Pointer meters can be divided into magnetoelectric and electromagnetic types. At present, most pointer meters are magnetoelectric, among which magnetoelectric meters are divided into three types: internal magnetic type, external magnetic type and internal and external magnetic type. MF-47 with external magnetic head is easily disturbed by external magnetic field, which leads to inaccurate measurement. Therefore, MF-47 with external magnetic head generally designs a metal shielding plate on the back cover of multimeter. The function of metal shielding plate is to shield the interference of external magnetic field and make the measurement of instrument better and more accurate. Generally, it is only used in the external magnetic meter MF-47, and the internal magnetic meter MF-47 is not designed. The function of metal shielding plate is to shield the interference of external magnetic field and make the measurement of instrument better and more accurate. Because the external magnetic meter is easy to introduce the interference of external electromagnetic field, the measurement is inaccurate, so a metal shielding plate is installed on the back cover of the multimeter for external magnetic shielding.

The following is the introduction of magnetoelectric meter:

The magnetoelectric instrument consists of a moving coil, a stationary coil, a damper, a spring hairspring and a pointer, wherein the moving coil and the stationary coil are mainly used for introducing current to generate magnetic field force, and the spring hairspring is mainly used for generating reaction moment to drive the pointer to deflect; Damper is used to generate certain inertia when the pointer is deflected by magnetic field force, and damper is used to absorb this inertia, so that the pointer can stop at a certain point as soon as possible and achieve the purpose of fast reading.

The working principle of mechanical instrument: the magnetic field force generated by the current flowing through the meter head drives the hairspring, and the hairspring drives the watch hand to deflect. The magnetic field force generated by the hairspring is different according to the current flowing through the meter, so the magnetic field force generated by the hairspring driving the watch hand is also different. The greater the current flowing through the meter, the stronger the magnetic field force, so the greater the deflection amplitude of the watch hand driven by the hairspring. The smaller the current flowing through the meter, the greater the magnetic field force.

Take domestic MF-47 as an example to illustrate the principle of pointer table;

The meter head of MF-47 pointer instrument is a micro-ampere (μA) DC ammeter, and its full deflection is 46.2 micro-amperes, which means the total current of the meter head is 46.2 micro-amperes. Its working principle is that when a current signal flows through the meter, the pointer will be deflected by the magnetic field force (because a magnetic field will be generated where there is current). According to the magnitude of the magnetic field force, the deflection amplitude of the pointer is also different. The greater the current flowing through the instrument, the stronger the magnetic field force, so the greater the amplitude of the spring hairspring that drives the pointer to deflect, and the weaker the magnetic field force, so the smaller the amplitude of the spring hairspring that drives the pointer to deflect, thus measuring the size of the measured signal.

Measuring principle: DC: DC: AC: AC.

Measuring principle of DCV DC voltage block: The measuring range can be expanded by series voltage dividing internal resistance, and the measuring range can be changed by changing the resistance value of series voltage dividing resistance in DC voltage block.

Measuring principle of DCA DC current block: the measuring range can be expanded by shunt internal resistance, and the measuring range can be changed by changing the resistance of shunt resistance in DC current block.

Measurement principle of AC voltage block of ACV: The measurement is extended by series internal resistance.

Measurement range: Half-wave rectifier circuit rectifies AC signal into DC signal, which flows through the meter for measurement. Because the meter head of the pointer instrument is a DC ammeter, the meter head cannot flow through the AC signal, so a half-wave rectifier circuit must be added to the AC voltage block to make the measured AC signal flow through the rectifier to be tested. Therefore, once AC is measured, it must be rectified by rectifier diode, and the AC voltage block must be equipped with rectifier to rectify the measured AC signal into DC signal, which flows through the meter for measurement.

Alternating current is rectified by D 1 in the positive half cycle, which changes the alternating current signal into a direct current signal and flows through the instrument for measurement. Alternating current is rectified by D2 in the negative half cycle, where D2 is used to protect the D 1 rectifier diode. In order to prevent alternating current from being rectified by D 1 in both positive and negative half cycles, a D2 rectifier diode is added, because the alternating voltage signal is too large to easily break through D 1. In this case, in the positive half cycle, the AC signal is converted into a DC signal and flows through the electric meter for measurement, and in the negative half cycle, the AC signal is rectified into a DC signal and flows through the electric meter for measurement.

Measuring principle of Ω barrier: The barrier is the only gear that works with the battery in the multimeter, while the DC voltage barrier, AC voltage barrier and DC current barrier do not work with the internal battery. There are two batteries in the indicator light, one is 1.5V and the other is 9V. Some 9v batteries are replaced by 15V barrier * * which is divided into five grades. RX 10K adopts internal 9vrx10rx10rx14th gear * * adopts internal 1.5V, and a closed loop is formed between the current flowing through the measured resistor and the external measured resistor. If the measured resistance is large, the current flowing through the resistance is small, so the amplitude of pointer deflection is small. (indicating that the measured resistance is large) If the measured resistance is small, the current flowing through the resistance will be large, and the amplitude of pointer deflection will be large (indicating that the measured resistance is small).

Multimeter shunt:

Open-circuit shunt: all shunt resistors are connected in parallel and series with the ammeter. In open-circuit shunt, a shunt resistor damages other range of gear or can be used normally.

Closed shunt: All shunt resistors are connected in parallel and series with the ammeter. In closed-circuit shunt, if a shunt resistor is damaged, all ranges of gear can't be used, because in closed-circuit shunt, all shunt resistors are connected in series, and a shunt resistor is damaged, which is equivalent to an open circuit, and no current is generated in the open circuit, so it can't be used. However, the current pointer multimeter design mostly adopts closed-circuit shunt, that is, DC voltage block, DC current block and voltage divider in AC voltage block are connected in series with shunt resistor. In this case, one shunt resistor or shunt resistor in the circuit is damaged, and all ranges of this gear are unavailable.

Explanation of MF-47 multimeter protection circuit;

1: meter protection: Two IN400 1 silicon rectifier diodes are connected in parallel to form a bidirectional limiter diode connected to the meter. The purpose is to block the measured voltage with current to prevent the meter from burning out. In this case, the input voltage signal will be limited to 0.7V by the bidirectional limiter diode, which is the turn-on voltage of the silicon diode, thus protecting the electric meter. Capacitors C 1 at both ends of the meter are used to filter the meter, and current-limiting protection resistor R 1 is used to prevent the meter from burning due to excessive current.

2. All gears adopt closed-circuit shunt: DC voltage gear. All voltage dividing resistors and shunt resistors of AC voltage module and DC current module are connected in series. In this case, if a voltage divider or shunt resistor damages the gear, all gears cannot be used.

3. Input fuse: the fuse is 250V/0.5A, and the input current value is greater than (AC /DC)0.5A, and the fuse will automatically blow to protect the subsequent circuit.

Experiment of MF-47 pointer instrument;

Experimental purpose: To prove that the current value of the instrument is 46.2 μ A..

First of all, we can use a micro-ampere DC ammeter (of course, we can also use a digital multimeter with a DC current block of 200μA) in series with the meter terminal of pointer multimeter (A+ and A- connect the red probe to the A+ terminal and the black probe to the A- terminal), and then turn the pointer multimeter to the resistance position, because only the resistance position in the multimeter can work with the internal battery. Short circuit stylus (equivalent to internal battery short circuit) A current signal flows through the stylus to generate a magnetic field force to push the stylus to deflect. At this time, the ammeter will indicate the deflection current value of pointer multimeter stylus. If the stylus of pointer multimeter does not return to zero, you can twist the resistance zeroing potentiometer to make the stylus point to the zero scale line. When pointing to the zero scale line, the current value of the ammeter should be between 46.2 μ A or 46.2μA and 46.7 μ A (because instrument measurement will produce. This experiment proves that the full deflection current of MF-47 pointer instrument is 46.2 μ A.