I would like to use capacitors salvaged from old electronics, but I currently don't have an LC meter.
What would be some really simple circuits, that would convert capacitance to something I can measure (voltage, current, frequency)?
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1\$\begingroup\$ Not wishing to go into the fundamental ways of how capacitance is defined, to measure capacitance (or inductance for that matter) you need a reference component to compare it against. The reference component can be a resistor, an inductor or another capacitor and if you have one that can be relied upon for its value then that is a starting point \$\endgroup\$– Andy akaCommented Apr 8, 2013 at 17:41
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2\$\begingroup\$ What test equipment do you have available? \$\endgroup\$– Matt YoungCommented Apr 8, 2013 at 17:45
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\$\begingroup\$ I only have a cheap multimeter and xprotolab scope (scope, voltmeter, frequency counter) \$\endgroup\$– nanaCommented Apr 8, 2013 at 17:51
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2 Answers
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There is a solution using a microcontroller. Microchip wrote AN611 on it, but I'd recommend you this blogpost instead.
How it's done: you start charging the capacitor and time how long it takes to get to \$\frac{1}{2}\cdot V_{in}\$.
The 16F628A isn't necessary at all, you can use any microcontroller with a comparator.
With that time (the time it takes to get to \$\frac{1}{2}\cdot V_{in}\$), you can calculate the RC value of the RC circuit. You know R, it's 22K in the circuit above, it's constant. So you can calculate the value of C.
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Another answer, given that you have a frequency counter, would be to build an oscillator using the 555 timer, and measure the frequency. Switch in a few standard resistors (1k, 10k, 100k etc) and check with a known capacitor how close the "frequency calculation" formula is to reality.