The structure and the principle of operation of a multimeter of the pointer type
The pointer type multimeter head is a DC ammeter, so the measured resistance, voltage, and current need to 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 there is current flowing through the wire, the coil will rotate.
When the current flows through the coil and causes it to rotate, the angle of rotation is proportional to the magnitude of the current. According to the left-hand rule of electromagnetic induction, when a current flows through a conductor located in a magnetic field, the conductor will move under the action of electromagnetic force. Based on this principle, an ammeter is made.
1. Internal circuit structure of the pointer multimeter:
The pointer type 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 splitter (to expand the measurement range of current), a multiplier (to expand the measurement range of voltage), a rectifier (to convert AC into DC), a battery (to provide power for measuring resistance), and a function knob,
2. Working principle of pointer type multimeter
When measuring resistance, current, and voltage with a pointer multimeter, the internal circuit structure of the multimeter will change accordingly. When detecting DC voltage, the internal circuit status of the pointer multimeter is shown in the following figure. From the graph, it can be seen that when the multimeter has a 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 measurement will not affect the measured voltage. The current flowing into the multimeter during voltage measurement is very small.
Measurement status of DC voltage
The internal circuit diagram of a pointer type multimeter for detecting AC voltage is shown in the circuit diagram. When the AC voltage is applied between the two terminals of the multimeter, a bridge rectifier circuit is installed inside the meter to convert the AC signal into DC current and then drive the meter head.
Measurement status of AC voltage
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 send current to the resistance. After passing through the resistance, it is then sent to the multimeter. A small resistance value will result in a larger current passing through, while a large resistance value will result in a smaller current passing through. The meter also has a shunt resistor, which makes the current flowing through the ammeter proportional to the measured resistance value. The deviation angle of the ammeter pointer corresponds to the value of the measured resistance.
