Capacitor Leakage

Capacitors will often read the correct value but exhibit leakage that may affect their operation in the circuit. Any type of capacitor may develop excessive leakage if the capacitor is subjected to excessive applied voltage or high-voltage spikes. These types of overloads may actually puncture the dielectric material and produce a short circuit or highresistance leakage path. Leakage often shows up only when the capacitor is charged to its normal operating voltage and may not show up under a low-voltage check such as you might make with an ohmmeter. To check a capacitor for leakage, perform the following steps:

1. Connect the capacitor to be tested to the test leads. If the capacitor is polarized, connect the positive capacitor terminal to the red test lead and the negative terminal to the black test lead.

2. Select the desired current range with the LEAKAGE RANGE switch. Use the ALL OTHER CAPACITORS (100 μA max) range for most small electrolytics and for paper, mica, film, and ceramic capacitors. Use the LARGE ALUM. ELECTROLYTICS (10K μA max) range for large electrolytics. Consult the leakage charts on the pullout tab under the analyzer (figs. 4-31 and 4-32) ^{to the other range. You can switch ranges of the} to determine which range should be used. ^{LEAKAGE RANGE switch while holding the} Start with the highest range (LARGE ALUM. ^{LEAKAGE button in.} ELECTROLYTICS) if you are not sure which 3. Select the normal dc working voltage of the range to use. If the display shows "000," switch ^{capacitor with the LEAKAGE VOLTAGE}

Figure 4-31.\Capacitor leakage chart for dipped solid tantalum capacitors.

Figure 4-32.\Capacitor leakage chart for standard aluminum electrolytic capacitors.

switch. If the capacitor's normal working voltage falls between switch ranges, select the next lower range. For example, if the capacitor's working voltage is 35 volts, select the 25-volt position of the LEAKAGE VOLTAGE switch.

4. Depress the LEAKAGE button and read the amount of leakage current in microamperes on the display. Capacitors take a certain amount of time to charge before a reading of the leakage current is displayed. Consult the leakage charts on the pullout tab under the analyzer (figs. 4-31 and 4-32) to determine which range should be used.

Ceramic, paper, film, and mica capacitors should not show any leakage at all. The maximum allowable leakage is below the sensitivity of the measuring circuit. If any of these capacitors exhibit leakage, they are defective.

Leakage measurements on nonpolarized electrolytics must be made in both directions. Simply make the leakage test, note the leakage current, and then reverse the leads and make the leakage test again. If both ends of the nonpolarized electrolytic are insulated from the case, the maximum allowable leakage is the same as listed in the leakage chart. If one end is connected to the case, the allowable leakage is doubled.