How to vary Capacitance using a variable resistor
How to vary Capacitance using a variable resistor
(OP)
I have to adjust the integral gain of a PID controller. One would normally use a capacitor decade box externally and select the optimum capacitor value for stable operation and solder that value ultimately.
In my case, I have to use a microcontroller to automatically tune and stabilise the system. I can only use digital potentiometers for changing the Capacitance value.
Any ideas ?
In my case, I have to use a microcontroller to automatically tune and stabilise the system. I can only use digital potentiometers for changing the Capacitance value.
Any ideas ?





RE: How to vary Capacitance using a variable resistor
RE: How to vary Capacitance using a variable resistor
A couple of thoughts.
1. You could use a varactor diode (although the capcitance values are small)
http://www.americanmicrosemi.com/tutorials/varactor.htm
for a preview
2. How about an analog switch which selects different capacitors
http://dbserv.maxim-ic.com/tarticle/view_article.cfm?article_id=90
for am introduction
Any help ?, yes no let me know.
Regards
RE: How to vary Capacitance using a variable resistor
RE: How to vary Capacitance using a variable resistor
Varacter diodes I ruled out because of too low a capacitance value
With analog switches I can only select a range but i can't continuosly vary the integral gain within that range.
I had long ago built a capacitance multiplier. The miller capacitance effect which is normally a nuisance, was effectively used in that circuit and was multipied by an op-amp. I built a PI controller using that and it worked beautifully. I could simulate high capacitance values using smaller value capacitors and vary the value with a pot.
I do not have that circuit with me. Any body who has done similar things ?
kps
RE: How to vary Capacitance using a variable resistor
There is a simple controlled cap circuit which can reduce
the effective value of C from the nominal:
The value of C = max. required. Uses 2 VF Voltage followers
Wire list (one node per line):
Input --- C.1 --- VF1 inp
Potmeter.hi --- VF1 outp.
Pot.low --- gnd
pot.wiper---VF2 input
VF2 output --- C.2
--------------------------------------
Op-amp integrator can be adjusted in wide range
Digital control is simple and cheap. NEC gives away
development SW.
<nbucska@pcperipherals.com>
RE: How to vary Capacitance using a variable resistor
So you blew out my varactor & switched capacitance ideas
Try
http://www.hep.upenn.edu/SNO/daq/parts/lm310.pdf
On page 12 is a variable capactor circuit using the multiplier principle.
or for millar intergrators see
http://lucas.elo.utfsm.cl/elo_b/an-32-2.pdf
Page 1
If you have any trouble with these pdfs I can send them to you if you wish.
Any help ?, yes no let me know.
Regards
RE: How to vary Capacitance using a variable resistor
Thank you nbucska.
For certain reasons I am not supposed to use digital control. The controller part of it has to be purely analog. But I am allowed to set all the parameters of the analog controller using digital pots. The microcontroller is used to change these pot settings under software control.
In a simple PI controller with one resistor and one capacitor in the feedback path, the resistor is varied to vary the P-Gain and the capacitor is varied to vary the integrating time. That is the reason why I was looking for a circuit idea which would enable me to vary the capacitance value with a pot.
I am also interested in PID controller circuits with multiple op-amps where each parameter can be independently varied without any interaction with other parameter.
Thank you guys for being so helpful.
KPS
RE: How to vary Capacitance using a variable resistor
I usually have a main capacitor and have one or two or many parallel variactors with it and can control the overall value of the cap with a D/A converter connected to the variactors.