Why can't a multimeter measure the polarity of an alternating current?
The so-called alternating current refers to its polarity is alternately changing, and the speed of change is very fast, whether it is a pointer or a little more advanced general digital multimeter, can not respond to its instantaneous polarity, its polarity is the number of times in a unit of time called frequency, its unit is Hertz, which is in memory of the famous physicists Mr Hertz and the name of the unit of physics. Because the polarity of this power supply alternates so with a general multimeter simply can not measure its polarity, if there is a need to understand the polarity of a moment in time, can also be said to be the polarity of its instantaneous the easiest way, that is specifically for the observation of alternating current or direct current pulses of electronic instruments oscilloscopes.
The direction of alternating current changes at any time and does not speak of polarity. If the said measurement of the fire line zero line, you can play the multimeter to the AC voltage of the highest grade, a hand pinch the black pen head, with the red pen to detect the wire, the pointer deflection is the fire line, does not deflect is the zero line. In addition, some digital multimeter nowadays, with the function of the power test pen.
Alternating current is the size and direction of the periodic changes in current, no polarity, only frequency, the frequency of our AC is 50 Hz, that is, the current changes back and forth 50 times per second, the direction changes 100 times, the question itself is a problem.
How to measure current with a multimeter? Multimeter method of measuring current
1, before measuring current, judge the nature of the current (AC and DC), current size range.
2,Choose the proper measuring tool (such as safety, accuracy and ease of use during on-site measurement) and measuring gear.
3,Operation steps:
(1) If it is a small electrical equipment (such as electronic equipment, etc.), you can read the product manual about the product power related description, the part of the instructions sometimes directly to the electrical equipment work through the current range marked in the manual. If the instruction manual only indicates the power consumption and voltage, the current can be calculated by the formula "A = W / V". When it is necessary to test the total current of several devices, add up the currents of the individual devices to calculate the total current on the main line.
If actual measurement is required:
(1) Select the appropriate gear (measured current, current value range)
(2) Disconnect the line to be measured (but electrical equipment and loads can not be disconnected, otherwise it is a short circuit line!) (2) Disconnect the line to be measured (but the electrical equipment and loads can not be disconnected, otherwise it is a short circuit!), the multimeter will be tampered with the line (note that the series), at this time, the resistance of the multimeter is almost zero, the current flowing through the multimeter is equal to the current flowing through the electrical equipment and loads.
(2) If it is a larger electrical equipment (operating voltage is generally 220V to 380V), it is recommended to use a clamp-on ammeter (multimeter). The principle is to form a closed electromagnetic field on the energised line, and changes in the electromagnetic field cause the multimeter to generate a current. This current has a proportional relationship with the measured line current, so that the line current can be read directly in the ammeter.
