MF47 pointer multimeter structure schematic diagram

Pointer-type mechanical multimeter, from the perspective of its structure, the meter head is a highly sensitive magnetoelectric microampere DC current meter head. The commonly used magnetoelectric microampere meter head is made based on the principle of the effect of magnetic field on the current-carrying wire, and its structure is shown in the figure below.

A set of cylindrical iron cores is embedded in the circular magnetic steel. The outside of the cylinder is a movable aluminum frame. The aluminum frame is wound with a coil made of fine enameled wire. There are sharp needles at both ends of the coil, a hairspring and a zero-adjustable rod. Its flexibility determines the sensitivity of the meter head, just like the pendulum needle of a watch. There is a positive pole head and a negative pole head at both ends of the coil. The red line represents the positive pole + and the black line represents the negative pole 1. It is an aluminum frame that can rotate around an axis. Two flat spiral springs and a pointer are installed on the rotating shaft of the aluminum frame. The two ends of the coil are connected to the two spiral springs respectively, and the measured current enters the coil through the spring. There is a pole shoe with a cylindrical inner wall on each pole of the horseshoe magnet. There is a fixed cylindrical iron core in the aluminum frame. The function of the pole shoe and the iron core is to make the magnetic flux lines between them along the radial direction and evenly distributed along the circumference.

When the coil moves in the magnetic field, no matter where it turns, its plane is parallel to the magnetic flux lines. When the current passes through the coil, the two sides of the coil parallel to the axis are affected by the magnetic field force, and the two forces act to make the coil rotate. When the variable meter coil rotates, the spiral spring is twisted, generating a force that hinders the rotation of the coil, and its torque force increases with the increase of the coil rotation angle. When this hindering effect increases to offset the rotation effect of the magnetic field force, the coil stops rotating. According to the principle that like charges repel and opposite charges attract, once the microampere-level coil magnetic field is reversed, the magnetic field it generates will become an opposite magnetic field and act on each other and deflect in the opposite direction. Furthermore, the current force passing through the coil is proportional to the current, so the larger the current in the coil, the greater the rotation effect of the magnetic field force, and the greater the angle of deflection of the coil and the pointer. Therefore, according to the size of the pointer deflection angle, the strength of the measured current can be known. When the direction of the current in the coil changes, the direction of the magnetic field force will also change, and the deflection direction of the pointer will also change. Therefore, according to the deflection direction of the pointer, the direction of the measured current can be known.

There is no reverse bias in AC voltage measurement, but for DC voltage measurement: set a switch of the multimeter to the DC voltage range V of the appropriate range, and connect the "+" probe (red probe) to the high potential, and the "-" probe (black probe) to the low potential, that is, let the current flow into the "+" probe and out of the "-" probe. If the probes are connected in reverse, the pointer of the meter will deflect in the opposite direction, which can easily bend the pointer.

When measuring DC current, set a switch of the multimeter to the appropriate range of 50uA to 500mA. The range selection and reading method of current are the same as those of voltage. When measuring, the circuit must be disconnected first, and then the multimeter is connected in series to the circuit being measured in the direction of the current from "+" to "-", that is, the current flows in from the red test lead and flows out from the black test lead. If the multimeter is mistakenly connected in parallel with the load, the internal resistance of the meter head is very small, which will cause a short circuit and burn the instrument. The reading method is as follows: actual value = indicated value × range/full deviation.