Capacitor is one of the most commonly used electronic components. The outline and circuit symbols of the capacitor are shown in the figure.
The common character symbol of capacitor is "C". The capacitor is mainly composed of metal electrode, dielectric layer and electrode lead, and the two electrodes are insulated from each other. Therefore, it has the basic function of "direct communication".
The following methods can be used to detect capacitors with digital multimeter.
First, use capacitance file for direct detection.
Some digital multimeters have the function of measuring capacitance, and their measuring ranges are divided into five grades: 2000p, 20n, 200n, 2μ and 20μ. When measuring, the two pins of the discharge capacitor can be directly inserted into Cx jack on the instrument panel, and the display data can be read after selecting the appropriate range.
2000p file, suitable for measuring capacitance less than 2000pF; 20n, suitable for measuring the capacitance between 2000 pf and 20 nf; 200n, suitable for measuring the capacitance between and 200nF; 2μ file, suitable for measuring the capacitance between 200 nf and 2 μ f; 20μ range, suitable for measuring capacitance between 2μF and 20 μ f.
Experience has proved that some models of digital multimeters (such as DT890B++) have large errors when measuring small capacitance below 50pF, and measuring capacitance below 20pF has almost no reference value. At this time, small capacitance can be measured by series method. The method is as follows: firstly, find a capacitor of about 220pF, measure the original actual capacity C 1 with a digital multimeter, and then measure the total capacity C2 of the small capacitor to be tested in parallel with it, and the difference between them (C 1-C2) is the capacity of the small capacitor to be tested. It is very accurate to measure the small capacitance of 1 ~ 20pf by this method.
Second, use resistance detection.
It is proved that the charging process of capacitor can also be observed with digital multimeter, and actually the reunion digital quantity is used to reflect the change of charging voltage. If the measuring rate of digital multimeter is n times per second, while observing the charging process of capacitor, you can see n independent and increasing readings per second. According to this display characteristic of digital multimeter, the quality of capacitor can be detected and the capacitance value can be estimated. The following introduces the method of using digital multimeter to detect capacitance, which has great application value for instruments without capacitance. This method is suitable for measuring large-capacity capacitors of 0. 1 μ f ~ several thousand microfarads.
1. Measuring operation method
As shown in Figure 5- 1 1(a), set the digital multimeter in a suitable resistance range, and the red stylus and the black stylus contact the two poles of the measured capacitor Cx respectively. At this time, the display value will gradually increase from "000" until the overflow symbol "1" is displayed. If "000" is displayed continuously, it means that there is a short circuit inside the capacitor; If overflow continues to be displayed, it may be an open circuit between the internal electrodes of the capacitor, or the selected resistance file is not suitable. When inspecting electrolytic capacitors, it should be noted that the red contact pin (positively charged) is connected to the positive electrode of the capacitor, and the black contact pin is connected to the negative electrode of the capacitor.
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2. Measuring principle
The measuring principle of measuring capacitance with resistance is shown in Figure 5- 1 1(b). During the measurement, the positive power supply charges the measured capacitor Cx by adjusting the resistor R0. At the beginning of charging, VC = 0, so "000" is displayed. As Vc becomes higher, the displayed value increases. When Vc =2VR, the overflow symbol "1" is displayed at the beginning of the instrument. Charging time t is the time required for the displayed value to change from "000" to overflow, and this time distance can be measured with a quartz watch.
3. Use DT830 digital multimeter to estimate capacitance measurement data.
When using DT830 digital multimeter to estimate the capacitance from 0.1μ f to several thousand microfarads, you can choose the resistance file according to Table 5- 1, which gives the range of measurable capacitance and the corresponding charging time. The data listed in the table are also of reference value to other types of digital multimeters.
The criteria for selecting the resistance range are: when the capacitance is small, the high resistance range should be selected, and when the capacitance is large, the low resistance range should be selected. If the high impedance file is used to estimate the large capacitance, the measurement time will last for a long time because of the slow charging process; If the low resistance is used to test the small capacitance, the meter will continuously display overflow and can't see the change process because of the short charging time.
Third, with voltage detection
It is a practical indirect measurement method to detect the DC voltage range of capacitors with digital multimeter. This method can measure the small capacitance from 220 pf to 1 μ f, and can accurately measure the leakage current of the capacitance.
1. Measurement method and principle
The measuring circuit is shown in figure 5- 12, and e is an external 1.5V dry battery. Set the digital multimeter at DC 2V, connect the red probe to one electrode of the measured capacitor Cx, and connect the black probe to the negative electrode of the battery. Input resistance rin =10Ω for 2V gear. After turning on the power supply, the battery E charges Cx through RIN and starts to set the voltage Vc. The relationship between Vc and charging time t is
Here, because the voltage across RIN is the instrument input voltage VIN, RIN actually acts as a sampling resistor. Obviously,
VIN(t)= E-Vc(t)= EeXP(-t/RINCx)(5-2)
Fig. 5- 13 is the change curve of input voltage VIN(t) and charging voltage Vc(t) on the measured capacitor. As can be seen from the figure, the changes of VIN(t) and Vc(t) are just the opposite. The curve of VIN(t) decreases with time, while Vc(t) increases with time. Although the instrument shows the change process of VIN-(t), it indirectly reflects the charging process of the measured capacitor Cx. During the test, if Cx is open (no capacity), the displayed value will always be "000", and if Cx is short-circuited, the displayed value will always be battery voltage E, which will not change with time.
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Equation (5-2) indicates that when the circuit is just turned on, T = 0 and VIN=E, the digital multimeter starts to display the battery voltage, and then with the increase of Vc(t), VIN(t) gradually decreases until VIN = 0V, thus completing the Cx charging process. At this time,
Using digital multimeter to detect capacitors in the voltage range can not only detect small-capacity capacitors from 220 pf to 1 μ f, but also measure the leakage current of capacitors at the same time. Let the leakage current of the measured capacitor be ID and the fixed value displayed at the beginning of the meter be VD (in V), then
Step 2: Example
Example 1:
The measured capacitance is a fixed capacitance of 1μF/ 160V, and the 2VDC range (rin =10 Ω) of DT830 digital multimeter is used. Connect the circuit according to Figure 5- 12. At first, the instrument displayed 1.543V, and then the display value gradually decreased for about 2min, and the display value was fixed at 0.003V Based on this, the leakage current of the measured capacitor was obtained.
The measured leakage current of capacitor is only 0.3nA, and the quality is good.
Example 2:
The measured capacitance is 0.022μF/63V polyester capacitor, and the measurement method is the same as that of 1. Due to the small capacitance, VIN(t) drops rapidly during the measurement, and the displayed value drops to 0.002V after about 3 seconds. Substituting this value into equation (5-3), the leakage current is calculated as 0.2nA.
3. Preventive measures
(1) Before the measurement, the two pins of the capacitor should be short-circuited and discharged, otherwise the reading change process cannot be observed.
(2) During the measurement, hands shall not touch the capacitor electrode to avoid meter tripping.
(3) In the process of measurement, the value of VIN(t) changes exponentially, and decreases rapidly at the beginning, and the decreasing speed will be slower and slower as time goes on. When the capacity of the measured capacitor Cx is less than several thousand picofarads, the initial display value of the instrument is lower than the battery voltage E, because the initial decline of VIN(t) is too fast, and the measurement rate of the instrument is too low to reflect the initial voltage value.
(4) When the measured capacitance Cx is greater than 1μF, in order to shorten the measurement time, the resistance file can be used for measurement. However, when the capacity of the measured capacitor is less than 200pF, it is difficult to observe the charging process because of the instantaneous change of the reading.
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First, use a buzzer for detection.
Using the buzzer of digital multimeter, the quality of electrolytic capacitor can be checked at high speed. The measurement method is shown in Figure 5- 14. Turn the digital multimeter to the buzzer position, and contact the two pins of the measured capacitor Cx with two probes respectively. You should be able to hear a quick beep, then the sound will stop and the overflow symbol "1" will be displayed. Then, the two probes switch to measure again, and the buzzer should ring again, and the overflow symbol "1" is displayed at the beginning, indicating that the measured electrolytic capacitor is basically normal. At this point, the leakage resistance of the capacitor can be measured and the quality can be judged by setting the capacitor to a high resistance of 20 mω or 200 mω.
The principle of the above measurement process is that at the beginning of the test, the charging current of the instrument to Cx is large, which is equivalent to the channel, so the buzzer rings. As the voltage across the capacitor rises from time to time, the charging current decreases rapidly, and the buzzer stops ringing at the beginning.
When testing, it is assumed that the buzzer keeps ringing, indicating that there is a short circuit inside the electrolytic capacitor; If the buzzer doesn't sound continuously, and the meter keeps displaying "1", it means that the measured capacitor is open or the capacitor disappears.
Secondly, measure the capacitance greater than 20μF with a digital multimeter.
The capacitance of the most common digital multimeter is 20μF, which sometimes fails to meet the measurement requirements. Therefore, the capacitance of more than 20μF can be measured by using the capacitance file of digital multimeter and the following simple method, and the measured capacitance can reach several thousand microfarads. When this method is used to measure the large capacity capacitance, it is not necessary to make any changes to the original circuit of the digital multimeter.
The measuring principle of this method is based on the series formula of two capacitors, and C string = C 1c2/(C 1+C2). Because two capacitors with different capacities are connected in series, the total capacity after series connection is smaller than that of the capacitor with small capacity. Therefore, assuming that the measured capacitance exceeds 20μF, it can be measured directly on the digital multimeter by connecting a capacitor less than 20μF in series. According to the series formula of two capacitors, C 1 = C2C series /(C2-C series) is simply deduced, and the capacitance value of the measured capacitor can be calculated by using this formula. Here is a test example to illustrate the detailed method of applying this formula.
The measured element is an electrolytic capacitor with a nominal capacity of 220μF, and let it be C 1. The electrolytic capacitor with the nominal value of 10μF is selected as C2, and the 20μF capacitor of digital multimeter is selected. The actual value of this capacitor is 9.5 μ f.. After connecting these two capacitors in series, the C string is 9.09μF f. If C2 = 9.5 μ f and C string = 9.09 μ f are substituted into the formula, then
C 1 = C2C chord /(C2-C chord) = 9.59.09/(9.5-9.09) ≈ 211(μ f)
Note that no matter what the capacity of C2 is, if it is less than 20μF, the capacitor with larger capacity should be selected, and C2 in the formula should be substituted into the original measured value instead of the nominal value to reduce the error. These two capacitors are connected in series and measured with a digital multimeter. Because of the capacitance error and measurement error of the capacitor itself, it is only necessary that the measured value is close to the calculated value to consider that the capacitor C 1 to be measured is good, and the actual capacity of C 1 can be further calculated according to the measured value.