ESR is the equivalent series resistance of capacitor. You can imagine it as a resistor connected
in series with an ideal capacitor. The value of this resistance is the ESR.
One of the most common faults of consumer electronics are just defective electrolytic capacitors. Using a multimeter or capacitance meter most of their faults can not be identified. Measuring ESR reliably finds defective electrolyte because failure to the electrolyte will appear first on increasing ESR. The great advantage is that you do not need to unsolder the capacitors, because the measuring voltage is small and thus the neighboring componentsdo not affect the measurement (unlike the capacity meter that can not operate on capacitors in PCB board). For electronics repairman, ESR meter is an absolute necessity. It is used even more than the multimeter!
The measuring principle is simple: The capacitor is connected to an alternating current of high frequency (typically 50-100kHz) and the voltage drop across it is measured. Higher voltage means higher ESR.
On the Internet there are many schematics of homemade ESR meters. Most of them, however, are very complex and utilize a microcontroller. Other indications have only one LED, which is not enough for the measurement of capacitors of many different sizes and values. I chose a simple and effective option. As a source of high frequency current, about 50 kHz, I used well known circuit 555. To measure the voltage drop I used a small analog meter. You can use a small microamp or millivolt meter, sufficient is a moving coil indicator, tuning indicator from an old radio or sound level indicator (VU meter) from the deck. The rectification diode D1 has a low voltage drop, it may be a germanium diode (from any older radio) or Schottky diode. D2 protects analog meter against excessive voltage at the moment when no capacitor is connecter. The values of resistors R1 and R2 must be selected according to your analog meter. R2 is selected so that the analog meter is not much over range, and R1 determines the sensitivity. Test leads should be short (max 30-40 cm / 1-1.33 feet) because their resistance and inductance may affect the measurement. Wires to the measured capacitor should be double (shown in the diagram).
Larger electrolytes have lower ESR, of course, so it is advisable to make a scale according to ESR values of brand new capacitors. Sufficient is logarithmic scale as 1uF, 10uF, 100uF, 1000uF. The meter is powered from 9V battery. At open circuit condition the ESR meter draws about 6 mA and max 16 mA during measurement.