# You're Welcome -- The capacitance of double clad copper glass-epoxy PCB between the sides.

Discussion in 'Wireless & RF Design' started by KL7AJ, Jul 8, 2016.

1. ### KL7AJ Thread Starter AAC Fanatic!

Nov 4, 2008
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A 4" x 5-1/2" x 1/16" piece of double clad copper glass-epoxy PCB has 975 pF between the sides. Now you know.

You're welcome.

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2. ### tracecom AAC Fanatic!

Apr 16, 2010
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Is there a linear relationship between the capacitance and the area? In other words, if the piece was 2' x 5 1/2", would the capacitance be 487 pF?

3. ### #12 Expert

Nov 30, 2010
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The basic equations for how to make a capacitor seem to confirm that idea.

That means, "yes" (but my supper was beeping so I had to run away for a minute.)

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4. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Interesting.... never thought of a PCB as a substitute for a capacitor... what would be its advantage, higher voltage capability?

5. ### #12 Expert

Nov 30, 2010
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Two conductors divided by an insulator is the definition of a capacitor. Circuit board capacitance is an inescapable fact caused by its shape. Area and distance were provided in the first post. The only thing missing to write the complete equation for circuit board capacitance is the dielectric constant of fiberglass.

6. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Yes, I understand all that, thank you. It's just that I think it's very easy to get a commercial capacitor of 975 pF (almost 1 nF), but it would probably have a voltage rating of a few hundred volts at most.
What I'm wondering is if using PCB's as caps could maybe work with much higher voltages. After all, when caps fail it's mainly because their insulator breaks down due to over voltage, isn't it?

7. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Better than some, worse than others. For even higher voltages, try a Leyden jar.

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8. ### #12 Expert

Nov 30, 2010
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9. ### cmartinez AAC Fanatic!

Jan 17, 2007
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MMMhhh.... a ceramic disk capacitor of 1,000 pF, rated at 15kV, for only \$24.81 ... I'd still like to know how much voltage a makeshift PCB on could take ...

Edit: and temperature stability would be interesting to know too

Last edited: Jul 8, 2016
10. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Man, what a memory... the Leyden jar was my first love in electronics when I was a kid. I just loved to play with it.

11. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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You can figure it out yourself.

12. ### #12 Expert

Nov 30, 2010
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I think joeyd helped invent them.

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13. ### cmartinez AAC Fanatic!

Jan 17, 2007
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I still wonder about temp stability, though.

14. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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You need to use dielectric strength. The breakdown voltage is a function of board thickness.

15. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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If I should only be so famous.

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16. ### cmartinez AAC Fanatic!

Jan 17, 2007
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I very much doubt that's what you really want... you've (wisely, imho) done everything possible to protect your privacy in this place

Apr 5, 2008
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18. ### atferrari AAC Fanatic!

Jan 6, 2004
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Wondering if not too prone to unpredictable variations due to changes in temperature, depending of next of what you assemble them, fans, etc. Maybe not hard to test, now that I think of it...

19. ### tcmtech Well-Known Member

Nov 4, 2013
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I made a few of those but most of what I played with required higher capacitance at lower voltages so I used whatever paper I could find and aluminum kitchen foil.

Oil soaked newspaper was good for at least a few hundred volts per layer.

20. ### #12 Expert

Nov 30, 2010
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The board might be good for 50KV, but don't forget the path around the edge of the board. That tiny air gap is not good for 50KV.

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