Methods for determining the capacitance of a capacitor
Using Cx Mode
After the contacts are short-circuited, it is possible to determine the resistance. If the element is fixed, then immediately after connecting it will begin to charge with direct current. In this case, the resistance will be minimal and will continue to grow.
If the capacitor is faulty, then the multimeter will immediately indicate infinity or will indicate zero resistance and at the same time squeak. Such verification is carried out if the design is polar.
In order to find out the capacitance it is necessary to have a multimeter with the function of measuring the “Cx” parameter.
To determine the capacitance using such a multimeter is simple: set it to “Cx” mode and indicate the minimum measurement limit that this capacitor should have. In such multimeters there are special sockets with certain measurement limits. A capacitor is inserted into these sockets according to its measurement limit and its parameters are determined.
If there are no such sockets in the tester, then you can determine the capacity using measuring probes, as shown in the photo below:
Important! In a separate article we talked about how to check the health of the capacitor. We also recommend that you familiarize yourself with this material!
Application of formulas
What should I do if I don’t have such a multimeter with measuring sockets at hand, and there is only an ordinary household appliance? In this case, it is necessary to recall the laws of physics that will help determine the capacity.
To begin with, we recall that in the case when the capacitor is charged from a constant voltage source through a resistor, there is a pattern according to which the voltage on the device will approach the source voltage and ultimately equal to it.
But in order to not expect this, the process can be simplified. For example, for a certain time, which equals 3 * RC, during charging the cell reaches a voltage of 95% applied to the RC circuit. Thus, by the current and voltage, you can determine the time constant. And more correctly, if you know the voltage in the power supply, the nominal value of the resistor itself, the time constant, and then the device capacity, is determined.
For example, there is an electrolytic capacitor, the capacity of which can be found by marking, where 6800 microfarads 50v are prescribed.But what if the device has long been idle, and the inscription is difficult to determine its working condition? In this case, it is better to check its capacity in order to know for sure.
To do this, do the following:
- Using a multimeter, measure the resistance of the resistor at 10 kOhm. For example, it turned out to be equal to 9880 Ohms.
- We connect the power supply. The multimeter is switched to the constant voltage measurement mode. Then we connect it to the power supply (through its conclusions). After that, 12 volts are set in the block (the number 12.00 V should appear on the multimeter). If it was not possible to adjust the voltage in the power unit, then we record the results that turned out.
- Using a capacitor and a resistor, we assemble an electric RC circuit. The diagram below shows a simple RC chain:
- Short circuit the capacitor and connect the circuit to power. Using the device, once again determine the voltage that is supplied to the circuit and record this value.
- Then you need to calculate 95% of the obtained value. For example, if it is 12 Volts, then it will be 11.4 V. That is, for a certain time, which equals 3 * RC, the capacitor will receive a voltage of 11.4 V. The formula is as follows:
- It remains to determine the time. To do this, we deploy the device and use the stopwatch to count down. The definition of 3 * RC will be calculated in this way: as soon as the voltage on the device is equal to 11.4 V, this will mean the right time.
- We make the definition. For this, the obtained time (in seconds) is divided by the resistance in the resistor and by three. For example, it turned out 210 seconds. This figure is divided by 9880 and 3. The result is a value of 0.007085. This value is indicated in farads, or 7085 microfarads. Permissible deviation can be no more than 20%. Given that 6800 microfarads are indicated on the product, our calculations are confirmed and fit into the norm.
And how to determine the capacity of a ceramic capacitor? In this case, a determination can be made using a mains transformer. To do this, connect the RC chain to the secondary winding of the transformer, and it is connected to the network. Then, with the help of a multimeter, voltage is measured on the capacitor and on the resistor. After that, it is necessary to make calculations: the current is calculated that passes through the resistor, then its voltage is divided by the resistance. It turns out the capacitive resistance Xc.
If there is a current frequency and Xc, you can determine the capacitance by the formula:
Also, the capacity can be determined using a ballistic galvanometer. For this, the formula is used:
- Cq is the ballistic constant of the galvanometer;
- U2 - voltmeter readings;
- a2 is the deflection angle of the galvanometer.
The determination of the value by the voltmeter ammeter method is as follows: the voltage and current in the circuit are measured, after which the capacitance value is determined by the formula:
The voltage with this method of determination should be sinusoidal.
Measurement of the value is also possible using a bridge circuit. In this case, the circuit of the AC bridge is indicated below:
Here, one shoulder of the bridge is formed by the element that needs to be measured (Cx). The next arm consists of a lossless capacitor and a resistance store. The remaining two shoulders consist of resistance stores. We connect the power source in one diagonal, and the zero indicator in the other. And we calculate the value according to the formula:
Finally, we recommend watching a useful video on the topic:
That's all we wanted to tell you about how to determine the capacitance of a capacitor with a multimeter. We hope the information provided was useful and interesting for you!
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