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Author Topic: Copper Foil and Glass Capacitors  (Read 6036 times)
K3AN
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« on: July 29, 2012, 08:31:31 PM »

A project I'm considering requires some fixed-value (less than 50 pf) capacitors, but a high breakdown voltage (2500 Volts minimum) is required. Air variable capacitors with the necessary plate spacing are large and also expensive. I'd have to series at least five 500 Volt mica caps (identical value, 2% tolerance) to get the necessary voltage rating

By searching the web I found that regular picture glass has two interesting properties- a breakdown voltage that's at least 5 times that of air for the same plate spacing, and a dielectric constant that's 7.5 or so vs a dlelectric constant of 1 for air. Using a parallel plate capacitor calculator I found on the web (and verified using the appropriate scientific formulas) I found that two conductive plates of 2" x 2" (4 square inches) each, separated by 1/8 inches of glass has a capacitance of 53 pf, which is more than I need. I can also use microscope slides with a thickness of .05" which would get the necessary plate area down to just 1.5 square inches or less.

Would a copper foil and glass capacitor have a suitable loss performance at HF frequencies? Any guess as to how thermally stable such a capacitor would be?

Thanks,
K3AN
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G3RZP
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« Reply #1 on: July 30, 2012, 12:39:47 AM »

A problem I had was the electric field at the plate edges literally punched through the glass. I think you need fairly thick material and rounded edges. That was about 1200 volts rms at 2 MHz by the way, and a 150pF capacitor.

I see Alpha make some padder caps in their amp with teflon.
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W4OP
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« Reply #2 on: July 30, 2012, 11:00:09 AM »

What is the loss factor "D" of plate glass? I think there may be quite a variation.

You might consider Rogers Duroid PCB. it has 750V/mil breakdown and is easily sheared. Available in 0.062" thicknesses.

Dale W4OP
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G3RZP
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« Reply #3 on: July 30, 2012, 12:19:42 PM »

Fair Radio Sales of Lima, OH., have a whole slew of small value 850 series 5kV ceramics below 50pF in various values at $8.25 each. Which would be smaller.
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KE3WD
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« Reply #4 on: July 30, 2012, 01:35:38 PM »

I used to make small high-voltage caps using glass and foil back when I was young and on a limited budget. 

It can be done, and it can work.  Matter of fact, copper foil on glass capacitors were used by the great Nilola Tesla, handling quite a bit of voltage.  In his case, many times he would immerse the large glass sheets in oil to prevent the arcing-at-edges-of-foil problem, but you don't have to do that for this kind of work. 

The trick is to have no rough or sharp edges or corners on the foil.  Serrated edge trimming scissors, such as the average "tinsnip" aren't good here, as they leave tiny burrs, each burr acting as an arc point where premature breakdown can occur. 

Also, rounding the four corners of each copper piece such that there isn't the sharp point of the ninety degree corner anymore, but a nicely radiused arc. 

Very sharp X-acto type knife blades were my tool of choice, applying the foil with adhesive to the glass first in an oversized for the task size, then using template edge to carve the undesired foil away, plus those rounded corners.  Then I would use a small wood spudger made from bamboo chopstick to further smooth down the edges. 

Small capacitors made in this fashion, you can solder leads to each foil side and then spray the whole deal with a couple coats of nonconformal clear electronic spray coating, and solder it in by the leads just as if it was a store bought disk or the likes. 

And yes, microscope slides make a great material source for the smaller stuff. 

But really.  These days, you might find it better to bite the bullet and just purchase high voltage caps, your call. 


73
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KB4QAA
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« Reply #5 on: August 04, 2012, 04:26:35 PM »

Window glass and foil capacitors were used by hams for HF/LF up into the early twenties.

Not hard to make.  If you want to improve the breakdown voltage performance, place the capacitor stack into a vessel, and cover it with mineral oil (from any pharmacy).
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W8JI
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« Reply #6 on: August 05, 2012, 05:33:05 PM »

Every higher voltage RF glass capacitor I've tried to make has shattered. I'm sure there are some quartz glasses that will work, but various types of common glass have always shattered.

Tesla coils were not at 5 MHz. :-)
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KE3WD
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« Reply #7 on: August 05, 2012, 07:13:42 PM »

Good to see yer gruff voice back on here, Tom.

Missed ya.


73
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G3RZP
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« Reply #8 on: August 06, 2012, 07:23:12 AM »

I got identical results to Tom.
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KD0REQ
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« Reply #9 on: August 07, 2012, 09:49:11 AM »

had a neighborhood buddy who made a half-million volt Tesla coil, and blew up two banks of foil/glass capacitors before sanding the edges and dunking them in mineral oil.  almost as bad a mess as his hydrogen plant.

I think that would be megalossy at RF... there are spikes up there, but not as many as I expected with a spark gap generating the magnetic field changes.  didn't mess up the AM band too badly.  on that basis, I think it's a low-dissipation dummy load you're building.
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ZL1BBW
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« Reply #10 on: August 12, 2012, 11:30:15 PM »

The ARRL Antenna Compedium vol 2 pages 182 ... 186  Remotely Controlled Antenna Coupler by W0RPV ha a design and usage of a glass plate variable C for a tuner unit.
« Last Edit: August 12, 2012, 11:31:46 PM by ZL1DAB » Logged

ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
G3RZP
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« Reply #11 on: August 13, 2012, 04:10:27 AM »

I seem to remember he wasn't running much power, though.

BTW, were you at GKA when G3ZRJ was there?
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ZL1BBW
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« Reply #12 on: August 13, 2012, 01:13:00 PM »

Yes, I know Tony well, well talk on Skype quite often.  I did not go to the other place that Tony went to, though.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
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