capacitor theory / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

capacitor theory

1 2 next

Move Thread

LAN_403

Andy

Fri Oct 04 2013, 03:47AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi, I've been thinking(dangerous :)) but what would happen if you put conductive metal between two plates of a capacitor as well as a insulator(50/50 mix), with the capacitor driving by a ac source?

radiotech

Fri Oct 04 2013, 03:53AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Then you would have 2 capacitors in series.

-------] [----] [------

Mattski

Fri Oct 04 2013, 03:58AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

As long as it's planar i.e. putting sheets of metal interspersed with sheets of dielectric between the outermost capacitor electrodes then it's just a number of capacitors in series each with a capacitance of epsilon*area/distance. You'd find that the capacitance at the outermost electrodes is just epsilon*area/(total dielectric thickness).

Andy

Fri Oct 04 2013, 04:04AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Wouldn't the voltage on the metal inner parts be zero, a cheap test showed voltage(more you separated the out most electrodes), or would they share the current? With the lower capacitance wouldn't that have more impedance, but the metal is conductive?

Thanks

Mattski

Fri Oct 04 2013, 05:27AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Look at radiotech's diagram, you can treat an inner plate as an a floating circuit node just as he has drawn. It's the same as connecting two physically separate parallel plate capacitors with a wire, except you're leaving out the wire and just connecting the plates directly.

That inner plate will have a voltage on it because it's a capacitor divider.

Ash Small

Fri Oct 04 2013, 08:31AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Andy wrote ...

Hi, I've been thinking(dangerous :)) but what would happen if you put conductive metal between two plates of a capacitor as well as a insulator(50/50 mix), with the capacitor driving by a ac source?

Andy wrote ...

Wouldn't the voltage on the metal inner parts be zero,
Thanks

If you have three identical plates, and one outer plate has a positive charge, and the other outer plate has an equal, but negative charge, then the charge on the centre plate will be zero (I think), but the voltage between the inner and outer plates will be half the voltage between the two outer plates (This doesn't apply for non-polar electrolytics, though, I think)

Andy

Fri Oct 04 2013, 07:40PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thats what I thought, the simulator shows zero voltage with three capacitors in series and measuring across the middle one. With the test there were four plates measuring the two inner plates showed a voltage, and more farther apart and when adding a high dconstant.
The capacitor divider makes sense, just curose how many amps would flow in the two inner plates, as the voltage could get quite high.

Ash Small

Fri Oct 04 2013, 08:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The charge on all capacitors in series will be the same, because all pass identical current. capacitors with a larger value will have less voltage across them. I forget the maths, but it's easy to google.

klugesmith

Sat Oct 05 2013, 03:30AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Andy wrote ...

Thats what I thought, the simulator shows zero voltage with three capacitors in series and measuring across the middle one. With the test there were four plates measuring the two inner plates showed a voltage, and more farther apart and when adding a high dconstant.
The capacitor divider makes sense, just curose how many amps would flow in the two inner plates, as the voltage could get quite high.

What kind of simulator are you talking about?

If you make a circuit model with two or more capacitors in series, creating an intermediate node with no DC path to ground, analog circuit simulators should give you an error message. The initial voltage on a floating node could be any value, but the simulator needs to be given a value to start with.

Simulators that let you get away with that, not even squawking, are in my opinion flawed. Kind of like LT SPICE giving inductors a finite resistance by default, that is invisible by default in the schematic capture view. User should have to choose when to model the non-ideal parasitics of any primitive circuit element.

Andy

Sat Oct 05 2013, 04:10AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

National instruments multisim, but its not just that, connecting two 1uF microwave caps at 90khz and reading the voltage across one, it shows 0.1volt from a 150volt source, but the 4 plate thingy is showing 40volt from the two middle ones. Wouldn't the impedance I think 1768ohms for a MOT cap at least show 75volt?

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob