Structure and Working Principle of Analog Multimeters
The pointer type multimeter head is a DC ammeter, so the measured resistance, voltage, and current must be converted into the current driving the ammeter through the internal circuit of the multimeter. The internal structure of the ammeter is shown in the figure, and its indicating part is to connect a coil wound in a magnetic field with the pointer. When current flows through the wire, the coil will rotate.
When current flows through a coil and rotates it, the angle of rotation is proportional to the magnitude of the current. According to the left-hand rule of electromagnetic induction, when current flows through a conductor located in a magnetic field, the conductor will be affected by electromagnetic force and move. Based on this principle, an ammeter has been developed.
1. Internal circuit structure of pointer multimeter:
A pointer multimeter mainly uses a sensitive magneto electric DC ammeter as the meter head. When a small current passes through the meter head, there will be a current indication. In addition, the multimeter is equipped with a shunt (to expand the measurement range of current), a multiplier (to expand the measurement range of voltage), a rectifier (to convert AC to DC), a battery (to provide power for measuring resistance), and function knobs. The following diagram shows the circuit composition of a pointer type multimeter.
2. Working principle of pointer multimeter
When using a pointer multimeter to measure resistance, current, and voltage, the internal circuit structure of the multimeter will change accordingly. The internal circuit status of the pointer multimeter when detecting DC voltage is shown in the following figure. From the figure, it can be seen that when the multimeter is in the range of 100V, the internal resistance of the meter is the sum of three resistors and the resistance of the meter head, which is about 2MQ, equivalent to 2kO/V. It can be seen that the internal resistance of the multimeter is very high, and generally does not affect the measured voltage during measurement. The current flowing into the multimeter when measuring voltage is very small.
When detecting AC voltage, as shown in the internal circuit diagram of the pointer multimeter, AC voltage is applied between the two terminals of the multimeter, and a bridge rectifier circuit is installed inside the meter to convert the AC signal into DC current before driving the meter head.
The internal circuit status of the pointer multimeter when detecting resistance is shown in the diagram. When measuring resistance, it is necessary to use the battery inside the multimeter to supply current to the resistor. After passing through the resistor, the current passing through the multimeter will be larger when the resistance value is small, and smaller when the resistance value is large. The multimeter also has a shunt resistor to make the current flowing through the ammeter proportional to the measured resistance value. The pointer deflection angle of the ammeter corresponds to the value of the measured resistance.
