A multimeter, also known as a multimeter, tri-meter, or multi-use meter, is a multifunctional, multi-range measuring instrument. Generally, a multimeter can measure direct current, direct voltage, alternating voltage, resistance, and audio level, while some can also measure alternating current, capacitance, inductance, and certain parameters of semiconductors (such as β).
Measuring AC voltage: As shown in Figure 1, the maximum range is 20V, which means your input voltage must not exceed this. Insert the red probe into the v/Ω hole and the black probe 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 value that can flow through these ranges, as shown in Figure 3.
Figure 3
These seven ranges are for measuring resistance values, and the markings indicate the maximum resistance that can be measured in each range, as shown in Figure 4.
Figure 4
This range is used to measure the condition of diodes and the continuity of circuits, as shown in Figure 5.
Figure 5
Figure 6 shows the range for measuring capacitance, and the markings also indicate the maximum capacitance that can be measured.
Figure 6
1. Structure of the Multimeter (Model 500)
A multimeter consists of three main parts: the meter head, measuring circuit, and selector switch.
(1) Meter Head
It is a highly sensitive magnetic electric 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 direct current flowing through it when the needle is deflected to full scale; the smaller this value, the higher the sensitivity. The higher the internal resistance when measuring voltage, the better the performance. The meter head has four scale lines, and their functions are as follows: The first line (from top to bottom) marked R or Ω indicates resistance; read this line when the selector switch is in the ohm position. The second line marked ∽ and VA indicates AC and DC voltage and DC current values; read this line when the selector switch is in the AC or DC voltage or current position, except for the AC 10V setting. The third line marked 10V indicates the AC voltage value of 10V; read this line when the selector switch is in the AC or DC voltage position and set to 10V. The fourth line marked dB indicates audio level.
(2) Measuring Circuit
The measuring circuit is used to convert various measurements into a suitable micro DC current for the meter head. It consists of resistors, semiconductor components, and batteries.
It can unify various different measurements (such as current, voltage, resistance, etc.) and different ranges into a specific limited micro DC current through a series of processes (such as rectification, shunting, and voltage division) to send to the meter head for measurement.
(3) Selector Switch
The function of the selector switch is to select various measuring circuits to meet the requirements of different types and ranges of measurements. There are usually two selector switches, each marked with different positions and ranges.
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 that the frequency range is below 1000 Hz, with a standard frequency range of 45-65Hz
(5) 2000Ω/V DC indicates that 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 written due to format issues, such as ‘indicating magnetic electric rectifying type with mechanical reaction force meter’ and ‘indicating three-level anti-external magnetic field’ and ‘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 zeroing.
(3) Based on the type and size of the measurement, select the appropriate position and range on the selector switch, and find the corresponding scale line.
(4) Choose the position for the probes.
(5) Measuring Voltage: When measuring voltage (or current), choose the appropriate range; if a small range is used to measure a high voltage, there is a risk of burning the meter; if a large range is used to measure a small voltage, the needle will deflect too little to read. The range selection should ideally make the needle deflect to about 2/3 of the full scale. If the voltage to be measured is not known in advance, start with the highest range and gradually reduce to an appropriate range.
(a) Measuring AC Voltage: Set one selector switch to the AC or DC voltage position and the other to the appropriate range for AC voltage. Connect the two probes in parallel with the circuit or load being measured.
(b) Measuring DC Voltage: Set one selector switch to the AC or DC voltage position and the other to the appropriate range for DC voltage, ensuring that the ‘+’ probe (red probe) is connected to the high potential point and the ‘-‘ probe (black probe) is connected to the low potential point, allowing current to flow from the ‘+’ probe to the ‘-‘ probe. If the probes are reversed, the needle will deflect in the opposite direction, which may bend the needle.
(6) Measuring Current: When measuring DC current, set one selector switch to the DC current position and the other to an appropriate range between 50uA and 500mA. The selection of current range and reading method is the same as for voltage. Before measuring, the circuit must be disconnected, and the meter must be connected in series with the circuit being measured, allowing current to flow from the red probe to the black probe. If the meter is mistakenly connected in parallel with the load, the internal resistance of the meter head is very low, which can cause a short circuit and burn out 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, the following actions should be taken:
(a) Select the appropriate multiplier range. The ohm scale on the multimeter is not uniform, so the selection of the multiplier range should keep the needle in the less dense part of the scale, and the closer the needle is to the middle of the scale, the more accurate the reading. Generally, the needle should point between 1/3 and 2/3 of the scale.
(b) Zeroing the ohm scale. Before measuring resistance, short the two probes and adjust the “ohm (electrical) zeroing knob” until the needle points just to the right of the zero position on the ohm scale. If the needle cannot be adjusted to the zero position, it indicates that the battery voltage is insufficient or there is a problem with the instrument. Moreover, every time the multiplier range is changed, zeroing should be repeated to ensure accurate measurements.
(c) Reading: The reading on the meter head multiplied by the multiplier gives the resistance value being measured.
(8) Precautions
(a) Do not change the range while measuring current or voltage under load.
(b) When selecting the range, always start with the largest range and then select smaller ranges, aiming to keep the measured value close to the range.
(c) Do not measure resistance while the circuit is powered. When measuring resistance, the multimeter is powered by its internal battery; measuring under power is equivalent to connecting an extra power source, which may damage the meter head.
(d) After use, set the selector switch to the maximum AC voltage position or the off position.
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, higher accuracy, clearer display, stronger overload capacity, are easier to carry, and are simpler to use. Below, we take the VC9802 digital multimeter as an example to briefly introduce its usage and precautions.
(1) Usage
a. Before use, carefully read the relevant instruction manual to familiarize yourself with the functions of 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 range for DCV (direct current) or ACV (alternating current) as needed. Insert the red probe into the V/Ω hole and the black probe into the COM hole, 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 range for DCA (direct current) or ACA (alternating current). Insert the red probe into the mA hole (for <200mA) or the 10A hole (for >200mA), and the black probe into the COM hole. Connect the multimeter in series with the circuit being measured. When measuring DC, the digital multimeter can automatically display polarity.
e. Measuring Resistance: Set the range switch to the appropriate range for Ω. Insert the red probe into the V/Ω hole and the black probe into the COM hole. If the measured resistance value 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 analog multimeters. Therefore, when measuring polarized components such as transistors and electrolytic capacitors, it is essential to pay attention to the polarity of the probes.
(2) Precautions
a. If you cannot estimate the size of the voltage or current to be measured in advance, first set it to the highest range and measure once, then gradually reduce the range to an appropriate position based on the situation. After measurement, set the range switch to the highest voltage setting and turn off the power.
b. When full scale is reached, the instrument will only display the digit “1” at the highest position, with all other positions disappearing; at this time, a higher range should be selected.
c. When measuring voltage, connect the digital multimeter in parallel with the circuit being measured. When measuring current, connect it in series with the circuit being measured; for DC measurements, polarity does not need to be considered.
d. If the AC voltage range is mistakenly used to measure DC voltage or vice versa, 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 damage to the switch contacts.
f. When the display shows ” ” or “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 connection terminals (L: line terminal, E: ground terminal, G: shield terminal).
1) Selection Principles for Shake Meters
(1) Selection of rated voltage level: 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 on the shake meter has two small black dots, and the area between the small dots is the accurate measurement area. Therefore, when selecting a shake meter, ensure that the insulation resistance value of the equipment being measured 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 needle should point to “∞”. Then short the two connection wires, and the needle should point to “0”. If it meets these conditions, it is good; otherwise, it cannot be used.
(2) Disconnect the device and circuit being measured, and discharge large capacitive devices.
(3) Select a shake meter with a voltage level that matches the measurement.
(4) When measuring insulation resistance, generally only use the “L” and “E” terminals. However, when measuring the insulation resistance of cables to ground or when the leakage current of the equipment being measured is severe, use the “G” terminal and connect it to the shielding layer or casing. After connecting the circuit, turn the crank in a clockwise direction, starting slowly and increasing speed; when the speed reaches about 120 revolutions per minute (for the ZC-25 model), maintain a uniform speed, read the value after one minute, and read while cranking without stopping to read.
(5) Discharge the circuit: After reading, slowly crank while disconnecting the wires, and then discharge the equipment being measured. The method of discharging is to remove the grounding wire used during measurement and short it with the device being measured (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 without induced voltage.
(2) During the shaking measurement process, no one should work on the device being measured.
(3) The shake meter wires should not be twisted together; they should be kept separate.
(4) Do not touch the shake meter before it stops turning or before the device being measured has been discharged. When disconnecting wires, do not touch the metal parts of the leads.
(5) After measurement, large capacitive devices should be discharged.
(6) Regularly check the accuracy of the shake meter.
6. Clamp Meter
A clamp meter is a device used to measure the current magnitude of electrical circuits while they are operational, allowing current measurement without interrupting the power supply.
1) Structure and Principle
A clamp meter essentially consists of a current transformer, a clamp handle, and a rectifying magnetic electric system with a reactive force meter.
2) Using the Clamp Meter
(1) Mechanical zeroing must be performed before measurement.
(2) Select the appropriate range, starting with the largest range and then moving to smaller ranges or estimating based on the nameplate value.
(3) When using the smallest range, if the reading is still not clear, the wire being measured can be wrapped several times; the number of wraps should be based on the central number of wraps at the clamp’s jaws, thus the reading = indicated value × range / full scale deflection × number of wraps.
(4) During measurement, ensure the wire being measured 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 position.
3) Precautions
(1) The voltage of the measured circuit must be lower than the rated voltage of the clamp meter.
(2) When measuring current in 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 under power.
Source: Chongqing Construction Training
Recent Highlights:
-
1 million super manufacturing factories, how to seek automation production…
-
“Golden Weld” zero rework, this welding team is not simple
-
Breaking! National standards will be fully publicly available
-
The generation of welding cracks and preventive measures
