Functional characteristics of multimeters and tips for using them
The basic principle of a multimeter is to utilize a sensitive magneto-electric DC ammeter (microammeter) as a meter head. When a small current passes through the head, a current is indicated. However, the head can not pass large currents, so the head must be connected in parallel with some resistors in series for shunt or voltage reduction, so as to measure the current, voltage and resistance in the circuit.
1, analog multimeter to identify the performance of the crystal transistor, generally should be used R × 100Ω or R × 1kΩ file, and should not be used R × 1Ω and R × 10kΩ file. Because R × 1Ω file is not easy to observe the leakage current of the tube; and R × 10kΩ file because of its internal high-voltage battery, which will inevitably lead to some of the lower withstand voltage tube by the high voltage breakdown and produce erroneous test results, and even cause damage to the tube under test.
Due to the high internal resistance of the digital multimeter ohm file, the test current provided by the extremely weak Because the internal resistance of digital multimeter is very high, the test current it can provide is extremely weak, which is not enough to overcome the dead zone voltage of PN junction when discriminating the semiconductor components, thus the measured resistance is much higher than that of analog multimeter, and there is no linear proportionality between the readings of two kinds of meters, so it does not constitute the basis for judging the performance of the tubes, and it should be changed to the diode test file for detection.
2, digital multimeter in the ohm file, diode test file and buzzer file position, the red pen and the table connected to the high potential and positively charged, the black pen is connected to the table virtual ground and negatively charged, which is obviously with the analog multimeter ohm file on the pen's charged polarity is completely the opposite of the detection of polarity components or related circuits, be sure to pay full attention to.
3, when the ohm file detection circuit components or circuit system, must first cut off the power supply of the device or system under test, if the object under test contains a large power storage capacitor, it must also be discharged in an appropriate manner, in order to confirm that the measured part of the premise of no power factors before measurement, otherwise, it is very easy to damage the multimeter, especially the analog multimeter.
4, in measuring the current of low internal resistance circuits (including networks containing low internal resistance power supply and low value load resistance), try to choose a larger current range; in measuring the voltage of high internal resistance circuits (or power supply), the analog multimeter should try to choose a higher voltage range, and the digital multimeter can easily satisfy the test requirements because of its high internal resistance.
5, shall not use the ohm file to test the internal resistance of various batteries, and shall not directly measure the internal resistance of the high sensitivity meter head. The former is very easy to damage the multimeter, the latter often cause the measured head to break the needle, and may even burn out the moving coil.
6, for digital multimeter, when the measured current is large (such as more than 200mA), should be changed to use the meter panel of the large current special jack (such as 10A or 20A, etc.) plug the pen, but the vast majority of the meter's large-current range is not set up to overcurrent protection measures, we must guard against overloading phenomenon. In addition, the meter should not be strung into the load line for a long time as a large range of ammeter use, the measurement time is generally not more than 15 seconds.
7, ordinary multimeter AC measurement file is only suitable for measuring sine wave voltage or current RMS value, it can not directly measure the sawtooth wave, triangle wave, square wave and other non-sinusoidal power. Even if it is sinusoidal power, its frequency parameter and waveform distortion must also comply with the technical conditions of the multimeter, otherwise, the measurement error will increase significantly. RMS value of non-sinusoidal voltage or current can generally be measured by electric, electromagnetic instruments or RMS digital multimeter.
8, in the process of measuring voltage and current, it is best not to change the selector switch gear, especially at higher voltages and higher currents, the selector switch in the switching process is very easy to produce arc and burn the switch contacts, and damage to the internal components and lines.
9, meet the table fuse fuse, according to the instructions specified in the specifications for replacement, do not arbitrarily expand or reduce.
10, for analog multimeter, in order to reduce the parallax of reading data, the eye line of sight must be right on the needle. For the dial equipped with a reflector, should adjust the line of sight to the needle and the mirror in the shadow of the needle overlap shall prevail, at this time the parallax is minimal. Multimeter must also be placed horizontally, the maximum angle of inclination shall not exceed 10 degrees.
