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Does a Variable capacitance switch exist?

  1. Apr 16, 2017 #1
    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
     

    Attached Files:

  2. jcsd
  3. Apr 16, 2017 #2

    phinds

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    I don't know about a switched bank, but continuously variable caps are readily available.
     
  4. Apr 16, 2017 #3
    Hmm, too small I think. thanks anyway
     
  5. Apr 16, 2017 #4

    davenn

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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 ?
     
  6. Apr 17, 2017 #5
    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
     
  7. Apr 17, 2017 #6

    berkeman

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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
    CDB-10-1.jpg
     
  8. Apr 17, 2017 #7
    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).
     
  9. Apr 17, 2017 #8
    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.

    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!
     
  10. Apr 18, 2017 #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.
     
  11. Apr 18, 2017 #10
    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
     
  12. Apr 18, 2017 #11
    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.
     
  13. Apr 18, 2017 #12

    berkeman

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    Thread closed temporarily for Moderation...
     
  14. Apr 18, 2017 #13

    berkeman

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    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).
     
  15. Apr 21, 2017 #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.
     
  16. Apr 21, 2017 #15
    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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