Detailed explanation of the multimeter’s structure and usage

Detailed Explanation of the Structure and Usage of a Multimeter. A multimeter, also known as a multi-functional meter, triple meter, or multipurpose meter, is a measuring instrument with multiple functions and measurement ranges. Typically, it can measure direct current, direct voltage, alternating voltage, resistance, and audio levels; some models can also measure alternating current, capacitance, inductance, as well as certain parameters of semiconductors such as β. 1. Structure of the multimeter (model 500) The multimeter consists of three main components: the meter head, the measurement circuit, and the selector switch. (1) Head: It is a high-sensitivity magnetoelectric DC ammeter; the main performance parameters of a multimeter essentially depend on the performance of this head. The sensitivity of the meter head refers to the value of the direct current flowing through it when the pointer reaches the full scale of deflection; the lower this value, the higher the sensitivity of the meter head. The higher the internal resistance when measuring voltage, the better its performance. There are four scale lines on the header, and their functions are as follows: the first one (from top to bottom) is marked with R or Ω, indicating the resistance value; this is the scale line that is read when the switch is set to the ohm setting. The second scale is marked with ∽ and VA; it indicates the values of AC and DC voltages as well as DC current. This scale is read when the selector switch is set to the AC/DC voltage or DC current setting, and the range is set to any value other than 10V AC. The third mark is labeled 10V, indicating a DC voltage value of 10V; when the selector switch is set to the AC/DC voltage mode and the range is set to 10V AC, this scale mark is read. Article 4 is marked with dB, indicating the audio level. (2) Measurement circuit: A measurement circuit is a circuit used to convert various quantities to be measured into small direct current values suitable for measurement by meters. It consists of resistors, semiconductor components, and batteries. It enables various different quantities to be measured (such as current, voltage, resistance, etc.), as well as different measurement ranges, to be transformed through a series of processes (such as rectification, current division, voltage division, etc.) into small direct current values within a specific range, which are then sent to the meter for measurement. (3) Changeover switch: Its function is to select various measurement circuits in order to meet the requirements of different types and ranges of measurements. A changeover switch usually has two, each marked with different settings and ranges. 2. Symbol meanings: (1) ∽ indicates alternating current and direct current. (2) V–2.5KV 4000Ω/V means that for alternating voltage and direct voltage at 2.5KV, 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 operating frequency range is below 1000 Hz, with the standard power frequency range being 45–65Hz. (5) 2000Ω/V DC means that the sensitivity for direct current is 2000Ω/V. The symbols on ammeters and megohmmeters are similar to those mentioned above; others cannot be listed all here due to formatting constraints – they represent instruments with electromagnetic rectification and mechanical feedback force, three-level protection against external magnetic fields, and horizontal placement respectively. 3. Use of multimeters: (1) Understand the meaning of various symbols on the dial, as well as the primary functions of each knob and selector switch. (2) Perform mechanical zeroing. (3) Depending on the type and size of the quantity being measured, select the gear and range of the selector switch, and identify the corresponding scale line. (4) Select the location for the probe jack. (5) Measuring voltage: When measuring voltage (or current), it is necessary to select the appropriate range; using a low range to measure a high voltage can pose a risk of damaging the meter ； If a large range is used to measure a small voltage, the pointer deflection is too small to be read. The range should be selected so that the pointer deflects to around 2/3 of the full scale. If the magnitude of the voltage to be measured is not known in advance, the highest range setting should be selected first, and then it should be gradually reduced to an appropriate range. a Measurement of AC voltage: Set one switch on the multimeter to the AC/DC voltage setting, and the other switch to the appropriate range for AC voltage. Then connect the two probes of the multimeter in parallel with the circuit or load being measured. b Measurement of DC voltage: Set one of the multimeter’s switches to the AC/DC voltage setting, and the other switch to the appropriate range for DC voltage. Connect the “+” probe (the red probe) to the higher potential, and the “-” probe (the black probe) to the lower potential; this allows current to flow in through the “+” probe and out through the “-” probe. If the test leads are connected reversely, the pointer of the meter will deflect in the opposite direction, which can easily bend the pointer. (6) Measuring current: When measuring direct current, set one of the multimeter’s switches to the direct current setting, and the other switch to an appropriate range of 50 uA to 500 mA. The selection of the current range and the method of reading it are the same as those for voltage. When making measurements, the circuit must first be disconnected, and then the multimeter should be connected in series with the circuit being tested in the direction of current from “+” to “–”, that is, the current flows in through the red test lead and out through the black test lead. If the multimeter is mistakenly connected in parallel with the load, its low internal resistance can cause a short circuit and destroy the instrument. The method of reading it is as follows: Actual value = Indicated value × Range/Full scale. (7) Measuring resistance: When using a multimeter to measure resistance, the following steps should be followed*: a. Select an appropriate multiplier setting. The scale markings on the ohm range of a multimeter are not uniform; therefore, the multiplier setting should be chosen so that the pointer stays in the areas where the markings are less dense. The closer the pointer is to the middle of the scale, the more accurate the reading will be. Under normal circumstances, the pointer should be positioned between 1/3 and 2/3 of the scale. b Ohm zero adjustment. Before measuring resistance, the two test leads should be shorted together, and the \"Ohm (Electrical) Zeroing Knob\" should be adjusted so that the pointer points exactly at the zero mark to the right of the ohm scale. If the pointer cannot be adjusted to zero, it indicates that the battery voltage is insufficient or there is a problem with the instrument itself. Moreover, every time the magnification setting is changed, an ohm zero adjustment must be performed again to ensure accurate measurements. c reading: The reading on the meter, multiplied by the multiplier factor, gives the resistance value of the resistor being measured. (8) Precautions: a. When measuring current and voltage, do not change the range while the device is still under voltage. b. When selecting a range, choose the larger one first and then the smaller one, so as to keep the value being measured as close as possible to that range. c. Do not measure resistance while the device is under voltage. When measuring resistance, the multimeter is powered by its internal battery; if measurement is carried out while it is still under voltage, it is equivalent to connecting an additional power source, which may damage the meter’s gauge. After use, the selector switch should be set to the highest AC voltage setting or to the off position. 4. Digital multimeters: Nowadays, digital measuring instruments have become the standard, and there is a trend toward replacing analog instruments with them. Compared to analog instruments, digital instruments have higher sensitivity and accuracy, provide clearer displays, offer better overload capacity, are easier to carry, and are simpler to use. Below, taking the VC9802 digital multimeter as an example, the usage method and precautions are briefly introduced. (1) Usage method: a Before use, carefully read the relevant instruction manual to familiarize yourself with the functions of the power switch, range switch, sockets, and special connectors. b Set the power switch to the ON position. c Measurement of DC and AC voltages: Set the range switch to the appropriate DCV (Direct Current) or ACV (Alternating Current) range as needed. Insert the red test lead into the V/Ω socket and the black test lead into the COM socket, then connect the leads in parallel with the circuit being measured; the reading will be displayed. Measurement of DC and AC currents: Set the range selector to the appropriate range for DCA (DC) or ACA (AC). Insert the red test lead into the mA socket (for currents < 200mA) or the 10A socket (for currents > 200mA), insert the black test lead into the COM socket, and then connect the multimeter in series with the circuit being tested. When measuring direct current, the digital multimeter can automatically display the polarity. Measurement of resistance: Set the range selector to an appropriate Ω range, insert the red test lead into the V/Ω socket, and insert the black test lead into the COM socket. If the resistance value being measured exceeds the maximum value of the selected range, the multimeter will display “1”; in such a case, a higher range should be selected. When measuring resistance, the red test lead is the positive terminal and the black test lead is the negative terminal, which is the opposite of what happens with an analog multimeter. Therefore, when measuring polar components such as transistors and electrolytic capacitors, attention must be paid to the polarity of the test leads. (2) Precautions for use: a If it is not possible to estimate in advance the magnitude of the voltage or current to be measured, start by setting the gauge to the highest range for a measurement, and then gradually reduce the range to an appropriate value as needed. After the measurement is complete, set the range switch to the highest voltage setting and turn off the power supply. At full scale, the instrument displays only the digit “1” in the highest position, with all other digits disappearing; in this case, a higher scale should be selected. c When measuring voltage, the digital multimeter should be connected in parallel with the circuit being tested. When measuring current, it should be connected in series with the circuit being tested; when measuring DC voltage, there is no need to consider positive or negative polarity. When the AC voltage range is used to measure DC voltage, or vice versa, the display will show “000”, or the digits in the lower position will flicker. It is prohibited to change the range when measuring high voltages (above 220V) or large currents (above 0.5A), in order to prevent arcs from forming and damaging the switch contacts. f When “ ”, “BATT”, or “LOW BAT” is displayed, it indicates that the battery voltage is below the operating voltage. II. Megohmmeter: Also known as a megger, a megohmmeter is an instrument used to measure the insulation resistance and high-value resistances of the equipment being tested. It consists of a hand-cranked generator, a meter gauge, and three terminals (namely L: line terminal, E: ground terminal, G: shield terminal). 1. Principles for selecting a megohmmeter: (1) Selection of the rated voltage class. Generally, for equipment with a rated voltage of 500V or less, a megohmmeter rated at 500V or 1000V should be used ； For equipment with a rated voltage of over 500V, use a megohmmeter with a range of 1000V to 2500V. (2) Selection of the resistance range. The dial of the megohmmeter has two small black dots, and the area between these dots is the region for accurate measurement. Therefore, when selecting a meter, the insulation resistance value of the equipment under test should be within the accurate measurement range. 2. Use of the megohmmeter (1) Calibrating the meter. Before measurement, the megohmmeter should be tested for open circuit and short circuit conditions to check whether it is in good condition. Open the two connection wires, turn the handle – the pointer should point at “∞”. Then short-circuit the two connection wires; the pointer should point at “0”. If these conditions are met, it is in good working order; otherwise, it cannot be used. (2) Disconnect the device under test from the circuit; for devices with large capacitances, discharge them as well. (3) Select a megohmmeter with a voltage rating that is suitable. (4) When measuring insulation resistance, only the “L” and “E” terminals are generally used. However, when measuring the insulation resistance of a cable with respect to ground or when the leakage current of the device under test is high, the “G” terminal must be used, with this terminal connected to the shielding layer or casing. After the wiring is completed, turn the crank in a clockwise direction; the speed of rotation should increase gradually. When the speed reaches around 120 revolutions per minute (for the ZC-25 model), maintain a constant speed of rotation. Take readings after 1 minute, and read the values while continuing to rotate the crank – do not stop to take the readings. (5) Unwinding discharge. After reading the values, slowly shake it while untangling the wires, and then discharge the device under test. The discharge method is to remove the ground wire used during measurement from the megohmmeter and short-circuit it to the device being tested (it is not the megohmmeter that is discharged). 4. Precautions: (1) It is prohibited to measure insulation resistance during thunderstorms or near high-voltage equipment; measurement can only be carried out when the equipment is not energized and there is no induced voltage. (2) No one should be working on the equipment being tested during the shaking test. (3) The megger wires must not be twisted together; they should be kept separate. (4) It is strictly prohibited to touch it with hands before the megohmmeter stops rotating or before the equipment under test is discharged. When removing the stitches, do not touch the metal part of the lead as well. (5) At the end of the measurement, large-capacitance devices must be discharged. (6) Its accuracy should be checked regularly. III. Clamp meter A clamp meter is an instrument used to measure the current level in electrical circuits that are in operation, allowing current to be measured without shutting down the circuit. 1. Structure and principle: A clamp meter essentially consists of a current transformer, a clamp, and a rectifying magnetoelectric instrument with a reactive force. 2. Usage method: (1) Perform mechanical zero adjustment before measurement. (2) Select the appropriate range; start with a larger range and then move to a smaller one, or estimate based on the values indicated on the nameplate. (3) When using the minimum range and the reading is still not clear, the wire to be measured can be wound several times; the number of turns should be based on those at the center of the clamps. In this case, the reading = indicated value × range / full scale value × number of turns. (4) During measurement, the wire to be measured should be placed at the center of the clamps, and the clamps should be closed tightly to reduce errors. (5) After the measurement is complete, set the selector switch to the maximum range. 3. Precautions: (1) The voltage of the circuit being tested must be lower than the rated voltage of the clamp meter. (2) When measuring the current in high-voltage lines, it is necessary to wear insulating gloves and insulating shoes, and stand on an insulating mat. (3) The jaws must be closed tightly; do not change the range while it is still powered on

It can be posted directly so that everyone can see it at a glance.