Functional characteristics and usage skills of multimeter
The basic principle of the multimeter is to use a sensitive magnetoelectric DC ammeter (microampere meter) as the meter head. When a small current passes through the meter head, there will be a current indication. However, the meter head cannot pass a large current, so some resistors must be connected in parallel or in series on the meter head to shunt or lower the voltage, so as to measure the current, voltage and resistance in the circuit.
1. When the analog multimeter judges the performance of the transistor, the R×100Ω or R×1kΩ file should generally be selected, and the R×1Ω and R×10kΩ file should not be used. Because the R×1Ω file is not convenient to observe the leakage current of the tube; and the R×10kΩ file is equipped with a high-voltage battery inside (MF24 type, 500 type is 9V; MF10 type, MF12 type and MF30 type are 15V; MF5 type, MF121 type 22.5V), it will inevitably cause some tubes with low withstand voltage to be broken down by high voltage and produce wrong test results, and even cause damage to the tubes under test.
Due to the high internal resistance of the ohm range of the digital multimeter, the test current that can be provided is extremely weak (for example, 20kΩ range: 75μA for DT-830; 60μA for DT-840D), which is not enough to overcome the PN junction when distinguishing semiconductor components. Therefore, 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, and it should be changed to the diode test file. to test.
2. When the digital multimeter is in the ohm position, diode test position and buzzer position, the red test lead is connected to the high potential inside the meter and is positively charged, and 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 scale of the multimeter is completely opposite. When testing polarized components or related circuits, full attention must be paid.
3. When using the ohm gear to detect circuit components or circuit systems, the power supply of the device or system under test must be cut off first. If the object under test contains a capacitor with a large storage capacity, it must also be discharged in an appropriate way. The measurement can only be carried out on the premise of confirming that the part under test has no power factor, 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 with a low internal resistance power supply and a network with a low value load resistance), try to select a larger current range; when measuring the voltage of a high internal resistance circuit (or power supply) , the analog multimeter should try to choose a higher voltage range, and the digital multimeter is easier to meet the test requirements because of its high internal resistance.
5. Do not use the ohm gear to test the internal resistance of various batteries, nor directly measure the internal resistance of the high-sensitivity meter. The former is very easy to damage the multimeter, and the latter often causes the meter head under test to damage the needle, and may even burn out the moving coil.
6. For digital multimeters, when the measured current is large (such as greater than 200mA), you should use the high-current special jack (such as 10A or 20A, etc.) There is no overcurrent protection measure for the measuring range, so we must beware of overloading. In addition, the ammeter should not be used as a large-range ammeter in series with the load line for a long time, and the measurement time should generally not exceed 15 seconds.
7. The AC measurement gear of ordinary multimeters is only suitable for measuring the effective value of sine wave voltage or current, and it cannot directly measure non-sinusoidal electricity such as sawtooth waves, triangle waves, and square waves. Even for sine wave power, 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-sine wave voltage or current can generally be measured by electric, electromagnetic instruments or effective value digital multimeters (such as DT-980).
8. In the process of measuring voltage and current, it is best not to change the gear position of the selector switch, especially in the case of higher voltage and larger current, the selector switch is easy to generate an arc during the switching process and burn the contact of the switch. point, and damage internal components and circuits.
9. When the fuse in the meter is blown, replace it according to the specifications specified in the manual, and do not enlarge or reduce it at will.
10. For the analog multimeter, in order to reduce the parallax of the read data, the eye sight must be facing the needle. For a dial equipped with a reflector, the line of sight should be adjusted until the pointer of the watch coincides with the shadow of the needle in the mirror, and the parallax is the smallest at this time. The multimeter must also be placed horizontally, with a maximum inclination of no more than 10 degrees.
