Structure and working principle of pointer multimeter
The pointer multimeter head is a DC ammeter, so the measured resistance, voltage and current are converted by the internal circuitry of the multimeter into the current that drives the ammeter. The internal structure of an ammeter is shown in the figure. The indicator part is a coil wound in a magnetic field connected to the pointer, and the coil will rotate when a current flows through the wire.
When a current flows through a coil, the angle of rotation is proportional to the size of the current. According to the left-hand rule of electromagnetic induction, when a current flows through a conductor in a magnetic field, the conductor is subjected to the action of the electromagnetic force and moves, and an ammeter is made based on this principle.
1. The internal circuit structure of the pointer multimeter:
Pointer multimeter is mainly the use of a sensitive magnetoelectric DC ammeter for the head, when a small current through the head, there will be current indication. In addition in the multimeter is also equipped with a shunt (to expand the current measurement range), multiplier (to expand the voltage measurement range), rectifier (AC into DC), battery (for the measurement of resistance when the power supply) and function knobs and other parts of the following figure for the pointer multimeter circuit composition diagram.
2. The working principle of the pointer multimeter
When using the pointer multimeter to measure resistance, current and voltage, the internal circuit structure of the multimeter will change accordingly, and the internal circuit of the pointer multimeter is shown in the following figure when detecting DC voltage. Visible from the figure in the range of the multimeter 100V, the table internal resistance for the three resistors and the head resistance and the sum of about 2MQ, equivalent to 2kO/V, inches see the internal resistance of the multimeter is very high, general measurement will not have an impact on the measured voltage. The current flowing into the multimeter during voltage measurement is very small.
Measurement state of DC voltage
When detecting AC voltage, the internal circuit diagram of the pointer multimeter shows that the AC voltage is added between the two terminals of the multimeter, and there is a bridge rectifier circuit inside the meter to turn the AC signal into a DC current and then drive the meter head.
Measurement status of AC voltage
The internal circuit of the pointer multimeter is shown in the figure. When measuring resistance, the internal battery of the multimeter is used to feed current into the resistor, and then into the multimeter after the resistor, and the current through the small resistance value will be large, and the current through the large resistance value will be small, and there is also a shunt resistor inside the meter to make the current value flowing through the ammeter proportional to the value of the resistance to be measured. The pointer of the ammeter deflects at an angle corresponding to the value of the measured resistance.
