1. Multimeter structure (500 type)
The multimeter consists of three main parts: the meter head, the measuring circuit and the changeover switch.
(1) Header: It is a high-sensitivity magneto-electric DC ammeter. The main performance indicators of the multimeter basically depend on the performance of the meter. The sensitivity of the meter head refers to the value of the DC current flowing through the meter head when the meter pointer is deflected at full scale. The smaller the value, the higher the sensitivity of the meter head. The greater the internal resistance when measuring voltage, the better its performance. There are four tick marks on the meter head. Their functions are as follows: The first bar (from top to bottom) is marked with R or Ω, indicating the resistance value. When the switch is in ohms, the tick mark is read. The second line is marked with ∽ and VA, indicating the AC, DC voltage and DC current value. When the transfer switch is in the AC, DC voltage or DC current block, and the range is other than AC 10V, read this scale. line. The third line is marked with 10V, indicating the AC voltage value of 10V. When the transfer switch is in the AC and DC voltage range and the range is 10V AC, the line is read. The fourth bar is marked with dB, indicating the audio level.
(2) Measuring line
The measuring circuit is a circuit for converting various measured DC currents suitable for meter measurement, which consists of a resistor, a semiconductor component and a battery.
It can change various measured (such as current, voltage, resistance, etc.), different ranges, through a series of processing (such as rectification, shunt, voltage division, etc.) into a certain amount of small DC current feed The meter is measured.
(3) Transfer switch
Its role is to select a variety of different measurement lines to meet the measurement requirements of different types and different ranges. There are generally two transfer switches, each marked with a different gear position and range.
2. Symbol meaning
(1) ∽ indicates AC and DC
(2) V-2.5KV 4000Ω/V means the sensitivity is 4000Ω/V for AC voltage and 2.5KV DC voltage block.
(3) A-V-Ω means measurable current, voltage and resistance
(4) 45-65-1000Hz means the frequency range is below 1000 Hz, and the standard power frequency range is 45-65Hz.
(5) 2000Ω/V DC means the sensitivity of the DC gear is 2000Ω/V
The symbols on the clamp meter and the dial are similar to the above symbols (others cannot be written in the same way because the symbol format is incorrect) "The magnetic resonance type has a mechanical reaction force" means that the three-stage external magnetic field "represents horizontal placement")
3. Use of the multimeter
(1) Familiarize with the meaning of the symbols on the dial and the main functions of each knob and selector switch.
(2) Perform mechanical zero adjustment.
(3) According to the type and size of the measurement, select the gear and range of the transfer switch to find the corresponding tick mark.
(4) Select the position of the test lead jack.
(5) Measurement voltage: When measuring voltage (or current), select a good range. If you use a small range to measure large voltage, there is a danger of burning the watch. If you use a large amount of process to measure small voltage, the pointer deflection is too small. Unable to read. The range should be chosen to deflect the pointer to about 2/3 of the full scale. If you do not know the magnitude of the voltage to be measured beforehand, you should first select the highest range and then gradually reduce it to the appropriate range.
a AC voltage measurement: put one switch of the multimeter in the AC and DC voltage, and the other switch is placed on the appropriate range of the AC voltage. The multimeter's two test leads and the circuit under test or load can be connected in parallel.
b DC voltage measurement: put one switch of the multimeter in the AC and DC voltage, the other switch is placed on the appropriate range of DC voltage, and the “+” meter (red pen) is connected to the high potential, “- The test pen (black pen) is connected to the low potential, that is, the current flows from the "+" test pen and flows out from the "-" test pen. If the test leads are reversed, the pointer of the watch will deflect in the opposite direction, which is easy to bend the pointer.
(6) Measuring current: When measuring DC current, put one switch of the multimeter in DC current block, and the other switch is placed on the appropriate range of 50uA to 500mA. The range selection and reading method of current are the same as voltage. The circuit must be disconnected first, and then the multimeter is connected in series to the circuit under test according to the current from "+" to "-", that is, the current flows from the red meter pen and flows out from the black meter pen. If the multimeter is connected in parallel with the load, the internal resistance of the meter head is small, which may cause the short circuit to burn the instrument. The reading method is as follows:
Actual value = indication value × range / full deviation
(7) Measuring resistance: When measuring resistance with a multimeter, it should be done according to the following method*:
a Select the appropriate override. The scale of the multimeter's ohmic gear is not uniform, so the selection of the override gear should be such that the pointer stays in the thinner part of the scale, and the closer the pointer is to the middle of the scale, the more accurate the reading. Under normal circumstances, the pointer should be pointed between 1/3~2/3 of the scale.
b ohms zero. Before measuring the resistance, short the two test leads and adjust the “ohmic (electrical) zero adjustment knob” so that the pointer just points to the zero position to the right of the ohmic scale. If the pointer cannot be adjusted to zero, the battery voltage is insufficient or there is a problem inside the meter. And every time you change the override gear, you must perform ohms zero again to ensure accurate measurement.
c Reading: The reading of the meter is multiplied by the magnification, which is the resistance of the measured resistance.
(8) Precautions
a When measuring current and voltage, it is not possible to charge range
b When selecting the range, first select the big one, then select the small one, try to make the measured value close to the range.
c When measuring resistance, it is not possible to measure with electricity. Because the multimeter is powered by the internal battery when measuring resistance, if the live measurement is equivalent to accessing an additional power supply, the meter may be damaged.
d After use, the switch should be in the maximum gear or neutral of the AC voltage.
4. Digital multimeter
Nowadays, digital measuring instruments have become the mainstream, and there is a trend to replace analog meters. Compared with analog instruments, digital meters have high sensitivity, high accuracy, clear display, strong overload capability, easy to carry, and simpler to use. Take the VC9802 digital multimeter as an example to briefly introduce its usage and precautions.
(1) How to use
a Before use, you should carefully read the relevant instruction manual, familiar with the role of power switch, range switch, jack, special socket.
bSet the power switch to the ON position.
c AC and DC voltage measurement: If necessary, set the range switch to the appropriate range of DCV (DC) or ACV (AC), insert the red test pen into the V/Ω hole, insert the black test lead into the COM hole, and connect the test lead to the line under test. The reading is displayed.
d AC and DC current measurement: Set the range switch to the appropriate range of DCA (DC) or ACA (AC), insert the red test pen into the mA hole (<200mA) or 10A hole (>200mA), and insert the black test lead into the COM hole. Connect the multimeter in series with the circuit under test. When measuring DC, the digital multimeter can automatically display the polarity.
Measurement of e resistance: Turn the range switch to the appropriate range of Ω, insert the red test lead into the V/Ω hole, and insert the black test lead into the COM hole. If the measured resistance value exceeds the maximum value of the selected range, the multimeter will display "1" and a higher range should be selected. When measuring resistance, the red test pen is positive and the black test pen is negative, which is the opposite of the pointer multimeter. Therefore, when measuring polar components such as transistors and electrolytic capacitors, you must pay attention to the polarity of the test leads.
(2). Precautions for use
a If you cannot estimate the magnitude of the measured voltage or current in advance, you should first dial the measurement to the highest range and then gradually reduce the range to the appropriate position. After the measurement is completed, the range switch should be turned to the highest voltage and the power should be turned off.
b When the scale is full, the meter only displays the number “1” in the highest position, and the other bits disappear. In this case, a higher range should be selected.
c 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. It is not necessary to consider positive and negative polarity when measuring DC.
d When the AC voltage is misused to measure the DC voltage, or the DC voltage is used to measure the AC voltage, the display will show “000”, or the number on the low position will jump.
e It is forbidden to change the range when measuring high voltage (above 220V) or high current (above 0.5A) to prevent arcing and burn switch contacts.
f When “ ”, “BATT” or “LOW BAT” is displayed, it indicates that the battery voltage is lower than the operating voltage.
Second, shake the table
The shaker, also known as the megohmmeter, is a meter used to measure the insulation resistance and high value resistance of the device under test. It consists of a hand crank generator, a meter head and three terminals (ie L: line end, E: ground). End, G: shield end).
1. Selection principle of shake table
(1) Selection of rated voltage level. Under normal circumstances, equipment with rated voltage below 500V should use 500V or 1000V shaker; for equipment with rated voltage above 500V, use 1000K~2500V shaker.
(2) Selection of the resistance range. There are two small black dots on the dial scale of the shake table, and the area between the small black dots is the accurate measurement area. Therefore, when selecting the meter, the insulation resistance value of the device under test should be within the accurate measurement area.
2. Use of shake table
(1) School table. Before the measurement, the shaker should be tested for open circuit and short circuit to check whether the shaker is good. Open the two connecting lines, shake the handle, the pointer should be at the “∞”, then short the two connecting lines, the pointer should be at “0”, it is good if the above conditions are met, otherwise it can not be used.
(2) The device under test is disconnected from the line, and the large capacitance device is also discharged.
(3) Select the shake table that meets the voltage level.
(4) When measuring the insulation resistance, generally only use the "L" and "E" terminals, but when measuring the insulation resistance of the cable to the ground or the leakage current of the device under test is serious, the "G" terminal is used, and The "G" terminates the shield or housing. After the line is connected, turn the handle clockwise. The speed of the shaking should be slow and fast. When the speed reaches 120 rpm (ZC-25 type), keep rotating at a constant speed, read after 1 minute, and While reading while shaking, you cannot stop reading.
(5) Disassembly and discharge. After the reading is completed, while slowly shaking, disconnect the line and then discharge the device under test. The discharge method is to take the ground wire used for the measurement from the shake table and short-circuit it with the device under test (not the shake table discharge).
4. Precautions
(1) It is forbidden to measure the insulation resistance during lightning or near high-voltage equipment, and only when the equipment is not charged or inductive.
(2) During the shaking test, no one can work on the device under test.
(3) The shaking table wires cannot be twisted together and should be separated.
(4) It is strictly forbidden to touch the hand before the shake table has not stopped rotating or before the device under test is not discharged. Do not touch the metal part of the lead when disconnecting the wire.
(5) At the end of the measurement, discharge is required for the large capacitance device.
(6) Regularly verify its accuracy.
Third, the clamp table
A clamp meter is a meter that measures the amount of current flowing on an electrical circuit that can measure current under uninterrupted power.
1. Structure and principle
The clamp meter consists essentially of a
Current Transformer, a clamp wrench and a rectifying magnetoelectric system with a reaction force meter.
2. Instructions
(1) Mechanical zeroing before measurement
(2) Select the appropriate range, first select the large, then select the small range or look at the nameplate value estimate.
(3) When the minimum range measurement is used and the reading is not obvious, the measured wire can be wound a few turns, and the number of turns should be based on the number of turns in the center of the jaw. Then the reading = indication value × range / full deviation × 匝number
(4) When measuring, the wire to be tested should be placed in the center of the jaws and the jaws should be closed tightly to reduce the error.
(5) After the measurement is completed, the transfer switch should be placed at the most range.
3. Precautions
(1) The voltage of the line under test is lower than the rated voltage of the clamp meter.
(2) When measuring the current of the high-voltage line, wear insulated gloves, wear insulated shoes, and stand on the insulating mat.
(3) The jaws should be closed tightly and cannot be charged for the range.
Pointer multimeters and digital multimeters have different advantages and disadvantages. The pointer multimeter is an average meter with an intuitive, visual reading. (The general reading value is closely related to the angle of the pointer swing, so it is very intuitive). Digital multimeters are instantaneous style meters. It takes 0.3 seconds to take a sample to display the measurement results. Sometimes the sampling results are only very similar, not exactly the same, which is not as convenient as the pointer. The pointer multimeter generally has no internal amplifier, so the internal resistance is small, such as the MF-10 type, and the DC voltage sensitivity is 100 kΩ/V. The MF-500 has a DC voltage sensitivity of 20 kΩ/V. Digital multimeters Due to the internal use of op amp circuits, the internal resistance can be made very large, often at 1M ohm or greater. (ie, you can get higher sensitivity). This makes the impact on the circuit under test smaller and the measurement accuracy higher. The pointer type multimeter has a small internal resistance and a discrete component is used to form a shunt voltage dividing circuit. Therefore, the frequency characteristics are not uniform (relative to digital), and the frequency characteristics of the pointer multimeter are relatively better. The pointer type multimeter has a simple internal structure, so the cost is low, the function is small, the maintenance is simple, and the overcurrent and overvoltage capability is strong. The digital multimeter uses a variety of internal oscillations, amplification, frequency division protection and other circuits, so there are many functions. For example, you can measure temperature, frequency (in a lower range), capacitance, inductance, signal generator, and so on. Due to the internal structure of the digital multimeter, the digital multimeter has a poor overload capability (although some of them can be automatically shifted, automatically protected, etc., but the use is more complicated), and it is generally not easy to repair after damage. The digital multimeter has a low output voltage (typically no more than 1 volt). It is inconvenient to test some components with special voltage characteristics (such as thyristors, light-emitting diodes, etc.). The pointer multimeter has a higher output voltage (10.5 volts, 12 volts, etc.). The current is also large (such as MF-500*1 ohms maximum 100 mA or so) can easily test thyristors, LEDs and so on. For beginners, a pointer multimeter should be used, and two instruments should be used for non-beginners.
