“Capacitors are basic components in various Electronic devices, and are widely used in bypassing, coupling, filtering, and tuning electronic circuits. However, in order to use a capacitor, it is necessary to understand its characteristics: including capacitance value, rated voltage value, temperature coefficient, and leakage resistance.
Capacitors are basic components in various electronic devices, and are widely used in bypassing, coupling, filtering, and tuning electronic circuits. However, in order to use a capacitor, it is necessary to understand its characteristics: including capacitance value, rated voltage value, temperature coefficient, and leakage resistance. Capacitor manufacturers test these parameters; end users also perform such tests. The application example discussed here is the measurement of 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 Megohms – Microfarads (the resistance value can be calculated by dividing the “IR” value by the capacitance). In other cases, leakage can be expressed in terms of leakage current at a given voltage (usually operating voltage).
Test method introduction
Capacitor leakage is measured by applying a fixed voltage to the capacitor under test and then measuring the resulting current. Leakage current decays exponentially with time, so it is often necessary to apply a voltage for a known period of time (wet time) before measuring the current.
Figure 1 is a general circuit for testing capacitor leakage. Among them, a voltage is applied to both ends of the capacitor (CX) during the soaking time, and its current is measured with an ammeter after the time has elapsed. In this test system, the resistor (R) in series with the capacitor is an important component. This resistor serves two purposes:
1 The resistor limits the amount of current in the case of a shorted capacitor.
2 As described 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 such that the product of RC is 0.5 to 2 seconds.
Adding a forward-biased diode to the circuit will give better results, as shown in Figure 2. The diode acts like a variable resistor. When the charging current of the capacitor is large, its resistance value is very low; while the current decreases with time, its resistance value increases. The series resistor can then be much smaller, since its only function is to prevent damage to the diode if 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 in antiparallel.
Figure 1. Simple Capacitor Leakage Test Circuit
Figure 2. Capacitor Leakage Test Circuit Using Diodes
From a statistical point of view, it is often necessary to test a large number of capacitors to obtain useful data. Obviously, it’s not practical to do these tests manually, so some type of automated test system is needed. Figure 3 shows such a system. The system uses a model 6487 picoammeter voltage source, a Model 7158 weak current scanner card, and a Model 7169A Class C switch card. These boards are installed in a programmable switch mainframe (eg Model 7002). Use a computer to control various instruments to automate the test.
In this test system, a Keithley Picoammeter 6487 is used to provide the functions of voltage source and weak current measurement. This instrument is particularly useful for this type of work as it can Display resistance or leakage current and can output DC voltages up to 500V. The Keithley Electrometer 6517B can also be used with this system when measuring lower currents.
Depending on the polarity of the voltage source, one of the two diodes (D) in parallel is used to reduce noise, while the other diode provides a discharge path. The normally closed contacts of the Model 7169A discharge the capacitor after the measurement is complete. 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 7169A card can be replaced by a 7111 type C switch card.
Figure 3: Capacitor Leakage Test System
One set of switches is used to apply the test voltage to each capacitor in turn, and the other set of switches connects each capacitor to the picoammeter after an appropriate soak time.
The conductivity of the solution is very sensitive to the presence of impurities. This means that the value of conductivity varies with the presence of impurities, not just a characteristic constant. So there is no need for high accuracy, and the test equipment does not need to be very delicate.
As in the case of pH measurement, the current should be kept as low as possible. Its polarity can also be alternated to avoid polarization of the electrodes.
The electrodes of the unit must be mounted securely to avoid noise and interference from vibration and movement. Also, shielding the leads will help reduce interference.
Each cell has its specific constant that is a function of the volume of conductive solution between the electrodes. Electrometers are useful when the electrode area is very small and the conductivity of the solution is very low. To make reliable measurements, temperature control is very important.
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)