A capacitor is an electrical component that stores energy in the form of an electrical charge, similar to a rechargeable battery. They are mainly used in electronic circuits to store voltage and are used frequently in power supplies, rectifiers and across microprocessor power inputs. They are also used as filters and in timing and oscillator circuits.
Capacitance is the measurement of a capacitor’s ability to collect and store that energy. The amount of capacitance in a capacitor depends upon the surface area and distance between its internal plates and the material that separates the plates.
The unit of measurement of capacitance is the Farad and uses the symbol ‘F’. The Farad is a relatively large unit of measurement so most capacitance is measured in smaller units such as micro-farads, nano-farads and pico-farads.
Capacitance Calibration Sources
In a calibration laboratory, capacitance can be sourced from many pieces of equipment and used to calibrate a wide variety of equipment such as Digital Multimeters, Capacitance meters and LCR meters. Multifunction Calibrators, single or variable air capacitors and decade capacitance boxes are available in a variety of ranges and terminal configurations to calibrate many different types of capacitance measuring equipment.
Capacitance Measurement Equipment
There are several types of electronic test equipment that are used to measure capacitance. Many Digital Multimeters contain a capacitance measuring function. Generally, multimeters have a capacitance accuracy of between ±1 to 3%. Digital Multimeters are useful as they are lightweight, portable and battery-operated, however, some of them are limited in the range of capacitance values that can be tested.
LCR Meters are the most widely used and accurate instruments used to measure capacitance. LCR meters are able to measure a wide range of capacitance values using a variety of frequencies and can measure accuracies down to 0.02% They also have advanced features such as programmable frequencies and can be used in computer automation.
LCR meters typically have a four-terminal connection that is used to measure the capacitance. The four-terminal connection helps to minimize measurement errors due to stray capacitance in the leads or other sources of interference. To properly correct for lead errors, LCR meters require an Open/Short compensation to be performed. The LCR meter first measures the leads during an ‘Open’ condition, then measures the leads after they have been shorted. For the best accuracies in the measurements, the Open/Short compensation should be performed after changing the measurement signal frequency or voltage, or when switching the measurement type.
Test Frequency Considerations
In most cases, capacitance should be tested at the test frequency of the capacitance standard or at the frequency where the capacitor will be used, if that can be determined. LCR meters with programmable frequencies are the best instruments to use. Common measurement frequencies are 60Hz, 120Hz, 1kHz, 10kHz and 100kHz.
Measuring capacitance at only one frequency could provide measurement errors, especially as the capacitance values get extremely large or small. As a general rule of thumb, a higher frequency in the range of 1kHz to 100 kHz should be used for measuring lower capacitance values, such as those in the nF and pF ranges and lower. A lower frequency of approximately 20 Hz to 120 Hz should be used for measuring higher capacitance values, such as those in the uF and mF ranges and higher.
Capacitance Dissipation Factor
The Dissipation Factor ‘D’ is another important part of a capacitance measurement. The Dissipation Factor is a dimensionless number which identifies a capacitor’s ‘Purity’ or the quality of the capacitor. The Dissipation Factor is especially important during the designing of electrical circuits. “Pure” capacitors, such as air capacitors, have low values of D, in the range of 0.0001 to 0.0010. Off the shelf capacitors or capacitors used in some low-end decade capacitors can have Dissipation Factors that exceed 0.0500.