to a 5uA/5mA = 0.1% change in the regulated
voltage. This order of effect is probably fairly acceptable in this class of
regulator.

The next to ideal if you can't have NO current is to have constant current
and this is what the IC designers strive for (while necessarily failing
heroically). It's still far better than if you use a regulator not designed
to be used in this way such as the LM340 / 7805


           Russell McMahon

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2002\11\03@080148 by Olin Lathrop

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> Yes, the adjuster pin for the 317 has only a few uA, but the switch
> application is for the actual output from the regulator, which will be
going
> up to 1.5A. I mentioned the adjustor pin because I needed advice on
> controlling the resistance digitally, hence a digital pot. Will the
IRFP260N
> act as a simple switch with +5V = on and 0V = off?

What are you trying to accomplish overall?  An adjustable voltage source?
If so, there are probably easier means than trying to tweak the adjust input
of an adjustable regulator with a digital pot.


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2002\11\03@085618 by Bruce Douglas

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On the second page of the LM317 datasheet they have a circuit with
bipolar transistors switching different resistors in order to
digitally control the output voltage.  I think this would do what
you want.

Bruce Douglas

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2002\11\03@090640 by Dave Tweed

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Russell McMahon <EraseMEapptechspamspamspamBeGonePARADISE.NET.NZ> wrote:
> National typically recommend a 240r resistor from Vout to Vref giving
> 1.2/240 = 5mA "regulation" current. A change of 5uA in the bias current
> mentioned above will lead to a 5uA/5mA = 0.1% change in the regulated
> voltage. This order of effect is probably fairly acceptable in this class of
> regulator.

There's a second reason for passing 5 mA through the feedback divider:
Since the LM317 dumps (nearly) all of its operating current to the OUT pin,
it has a minimum load requirement of about 5 mA worst case (2-3 mA typical)
in order to maintain regulation. Putting this current through the divider
means that you don't need a separate load to drain the quiescent current.

-- Dave Tweed

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2002\11\03@133623 by Larry Bradley

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Haven't paid too much attention to this thread, but Olin's comments tweaked
a memory of something I played with at one time - a voltage regulator
controlled by a Basic Stamp.

Originally I tried a purely software regulator - I used a serial D to A
chip driving the pass transistor, and an A to D chip reading the o/p
voltage. It actually worked quite well, although it's transient response
was lousy since the Stamp was a slow processor. Using a faster PIC like an
877 would work better.

But then I realized I didn't need to have the processor run the control
loop - the processor just needed to supply the reference voltage for a
traditional regulator. Rather than use an LM317 or the like, I used a
simple op-amp (LF411) as the error amp driving the pass transistor. One of
the op amp inputs was driven from the DtoA converter, the other was the
feedback. This worked like a charm.

I had a small serial DAC to use, but you could use the PWM output and
filter it.

The advantage of this scheme over the idea of trying to control an LM317 is
that the o/p voltage of the regulator is the same as the DtoA output
voltage - no strange calculations are needed to determine what the o/p
voltage would be.

Larry

At 08:00 AM 11/3/2002 -0500, you wrote:
{Quote hidden}

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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2002\11\03@141408 by Jai Dhar

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I'm don't understand your idea 100%, yet, but from what I see, wouldn't that
limit me to a regulation between 0 and +Vcc (ie: 5V)?? The 317/337 combo I am
using lets me have 0-25 and -25 - 0 (since I am using a stepdown transformer
to 50V). I couldn't accomplish this with a DAC, could I?
Quoting Larry Bradley <RemoveMElhbradleyKILLspamspamIGS.NET>:

{Quote hidden}

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2002\11\03@144122 by Larry Bradley

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Sure. My app was in a 12 volt environment.

The PIC/DAC are providing the REFERENCE voltage for the regulator. You
normally use a voltage divider from the regulator o/p to the error amp
input. For example, using a PIC/DAC to get a range of reference voltage
from 0 to 4 volts, if you use a 6 to 1 voltage divider, the output voltage
can vary from 0 to 24 volts.

I'm just suggesting an approach - as I said, I haven't followed the thread
in detail so I'm don't recall the exact requirements.

The resolution of the DAC (8 bit, 10 bit, 12 bit, etc) determine the step
size of your output voltage.

You can have a supply that goes from -25 to + 25, or any other range - by
adjusting the voltage divider, and possibly offsetting the DAC output (from
-2 to +2 volts, using another op amp).

Larry

At 02:11 PM 11/3/2002 -0500, you wrote:
>I'm don't understand your idea 100%, yet, but from what I see, wouldn't that
>limit me to a regulation between 0 and +Vcc (ie: 5V)?? The 317/337 combo I am
>using lets me have 0-25 and -25 - 0 (since I am using a stepdown transformer
>to 50V). I couldn't accomplish this with a DAC, could I?

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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2002\11\03@152731 by Russell McMahon

face
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> Thank you for all your suggestions. But in regards to the current capacity
of
> the dpot, I do'nt think that will be a problem. Reading the lm317
datasheet,
> it appears that only a few uamps pass through the adjustment pin, which
should

> > I am not sure if the ADG736 can handle the [adj] current
> LM317 adj current is only 50-60uA for 10mA < Iout < Imax
> www.analog.com/productSelection/pdf/ADG736_0.pdf
> shows through-current for ADG736 is 30mA max (4066 is 25mA)

> Wherever you get that. LM317 adj current is exactly 120uA no matter what
> the load.

All above are in the order of right but all are wrong in the context (my
turn will come (or may have already :-) )).

While the ADJUST PIN current is in the order of 50 to 100 uA this is not
solely what the control element must handle. It must also handle the current
through the upper resistor of the voltage setting divider which is typically
designed to be 5 mA. Still well within spec for the control device
mentioned. Also note that the voltage at the adjust pin will be 1.2v less
than full output voltage.


           RM

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2002\11\03@182857 by Olin Lathrop

face picon face
> I'm don't understand your idea 100%, yet, but from what I see, wouldn't
that
> limit me to a regulation between 0 and +Vcc (ie: 5V)??

The amplifier circuit driving the final pass element is allowed to have some
voltage gain.  Think of it as a DC amplifier that may not be particularly
fast but can source a hefty current (and probably not sink any at all).  If
it has a gain of 5, then the output can be adjusted from 0 to 25 volts with
an input signal from 0 to 5 volts.  This in turn is easy to produce by low
pass filtering a digital PWM signal from a PIC.


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2002\11\04@051131 by Alan B. Pearce

face picon face
>Reading the lm317 datasheet, it appears that only a few uamps
>pass through the adjustment pin, which should mean that the
>range across the dpot will be between 0 and GND.


Most of the current will be through the 240 ohm resistor from output to
adjustment terminal.

At 1.25V this represents about 2mA. :))))

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2002\11\04@114114 by Peter L. Peres

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On Sun, 3 Nov 2002, Jinx wrote:

*>> Wherever you get that.

The trick to use a 317 is to set up the current through Radj to at least 3
times Iadj. This way the errors caused by temperature etc become
negligible.  This is done by choosing an appropriate resistor between out
and adj. Since the voltage is constant (nominal 1.2V) you set a current
using the resistor which will flow through Radj. F.ex. 240 ohms is very
popular (5mA nominal ~= 100 times Iadj). Also 2k4 or 2k2 (10 times Iadj).
The bottom line is that the current through Radj will be constant. Note
also the minimum load requirement. 240 and 470 ohms between out and adj
fulfill this requirement, 2k4 does not.

Peter

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2002\11\04@223933 by Jai Dhar

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From what I see of your post, are you not suggesting the 'recommended' setup
from National on the LM317 datasheet? (check out http://www.national.com). I'm just
not sure if what you told me is something that I should be applying to my
design or not, considering I planned on having a 240 ohm resistor there
already (as recommended per the datasheet), and either using a digital pot for
adjusting the current to the adj pin, or some other method (which is what I
have been asking, but I think I have it answered). I hope I am not mistaking
your post.
Quoting "Peter L. Peres" <plpSTOPspamspamspam_OUTACTCOM.CO.IL>:

{Quote hidden}

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2002\11\04@230031 by William Chops Westfield

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> The trick to use a 317 is to set up the current through Radj to at least 3
> times Iadj. This way the errors caused by temperature etc become
> negligible.  This is done by choosing an appropriate resistor between out
> and adj.

Well, of course.  The typical application uses a volage divider between the
output and ground, with the middle going off the the "adj" input of the 317
to provide the voltage set point.  In order for this to work easilly, you
need the current through the voltage divider to be much larger than Iadj, or
you'll introduce a lot of error.  Since Iadj isn't tightly spec'ed, or even
(as far as I can tell) guaranteed to be constant over assorted extremities
of operation, I don't think you can just treat it as part of the voltage
divider...

BillW

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2002\11\05@010921 by Dwayne Reid

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At 09:43 AM 11/4/02 +0000, Alan B. Pearce wrote:

>Most of the current will be through the 240 ohm resistor from output to
>adjustment terminal.
>
>At 1.25V this represents about 2mA. :))))

Umm. . . 1.25 / 240 =~ .005

dwayne

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2002\11\05@054646 by Alan B. Pearce

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>>At 1.25V this represents about 2mA. :))))
>
>Umm. . . 1.25 / 240 =~ .005

Yeah this is what comes of trying to do mental arithmetic with a head cold
:)) I knew the current was quite a bit more than the Iadj current though.

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2002\11\05@162646 by Peter L. Peres

picon face
On Mon, 4 Nov 2002, Jai Dhar wrote:

*>>From what I see of your post, are you not suggesting the 'recommended' setup
*>from National on the LM317 datasheet? (check out http://www.national.com). I'm just
*>not sure if what you told me is something that I should be applying to my
*>design or not, considering I planned on having a 240 ohm resistor there
*>already (as recommended per the datasheet), and either using a digital pot for
*>adjusting the current to the adj pin, or some other method (which is what I
*>have been asking, but I think I have it answered). I hope I am not mistaking
*>your post.

The usual way to interface a AD to a 317 is to use an opamp. The opamp
will drive the ADJ pin of the 317, the out-adj resistor will be higher
than 240Ohms (probably 2k2) and the opamp will be connected as dc
amplifier with feedback from out. This way you can control a 40V supply
with a 0-5V standard AD. The 317 becomes a high quality pass transistor
with SOA and current limit (like a LM195 but different) with the opamp
doing the regulation. Pick an opamp with a lot of input voltage range
(rail to rail preferrably). This is not so easy.

The standard application of the 317 is not directly suitable for
electronic control imho. Unless you find a mosfet transistor and map its
individual characteristic probably. The circuit would not be reproducible.

hope this helps,

Peter

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