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Here's the project: I'm building a capacitor by hand to essentially measure the permittivity constant. I am doing so by building a capacitor of known dimensions, setting up an RC circuit and plotting the cap. discharge. With the known resistance, the cap. should discharge according to the model

V(t) = V0e^-t/RC

where V0 = the battery voltage (i'm using a 1.5V Dcell), R = the resistance of the circuit (i'm using megaohm resistors - between 3-22MΩ - i have 10 of each), and C = the capacitance = KEA/d where K = dielectric constant (3.85 for paper), A = plate surface area, d = distance between the plates (im separating the plates with paper but may try other materials) and E = the permittivity of free space.

I'm measuring v(t) with a program called logger pro. It basically does exactly what i need; it plots voltage over time.

So here's my problem. I can get everything to work perfectly with a pre-built capacitor. The circuit works, the resistors work as predicted, etc.

But when I hook my one of my many attempts at a home-made resistor (one in which i can actually know the dimensions), I start seeing really weird things.

I hook up the cap. to the battery, and the voltage across the cap goes up to about 1.5V so the cap is storing charge. But as soon as I disconnect it from the battery, the voltage drops instantly. Then rises. Sometimes it rises to a voltage greater than 1.5V! It goes through these really random curves. Keep in mind, at this point, the circuit isnt even completed! its just a cap with a voltage meter and open wires!

The actual circuit design has resistors on a switch so that I can charge the cap, flip the switch, and once the switch is flipped, current flows through the resistors and i can track the discharge. Whenever i try that, again i see really random patterns. As soon as i flip the switch, the voltage across the cap drops into the negatives, then starts gaining voltage, and even gains positive voltage!

I've tried a few different cap designs, but i think I'm clearly missing something. My current model has two small strips of copper tape across a piece of paper. I made sure that the copper sheets are not in contact, and since there is a voltage across the cap, it must work.

I've also tried aluminum foil sheets over paper; similar results. but again, the curves and voltage fluctuations are different every time.

Can anyone think of solutions? Perhaps a better cap design, or a better circuit design?

thank you guys so much! again, i dont know if this really belongs on this forum but i appreciate any help u guys can offer!!

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    \$\begingroup\$ Paper is a really poor choice for the insulator (dielectric) as it absorbs water and becomes conductive. If only there was a very thin, self sticking plastic material sold in rolls for wrapping sandwiches. \$\endgroup\$
    – JIm Dearden
    Feb 20, 2017 at 11:50
  • \$\begingroup\$ Crude film-foil capacitors can have enough trapped air to throw you numbers off and made them unstable. This is true to a small extent even in store-bought capacitors. \$\endgroup\$
    – Robert Endl
    Feb 21, 2017 at 19:52

1 Answer 1

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I suspect it is a combination of a few things: 1) The capacitance is much lower than when you are using a reference capacitor. 2) The discharge time is much shorter with your self-built capacitor 3) You are picking up 60/50Hz noise that is compromising your measurements.

Have you calculated how much capacitance you should be getting?

What value are you using when you try it with a reference capacitor?

Try shielding the system, for example putting it into a metal box. Even a grounded metal plate under the test system may help.

Why are you only using 1.5V? I would use a much higher voltage to reduce the effects of noise. What voltage does your data acquisition system tolerate?

What is the input impedance of the acquisition system.

How long does the capacitor discharge take with the reference capacitor. What is the sample rate of the acquisition system?

kevin

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  • \$\begingroup\$ The hand built cap that i'm using right now has a surface area of around 0.041249m^2 so i'm expecting C = 3.85*(8.85*10^12)*0.041249/(1*10^-4) = ~1.4*10^-8. The reference cap is a 2.2µF cap. The discharge takes around 16 seconds with a 10MΩ resistor. The volt meter can handle up to 6V so I will try a bigger battery. Thank you for the advice! i'll give those things a try!! \$\endgroup\$
    – Alex Ezzit
    Feb 20, 2017 at 2:14
  • \$\begingroup\$ I can't make your calculation work, I get 1.4*110^16. What is the dielectric and how thick is it? It is about 200mm square, right? \$\endgroup\$
    – Kevin White
    Feb 20, 2017 at 2:38
  • \$\begingroup\$ oh sorry it should be 8.85*10^-12 (permittivity of free space). The dielectric is just photocopy paper, about 0.1mm thick \$\endgroup\$
    – Alex Ezzit
    Feb 20, 2017 at 2:40
  • \$\begingroup\$ From daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml I only get a capacitance of 0.008uF (er = 2.3) which is less than 1/250 of your reference and will discharge in only ~50ms. The large area will also pick up noise more easily. You could try multiple layers of paper/foil interleaved to use both sides of the foil and increase the total area. \$\endgroup\$
    – Kevin White
    Feb 20, 2017 at 2:48
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    \$\begingroup\$ First make your electronics work with a known good 10nf capacitor, since that is in the same range of values as your test cell. \$\endgroup\$
    – user16324
    Feb 20, 2017 at 10:13

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