Identification and use of multimeters
1. The shape of the multimeter. 
A multimeter is a type of instrument that measures parameters such as voltage, current, and resistance, and is available in two types: pointer and digital, as shown in Figure 1.
Figure 1 Multimeter.
2. Prepare before the use of the multimeter. (1) Horizontal placement. Place the multimeter horizontally.
2) Check the pointer. Check that the multimeter hand stops at the "zero" position at the left end of the dial. If it is not in the "zero" position, use a small screwdriver to gently turn the mechanical zeroing knob on the meter head so that the pointer points to "zero", as shown in Figure 2.
Figure 2: Mechanical zeroing of a multimeter.
3) Insert the pen. Insert the red and black pen into the pen jack respectively.
4) Check the battery. Screw the range selector switch to the resistance R 1 gear, and short the red and black watch pens, such as after "ohm zeroing", the multimeter pointer still cannot turn to the zero position at the right end of the scale mark, indicating that the voltage is insufficient and the battery needs to be replaced.
5) Select the measurement item and range. Screw the range selector switch to the appropriate item and range. It is forbidden to switch the range selector switch in the energized measurement state, so as to avoid possible arcing damage to the switch contacts.
3. Multimeter measures current, voltage and resistance(1) Current measurement. Select a range. The current block of the multimeter is marked with "MA", and there are different ranges such as 1mA, 10mA, 100mA, 500mA, etc. The appropriate range should be selected according to the size of the current to be measured. If you don't know the size of the current, you should first measure it with the maximum current stop, and gradually change to the appropriate current stop.
Measurement method. To connect the multimeter in series with the circuit under test, the corresponding part of the circuit should be disconnected, and the multimeter pen should be connected to both ends of the breakpoint. In the case of DC current, the red watch pen is connected to the break point connected to the positive pole of the circuit, and the black watch pen is connected to the break point connected to the negative pole of the circuit, as shown in Figure 37.
Correct reading. Carefully observe the dial, find the corresponding tick mark and read out the measured voltage value. Note: When reading, the eye should be directly on the pointer.
2) Voltage measurement. Select a range. The DC voltage block of the multimeter is marked with "V", and there are 2For different ranges such as 5V, 10V, 50V, 250V and 500V, the appropriate range should be selected according to the size of the voltage to be measured. If you don't know the voltage, you should first measure it with the highest voltage block, and gradually change to the appropriate voltage block.
Measurement method. Connect the multimeter in parallel at both ends of the circuit under test. If the DC voltage is measured, the red pen is connected to the positive pole of the circuit under test, and the black pen is connected to the negative pole of the circuit under test, as shown in Figure 4.
Figure 3: A multimeter measures the DC current.
Figure 4: A multimeter measures the DC voltage.
Correct reading. Carefully observe the dial, find the corresponding tick mark and read out the measured voltage value. Note: When reading, the eye should be directly on the pointer.
3) Measure resistance. Select a range. The multimeter block is marked with " " There are different ranges such as r 1, r 10, r 100, r 1k, r 10k, etc., the range selector switch should be dialed to the appropriate gear according to the size of the measured resistance, so that the pointer stays near the center as much as possible, because the error is the smallest at this time, as shown in figure 5(a).
Ohmic zeroing. If the red and black watch pens are shorted, if the multimeter hand cannot be fully offset (the pointer cannot be deflected to the zero position at the right end of the scale mark), "ohmic zeroing" can be performed, as shown in Figure 5(b).
Measurement method. The resistor to be measured is disconnected from other components or power supplies, and the pen is held in one hand and crossed across both ends of the resistor, as shown in Figure 5(c).
Figure 5: A multimeter measures resistance.
Correct reading. When reading, the minimum scale value should be determined according to the position of the pointer, and then multiplied by the magnification, which is the actual resistance value of the resistor. If the value indicated by the hand is 181. If the selected range is r 100, the measured resistance value is 1810.
After each gear shift, the Ohmic Zero knob should be adjusted again before measuring again.
4. Maintenance of multimeter. (1) After each use, the watch pen should be pulled out.
2) Dial the range selector switch to "OFF" or the highest AC voltage to prevent the multimeter from accidentally burning out when you start measuring next time.
3) If it is not used for a long time, the battery in the multimeter should be taken out to prevent the battery electrolyte leakage and corrode the internal circuit, as shown in Figure 6.
Figure 6: Maintenance of a multimeter.
4) Keep the multimeter dry and clean at all times, and vibration and mechanical impact are strictly prohibited.
Knowledge developmentMultimeter measurement of potentiometers and capacitors
1. Measuring potentiometer. Potentiometers, also known as potentiometers, refer to resistors whose resistance values can be freely adjusted within a certain range. The nominal resistance and resistance change of a potentiometer can be measured with a multimeter, as shown in Figure 7.
Figure 7: How to use a multimeter to measure potentiometers.
1) Select the range and turn the range selection transfer switch to the appropriate resistor.
2) Measure the nominal resistance. The resistance at the pinout of the potentiometer is measured, which is the nominal resistance of the potentiometer, as shown in Figure 7(a).
If the resistance value is infinite, the internal open circuit of the potentiometer can be determined.
3) Measure resistance changes. While slowly rotating the rotation axis of the potentiometer, the end or end resistance value is measured to see if there is a continuous and uniform change in the end, as shown in Figure 7(b). If it is found that the resistance value changes intermittently or there is a runout phenomenon, it can be preliminarily judged that the potentiometer has the problem of uneven resistance value change or poor contact.
4) The type of potentiometer. When the potentiometer rotates evenly, if the pointer of the multimeter is also deflected uniformly, it indicates that it is a linear potentiometer; When the potentiometer rotates evenly, if the pointer deflection of the multimeter starts faster (or slower) and is about to end slower (or faster), it indicates that it is an anti-digital or logarithmic potentiometer.
2. Multimeter measuring capacitors. A capacitor is a commonly used component consisting of two metal plates sandwiched between them with an insulating material (dielectric). A multimeter can be used for qualitative and semi-quantitative quality inspection of capacitors. The basic method for measuring capacitors with a multimeter is shown in Figure 8.
Select a range. Turn the range selection transfer switch to the appropriate resistor (R 1k or R 10k).
Measure common capacitors. When measuring a capacitor with a large capacitance (5000 pF or more), the multimeter pointer will quickly swing to the right and then gradually return to the left end, and the resistance value indicated when the pointer stops is the capacitor insulation resistance. The greater the insulation resistance, the better, and generally should be close to , as shown in Figure 8(a). When measuring capacitors with small capacitances (5000 pF or less), the multimeter pointer is basically unchanged.
Figure 8: A basic method for measuring capacitors with a multimeter.
Measuring electrolytic capacitors. Electrolytic capacitors are polarized capacitors, and the red pen is connected to the negative pole of the electrolytic capacitor and the black pen is connected to the positive pole during the test, the larger the capacitance, the greater the swing of the watch hand, as shown in Figure 8(b). After each measurement, the capacitor should be shorted at both ends with a table pen to discharge the capacitor.
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