TESTING CAPACITORS

A capacitor is nothing more than two metal plates separated by an insulator called the dielectric. The common causes of capacitor failures are excessive dielectric leakage, change in capacity value, and an increase in dielectric absorption (battery action). All capacitors should be removed from the circuit before testing. Impedances found in the circuit will result in false meter readings.

Capacitor Value

All types of capacitors change in value with age. A reliable value test is important when you suspect that a capacitor may have changed value or is the wrong value when new. To check capacitors for capacity value, perform the following steps:

1. Connect the test leads to the capacitor to be tested. Polarity of the test leads is important only if you are checking a polarized capacitor. When checking polarized capacitors, be sure to connect the red test lead to the positive capacitor terminal.

Figure 4-29.\Typical schematic diagram with oscilloscope waveforms.

^{2. Depress the CAPACITOR VALUE push} indicator is lit, or in picofarads if the LED in front ^button. of the pF indicator is lit.

3. Read the value of the capacitor on the display. The value. of capacity will be in NOTE: Most capacitor values are displayed ^{microfarads if the LED in front of the μF} quickly, but extremely large electrolytic capacitors

Figure 4-30.\Sencore LC75 capacitor-inductor analyzer.

(over 50,000 μF) may take a few seconds to display a reading. For example, a 50,000-μF capacitor will take about 5 seconds before a reading is seen on the digital readout, and a 100,000 μF electrolytic capacitor may take 10 seconds. If the value does not appear in the time listed above, the capacitor is either shorted or quite leaky. In either case, it is probably defective.