Identification and use of digital multimeter

Identification and use of multimeter

1. The appearance of the multimeter.

A multimeter is an instrument for measuring parameters such as voltage, current and resistance. It has two types: pointer type and digital type. Its appearance is shown in Figure 1.

2. Prepare the multimeter before use.

(1) Place horizontally.

Place the multimeter horizontally.

(2) Check the pointer.

Check whether the multimeter pointer is at the "zero" position on the left end of the dial. If it is not at the "zero" position, use a small screwdriver to gently turn the mechanical zero adjustment 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 test leads.

Insert the red and black test leads into the test lead jacks respectively.

(4) Check the battery.

Turn the range selection switch to the resistance R×1 position and short-circuit the red and black test leads. If the multimeter pointer still cannot move to the zero position on the right end of the scale line after performing "Ohm zero adjustment", it means that the voltage is insufficient and the battery needs to be replaced.

(5) Select the measurement item and range.

Turn the range selector switch to the corresponding item and range. Do not switch the range selector switch when the power is on to prevent the switch contacts from being damaged by the arc.

3. Use a multimeter to measure current, voltage and resistance

(1) Measure current.

① Select the range. The current range of the multimeter is marked with "mA", and there are different ranges such as 1mA, 10mA, 100mA, 500mA, etc. You should select the appropriate range according to the size of the current being measured. If you don't know the current size, you should first measure it with the maximum current range, and then gradually switch to the appropriate current range.

② Measurement method. Connect the multimeter in series with the circuit to be measured, disconnect the corresponding part of the circuit, and connect the multimeter probes to both ends of the breakpoint. If it is a DC current, the red probe is connected to the breakpoint connected to the positive pole of the circuit, and the black probe is connected to the breakpoint connected to the negative pole of the circuit, as shown in Figure 3.7.

③ Read the value correctly. Carefully observe the dial, find the corresponding scale line and read the measured voltage value. Note that when reading, your sight should be directly facing the pointer.

(2) Measure voltage.

① Select the range. The DC voltage range of the multimeter is marked with "V", and there are different ranges such as 2.5V, 10V, 50V, 250V and 500V. You should select the appropriate range according to the voltage being measured. If you don't know the voltage, you should first measure it with the highest voltage range, and then gradually switch to the appropriate voltage range.

②Measurement method. Connect the multimeter in parallel to the two ends of the circuit under test. If measuring DC voltage, connect the red test lead to the positive pole of the circuit under test and the black test lead to the negative pole of the circuit under test, as shown in Figure 4.

Figure 3 Multimeter measuring DC current

Figure 4 Multimeter measuring DC voltage

③ Read the value correctly. Carefully observe the dial, find the corresponding scale line and read the measured voltage value. Note that when reading, your sight should be directly facing the pointer.

(3) Measure resistance.

① Select the range. The resistance block of the multimeter is marked with "Ω", and there are different ranges such as R×1, R×10, R×100, R×1k, and R×10k. The range selection switch should be turned to the appropriate position according to the size of the resistance to be measured, so that the pointer stays as close to the center as possible, because the error is minimized at this time, as shown in Figure 5 (a).

②Ohm zero adjustment. Short-circuit the red and black test leads. If the multimeter pointer cannot fully deflect (the pointer cannot deflect to the zero position on the right end of the scale line), you can perform "Ohm zero adjustment", as shown in Figure 5 (b).

③ Measurement method: Disconnect the resistor to be measured from other components or power supplies, hold the test lead with one hand and connect it across the two ends of the resistor, as shown in Figure 5 (c).

Figure 5 Multimeter measuring resistance

④ Read correctly. When reading, you should first determine the minimum scale value according to the position of the pointer, and then multiply it by the magnification to get the actual resistance value of the resistor. If the value indicated by the pointer is 18.1Ω, and the selected range is R×100, the measured resistance value is 1810Ω.

⑤After each gear change, adjust the "Ohm Zero Adjustment" knob again and then measure again.

4. Maintenance of the multimeter.

(1) Remove the test leads after each use.

(2) Turn the range selector switch to "OFF" or the highest AC voltage position to prevent the multimeter from being accidentally burned out when you start measuring next time.

(3) If the multimeter is not used for a long time, the battery should be removed to prevent the battery electrolyte from leaking and corroding the internal circuit, as shown in Figure 6.

Figure 6 Maintenance of multimeter

(4) Keep the multimeter dry and clean at all times, and avoid vibration and mechanical shock.

Knowledge Development

Multimeter measurement of potentiometer and capacitor

1. Measure the potentiometer.

Potentiometer, also known as variable resistor, refers to a resistor whose resistance value can be freely adjusted within a certain range. The nominal resistance and resistance change of the potentiometer can be measured with a multimeter. The specific usage is shown in Figure 7.

Figure 7 Specific method of using a multimeter to measure the potentiometer

(1) Selection

Range

。

Turn the range selection switch to the appropriate range.

(2) Measure the nominal resistance.

Measure the resistance of the 1 and 3 terminals of the potentiometer, which is the nominal resistance of the potentiometer, as shown in Figure 7 (a).

If the resistance is infinite, it can be determined that there is an open circuit inside the potentiometer.

(3) Measure the resistance change.

While slowly rotating the potentiometer's shaft, measure the resistance of terminals 1 and 2 or terminals 2 and 3 to see if they change continuously and evenly, as shown in Figure 7 (b). If the resistance changes intermittently or jumps, it can be preliminarily determined that the potentiometer has problems such as uneven resistance change or poor contact.

(4) Determine the type of potentiometer.

When the potentiometer rotates evenly, if the deflection of the multimeter pointer is also even, it indicates that it is a linear potentiometer; when the potentiometer rotates evenly, if the deflection of the multimeter pointer is faster (or slower) at the beginning and slower (or faster) at the end, it indicates that it is an anti-logarithmic or logarithmic potentiometer.

2. Use a multimeter to measure the capacitor.

Capacitors are commonly used components consisting of two metal plates with an insulating material (dielectric) in between. A multimeter can be used to perform qualitative and semi-quantitative quality tests on capacitors. The basic method of measuring capacitors with a multimeter is shown in Figure 8.

①Select the range.

Turn the range selection switch to the appropriate resistance range (R×1k or R×10k).

②Measure ordinary capacitors.

When measuring a capacitor with a large capacity (above 5000pF), the multimeter pointer will quickly swing to the right and then gradually return to the left. The resistance value indicated by the pointer when it stops is the insulation resistance of this capacitor. The larger the insulation resistance, the better, and it should generally be close to ∞, as shown in Figure 8 (a). When measuring a capacitor with a small capacity (below 5000pF), the multimeter pointer basically does not move.

Figure 8 Basic method of measuring capacitors with a multimeter

③Measure electrolytic capacitors.

Electrolytic capacitors are polarized capacitors. When testing, connect the red test lead to the negative electrode of the electrolytic capacitor and the black test lead to the positive electrode. The larger the capacitance, the greater the swing of the meter needle, as shown in Figure 8 (b). After each measurement, short-circuit the two ends of the capacitor with the test lead to discharge the capacitor.