Does a Variable capacitance switch exist?

  • #1
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Hi,
I want to be able to vary the amount of capacitance in a circuit, ideally I'd like to be able to turn a knob to tap a bank of capacitors, something like the attached picture.


Is there some sort of switching or device available that anyone can think of which does this?

Cheers
 

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  • #2
phinds
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I don't know about a switched bank, but continuously variable caps are readily available.
 
  • #3
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I don't know about a switched bank, but continuously variable caps are readily available.
Hmm, too small I think. thanks anyway
 
  • #4
davenn
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ideally I'd like to be able to turn a knob to tap a bank of capacitors, something like the attached picture.
not too difficult to build a switched bank of caps
you labelled your image ... "capacitance modulation" .... what do you mean by that ?
what are you trying to achieve ?
 
  • #5
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not too difficult to build a switched bank of caps
you labelled your image ... "capacitance modulation" .... what do you mean by that ?
what are you trying to achieve ?
Well I wasn't really thinking about the name, anything's better than 'Untitled', but where the need arose was from an idea for a filter I've been thinking about. I wanted to easily tweak the amount of bipolar capacitance across something (something like 1uF to 500uF for instance). At this point turning a knob would be best, but even if I could do it electronically that would be good (but I wouldn't want any semiconductor voltage drop because minimal series resistance etc. is what I'm after).
Thanks
 
  • #6
berkeman
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I wanted to easily tweak the amount of bipolar capacitance across something (something like 1uF to 500uF for instance)
For non-polar capacitance, that's pretty big. Are you familiar with how to use 2 polar caps back-to-back to make a non-polar capacitor (of half the value)? That is done in audio circuits pretty commonly, since audio frequencies typically require fairly high capacitance.

And a mechanical switch sounds like your best bet for changing the capacitance. Be careful of large current transients if you switch capacitors at non-zero voltages. You probably want to use multiple values that can be switched in and out (via relays?), so you can vary the total value fairly smoothly...

http://www.mouser.com/images/globalspecialties/lrg/CDB-10-1.jpg
CDB-10-1.jpg
 
  • #7
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Digitally with Relays? Rotary switch - there are lots of ways to do this, still a lot to be defined. Operating voltage, load (current), speed, resolution ( 1 to 500uF -in what size steps - 1uF resolution would take about 10 steps).
 
  • #8
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For non-polar capacitance, that's pretty big. Are you familiar with how to use 2 polar caps back-to-back to make a non-polar capacitor (of half the value)? That is done in audio circuits pretty commonly, since audio frequencies typically require fairly high capacitance.

And a mechanical switch sounds like your best bet for changing the capacitance. Be careful of large current transients if you switch capacitors at non-zero voltages. You probably want to use multiple values that can be switched in and out (via relays?), so you can vary the total value fairly smoothly...

http://www.mouser.com/images/globalspecialties/lrg/CDB-10-1.jpg
View attachment 195300
It had occurred to me to use two back to back polar caps to make a bipolar, but I was worried it might still fatigue them. Does it?

Yeah, a mechanical switch was what I was after, like a sliding bar, but I want to avoid relays and extra complication.

Digitally with Relays? Rotary switch - there are lots of ways to do this, still a lot to be defined. Operating voltage, load (current), speed, resolution ( 1 to 500uF -in what size steps - 1uF resolution would take about 10 steps).
If there was a rotary switch that maintained connection with the previous taps that would be great, rather than just taping a new section. Do you know of one?
Thanks!
 
  • #9
dlgoff
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You haven't said at what voltages the caps will be experiencing and at what kind of load. Switch contacts are voltage rated; think arc welding.
 
  • #10
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You haven't said at what voltages the caps will be experiencing and at what kind of load. Switch contacts are voltage rated; think arc welding.
I haven't really decided, for arguments sake say 240VAC but no more current than an Ampere, probably a lot less. I really would like some sort of rotary tap selector switch or rotary contact bar preferably.

Thanks
 
  • #11
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Search for Programmable Rotary Switch, EXAMPLE - when you see the price tag, you may head back to relays - really not that difficult. They do make Power Factor correction systems with film caps and relays like that.
 
  • #12
berkeman
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Thread closed temporarily for Moderation...
 
  • #13
berkeman
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I haven't really decided, for arguments sake say 240VAC but no more current than an Ampere, probably a lot less
Holy crap, @tim9000 you hadn't mentioned using AC Mains voltages up until now. That can be very dangerous, especially since you don't have much experience with circuits yet. please send me a PM describing exactly what you are wanting to build this for, so I can try to make a decision about letting this thread go on. At the very least, you will need to synchronize any disconnection/connection of capacitors with the zero-crossing of the AC Mains voltage waveform (assuming a non-inductive load).
 
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  • #14
berkeman
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After a PM discussion with @tim9000 this thread is re-opened. Tim understands some of the issues in working with AC-Mains powered projects, and he understands that it is a good idea to power off his circuit before switching in different capacitors.
 
  • #15
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I will post an update on the progress of my project and design details in due course for anyone interested; I have some prototyping to do and some things to buy.
 

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