A multimeter, also known as a multimeter, three-in-one meter, or multifunction meter, is a versatile measuring instrument that can measure direct current (DC) and alternating current (AC) voltage, resistance, and audio levels. Some models can also measure AC current, capacitance, inductance, and certain parameters of semiconductors (such as β).
AC Voltage Measurement: As shown in Figure 1, the maximum range is 20V, meaning your input voltage must not exceed this. The red probe is inserted into the v/Ω hole, and the black probe is inserted into the com hole. The value displayed on the meter is the AC voltage you are measuring.
Figure 1
Figure 2 shows five ranges used for measuring DC voltage.
Figure 2
The numbers on the top four ranges for DC measurement represent the maximum current values that can flow through these ranges, as shown in Figure 3.
Figure 3
These seven ranges are for measuring resistance values, with the maximum resistance values indicated for each range, as shown in Figure 4.
Figure 4
This range is used to test the condition of diodes and check for continuity, as shown in Figure 5.
Figure 5
Figure 6 shows the range for measuring capacitance, with the maximum capacitance values indicated for each range.
Figure 6
1. Structure of the Multimeter (Model 500)
The multimeter consists of three main parts: the meter head, measurement circuit, and selector switch.
(1) Meter Head
It is a highly sensitive moving coil DC ammeter, and the main performance indicators of the multimeter largely depend on the performance of the meter head. The sensitivity of the meter head refers to the value of the DC current flowing through the meter head when the pointer deflects to full scale; the smaller this value, the higher the sensitivity of the meter head. The internal resistance when measuring voltage should be as high as possible for better performance. The meter head has four scale lines, which function as follows: The first line (from top to bottom) is marked R or Ω, indicating resistance values, and is read when the selector switch is in the ohm range. The second line is marked ∽ and VA, indicating AC and DC voltage and DC current values, and is read when the selector switch is in the AC or DC voltage or current range, except for the AC 10V position. The third line is marked 10V, indicating the AC voltage value of 10V, and is read when the selector switch is in the AC or DC voltage range at the AC 10V position. The fourth line is marked dB, indicating audio levels.
(2) Measurement Circuit
The measurement circuit is used to convert various measurements into a suitable small DC current for the meter head. It consists of resistors, semiconductor components, and batteries.
It can convert various measurements (such as current, voltage, resistance, etc.) and different ranges into a small DC current within a certain limit through a series of processes (such as rectification, shunting, and voltage division) for measurement by the meter head.
(3) Selector Switch
The selector switch is used to choose different measurement circuits to meet the requirements for various types and ranges of measurements. There are generally two selector switches, each marked with different ranges and settings.
2. Symbol Meanings
(1) ∽ indicates AC and DC
(2) V-2.5KV 4000Ω/V indicates that for the AC voltage and 2.5KV DC voltage ranges, the sensitivity is 4000Ω/V
(3) A-V-Ω indicates that it can measure current, voltage, and resistance
(4) 45-65-1000Hz indicates the operating frequency range is below 1000 Hz, with a standard frequency range of 45-65Hz
(5) 2000Ω/V DC indicates the sensitivity for the DC range is 2000Ω/V
The symbols on clamp meters and shake meters are similar to the above symbols (others cannot be fully listed due to format issues, such as indicating a moving coil rectifier type with mechanical feedback, indicating three levels of anti-magnetic field, indicating horizontal placement).
3. Using the Multimeter
(1) Familiarize yourself with the meanings of the symbols on the dial and the main functions of each knob and selector switch.
(2) Perform mechanical zero adjustment.
(3) Based on the type and size of the measurement, select the appropriate range and find the corresponding scale line.
(4) Choose the position of the test lead sockets.
(5) Measuring Voltage: When measuring voltage (or current), select the appropriate range. If a small range is used to measure a large voltage, there is a risk of burning out the meter; if a large range is used to measure a small voltage, the pointer deflection will be too small to read. The range selection should aim to make the pointer deflect to about 2/3 of full scale. If the size of the voltage to be measured is unknown, start with the highest range and gradually decrease to an appropriate range.
(a) Measuring AC Voltage: Set one selector switch to the AC/DC voltage range and the other to the appropriate AC voltage range. Connect the two probes in parallel with the circuit or load being measured.
(b) Measuring DC Voltage: Set one selector switch to the AC/DC voltage range and the other to the appropriate DC voltage range, ensuring the “+” probe (red probe) is connected to the high potential and the “-” probe (black probe) to the low potential, allowing current to flow from the “+” probe to the “-” probe. If the probes are reversed, the pointer will deflect in the opposite direction, which may bend the pointer.
(6) Measuring Current: When measuring DC current, set one selector switch to the DC current range and the other to an appropriate range between 50μA and 500mA. The current range selection and reading method are the same as for voltage. Before measuring, the circuit must be disconnected, and the multimeter must be connected in series with the circuit being measured, allowing current to flow from the red probe to the black probe. If the multimeter is mistakenly connected in parallel with the load, the low internal resistance of the meter head may cause a short circuit and damage the instrument. The reading method is as follows:
Actual Value = Indicated Value × Range / Full Scale Deflection
(7) Measuring Resistance: When measuring resistance with a multimeter, follow these steps:
(a) Select the appropriate range. The ohm scale on the multimeter is not uniform, so the range selection should aim to keep the pointer in the less dense part of the scale, and the closer the pointer is to the middle of the scale, the more accurate the reading. Generally, the pointer should be between 1/3 and 2/3 of the scale.
(b) Zero adjustment for ohms. Before measuring resistance, short the two probes and adjust the “ohm (electrical) zero adjustment knob” so that the pointer just points to the zero position on the ohm scale. If the pointer cannot be adjusted to zero, it indicates insufficient battery voltage or an internal problem with the instrument. Additionally, every time the range is changed, the ohm zero adjustment should be performed again to ensure accurate measurements.
(c) Reading: The reading on the meter head multiplied by the range gives the measured resistance value.
(8) Precautions
(a) Do not change the range while measuring current or voltage when the circuit is live.
(b) When selecting a range, start with a larger range and then select a smaller one, aiming to keep the measured value close to the range.
(c) Do not measure resistance while the circuit is live. When measuring resistance, the multimeter is powered by its internal battery, and measuring with a live circuit is equivalent to connecting an additional power source, which may damage the meter head.
(d) After use, set the selector switch to the maximum AC voltage position or off.
4. Digital Multimeter
Currently, digital measuring instruments have become mainstream and are trending to replace analog instruments. Compared to analog instruments, digital instruments have higher sensitivity, accuracy, clear display, strong overload capacity, are portable, and easier to use. Below, we will briefly introduce the usage and precautions of the VC9802 digital multimeter as an example.
(1) Usage
a. Before use, carefully read the relevant user manual to familiarize yourself with the power switch, range switch, sockets, and special sockets.
b. Set the power switch to the ON position.
c. Measuring AC and DC voltage: Set the range switch to the appropriate DCV (DC) or ACV (AC) range as needed, insert the red probe into the V/Ω socket, the black probe into the COM socket, and connect the probes in parallel with the circuit being measured. The reading will be displayed.
d. Measuring AC and DC current: Set the range switch to the appropriate DCA (DC) or ACA (AC) range, insert the red probe into the mA socket (for <200mA) or the 10A socket (for >200mA), the black probe into the COM socket, and connect the multimeter in series with the circuit being measured. When measuring DC, the digital multimeter can automatically display the polarity.
e. Measuring resistance: Set the range switch to the appropriate Ω range, insert the red probe into the V/Ω socket, the black probe into the COM socket. If the measured resistance exceeds the maximum value of the selected range, the multimeter will display “1”; at this point, a higher range should be selected. When measuring resistance, the red probe is positive, and the black probe is negative, which is the opposite of the analog multimeter. Therefore, when measuring polarized components such as transistors and electrolytic capacitors, attention must be paid to the polarity of the probes.
(2) Usage Precautions
a. If the size of the voltage or current to be measured cannot be estimated in advance, first set it to the highest range and measure once, then gradually reduce the range to an appropriate position. After measurement, set the range switch to the highest voltage range and turn off the power.
b. When at full scale, the instrument will only display the digit “1” at the highest position, and other positions will disappear; at this point, a higher range should be selected.
c. When measuring voltage, the digital multimeter should be connected in parallel with the circuit being measured. When measuring current, it should be connected in series with the circuit being measured; when measuring DC, polarity does not need to be considered.
d. If the AC voltage range is mistakenly used to measure DC voltage, or the DC voltage range is mistakenly used to measure AC voltage, the display will show “000” or the digits at the lower position will fluctuate.
e. It is prohibited to change the range while measuring high voltage (above 220V) or large current (above 0.5A) to prevent arcing and burning the switch contacts.
f. When the display shows “ ”, “BATT”, or “LOW BAT”, it indicates that the battery voltage is below the operating voltage.
5. Shake Meter
A shake meter, also known as a megohmmeter, is used to measure the insulation resistance and high resistance of the equipment being tested. It consists of a hand-cranked generator, a meter head, and three terminals (L: line terminal, E: ground terminal, G: shield terminal).
1) Selection Principles for Shake Meters
(1) Selection of rated voltage levels. Generally, for equipment rated below 500V, a shake meter rated at 500V or 1000V should be selected; for equipment rated above 500V, a shake meter rated at 1000V to 2500V should be selected.
(2) Selection of resistance range. The scale line of the shake meter has two small black dots, and the area between the dots is the accurate measurement area. Therefore, when selecting a meter, ensure that the insulation resistance value of the equipment being tested falls within the accurate measurement area.
2) Using the Shake Meter
(1) Calibration. Before measurement, perform an open-circuit and short-circuit test on the shake meter to check if it is functioning properly. With the two connection wires open, crank the handle, and the pointer should point to “∞”. Then short the two connection wires, and the pointer should point to “0”. If these conditions are met, the meter is good; otherwise, it should not be used.
(2) Disconnect the equipment and lines being tested, and discharge large capacitive devices.
(3) Select a shake meter that meets the voltage level requirements.
(4) When measuring insulation resistance, generally only the “L” and “E” terminals are used, but when measuring the insulation resistance of cables to ground or when the leakage current of the equipment being tested is significant, the “G” terminal should be used, connecting it to the shielding layer or housing. After connecting the lines, turn the crank in a clockwise direction, starting slowly and then increasing speed. When the speed reaches about 120 revolutions per minute (for the ZC-25 model), maintain a constant speed for one minute while reading the value, and read while cranking without stopping.
(5) Discharge the lines after reading. After reading, slowly crank while disconnecting the lines, then discharge the equipment being tested. The discharge method is to remove the ground wire used during measurement from the shake meter and short it to the equipment being tested (not discharging the shake meter).
3) Precautions
(1) It is prohibited to measure insulation resistance during thunderstorms or near high-voltage equipment; measurements can only be made when the equipment is not powered and there is no induced voltage.
(2) During the shaking measurement process, no one should work on the equipment being tested.
(3) The shake meter wires should not be twisted together; they should be kept separate.
(4) It is strictly prohibited to touch the equipment before the shake meter stops turning or the equipment is discharged. When disconnecting the wires, do not touch the metal parts of the leads.
(5) At the end of the measurement, large capacitive devices should be discharged.
(6) Regular calibration of accuracy is required.
6. Clamp Meter
A clamp meter is an instrument used to measure the current in an electrical circuit without interrupting the circuit.
1) Structure and Principle
A clamp meter essentially consists of a current transformer, a clamp-type wrench, and a rectifier-type moving coil instrument with feedback.
2) Usage
(1) Perform mechanical zero adjustment before measurement.
(2) Select the appropriate range, starting with a larger range and then selecting a smaller one or estimating based on the nameplate value.
(3) When using the smallest range for measurement, if the reading is still not clear, the measured wire can be looped several times, with the number of loops based on the central loop of the clamp; the reading = indicated value × range / full scale deflection × number of loops.
(4) During measurement, ensure the measured wire is centered in the clamp and that the clamp is tightly closed to reduce errors.
(5) After measurement, set the selector switch to the maximum range.
3) Precautions
(1) The voltage of the measured circuit must be lower than the rated voltage of the clamp meter.
(2) When measuring the current of high-voltage circuits, wear insulated gloves, insulated shoes, and stand on an insulated mat.
(3) The clamp must be tightly closed, and the range should not be changed while the circuit is live.
-End-
Source: Chongqing Construction Training.
