Guest columnist Dale Cigoy, lead applications engineer at Keithley Instruments, outlines how an understanding of the challenges for testing capacitors can be invaluable to engineers.
Capacitors are essential electrical components that are incorporated into just about every type of electronic hardware manufactured. They are widely used for bypassing, coupling, filtering, and tunnelling electronic circuits. However, to be useful, their capacitance value, voltage rating, temperature coefficient, and leakage resistance must be characterised. Although capacitor manufacturers perform these tests, many of the electronics manufacturers who build them into their products also perform some of these tests as quality checks.
What is a capacitor?
A capacitor is somewhat like a battery in that both store electrical energy. Inside a battery, chemical reactions produce electrons on one terminal and store electrons on the other. However, a capacitor is much simpler than a battery, because it can’t produce new electrons—it only stores them. Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance known as a dielectric.
A capacitor’s storage potential, or capacitance, is measured in farads. A one-farad (1F) capacitor can store one coulomb (1C) of charge at one volt (1V). A coulomb is 6.25 x 1018 electrons. One amp represents a rate of electron flow of 1C of electrons per second, so a 1F capacitor can hold one amp-second (1A/s) of electrons at 1V.
Capacitance Measurements
The coulombs function of an electrometer can be used with a step voltage source to measure capacitance levels ranging from <10pF to hundreds of nanofarads. The unknown capacitance is connected in series with the electrometer input and the step voltage source.
The calculation of the capacitance is based on this equation: C=Q/V
Figure 1. Capacitance measurement using an electrometer with an integrated voltage source.
Figure 1 illustrates a basic configuration for measuring capacitance with an electrometer with an internal voltage source. The instrument is used in the charge (or coulombs) mode and its voltage source provides the step voltage.
Just before the voltage source is turned on, the meter’s zero check function should be disabled and the charge reading suppressed by using the REL function to zero the display. Then, the voltage source should be turned on and the charge reading noted immediately. The capacitance can then be calculated from this equation: C= (Q2-Q1)/(V2-V1)
where: Q2 = final charge
Q1 = initial charge (assumed to be zero)
V2 = step voltage
V1 = initial voltage (assumed to be zero)
After the reading has been recorded, reset the voltage source to 0V to dissipate the charge from the device. Before handling the device, verify the capacitance has been discharged to a safe level. The unknown capacitance should be in a shielded test fixture. The shield is connected to the LO input terminal of the electrometer. The HI input terminal should be connected to the highest impedance terminal of the unknown capacitance.
If the rate of charge is too great, the resulting measurement will be in error because the input stage becomes temporarily saturated. To limit the rate of charge transfer at the input of the electrometer, add a resistor in series between the voltage source and the capacitance. This is especially true for capacitance values >1nF. A typical series resistor would be 10kΩ to 1MΩ.
Capacitor Leakage
Leakage is one of the less-than-ideal properties of a capacitor, which is expressed in terms of its insulation resistance (IR). For a given dielectric material, the effective parallel resistance is inversely proportional to the capacitance. This is because the resistance is proportional to the thickness of the dielectric, and inverse to the capacitive area. The capacitance is proportional to the area and inverse to the separation.
Therefore, a common unit for quantifying capacitor leakage is the product of its capacitance and its leakage resistance, usually expressed in megohms-microfarads (MΩ•μF). Capacitor leakage is measured by applying a fixed voltage to the capacitor under test and measuring the resulting current. The leakage current will decay exponentially with time, so it’s usually necessary to apply the voltage for a known period (the soak time) before measuring the current.
Figure 2 illustrates a general circuit for testing capacitor leakage. Here, the voltage is placed across the capacitor (CX) for the soak period, then the ammeter measures the current after this period has elapsed. The resistor (R), which is in series with the capacitor, serves two important functions.
First, it limits the current in case the capacitor becomes shorted. Second, the decreasing reactance of the capacitor with increasing frequency will increase the gain of the feedback ammeter. The resistor limits this increase in gain to a finite value. A reasonable value is one that results in an RC product from 0.5 to 2 seconds. The switch (S), while not strictly necessary, is included in the circuit to allow control over the voltage to be applied to the capacitor.
Alternate Test Circuit
Greater measurement accuracy can be achieved by including a forward-biased diode (D) in the circuit, as shown in Figure 3. The diode acts like a variable resistance, low when the charging current to the capacitor is high, then increasing in value as the current decreases with time.
This allows the series resistor used to be much smaller because it is only needed to prevent overload of the voltage source and damage to the diode if the capacitor is short-circuited. The diode used should be a small signal diode, such as a 1N914 or a 1N3595, but it must be housed in a light-tight enclosure to eliminate photoelectric and electrostatic interference. For dual-polarity tests, two diodes should be used back to back in parallel.
Figure 3. Alternative test circuit that incorporates a small-signal diode
Test Hardware Considerations
A variety of considerations should go into the selection of the instrumentation used when measuring capacitor leakage. Although it is certainly possible to set up a system with a separate voltage source, an integrated one simplifies the configuration and programming process significantly, so look for an electrometer or picoammeter with a built-in variable voltage source.
As temperature and humidity can have a significant effect on high resistance measurements, monitoring, regulating, and recording these conditions can be critical to ensuring measurement accuracy. Some newer electrometers, such as Keithley’s Model 6517B electrometer/source, have the ability to monitor temperature and humidity simultaneously.
This provides a record of conditions, and allows for easier determination of temperature coefficients. Automatic time-stamping of readings provides a further record for time-resolved measurements.
Incorporating switching hardware into the test setup allows automating the testing process. For small batch testing in a lab with a benchtop test setup, consider an electrometer that offers the convenience of a plug-in switching card. For testing larger batches of capacitors, look for an instrument that can integrate easily with a switching system capable of higher channel counts.