This ESR meter device provides the measurement of equivalent series resistance (ESR) of capacitors. The circuit diagram is very simple (See Fig. 1.), there is only two bipolar transistors of n-p-n conductivity type are used. The circuit was described in July 2008 issue of "Radio" magazine by Y. Kurakin. The device provides measurements of the equivalent series resistance in the range of 0.1 Ω to 23 Ω.
The heart of the circuit is a common collector Colpitts oscillator with transistor Q1. The frequency of the oscillator is determined by the components L1, C1 and C2. The operating frequency is about 12..17 kHz. The capacitor under test Cx is connected in series with C1. Because the capacitance of Cx is much higher than the capacitance of C1, the capacitance of Cx doesn't affect the oscillator.
The oscillator produces oscillations only if a series resistance of the capacitor under test Cx is low. With the growth of the series resistance of Cx, an amplitude of oscillations decreases. At some point, if the series resistance is too high, there will be no oscillations. The series resistance of Cx is inversely proportional to the amplitude of oscillations.
Diodes D1..D4 are used to discharge the tested capacitor if it charged, it protects the circuit from damage if the tested capacitor is charged with high voltage.
The transistor Q2 works as a rectifier, the capacitor C4 suppresses the ripple voltage at the collector of Q2. The milliammeter PA1 provides an indication of the series resistance of the capacitor under test. Any suitable milliammeter with a full scale deflection rating of 0.5 mA...15 mA can be used as PA1. Instead of the milliammeter, a digital multimeter (DMM) configured as an ammeter can be used for measurements.
The inductance coil L1 is wound on a toroidal core FT-37-43 with 77 turns of 0.2 mm (32 AWG) enameled copper wire, its inductance is 2.5 mH. You can use another type of core, but you have to recalculate the parameters of the inductance coil L1 using Toroidal core inductance Calculator.
To provide the measurement of the ESR, you have to short the probes X1. Use the potentiometer R2 to deflect the needle of PA1 to the end of the scale - it's a calibration. Now connect a capacitor to the probes, and see the reading of the ESR. The closer the needle to the last mark on the scale, the lower the ESR value. If the needle is in the last third part of the scale (see the Figure 2), then the tested capacitor is good for using. If the needle is in first two of three parts of the scale, then the tested capacitor is not good. You also can connect resistors of 1..30 Ω to calibrate the scale of the ESR meter.
The current consumption of the circuit is about 1..2 mA.