Metering and Instrument Circuits

Measuring Electrolytic Capacitors

Most capacitance meters have no facility for measuring large electrolytic capacitors. The circuit described here makes it possible for such capacitors to be measured with some degree of accuracy, in spite of the large tolerances these components normally have.Measuring Capacitors

Opamp IC-1a is arranged as an astable. Capacitor C2 is charged via R2; as soon as the potential across it reaches the level of that at the non-inverting input of IC-1a, which is determined by voltage divider R1-R3-R4, the opamp toggles and C2 is discharged till the voltage across it reaches the new level at the + input of IC-1a. The measuring circuit consists of switched The capacitor on test is charged via T2 and discharged rapidly via T1. Comparator IC H, compares the level (0.65 V) at its non-inverting input with that at its inverting input. When the capacitor on test is connected across the input terminals, P2 is adjusted till the LED just lights. The potentiometer must be given a scale to enable the value of the electrolytic capacitor to be read directly. The scale can be calibrated with the aid of a capacitor of known value for each range (1-4.7 pF; 4.7-47 pF; 47-470 pF; and 470-4700 pF). Basically, it is linear, but it may be necessary to make a scale for range 1 empirically. To ensure the best possible accuracy, it is advisable to use a regulated power supply. The circuit draws a current of about 20 mA (almost all of it through the LED).

Some additional factors involving capacitance are worth knowing:

  • Capacitors have a limited life and are often the cause of a malfunction.
  • Faulty capacitors may have a short circuit, an open circuit, or may physically deteriorate to the point of failure.
  • When a capacitor short circuits, a fuse may blow or other components may be damaged.
  • When a capacitor opens or deteriorates, the circuit or circuit components may not operate.
  • Deterioration can also change the capacitance value of a capacitor, which can cause problems.

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