Capacitor leakage measurement method
Time：2021.09.27
View：
Capacitors are the main components used in almost all electrical equipment. Leakage resistance is one of many electrical characteristics of capacitors being tested. Leakage resistance is usually called "IR" (Insulation Resistance, insulation resistance), expressed in "Megaohm-microfarad". 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 before measuring the current, the applied voltage must reach a known period of time (holding time).
For statistical purposes, a certain number of capacitors must be tested to generate useful data. In order to test, an automatic switching system is needed.
Figure 1 shows a set of capacitor leakage test system, which uses Keithley 6517A electrometer/source, 7158 low current scanning card and C switch card, such as 7111-S or 7169A. The plug-in card is installed in the 7002 switch host.
Figure 1 Capacitor leakage test system
In this test system, a set of switches (Type 7111-S or Type 7169A) are used to apply a 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 activated, the capacitor is connected to the voltage source. The switches are usually staggered (for example, 2 seconds apart), so that each capacitor can be charged for the same period of time before measuring the leakage. If the maximum test voltage is 110 V or lower, you can use the 7111-S switch card; otherwise, you can use the 7169A switch card to test voltages up to 500 V. If a voltage higher than 500 V must be applied, a switch with the corresponding rating should be used.
The second set of switches (type 7158) connect each capacitor to a picoammeter after a suitable plateau time has elapsed. Note that before the capacitor is switched to the picoammeter, it is connected to the circuit LO. In this way, the leakage current can also circulate continuously in the charging device.
For this application, a single instrument provides voltage source and small current measurement functions. Model 6517A can display resistance or leakage current, and can provide voltages up to 1000 VDC, so it is particularly suitable for this application.
After testing the capacitor, the voltage source should be set to zero. Sometimes the capacitor must be discharged before removing it from the test fixture. Note that the capacitor (C) in Figure 1 forms a discharge path through the normally closed contact of the relay. The test sequence synchronization is as follows:
Static——The 7169A relay is normally closed, and the 7158 relay is normally closed.
Applied voltage (holding pressure time)-7169A relay is connected to the normally open contact, and the 7158 relay remains normally closed.
