Post by thread:Controlling Resistance with PIC

> 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! -- Clyde Smith-Stubbs | HI-TECH Software, | Voice: +61 7 3300 5011 spam_OUTclydeTakeThisOuThitech.com.au | P.O. Box 103, Alderley, | Fax: +61 7 3300 5246 http://www.hitech.com.au | QLD, 4051, AUSTRALIA. | BBS: +61 7 3300 5235 ---------------------------------------------------------------------------- FREE! Download our shareware (FREE for noncommercial use) MS-DOS C Compiler! Point your Web browser at http://www.hitech.com.au/

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 <.....PICLISTKILLspam@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.

In message <819601260.23810.0KILLspamvms.dc.lsoft.com> .....PICLISTKILLspam.....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

mike@D-M-G.DEMON.CO.UK wrote: >In message <EraseME819601260.23810.0spam_OUTTakeThisOuTvms.dc.lsoft.com> PICLISTspam_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

>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.

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@daveKILLspamdave-ltd.co.uk