The main characteristic of a capacitor is its capacitance. Very often, capacitance measurements are required to be carried out in an electrolytic capacitor. Unlike ceramic and oxide capacitors, which rarely fail (except as a result of dielectric breakdown), electrolytic parts tend to lose capacity due to the drying of the electrolyte. Since the operation of electronic circuits is highly dependent on capacitive characteristics, it is necessary to know how to determine the capacitance of a capacitor.
There are different ways to determine capacity:
- By code or color marking of parts;
- Using measuring instruments;
- Using a formula.
The easiest way to measure capacitance is with a C and ESR meter. To do this, the contacts of the measuring probes are connected to the terminals of the capacitor, observing the polarity of the electrolytic parts. In this case, the measurement results are displayed. Radio amateurs often have to take measurements to purchase such a device or make it themselves.
Using a multimeter and formulas
If you have a multimeter with the function of measuring the “CX” parameter at your disposal, then measuring the capacitance of the capacitor is quite simple: you should switch the device to the “CX” mode, and then select the optimal measurement range corresponding to the parameters of the ac capacitor. We insert the legs of the capacitor into the appropriate socket (observing the polarity of the connection) and read its parameters.
Less accurately, you can determine the capacity using a tester that does not have the “CX” mode. This will require a power source to which the capacitor is connected in a simple way.
The measurement algorithm is as follows:
- Measure the voltage of the power supply with the contact probes of the meter.
- Form an RC circuit with a capacitor and resistor leads with a value of 1 – 10 kOhm.
- Short the capacitor leads and connect sct the RC circuit to the power supply.
- Measure the voltage of the formed circuit with a multimeter.
- If the voltage has changed, you need to adjust it to a value close to the one you received at the output of the power supply.
- Calculate 95% of the received value. Write down the measurements.
- Take a stopwatch and turn it on at the same time as removing the shorts.
- Once the multimeter shows the voltage you calculated (95%), stop the stopwatch.
- According to the formula C \u003d t / 3R, where T is the voltage drop time, we calculate the capacitance of the capacitor in farads, if the units of measurement of the resistor resistance are expressed in ohms, and the time is in seconds.
We emphasize once again that the accuracy of capacitance measurement by this method is not too high, but it is quite possible to determine the performance of a radio element based on such a measurement. Some components of electronic devices work properly if there are small deviations from the nominal capacities, the main thing is that there is no electrical breakdown.
The same method can be used to calculate the parameters of a ceramic radioelement. To do this, you need to connect the RC circuit through a transformer and apply alternating voltage. The capacitance value, in this case, is determined by the formula: C \u003d 0.5 * π * f * X c, where f is the frequency of the current, and XC is the capacitance.
Oscilloscope
With acceptable accuracy, you can determine the capacitance of a capacitor using a digital or conventional electronic oscilloscope. The principle is similar to the method of measuring capacitance with a tester. The only difference is that you do not need a stopwatch, since the capacitor charging time is displayed on the oscilloscope screen with high accuracy. If we use a frequency generator and a serial RC circuit, then the capacitance can be calculated using a simple formula: C = U R / U C * (1 / 2*π*f* R).
The calculation algorithm is simple:
- Connect the oscilloscope to the electrical circuit. When connecting the probes of the device to electrolytes, observe the polarity of the electric current.
- Measure the voltage amplitudes across the capacitor and across the resistor.
- By adjusting the frequency of the generator, make sure that the amplitude values on both elements are equal (at least approximately).
- Substitute the obtained values into the formula and calculate the capacitance of the capacitor.
When measuring capacitances of non-polar capacitors, a bridge circuit with a frequency generator, as well as other assemblies, is often assembled instead of an RC circuit. The resistance of the resistors is selected depending on the parameters of the nominal voltages of the measured parts. The capacity is calculated from the ratio: r 4 / C x = r 2 / C 0.
