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Electrometer Solutions for Capacitor Testing
Capacitors are essential electrical components that are incorporated into just about every type of electronic product. They are widely used for such things as timing circuits, sample and hold applications, and for bypassing, coupling, and filtering. However, for them to be useful, capacitance value, voltage rating, temperature coefficient, and leakage resistance measurements must be performed. Although capacitor manufacturers perform these tests, many electronic system manufacturers who build them into their products also perform some of these tests as quality assurance checks. This article explains some capacitor fundamentals, and then presents challenges associated with their testing, along with test and measurement techniques that help overcome these challenges.
Capacitor Fundamentals. A capacitor is somewhat like a battery. Both devices can both store electrical energy. However, a capacitor is much simpler because it can't produce new electrons through chemical reactions like a battery. Capacitors can only store electrons originating from elsewhere in an electrical circuit. Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting insulator called the dielectric.
A capacitor's storage potential, or capacitance, is measured in farads. A one-farad (1F) capacitor can store one coulomb (1C) of electrical charge at one volt (1V). A coulomb is 6.251018 electrons. A current flow of one ampere represents electron flow at the rate of 1C of electrons per second, so a 1F capacitor can hold one ampere-second (1A/s) of electrons at 1V. In terms of a capacitor's construction, its capacitance value is directly proportional to the area of its metal plates and the dielectric constant of the insulator separating them. The capacitance value is inversely proportional to the thickness of the dielectric, i.e., the separation distance between the plates.
In many electronic capacitors the plates are actually sheets of metallic foil, such as aluminum or tantalum separated by a polymer dielectric sheet, and this "sandwich" is wound into a cylindrical shape with enough windings to get the capacitance value needed. An electrolyte may also be used within this assembly. Such assemblies are called electrolytic capacitors. There are many other capacitor designs, such as ceramic types.
Capacitor Characteristics. Circuit designers depend on capacitors to act in a nearly ideal fashion. It's probably rare for a new capacitor not to meet the manufacturer's stated tolerances for capacitance value and breakdown voltage. However, with repeated cycles of voltage and temperature stress, complex electrochemical interactions can cause capacitors to fall short of perfection. One of the less than ideal properties of a capacitor is its insulation resistance or leakage.
For a given dielectric material, the capacitor' effective shunt resistance is inversely proportional to the capacitance. This is because the resistance is proportional to the thickness of the dielectric, and inversely proportional to the capacitive area. Thus, a common unit for qualification of capacitor leakage is the product of its capacitance and leakage resistance measurements under known conditions, usually stated in megohm-microfarads (MΩ-μF).
Capacitance Measurements. The charge or coulombs function of an electrometer can be used with a step voltage source to measure capacitance levels typically ranging from <10pF to hundreds of nanofarads. Some electrometers, such as the Keithley Model 6517B contain an internal voltage source with that simplifies these measurements. The unknown capacitance is connected in series with the electrometer input and the voltage source. The calculation of the capacitance is based on the 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)
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.
Some preliminary adjustments should be performed on the electrometer. 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 (relative) function to zero the display. The purpose of zero check is to protect the input FET from overload and to zero the instrument. When zero check is enabled, the input of the electrometer is a resistance from roughly 10 mega-ohms to 100 mega-ohms, depending on the electrometer used. Zero check should be enabled when changing conditions on the input circuit, such as changing functions and connections. The REL function subtracts a reference value from actual readings. When REL is enabled, the instrument uses the present reading as a relative value. Subsequent readings will be the difference between the actual input value and the relative value.)
After performing preliminary adjustments, the voltage source should be turned on and the charge reading noted immediately. The capacitance can then be calculated from this equation above.
CAUTION! After the reading has been recorded, reset the voltage source to 0V to dissipate the charge from the capacitor. Before handling it, verify that the residual voltage has been discharged to a safe level using a voltmeter. Do not attempt to discharge the capacitor by shorting its terminals, as a dangerous current level could flow through the shorting connection. This can cause a dangerous arc flash, and even raise the temperature of the shorting wire or bar to its melting point.
Leakage Resistance Measurements. Leakage resistance is a function of the dielectric material and capacitor design. These details depend on a capacitor's intended function. For example, the insulation resistance of polymer dielectrics such as polystyrene, polycarbonate, or Teflon® can range from 104MΩ-μF to 108MΩ-μF, depending on specific materials used and their purity. The insulation resistance of ceramics such as X7R or NPO can be anywhere from 103MΩ-μF to 106MΩ-μF. Electrolytic capacitors such as tantalum or aluminum have much lower leakage resistances, typically ranging from 1MΩ-μF to 100MΩ-μF.
Usually, a minimum insulation resistance is guaranteed by the manufacturer. For instance, a 4.7μF aluminum cap with a specified leakage resistance of 50MΩ-μF would be guaranteed to have at least 10.6MΩ of insulation resistance (50MΩ-μF / 4/7μF).
Leakage resistance measurements are conducted by applying a fixed voltage to the capacitor 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. An electrometer's voltage source and picoammeter functions can be used for this test. The capacitor is placed in series between the voltage source and the picoammeter.
Generally, a switch and resistor are placed in series with the capacitor, which serves three important functions. First, the resistor 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 in the electrometer. The resistor limits this increase in gain to a finite value. A reasonable resistor value is one that results in an RC product from 0.5 to 2 seconds. Third, the switch, while not strictly necessary, is included in the circuit to allow control over the voltage to be applied to the capacitor.
On the other hand, the series resistor also adds Johnson noise (the thermal noise created by any resistor) to the measurement. At room temperature, this is roughly 6.5×10 10 amps, p-p. The current noise in a 1TΩ feedback resistor at a typical 3Hz bandwidth would be ~8×10-16A. When measuring an insulation resistance of 1016Ω at 10V, for example, the noise current would be 80% of the measured current.
An alternate test configuration for better measurement accuracy is to place a forward-biased diode in the measurement circuit between the resistor and the capacitor under test. 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 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.
Test Hardware Configurations. A variety of considerations should go into the selection of instrumentation and its configuration in capacitor leakage measurements:
- 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. A continuously variable voltage source allows calculating voltage coefficients easily. For making high resistance measurements on capacitors with high voltage ratings, a 1000V source with built-in current limiting is best. For a given capacitor, a larger applied voltage within the voltage rating of the capacitor will produce a larger leakage current. Measuring a larger current with the same intrinsic noise floor produces a greater signal-to-noise ratio and, therefore, a more accurate reading.
- Temperature and humidity can have a significant effect on high resistance measurements, so monitoring, regulating, and recording these conditions can be critical to ensuring measurement accuracy. Some newer electrometers (for example, the Keithley Model 6517B) can simultaneously take humidity and temperature measurements. 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 a switching system 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 switch card. For testing larger batches of capacitors, look for an instrument that can integrate easily with a switching system capable of higher channel counts.
Testing for statistical analysis purposes requires the rapid collection of a large amount of measurement data from a large number of capacitors. Performing these tests manually is impractical, so an automated test system is required. Such a system can use an electrometer with built-in voltage source, and a switching system that houses a low current scanner card and a Form C switch card. In this test setup, a single instrument provides both the voltage sourcing and low current measurement functions. A computer controls the instruments to perform the tests automatically.
One set of switches on the switch card is used to apply the test voltage to each capacitor in turn; a second set of switches connects each capacitor to the electrometer's picoammeter input after a suitable soak period. After the capacitors have been tested, the voltage source should be set to zero and then some time allowed so the capacitors can discharge before they are removed from the test fixture.
Typically, the capacitors will have a discharge path through the normally closed contacts of the switch card relays. To prevent electric shock, test connections must be configured in such a way that the user cannot come in contact with the conductors, connections, or the DUT. Safe installation requires proper shielding, barriers, and grounding to prevent contact with conductors.
More complex test systems can be designed that combine leakage measurement with capacitance measurements, dielectric absorption and other tests. Test systems of this sort can use a switching mainframe along with a voltage source, LCZ bridge and picoammeter.
General Test System Safety Practices. Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels. It is also possible, under single fault conditions (e.g., a programming error or an instrument failure), to output hazardous levels even when the system indicates no hazard is present. These high voltage and power levels make it essential to protect operators from any of these hazards at all times. It is the responsibility of the test system designers, integrators, and installers to make sure operator and maintenance personnel protection is in place and effective. Protection methods include:
- Design test fixtures to prevent operator contact with any hazardous circuit.
- Make sure the device under test is fully enclosed to protect the operator from flying debris. Double insulate all electrical connections that an operator could touch.
- Double insulation ensures the operator is still protected, even if one insulation layer fails.
- Use high-reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened.
- Where possible, use automated handlers so operators do not require access to the inside of the test fixture.
- Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury.
References. For more information on capacitor measurement techniques, download Keithley's Application Note #315, "Capacitor Leakage Measurements…" available at www.keithley.com/data?asset=6076.
About the Author
Dale Cigoy is a Senior Application Engineer at Keithley Instruments in Cleveland, OH. With 25 years of experience in instrument applications, his major responsibility is helping customers with electrical measurements that include Keithley equipment. Prior to this he wrote technical instruction manuals for Keithley products. Cigoy joined Keithley in 1976 after earning a Bachelor of Science degree in Electronic Technology from Capitol College in Laurel, MD.
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