Digital multimeter principle send note

One of the most basic functions of a digital multimeter is to measure the voltage. A typical DC voltage source is a battery, such as a car battery. The AC voltage is usually generated by a generator. The most common AC voltage source is a wall outlet on the home. Some devices convert AC to DC, such as televisions, stereos, video recorders, computers, and other electronic devices, and convert AC to DC through rectifiers plugged into wall outlets; DC voltage is the power required by these electronic devices.

A multimeter is an indispensable measuring instrument for power electronics and other departments. It is generally used to measure voltage, current, and resistance. The multimeter is divided into a pointer multimeter and a digital multimeter according to the display mode. Digital multimeters are instantaneous take-out style meters. It takes a sample of 0.3 seconds to display the measurement results. Sometimes the results of each sampling are very similar and not exactly the same. Digital multimeters use internal op amp circuits and internal resistance can be very large, often in the 1M Euro or larger. (ie, higher sensitivity can be achieved). This makes the impact on the circuit under test smaller and the measurement accuracy higher. Digital multimeter uses a variety of oscillation, amplification, frequency division protection circuit, so the function is more. For example, temperature, frequency (in a lower range), capacitance, inductance, signal generator, etc. can be measured. Digital multimeters have poor overload capability due to their multiple internal circuits using integrated circuits (though some of them have been able to automatically shift gears and protect them automatically, but they are more complex to use), and are generally not easily repaired after damage. The digital multimeter output voltage is low (usually no more than 1 volt). It is inconvenient to test some special voltage components (such as thyristors, light emitting diodes, etc.).

In the current measurement must pay attention to: the circuit must be disconnected or disconnected, you must disconnect the power circuit to be tested, a small current can also cause danger; do not test the voltage when the test lead is inserted in the current hole, this will cause the number The multimeter is damaged or endangers personal safety. Digital multimeters are most likely to burn out when measuring current. A common mistake is to place the test leads in the current jacks while attempting to test the voltages, as well as the common faulty test leads (see the Digital Multimeter Using Tips (1) article). At this time, the small value resistance in the digital multimeter will cause the short circuit of the voltage source, and the large current will flow through the digital multimeter. If the digital multimeter does not have enough protection, it will not only damage the table and the circuit, but also hurt the operator. If it is a high-voltage circuit (480 volts or more), it will lead to greater danger. Therefore, the digital multimeter should have a sufficiently large capacity current input protection fuse. Digital multimeters without current input fuses cannot be used in high energy circuits (240V ac). For digital multimeters that use fuses, the fuses should have enough capacity to remove high-energy faults. The rated voltage of the fuse should be greater than the maximum voltage you expect. For example, a 20A/250V fuse in a digital multimeter cannot protect the digital multimeter when measuring a 480V circuit; a 20A/600V fuse can be used in digital The multimeter provides protection when measuring 480V circuits. Some digital multimeters will have a watchdog line plugging in and out alarms when measuring current, such as OI857 and OI859CF.