Ammeter The ammeter is connected in series to the circuit being measured and its current value is measured. According to the nature of the measured current, it can be divided into DC current meter, AC current meter and AC dual current meter. In terms of its measurement range, there are micro-ampere meters, milliampere meters and ammeters. According to the principle of operation, it is divided into magnetoelectric, electromagnetic and electric. Portable clamp type ammeter↓ 1, the choice of ammeter When measuring DC current, it is more common to use a magnetoelectric meter or an electromagnetic or electric meter. When measuring AC current, more electromagnetic instruments are used, and electric meters can also be used. For the occasions with high measurement accuracy and high sensitivity, magnetoelectric meters should be used; for the occasions where the measurement accuracy is not strict and the measurement is large, electromagnetic instruments with low price and strong overload capability are often selected. The range selection of the ammeter should be determined according to the magnitude of the current to be measured. The measured current value should be within the range of the ammeter. When the current is not specified, the current meter should be tested with a larger range to avoid damage to the meter due to overload. 2, use methods and precautions (1) Be sure to connect the ammeter to the circuit under test. (2) When measuring DC current, the polarity of “+†and “—†at the terminal of the ammeter cannot be connected incorrectly, otherwise the instrument may be damaged. Magnetoelectric ammeters are generally only used to measure DC current. (3) The appropriate range should be selected according to the magnitude of the current to be measured. For an ammeter with two ranges, it has three terminals. In use, it is necessary to see the terminal range mark, and connect the common terminal and a range terminal in the circuit under test. (4) Select the appropriate accuracy to meet the needs of the measurement. The ammeter has an internal resistance, and the smaller the internal resistance, the closer the measured result is to the actual value. In order to improve the accuracy of the measurement, an ammeter with a small internal resistance should be used as much as possible. (5) When measuring a large alternating current, the current transformer is often used to expand the range of the AC ammeter. The current rating of the secondary winding of the current transformer is generally designed to be 5 amps, and the AC ammeter range used with it should also be 5 amps. The current meter indication value is multiplied by the current transformer's current conversion ratio, which is the value of the actual current measured. The current transformer should be used to reliably ground the secondary coil and core of the transformer. The fuse should not be installed at one end of the secondary coil. It is strictly prohibited to open the circuit when using it. Voltmeter The voltmeter is connected in parallel to the circuit under test to measure the voltage value of the circuit under test. According to the nature of the measured voltage, it is divided into DC voltmeter, AC voltmeter and AC/DC voltmeter. In terms of its measurement range, there are also millivoltmeters and voltmeters. According to the principle of operation, it is divided into magnetoelectric, electromagnetic and electric. 1, the choice of voltmeter The selection principle and method of the voltmeter are basically the same as the selection of the ammeter, mainly considering several aspects such as measurement object, measurement range, required accuracy and instrument price. The measurement accuracy is not high, and electromagnetic voltmeters are generally used. For the measurement accuracy and sensitivity requirements, magnetic multi-range voltmeters are often used, and the multimeter's voltage file is commonly used. 2, use methods and precautions (1) Make sure that the voltmeter is connected in parallel with both ends of the circuit under test. (2) The voltmeter range should be larger than the voltage of the circuit under test to avoid damage to the voltmeter. (3) When measuring the DC voltage using a magnetoelectric voltmeter, pay attention to the “+†and “—†polarity marks on the terminals of the voltmeter. (4) The voltmeter has an internal resistance, and the larger the internal resistance, the closer the measured result is to the actual value. In order to improve the accuracy of the measurement, a voltmeter with a large internal resistance should be used as much as possible. (5) Use a voltage transformer when measuring high voltage. The primary coil of the voltage transformer is connected to the circuit under test. The secondary coil is rated at 100 volts and is connected to a voltmeter with a range of 100 volts. The voltmeter indication value is multiplied by the voltage transformer's transformation ratio. The value of the actual voltage is measured. The voltage transformer must be protected against short-circuiting of the secondary coil during operation. Usually, a fuse is placed in the secondary coil for protection.
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