Capacitor leakage measurement solution

Overview

Capacitors are basic components in a variety of electronic devices and are widely used for bypassing, coupling, filtering, and tuning electronic circuits. However, to use capacitors, it is necessary to 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 here is to measure the leakage resistance of a capacitor using the Keithley Picoammeter 6487 or Keithley Electrometer 6517B. This leakage resistance can 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 can 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 1 is a general circuit for testing capacitor leakage. In this circuit, a voltage is applied to both ends of a 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 In the event of a capacitor short circuit, the resistor limits the amount of current.

2 As mentioned in Section 2.3.2, the capacitive reactance of the capacitor decreases with increasing frequency, which increases the gain of the feedback ammeter. This resistor limits the gain to a finite value. A reasonable value for this 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 2. The diode acts like a variable resistor. When the charging current of the capacitor is large, its resistance is very low; and when the current decreases over time, its resistance increases. In this case, the series resistor can be much smaller, because its role 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 IN914 or 1N3595, and must have a light-tight package. When making bipolar measurements, two diodes should be used and connected in anti-parallel.

Figure 1. Simple capacitor leakage test circuit.

Figure 2. Capacitor leakage test circuit using a diode

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 3 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 controls the various instruments to automatically perform the test.

In this test system, a single instrument, the Keithley Picoammeter 6487, 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 500 V DC. When measuring lower currents, the system can also use the Keithley Electrometer 6517B.

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 limitations 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.

Figure 3: Capacitor leakage test system

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.

The conductivity of a solution is very sensitive to the presence of impurities. This means that the value of the conductivity varies with the presence of impurities, rather than being a characteristic constant. So high accuracy is not required, and the test equipment does not need to be very sophisticated.

As in the case of pH measurements, the current should be kept as low as possible. It is also possible to alternate the polarity to avoid polarization of the electrodes.

The electrodes of the unit must be securely mounted to prevent them from vibrating and moving, which would cause noise and interference. Shielding the leads also helps reduce interference.

Each cell has its own specific constant which is a function of the volume of the conductive solution between the electrodes. Electrometers are very useful when the electrode area is very small and the conductivity of the solution is very low. Temperature control is very important for reliable measurements.

Conductivity can be calculated from the known current value (I), voltage reading (V), the area of ​​the electrodes and the distance between them:

Where: s = conductivity (Siemens/cm)

A = Surface area of ​​the electrode (cm2)

L = distance between electrodes (cm)