Measuring the leakage resistance of a capacitor using the Model 6487 Picoammeter Source or the Model 6517A Electrometer

Capacitors are basic components in various electronic devices and are widely used for bypassing, coupling, filtering , tuning, etc. in electronic circuits. However, to use capacitors, you must understand their characteristics: including capacitance, rated voltage, temperature coefficient, and leakage resistance. Capacitor manufacturers test these parameters; end users also perform such tests.

The application example discussed in this article is the measurement of the leakage resistance of a capacitor using a Model 6487 Picoammeter Source or Model 6517A Electrometer . This leakage resistance may be represented by "IR" (Insulation Resistance) and expressed in megohm-microfarads (the resistance value can be calculated by dividing the "IR" value by the capacitance). In other cases, leakage current may be expressed as leakage current at a given voltage (usually the operating voltage).

Test method introduction

The method of measuring capacitor leakage is to apply a fixed voltage to the capacitor under test and then measure the resulting current. Leakage current decays exponentially with time, so it is usually necessary to apply the voltage for a known period of time (the soak time) and then measure the current.

Figure 4-7 is a general circuit for testing capacitor leakage. In this circuit, voltage is applied to both ends of the capacitor (CX) during the soaking time, and the current is measured with an ammeter after the time has passed. In this test system, the resistor (R) connected in series with the capacitor is an important component. This resistor has two functions:

1. When the capacitor is short-circuited, the resistor limits the current.

2. The capacitive reactance of the capacitor decreases with increasing frequency, which increases the gain of the feedback ammeter. The resistor limits the gain to a finite value. A reasonable value for the resistor is one that gives an RC product of 0.5 to 2 seconds.

Better results can be obtained by adding a forward biased diode to the circuit, as shown in Figure 4-8. The diode acts like a variable resistor. When the capacitor charging current is high, its resistance is low; when the current decreases over time, its resistance increases. The series resistor can be much smaller because its function is only to protect the diode from damage when the voltage source is overloaded and the capacitor is shorted. The diode should be a small signal diode, such as 1N914 or 1N3595, and must have a light-tight package. When making bipolar measurements, two diodes should be used and connected in anti-parallel.

Test Circuit

From a statistical point of view, it is often necessary to test a large number of capacitors to obtain useful data. Obviously, it is not practical to perform these tests manually, so some type of automatic test system is required. Figure 4-9 shows such a system. The system uses a 6487 picoammeter voltage source, a 7158 low current scanner card, and a 7169A Class C switch card. These boards are installed in a programmable switch host (such as the 7002). A computer is used to control the various instruments to automatically perform the test. [page]

In this test system, one instrument, the Model 6487 Picoammeter Voltage Source, is used to provide both voltage sourcing and low current measurement capabilities. This instrument is particularly useful for this application because it can display resistance or leakage current and can source up to 500V 直流电\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> DC . For measuring lower currents, the Model 6517A can also be used in this system.

Depending on the polarity of the voltage source, one of the two diodes (D) connected in parallel is used to reduce noise, while the other diode provides a discharge path. After the measurement is completed, the normally closed contact of the 7169A discharges the capacitor. Due to the limitation of the 7169A card, the output voltage of the voltage source cannot exceed 500V. If the maximum test voltage is only 110V, the 7111 Type C switch card can be used instead of the 7169A card.

One set of switches is used to apply the test voltage to each capacitor in turn, and another set of switches connects each capacitor to the picoammeter after the appropriate soak time.

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