Capacitor leakage measurement

Capacitors are a major component used in almost all electrical equipment. Leakage resistance is one of the many electrical characteristics that capacitors are tested for. Leakage resistance is often referred to as "IR" (Insulation Resistance) and is expressed in "megohm-microfarads". In other cases, leakage may be expressed as leakage current at a specific voltage (usually the operating voltage).

The leakage of a capacitor is measured by applying a fixed voltage to the capacitor and measuring the resulting current. The leakage current will decay exponentially with time, so the voltage must be applied for a known period of time (the dwell time) before the current is measured.

For statistical purposes, a certain number of capacitors must be tested to generate useful data. To perform the tests, an automatic switching system is required.

Figure 1 shows a capacitor leakage test system that uses a Keithley Model 6517A electrometer/source, Model 7158 low current scanner card, and a C-type switch card, such as the 7111-S or 7169A. The card is installed in a Model 7002 switch mainframe.

Figure 1, Capacitor leakage test system

In this test system, a set of switches (Model 7111-S or Model 7169A) is used to apply the test voltage to each capacitor. In the normally closed position, one end of the capacitor is connected to the circuit LO. When the switch is actuated, the capacitor is connected to the voltage source. The switches are usually actuated in staggered intervals (e.g., 2 seconds apart) so that each capacitor can be charged for the same period of time before leakage is measured . If the maximum test voltage is 110 V or less, the Model 7111-S switch card can be used; otherwise, the Model 7169A switch card can be used to test voltages up to 500 V. If voltages higher than 500 V must be applied, switches with corresponding ratings should be used.

The second set of switches (Model 7158) connects each capacitor to the picoammeter after a suitable settling time . Note that before the capacitor is switched to the picoammeter, it is connected to the circuit LO. This allows the leakage current to continue to flow through its charging device.

For this application, a single instrument provides both voltage sourcing and low current measurement capabilities. The Model 6517A is particularly well suited for this application because it can display resistance or leakage current and source voltages up to 1000 VDC.

After testing the capacitor , the voltage source should be set to zero. Sometimes the capacitor must be discharged before it is removed from the test fixture. Note that the capacitor (C) in Figure 1 forms a discharge path through the normally closed contacts of the relay. The test sequence is synchronized as follows:

Static - Model 7169A relay is normally closed, Model 7158 relay is normally closed.

Apply voltage (dwell time) - 7169A relay connected to normally open contacts, 7158 relay remains normally closed.

Measuring current - Model 7169A relay remains in the normally open position and Model 7158 relay is connected to the normally open contacts.

Discharge Capacitors - 7169A relays connected to normally closed contacts, 7158 relays connected to normally closed contacts.

Because the isolation switches on the Model 7169A Form C switch card remain energized during the current measurement, any offset current from the switch card is irrelevant to the measurement.

The resistor (R) in series with the capacitors is an important element in this test system. It limits the charging current of each capacitor and protects the relay in the event of a capacitor short circuit. In addition, the resistor limits the AC gain of the feedback ammeter. Generally speaking, as the source capacitance increases, the noise gain also increases. This resistor limits the gain to a finite value. A reasonable limit is to make the RC time constant 0.5 to 2 seconds. The forward biased diode in series with the HI terminal of the electrometer (pA) also serves to limit the AC gain.

A "coax tri-pole to BNC" converter (model 7078-TRX-BNC) is used to connect the 6517A to the 7158 switch card. The capacitors are connected to the 7158 switch card via low noise coaxial cable. Insulated wire can be used to connect the 7111-S switch card to the capacitors . The 7169A is connected via a terminal block connector.