The Capmeter

measuring capacitances with an AVR

Motivation

It is often useful to know the capacitance between conductors such as wires, PCB traces or the legs of a capacitor. Being able to measure such capacitances is a great tool to verify the value of capacitors or other components, to test cables or to analyse circuits.

It is perfectly possible to buy multimeters able to measure capacitance and to buy dedicated capacitance meters. But these devices tend to be expensive, so I decided to make one myself. Besides, the capmeter presented here has a special feature that makes it stand out from at least the more inexpensive commercial meters.

AVR capmeter measuring 1000 uF electrolyte

Features

The capmeter is able to measure capacitances with a resolution of 1 pF in the lower end. Max measuring capacitance is about 10000 uF. Actual measurement precision is not known, but linearity errors seems to be about 0.5 % worst case, and < 0.1% typical (measured by checking if parallel capacitors measures as the sum of each). Large electrolytes are the most problematic to measure accurately.

The meter has two ranges accessible through an auto detect mode, and modes for forcing low or high range. The meter also has two different measurement thresholds, giving two readouts for the same capacitor. This special feature enables checking if the measurement is credible and if the measured device is behaving like a capacitor should. Electrolytes will display their inherent non linearity by giving differing readouts.

The meter has a menu system that among other things allow calibration of zero offset and 1 uF. The calibration can be saved to EEPROM.

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By Lars Ole Pontoppidan

Hardware done: Jan 2006, article written: Nov 2006

 

Next: Design considerations