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'Controlling Resistance with PIC'
1995\12\21@185739 by Gerry Smith

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I am looking for a way to use the pic to control a resistor in a circuit
(to use as a microcontroller controlled variable power supply).  Would
anyone have any ideas how?  Thanks in advance.

1995\12\21@190821 by Clyde Smith-Stubbs

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> I am looking for a way to use the pic to control a resistor in a circuit
> (to use as a microcontroller controlled variable power supply).  Would

Rather than controlling a resistor as such, what you should do is use a PWM
output, either with a PIC that has PWM hardware or by bit-bashing under
interrupt control, filter this to a stable DC voltage and use it as a
reference voltage for your power supply regulator. I'm not sure if this
will work well with the three-terminal regulators like the LM317, though
other people may have ways of using them, but one of the more sophisticated
power supply regulator chips would work fine. Rather than using a fixed
reference, and a variable voltage divider from the output, you would
use a fixed divider with the variable reference generated by the PIC.

Even a simple emitter follower setup - just like the old resistor,
zener and 2N3055 - would work. Just feed the filtered PWM voltage to
the base of the transistor, and the emitter voltage will track it
reasonably well.

Merry Christmas!



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1995\12\21@194008 by Ed VanderPloeg

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      Try a Dallas DS1267 digital pot, @~$5, which has a serial input
      to set two output pots, scalable seperately in 256 increments
      of the nominal value.  10K, 50K, and 100K nominal values
      available.  Takes two PIC i/o pins, can be sync serial module
      or regular i/o pins, either way is <30 lines of code to write a
      new value into the digital pot.

      If budget is really tight, try using PWM control of a FET from
      a PIC i/o pin or two, by which you would essentially be
      creating a switching power supply.  Control is pretty basic,
      but have fun with the noise filtering...


______________________________ Reply Separator _________________________________
Subject: Controlling Resistance with PIC
Author:  pic microcontroller discussion list <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU> at
InterNet
Date:    12/21/95 3:59 PM


I am looking for a way to use the pic to control a resistor in a circuit
(to use as a microcontroller controlled variable power supply).  Would
anyone have any ideas how?  Thanks in advance.

1995\12\22@054310 by mike

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In message <819601260.23810.0spamKILLspamvms.dc.lsoft.com> .....PICLISTKILLspamspam.....mitvma.mit.edu writes:
> I am looking for a way to use the pic to control a resistor in a circuit
> (to use as a microcontroller controlled variable power supply).  Would
> anyone have any ideas how?  Thanks in advance.
>
You could look at multipliying DACs. Maxim do some 12 bit 8 pin
packages. Analog Devices do some. I'm afraid I haven't got the
device number to hand though.

Regards,

Mike Watson
--
Mayes uk

1995\12\22@102155 by Markus Imhof

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mike@D-M-G.DEMON.CO.UK wrote:

>In message <EraseME819601260.23810.0spam_OUTspamTakeThisOuTvms.dc.lsoft.com> PICLISTspamspam_OUTmitvma.mit.edu writes:
>> I am looking for a way to use the pic to control a resistor in a circuit
>> (to use as a microcontroller controlled variable power supply).  Would
>> anyone have any ideas how?  Thanks in advance.
>>
>You could look at multipliying DACs. Maxim do some 12 bit 8 pin
...
Haven't read the original post, but since I hope that the PIC will be part
of the feedback loop, what about a LED/LDR approach, with PWM and a filter
cap to drive the LED ? Slower, and you need some feedback, but cheap.

Or even an ordinary LED-Transistor optocoupler - again with PWM and
filtering for the LED.

Just gives me an idea - what about a switched mode power supply in this way
? Although output filtering at high currents may be a bugger. Any ideas how
high one could push the switching frequency with a PIC and PWM regulation ?
No additional parts except power transistor (and necessary drivers) and
output filter, adjustment via two buttons (up/down), and a LCD dot-matrix
as display :-)

Would need a 17Csomething, though, with built-in ADCs.

Bye
 Markus

1995\12\22@161905 by Scott Stephens

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>I am looking for a way to use the pic to control a resistor in a circuit
>(to use as a microcontroller controlled variable power supply).  Would
>anyone have any ideas how?  Thanks in advance.
>
You could pulse-width modulate an RC integrator to develop the reference
voltage for a regulator, use a CMOS analog switch pulse modulated with an
integrateing capacitor to form a variable voltage divider, an R-2R network
to source current to a current-mirror controlled regulator or pulse modulate
a MOSFet and filter its output.

1995\12\23@035913 by David Knell

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At 15:16 22/12/95 -0600, you wrote:
>>I am looking for a way to use the pic to control a resistor in a circuit
>>(to use as a microcontroller controlled variable power supply).  Would
>>anyone have any ideas how?  Thanks in advance.
>>
>You could pulse-width modulate an RC integrator to develop the reference
>voltage for a regulator, use a CMOS analog switch pulse modulated with an
>integrateing capacitor to form a variable voltage divider, an R-2R network
>to source current to a current-mirror controlled regulator or pulse modulate
>a MOSFet and filter its output.

...and then we'll just reverse the polarity of the neutron flow and connect
the input of the high-frequency tachyon generator to the subatomic particle
spin mirror and..blam! Antimatter!

On a more serious note, you can, provided the voltages around the rest of
the circuit can be made to fit, use a resistor and a CMOS switch in series.
Turn the switch on/off with a variable duty cycle at a sufficiently high
frequency, and the resistor value will appear to change from R (with the
switch on all the time) to infinity, with it off all the time.  I've used
this technique for digitally controlled pots in a guitar amplifier, where
it worked a treat with the switches going at 40kHz or so.  A 'sufficiently
high frequency' is one where the rest of the circuitry doesn't notice
what you're doing too much.

Dave
-----------------
David Knell M.A.
Tel: 01843 846558
Fax: 01843 846608
E-mail: @spam@daveKILLspamspamdave-ltd.co.uk

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