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

#### KL7AJ

Joined Nov 4, 2008
2,229
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.

#12

#### tracecom

Joined Apr 16, 2010
3,944
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?

#### #12

Joined Nov 30, 2010
18,224
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?
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.)

#### cmartinez

Joined Jan 17, 2007
8,252
Interesting.... never thought of a PCB as a substitute for a capacitor... what would be its advantage, higher voltage capability?

#### #12

Joined Nov 30, 2010
18,224
Interesting.... never thought of a PCB as a substitute for a capacitor... what would be its advantage, higher voltage capability?
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.

#### cmartinez

Joined Jan 17, 2007
8,252
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.
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?

#### joeyd999

Joined Jun 6, 2011
5,283
What I'm wondering is if using PCB's as caps could maybe work with much higher voltages.
Better than some, worse than others. For even higher voltages, try a Leyden jar.

#### cmartinez

Joined Jan 17, 2007
8,252
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#### cmartinez

Joined Jan 17, 2007
8,252
Better than some, worse than others. For even higher voltages, try a Leyden jar.
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.

#### #12

Joined Nov 30, 2010
18,224
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.
I think joeyd helped invent them.

#### cmartinez

Joined Jan 17, 2007
8,252

I still wonder about temp stability, though.

#### joeyd999

Joined Jun 6, 2011
5,283
I still wonder about temp stability, though.
You need to use dielectric strength. The breakdown voltage is a function of board thickness.

#### joeyd999

Joined Jun 6, 2011
5,283

#### cmartinez

Joined Jan 17, 2007
8,252
If I should only be so famous.
I very much doubt that's what you really want... you've (wisely, imho) done everything possible to protect your privacy in this place

#### atferrari

Joined Jan 6, 2004
4,768
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?
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...

#### tcmtech

Joined Nov 4, 2013
2,867
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.
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.

#### #12

Joined Nov 30, 2010
18,224