Building a HV AC capacitor

[salter]

In previous threads, i have mentioned that i am building a tesla coil with no physics/electronics knowledge beyond what I have taught myself. So if you see something in the paper about a kid blowing himself up it's probably me. lol.

But anyway, I need to build an HV capacitor somewhere between 1 and 40 nF rated for close to 28k volts to account for surges and gains in the capacitor. so i did some research... eventually i chose plexiglass for my dielectric. I picked up some .093 inch plexiglass at Home Depot, and proceded to cut out 3 inch squares. i then made 2.5 inch circles of aluminum foil using the bottom of a can and alternated between aluminum and plexiglass, lining up the circles to the best to my ability, putting out tabs of aluminum to the right side for the first aluminum circle and then to the left for the second, etc. I hot glued around each plate to prevent contact between plates of the opposite charge. I used 20 plates. I then linked all the tabs on either side by folding them together. i attached the leads of my 10k volt 30mA 60Hz neon transformer to the tabs, then added a second set of wires going off that i could short to discharge the capacitor. I submerged the entire apparatus in mineral oil as an additional precation against arcing.

Well, as I'm sure anyone with any knowledge of basic electronics (not myself) has figured out, it didn't work. I got small sparks off of the secondary wires when i touched them (while the apparatus was plugged in), but not even enough to make an arc. There was nothing after i unplugged it. The capacitor retained no charge. I figure that, of all the possible reasons, two of the most likely are that either: a. i accidentially made a short when building the capacitor (no visible arcs existed) or b. the aluminum foil isn't conductive enough and i should get copper sheets.

i suppose the AC could have something to do with it...

But the bottom line is i dont know why it didn't work. So any help would be appreciated.

 

P.S. i understand that the capacitor doesn't match my goal of 1 to 40nf, but that was because i decided to test it before i finished. The one i built should have been about .9nF according to my calculations.

 

Thanks

[Xittenn]

Guelph's cool.............

 

http://www.uoguelph.ca/~antoon/circ/hv/hvcap/hvcap.html

 

ummmmmmmmmmmmmm if you want a proper charge/discharge scenario you should probably go DC. I mean think of this logically if you use an AC current you charge in one direction and then discharge and charge in the other. When you disconnect the chances are pretty good you'll be at a voltage less then your peak or RMS. I'm sure if you repeated a bunch of times you'd get it at a good point so that's pretty insignificant.

 

.000000001*28000= <28 micro-amps

 

I'm pretty sure that would discharge faster than your eye would have the time to actually see it........................I've never tried myself, 240v three phase was enough to make me fear the reaper.............

[UC]

A capacitor is effectively a break in the circuit, AC is a constant shuffling back and forth of electrons in the wires. There is no push against the capacitor to build up any charge for the reasons buttacup stated above. You need to use DC current if you want anything to happen.

 

Fundamentally, capacitors block DC current (and build charge) and pass AC current. Some kinds are unidirectional and blow up if you wire them backwards while others will work in either direction.

 

You should use copper sheeting anyway. It's quite a bit sturdier

[Xittenn]

A capacitor is effectively a break in the circuit, AC is a constant shuffling back and forth of electrons in the wires. There is no push against the capacitor to build up any charge for the reasons buttacup stated above. You need to use DC current if you want anything to happen.

 

Fundamentally, capacitors block DC current (and build charge) and pass AC current. Some kinds are unidirectional and blow up if you wire them backwards while others will work in either direction.

 

You should use copper sheeting anyway. It's quite a bit sturdier

 

Then why is Xc = 1/(2*pi*f*C) ?

 

Assuming 28000VAC @ 60hz you're current curent is aprx. 10.5mA. What I had originally posted as 28micro-Amps was the coulombs of charge within the capacitor after disconnect assuming full charge(actually it could be as high as 40kV in an AC circuit due to peak voltage.) The total Impedance of your circuit assuming nothing else is in it is apx. 2.5MOhms.

 

I don't know what you are using for a transformer(I needed to read again) but if you're planning on obtaining any kind of capacitance in the range of 30nF @ 28000V don't. You are looking at roughly 90kOhms which is like .316Amps notice no mili micro or nano which is like you do the math 9000Watts of pumpin'. Not your standard household appliance.

 

I'm not exactly sure what UC means by that.......sort of but like.....A capacitor still charges under AC conditions it just charges back and forth for short periods of time(relative to frequency.)

 

40nF ummmmmmmmmmmmmmmmmm................................... try 2 no more than 4; what's wrong with 1!

 

I could redo that for 10kV

 

You max out your little transformer @ 7nF just so you know; generally you want to go a little bit less than max..........

Edited February 10, 2009 by buttacup