1. structure of multimeter (model 500) The multimeter consists of three main parts: meter head, measuring circuit and transfer switch. (1) ammeter: It is a high-sensitivity magnetoelectric DC ammeter, and the main performance indexes of multimeter basically depend on the performance of ammeter. The sensitivity of the instrument refers to the DC current value flowing through the instrument when the pointer of the instrument deflects at full scale. The smaller the value, the higher the sensitivity of the blood glucose meter. When measuring voltage, the greater the internal resistance, the better its performance. There are four scale lines on the meter head, and their functions are as follows: The first line (from top to bottom) is marked with R or Ω, indicating the resistance value. When the changeover switch is in the ohm range, read the scale line. The second line is marked with ∽ and VA, indicating the values of AC and DC voltages and DC currents. When the change-over switch is in AC, DC voltage or DC current block, and the measuring range is in other positions except AC 10V, read the scale line. The third line is marked with 10V, indicating that the AC voltage value is 10V. When the change-over switch is in the AC and DC voltage range, read the scale line. The fourth mark is dB, which indicates the audio level. (2) Measuring circuit The measuring circuit is used to convert various measured signals into tiny DC current suitable for electric meter measurement. It consists of resistors, semiconductor components and batteries. It can convert various measured signals (such as current, voltage, resistance, etc.). ) and different measurement ranges, after a series of treatments (such as rectification, shunt, voltage division, etc.), it becomes a certain amount of tiny DC current. ) and send them to the meter for measurement. (3) The change-over switch is used to select various measuring lines to meet the measurement requirements of different types and ranges. Generally, there are two transfer switches marked with different gears and ranges. 2. Symbol meaning (1)∽ indicates AC/DC (2) V-2.5kV4000Ω/V indicates AC voltage and 2.5 kV DC voltage. Its sensitivity is 4000 Ω/v (3) A-V-Ω means measurable current, voltage and resistance (4) 45-65- 1000 Hz means the frequency range is below 1000 Hz. The standard power frequency range is 45-65hz (5) 2000Ω/V DC, which means that the sensitivity of DC gear is 2000Ω/V, and the symbols on the pliers table and dial are similar to the above symbols (others can't be written completely because of the wrong symbol format, which means that the electromagnetic rectifier instrument has mechanical reaction force, which means that the external magnetic field is placed horizontally)) 3. Use a multimeter (1) to get familiar with the dial. (2) Perform mechanical zero adjustment. (3) According to the type and size of measurement, select the gear and range of transfer switch, and find out the corresponding scale line. (4) Select the position of the probe jack. (5) Measuring voltage: Choose a good range when measuring voltage (or current). If the large voltage is measured with a small range, there is a danger of burning the meter; If a small voltage is measured in a large range, the pointer deflection is too small to read. When selecting the measuring range, the pointer should be deflected to about 2/3 of the full scale. If the measured voltage is not known in advance, the highest range block should be selected first, and then gradually reduced to the appropriate range. A. Measurement of AC voltage: Put one change-over switch of multimeter in AC and DC voltage range, and the other change-over switch in proper AC voltage range. The two probes of the multimeter are connected in parallel with the tested circuit or load. B. DC voltage measurement: put one switch of the multimeter in the range of AC and DC voltage, the other switch in the appropriate range of DC voltage, connect the "+"pin (red pin) to high potential, and connect the "-"pin (black pin) to low potential, that is, let the current flow in from the "+"pin and out from the "-"pin. If the stylus is connected backwards, the pointer on the meter will deflect in the opposite direction, and it is easy to bump the pointer. (6) Current measurement: When measuring DC current, put one change-over switch of multimeter in DC current range and the other change-over switch in the appropriate range of 50uA to 500mA. The range selection and reading method of current are the same as that of voltage. When measuring, the circuit must be disconnected first, and then the multimeter is connected in series to the circuit under test in the direction from "+"to "-",that is, the current flows in from the red stylus and out from the black stylus. If the multimeter is mistakenly connected in parallel with the load, the internal resistance of the meter is very small, which will cause short circuit and burn out the instrument. The reading method is as follows: actual value = indicated value × range/full bias (7) Resistance measurement: When measuring resistance with a multimeter, the following methods should be adopted: A Select the appropriate magnification. The scale line of the multimeter's ohm gear is uneven, so choose the multiplier gear to make the pointer stay in the thinner part of the scale line. The closer the pointer is to the middle of the scale, the more accurate the reading will be. Generally speaking, the pointer should be between 1/3 and 2/3 of the scale. B ohm zero. Before measuring the resistance, the two probes should be shorted together, and at the same time, the "ohm (electric) zero adjustment knob" should be adjusted so that the pointer just points to the zero position on the right side of the ohm scale line. If the pointer is not set to zero, it means that the battery voltage is insufficient or there is a problem inside the instrument. Moreover, every time the amplification gear is changed, ohmic zero adjustment should be carried out again to ensure the accuracy of measurement. C reading: the reading of the instrument multiplied by the magnification is the resistance value of the measured resistor. (8) Precautions A When measuring current and voltage, you can't change the range with electricity. B. When choosing the range, choose the large one first, then the small one, and try to make the measured value close to the range. C. When measuring resistance, you can't use electricity to measure. Because when measuring resistance, the multimeter is powered by an internal battery. If it is charged, it is equivalent to connecting an extra power supply, which may damage the meter. D after use, the change-over switch should be at the maximum position or neutral position of AC voltage. 4. Digital Multimeter Nowadays, digital measuring instruments have become the mainstream and tend to replace analog instruments. Compared with analog instruments, digital instruments have the advantages of high sensitivity, high precision, clear display, strong overload capacity, convenient carrying and simpler use. Taking VC9802 digital multimeter as an example, this paper briefly introduces its usage and matters needing attention. (1) Usage A Before use, please read the relevant operating instructions carefully and be familiar with the functions of power switch, travel switch, jack and special socket. Turn the power switch to the ON position. C AC /DC voltage measurement: adjust the range switch to the appropriate range of DCV (DC) or ACV (AC) as required, insert the red stylus into the V/Ω hole, insert the black stylus into the COM hole, connect the stylus in parallel with the tested line, and the reading will be displayed. D. Measurement of AC /DC current: Turn the range switch to the appropriate range of DCA (DC) or ACA (AC), insert the red probe into the mA hole ( 200ma), insert the black probe into the COM hole, and connect the multimeter in series in the circuit under test. When measuring DC flow, the digital multimeter can automatically display polarity. E resistance measurement: set the range switch to the appropriate ω range, insert the red stylus into the V/ω hole, and insert the black stylus into the COM hole. If the measured resistance exceeds the maximum value of the selected range, the multimeter will display "1", so a higher range should be selected. When measuring resistance, the red stylus is the positive electrode and the black stylus is the negative electrode, which is just the opposite of pointer multimeter. Therefore, when measuring polar components such as transistors and electrolytic capacitors, we must pay attention to the polarity of the stylus. (2) Precautions for Use A If it is impossible to estimate the measured voltage or current in advance, you should dial the highest range to measure it once, and then gradually narrow the range to a suitable position as appropriate. After measurement, set the range switch to the highest voltage range and turn off the power supply. B at full scale, the instrument only displays the number "1" in the highest bit, and all other bits disappear. You should choose a higher range at this time. When measuring voltage, the digital multimeter should be connected in parallel with the circuit under test. When measuring current, it should be connected in series with the circuit under test, and the positive and negative polarities need not be considered when measuring DC. D. When the AC voltage block is misused to measure DC voltage, or the DC voltage block is misused to measure AC voltage, "000" will be displayed on the display screen, or the low digit will jump. E. when measuring high voltage (above 220V) or high current (above 0.5A), it is forbidden to change the range to prevent arc discharge and burn out the switch contacts. F when "","BATT" or "LOW BAT" is displayed, it means that the battery voltage is lower than the working voltage. Second, the shaking table, also known as megohmmeter, is an instrument used to measure the insulation resistance and high resistance of the equipment under test. It consists of a hand-operated generator, a meter head and three terminals (namely, L: line end, E: grounding end and G: shielding end). Selection principle of rated voltage level of 1. shaking table (1). In general, for equipment with rated voltage below 500V, 500V or 1000V shaking table should be selected; For equipment with rated voltage above 500V, choose 1000V~2500V shaking table. (2) Selection of resistance range. There are two small black dots on the dial scale line of the shaking table, and the area between the small black dots is the accurate measurement area. Therefore, when selecting instruments, the insulation resistance value of the tested equipment should be within the accurate measurement area. 2. Use a shaking table (1) to calibrate the watch. Before measurement, the shaking table should be tested for open circuit and short circuit to check whether the shaking table is in good condition. Open two connecting wires and shake the handle. The pointer should point to ∞, and then short two connecting wires. The pointer should point to "0", so long as the above conditions are met, otherwise it cannot be used. (2) The equipment under test is disconnected from the line, and the equipment with large capacitance needs to be discharged. (3) Choose a shaking table with voltage grade. (4) When measuring the insulation resistance, generally only "L" and "E" terminals are used, but when measuring the insulation resistance of cables to the ground or the leakage current of the tested equipment is serious, the "G" terminal should be used and connected to the shielding layer or shell. After the wire is connected, you can rotate the crank clockwise, and the speed of shaking should be fast and slow. When the rotating speed reaches about 120 revolutions per minute (ZC-25 model), keep rotating at a constant speed, read after 1 minute, and read while shaking, without stopping reading. (5) Take out stitches and leave the hospital. After reading it, shake it slowly, take out the suture, and then discharge the device under test. The discharge method is to remove the grounding wire for measurement from the shaking table and simply connect it with the measured equipment (not shaking table discharge). 4. Precautions (1) It is forbidden to measure insulation resistance when lightning strikes or near high-voltage equipment, and it can only be measured when the equipment is not charged and there is no induced electricity. (2) During the shaking test, no one is allowed to work on the tested equipment. (3) The shaking table wires should not be twisted together, but should be separated. (4) It is forbidden to touch the shaking table by hand before the shaking table stops rotating or before the device under test discharges. Don't touch the metal part of the wire when removing it. (5) At the end of measurement, discharge the equipment with large capacitance. (6) Check its accuracy regularly. Third, the clamp table The clamp table is an instrument used to measure the current of the operating electrical circuit, which can continuously measure the current. 1. Structure and principle The clamp meter is mainly composed of current transformer, clamp wrench and rectifier magnetoelectric reaction instrument. 2. Use the method (1) to perform mechanical zero adjustment before measurement. (2) Choose a suitable range, first choose a large range, then choose a small range or look at the nameplate value estimation. (3) When the minimum range measurement is adopted and the reading is not obvious, the measured wire can be wound several times, and the number of turns shall be subject to the number of turns in the center of the jaw, then the reading = indicated value × range/full bias × number of turns (4) When measuring, the measured wire should be located in the center of the jaw and the jaw should be closed to reduce the error. (5) After measurement, place the transfer switch at the maximum position of the measuring range. 3. Note (1): The voltage of the tested line should be lower than the rated voltage of the clamp meter. (2) When measuring the high voltage line current, wear insulating gloves, shoes and stand on the insulating mat. (3) The jaws should be closed, and the range cannot be changed by charging. Comparison between pointer multimeter and Digital Multimeter pointer multimeter and Digital Multimeter have their own advantages and disadvantages. Pointer multimeter is an ordinary instrument, and its reading indication is intuitive and vivid. The general reading is closely related to the swing angle of the pointer, so it is more intuitive. Digital multimeter is an instantaneous sampling instrument. It samples every 0.3 seconds to display the measurement results, sometimes the results of each sampling are only very close, not exactly the same, and the reading results are not as convenient as the pointer. Pointer multimeter generally has no amplifier, so the internal resistance is small. For example, the DC voltage sensitivity of MF- 10 model is100 kω/V ... The DC voltage sensitivity of MF-500 model is 20 kω/V. Due to the internal operational amplifier circuit, the internal resistance of the digital multimeter can be very large, usually 1mω or more. (i.e. higher sensitivity can be obtained). This makes the influence on the tested circuit smaller and the measurement accuracy higher. Because of the low internal resistance, pointer multimeter mostly uses discrete components to form shunt and voltage divider circuits. Therefore, the frequency characteristics are not uniform (relative to numbers), while the frequency characteristics of pointer multimeter are relatively better. Pointer multimeter has simple internal structure, so it has lower cost, fewer functions, simple maintenance and strong over-current and over-voltage capability. Digital multimeter adopts oscillation, amplification, frequency division protection and other circuits, so it has many functions. For example, it can measure temperature, frequency (in a low range), capacitance and inductance, and can be used as a signal generator and so on. Digital multimeters have poor overload capability due to the internal structure of integrated circuits (although some of them can automatically shift gears and protect themselves, but it is more complicated to use), and it is generally not easy to repair after being damaged. The output voltage of digital multimeter is low (generally less than 1V). It is not convenient to test some components with special voltage characteristics (such as SCR, LED, etc.). ). The output voltage of pointer multimeter is relatively high (10.5V, 12V, etc. ). The current is also large (for example, MF-500* 1 Euromax 100 mA), which is convenient for testing silicon controlled rectifier and light emitting diode. Beginners use pointer multimeter, and non-novices use two kinds of watches.