# How do I test 10-12 pF range capacitor

Here is the situation: I have few unknown 1% high precision capacitors and I know they are either 10pF or 12pF, and I have few known 12pF 1% capacitors.

Q1, How can I test those unknown 10-12 pF capacitors?

Q2, Can I use a MCU to test them? I don't have high end equipment.

Q3, Since I have a known 12pF capacitor, can I use it as a reference to test the other unknown capacitor? HOW?

I need to use them in a RTC circuit. I know if I put them in my RTC board, and have the board run for a week, then I can be sure which one is which, but please tell me that there is a smarter way to test those little guys.

• Do you have precise resistors around as well? Function generator? Oscilloscope? – Eugene Sh. Jun 18 '18 at 21:16
• @Eugene Sh. Yes, I do. but I don't have a Oscilloscope – Atmega 328 Jun 18 '18 at 21:23
• That would be tough then. – Eugene Sh. Jun 18 '18 at 21:24
• @Eugene Sh. I don't need to measure the exact value. I just need to know which one is larger. Can a simple comparator circuit work? – Atmega 328 Jun 18 '18 at 21:27
• This statement of the question is much more interesting. You can connect a resistor and two of the caps in series and see the voltage drop on each. It is proportional to the reciprocal of the capacitance. So you can figure out which one is bigger. – Eugene Sh. Jun 18 '18 at 21:32

You can use an Arduino board to measure rise time of an RC network built with a precise resistor and your unknown capacitor. And simply you can measure $τ$

The RC time constant, also called tau, the time constant (in seconds) of an RC circuit, is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e.

$τ = RC$

(Source: Wikipedia - RC time constant)

Well, with a simple measurement you have to rely on the threshold voltage of your Arduino (or any other microcontroller that you use) from low to high.

Be sure that you use at least 1M ohm as series resistor. Even when your resistor is not precise, you can measure all your capacitors with the same resistor and finally you will clearly see 2 different groups.

Some years ago, I wrote a very simple Arduino program that does exactly what you ask, but I don't keep it somewhere. You can find similar projects on internet that you can use without coding.

• Indeed, on wouldn't think this would work, but done right it actually does have picofarad resolution. Try to use the ATmega hardware comparator to directly control the hardware timer. To make something more useful, add a zero button to calibrate (subtract) out the stray capacitance. Beware of leakage paths - I soldered one together and it worked, then cleaned it with alcohol spreading around flux residue and it wouldn't work again until I got it super clean. Breadboard works for a quick setup though. – Chris Stratton Jun 18 '18 at 22:32
• nice try. My friend, but this is not even close to good enough for me. First of all, why do you need to use at least 1M ohm? Second of all, I am seeking for an answer to my problem, not your past romantic history. Last but not least, I would have been tired if I have an arduino board like that with that kind of fancy display and all in the site that you have directed me to, so your answer for my simple testing a tiny capacitor question is kinda giving me more headache than before. – Atmega 328 Jun 19 '18 at 23:33