Does a Variable capacitance switch exist?

[tim9000]

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

 

[phinds]

I don't know about a switched bank, but continuously variable caps are readily available.

 

[tim9000]

I don't know about a switched bank, but continuously variable caps are readily available.

Hmm, too small I think. thanks anyway

 

[davenn]

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 ?

 

[tim9000]

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

 

[berkeman]

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

 

[Windadct]

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).

 

[tim9000]

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!

 

[dlgoff]

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.

 

[tim9000]

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

 

[Windadct]

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.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

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).

 

[berkeman]

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.

 

[tim9000]

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.