The structure and working principle of the pointer multimeter
The head of the pointer multimeter 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. Its indicating part is to connect a coil wound in a magnetic field with the pointer. When a current flows through the wire, the coil will rotate.
When current flows through the coil to make it 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, and According to this principle, an ammeter is made.
1. The internal circuit structure of the pointer multimeter:
The pointer multimeter mainly uses a sensitive magnetoelectric 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 also has a shunt (to expand the measurement range of the current), a multiplier (to expand the measurement range of the voltage), a rectifier (to convert AC into DC), a battery (to provide power when measuring resistance) and The function knob and other parts, the figure below is a schematic diagram of the circuit composition of the pointer multimeter.
2. The 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 state of the pointer multimeter when detecting DC voltage is shown in the figure below. It can be seen from the figure that when the multimeter has a range of 100V, the internal resistance of the meter is the sum of the three resistors and the resistance of the meter head, which is about 2MQ, which is equivalent to 2kO/V. It can be seen that the internal resistance of the multimeter is very high, and the general measurement will not affect the measured value. voltage is affected. Very little current flows into the multimeter when measuring voltage.
Measurement status of DC voltage
As shown in the circuit diagram inside the pointer multimeter when detecting the AC voltage, the AC voltage is applied between the two terminals of the multimeter, and a bridge rectifier circuit is installed in the meter to convert the AC signal into a DC current and then drive the meter head.
Measurement status of AC voltage
The internal circuit status of the pointer multimeter is as shown in the figure when detecting resistance. When measuring resistance, it is necessary to use the battery inside the multimeter to send current to the resistance, and then send it to the multimeter after passing through the resistance. The current passing through the small resistance value will be large, and the current passing through the large resistance value will be small. There is also a shunt in the meter. Resistance, so that the current value flowing through the ammeter is proportional to the measured resistance value. The pointer deflection angle of the ammeter corresponds to the value of the measured resistance.
Measure the state of the resistance
