Capacitance is measured using a multimeter, LCR meter and oscilloscope.
Ordinarily, capacitors are provided with the metrics – capacitance and working volts – printed or color-coded on the component. The capacitance as labeled is generally accurate to a reasonable tolerance. For most applications a wide range of values will perform as expected. As for working volts, you never know until it is too late, and the only way to test it is destructively.
There are several ways to test capacitance. If you have a multimeter with capacitor test mode, that is the way to go. Otherwise you have to use the ohms mode, by which a certain amount of information but not the exact capacitance value can be acquired.
This is a Fluke 287 True RMS Multimeter, a very high-quality instrument. To do a capacitor measurement, plug the black probe into the common port and the red probe into the combination diode-capacitor port. With the instrument turned on, press F1, which, as you can see in the display, corresponds to Menu. Use the Up or Down arrow to highlight Diode-Capacitor. Then press F2, corresponding to Capacitor. In Capacitor Mode, the instrument is auto-ranging, so it will take care of itself.
Now we’re ready to take some capacitance measurements. Here are some examples.
This is an AC receptacle multiplier, commonly called a cube. We might expect that it has a measurable capacitance because there are two electrodes a finite distance apart, the air between them constituting the electrolyte layer. Since the plates are small and relatively far apart and the electrolytic constant of air is not very high, the capacitance is going to be quite small. And as you can see, it does not even make a reading.
Another experiment is to wrap one of the probes in paper and bring it in contact with the other probe, positioning them parallel to one another. And here there is a definite reading.
Electrolytic capacitors can be checked using a multimeter in the ohms mode. If the meter is not auto-ranging, set it to a megohm range and connect the probes to the leads. If the device is shorted or open, of course it is defective. Otherwise, depending on the polarity of your hookup, the resistance in ohms will appear to either rise or fall in a distinctive fashion. The rate appears at first very steady, decelerating noticeably only as the endpoint is approached. What is happening is the capacitor charges or discharges due to the meter’s internal battery that through a voltage divider supplies something like a three-volt bias that is used to measure resistance. Electricians, testing a motor capacitor, call this strange phenomenon “counting”, and it indicates that the capacitor is good.
If these types of measurements are not sufficient for your application, the next step up is an LCR meter. Using this instrument, the procedure is the same as for the multimeter in Capacitance Mode. Just clip onto the leads and take the reading.
The LCR meter is a highly sophisticated instrument. For one thing, it has the capability to measure inductance in a component, in electronic equipment or distributed throughout a communication or power network.
The LCR meter works by imposing an AC voltage on the device in question. Then, voltage across and current through the component are measured. Additionally, in high-end instruments, the LCR meter calculates the phase angle between voltage and current, thereby displaying capacitance or inductance with a high degree of repeatability.
The oscilloscope can likewise be used to measure capacitance. To do this, a square wave from an arbitrary function generator is applied to a known resistance in series with the unknown capacitance.
Use cursors to find the elapsed time. The Y-cursor is placed at 63.2 percent of the waveform peak value, which by definition is the amplitude that corresponds to the time constant of the circuit that consists of the resistor and capacitor in series. Then, the X-cursor is dropped down to intersect the X-axis, and the distance between this point and the intersection of X- and Y-Axes (known as the origin) is the elapsed time.
The elapsed time along with the known resistance are inserted into the well-known equation:
C = R/t
Where C is capacitance, R is the value of the resistor and t is the elapsed time. To facilitate the computation, a 1K resistor should be chosen and the square wave in the arbitrary function generator should be set at one volt peak-to-peak, which for a square wave is the same as RMS.
There are other methods for determining capacitance or inductance using an oscilloscope. For example, a resonant circuit can be constructed with known and unknown devices and the peak frequency can be measured.
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