Currently available cheap (< $100) ESR meters on Ebay only measure the equivalent series resistance of a capacitor. However, they do not measure the actual capacitance of the capacitor, so they will fail to identify open capacitors or capacitors with a reduced capacitance as defective.
On the other hand, cheap capacitance meters cannot operate in-circuit, which requires to unsolder a capacitor in order to measure its capacitance value.
I propose a meter that can simultaneously measure capacitance and ESR in-circuit as a useful troubleshooting device. It is meant to be a troubleshooting tool and not a precision instrument. Given that typical capacitor tolerances are quite large, high precision is not required for a pass/fail test, therefore many design choices will prioritize price over precision.
The meter will work as follows:
The meter generates an oscillating signal. Different frequencies can be used in order to account for different capacitance ranges. (1 kHz, 10 kHz, 100 kHz).
The device under test is subjected to the oscillating signal, which should have a peak to peak voltage of 300 mV or less to avoid triggering semiconductors and most Schottky diodes.
The current flowing through the capacitor under test is measured with a shunt. The resulting signal will have a peak to peak voltage that is a function of the capacitor impedance at the oscillating frequency plus the esr.
The phase shift between the signal measured in step 3 and the signal generated in step 1 is measured in order to calculate the capacitance. Alternatively, the sine wave component 90º out of phase with that generated in step 1 can be measured to calculate the capacitance.
The esr can be measured in a similar way, measuring the component of the sine wave that is in phase with the oscillating signal generated in step 1.