Functional features and usage tips of multimeters
The basic principle of a multimeter is to use a sensitive magnetoelectric DC ammeter (microampere) as the meter head. When a small current passes through the meter, there will be a current indication. However, the meter head cannot pass large current, so some resistors must be connected in parallel or series with the meter head to shunt or reduce the voltage, so as to measure the current, voltage and resistance in the circuit.
1. When using an analog multimeter to judge the performance of a transistor, you should generally use the R×100Ω or R×1kΩ gear, rather than the R×1Ω and R×10kΩ gears. Because the R×1Ω gear is not convenient for observing the leakage current of the tube; and the R×10kΩ gear is equipped with a high-voltage battery (MF24 type, 500 type is 9V; MF10 type, MF12 type and MF30 type is 15V; MF5 type, MF121 type is (22.5V), it will inevitably cause some tubes with lower withstand voltage to be broken down by high voltage, resulting in erroneous test results, or even damage to the tube under test.
Since the internal resistance of the ohmic range of the digital multimeter is very high, the test current it can provide is extremely weak (such as the 20kΩ range: 75μA for the DT-830 model; 60μA for the DT-840D model), which is not enough to overcome the PN junction when identifying semiconductor components. Dead zone voltage, so the measured resistance value is much higher than that of the analog multimeter, and there is no linear proportional relationship between the readings of the two meters, so it cannot be used as a basis for judging the performance of the tube. It should be switched to the diode test mode. Perform testing.
2. When the digital multimeter is at the ohm level, diode test level and buzzer level, the red test lead is connected to the high potential in the meter and is positively charged, while the black test lead is negatively charged because it is connected to the virtual ground in the meter. This is obviously different from the analog type. The charged polarity of the test leads on the ohm range of the multimeter is completely opposite. When testing polarized components or related circuits, be sure to pay full attention.
3. When using the ohm level to test circuit components or circuit systems, you must first cut off the power supply of the device or system under test. If the object under test contains a capacitor with a large storage capacity, it must be discharged in an appropriate manner. Measurement can only be carried out after confirming that there is no power factor in the part being measured. Otherwise, the multimeter, especially the analog multimeter, will be easily damaged.
4. When measuring the current of a low internal resistance circuit (including a network containing a low internal resistance power supply and a network with a low value load resistor), try to choose a larger current range; when measuring the voltage of a high internal resistance circuit (or power supply) When testing, the analog multimeter should try to choose a higher voltage range. The digital multimeter is easier to meet the test requirements because of its higher internal resistance.
5. Do not use the ohm level to detect the internal resistance of various batteries, nor directly measure the internal resistance of high-sensitivity meters. The former can easily damage the multimeter, while the latter often causes the head of the meter being measured to break the needle, and may even burn out the moving coil.
6. For digital multimeters, when the measured current is relatively large (such as greater than 200mA), the high-current dedicated jack on the meter panel (such as 10A or 20A, etc.) should be used to plug in the test leads. However, the large current of most meters There is no overcurrent protection measure in the measuring range, so beware of overload. In addition, the meter must not be connected to the load line for a long time to be used as a large-range ammeter, and the measurement time should generally not exceed 15 seconds.
7. The AC measurement mode of an ordinary multimeter is only suitable for measuring the effective value of sine wave voltage or current. It cannot directly measure non-sinusoidal electricity such as sawtooth wave, triangle wave, square wave and so on. Even for sine wave electricity, its frequency parameters and waveform distortion must meet the technical conditions of the multimeter, otherwise, the measurement error will increase significantly. The effective value of non-sinusoidal voltage or current can generally be measured with an electric or electromagnetic instrument or an effective value digital multimeter (such as DT-980).
8. During the process of measuring voltage and current, it is best not to change the gear of the selector switch, especially in the case of higher voltage and larger current. The selector switch can easily generate arc during the switching process and burn the switch contacts. point, and damage internal components and circuits.
9. When the fuse in the watch is blown, it must be replaced according to the specifications specified in the instruction manual. Do not expand or reduce it at will.
10. For analog multimeters, in order to reduce the parallax of reading data, the eyes must be facing the needles of the meter. For a dial equipped with a reflector, the line of sight should be adjusted until the needle shadow of the watch hand coincides with the needle shadow in the mirror. At this time, the parallax is minimal. The multimeter must also be placed horizontally, with a maximum inclination of no more than 10 degrees.
