MF-47 pointer multimeter fault maintenance, please correct me if there is anything wrong.
There are two kinds of pointer instruments: magnetoelectric and electromagnetic. At present, pointer multimeter is dominated by magnetoelectric instruments, among which magnetoelectric instruments are divided into three types according to different magnetic circuits: internal magnetic type, external magnetic type and internal magnetic type. Internal magnetic meter: magnetic steel is called internal magnetic meter outside the coil, and external magnetic meter: magnetic steel is called external magnetic meter outside the coil. pointer multimeter with external magnetic meter is easy to introduce the interference of external electromagnetic field, resulting in inaccurate measurement. Therefore, pointer multimeter, who has an external magnetic meter, usually designs a metal shielding plate on the back cover of multimeter. The function of metal shielding plate is to shield the interference of external electromagnetic field, so that the instrument measurement is more accurate, while the internal magnetic meter head will not be designed, because the internal magnetic meter head is not easy to introduce the interference of external electromagnetic field and has strong anti-interference ability.
The following is the composition of the magnetoelectric instrument:
Magnetoelectric instrument is composed of magnetic steel (permanent magnet), moving coil (coil), spring hairspring (generating reaction moment) and pointer, in which the role of magnetic steel is mainly to generate magnetic field force by introducing current, the role of spring hairspring is to generate reaction moment device, the role of moving coil is to drive pointer to deflect, and the role of pointer is to display the size of measured signal.
Let's talk about the working principle of magnetoelectric instrument:
When the magnetic steel (permanent magnet) inside the instrument is electrified, the current cutting the magnetic induction line will produce magnetic field force, which is what we call rotating torque. This magnetic field force, that is, this rotating torque, will drive the moving coil inside the instrument to drive the pointer to deflect. The generated magnetic field force is different according to the current flowing into the magnetic steel inside the instrument, so the deflection amplitude of the moving coil driving pointer is also different. That is to say, the greater the current flowing into the magnetic steel inside the electric meter, the stronger the magnetic field force generated, and the greater the deflection amplitude of the pointer driven by the moving coil, which means that the measured signal is large. On the other hand, the smaller the magnetic steel current flowing into the meter, the weaker the magnetic field force, so the smaller the amplitude of the pointer deflection driven by the moving coil, which means that the measured signal is small. Based on this principle, the magnitude of the measured signal is realized, and the function of the spring hairspring is mainly to generate the device of counter torque. We know that pointer deflection is deflected by the rotating moment of magnetic field force. The hairspring mainly produces the reaction moment. Simply put, the hairspring mainly produces a moment opposite to the magnetic field force, which we call the reaction moment. When the magnetic force (rotating moment) is equal to the reaction moment generated by the hairspring, the pointer stops reading. If there is no reaction moment generated by the hairspring, no matter how big the measured signal is, the pointer will deflect to the end, the magnetic field force drives the pointer to deflect to the right, and the hairspring itself generates reaction moment to pull to the left.
MF47 multimeter protection circuit:
Header protection: two 1N400 1 parallel bidirectional limiter diodes are connected to the header for header limiter protection.
Circuit protection: When the input current of 250V/0.5A is greater than 0.5A, the fuse will be blown automatically. ..
Circuit protection 2: a new MF-47 electrical block, and the current block is designed with voltage-sensitive/thermistor overvoltage protection.
Current block protection: two 1N400 1 parallel devices are used to form current limiting protection, which is connected to the current block to prevent the shunt resistor from burning out.
Explain pointer multimeter's two zero adjustment methods and zero adjustment principle;
Mechanical zeroing: the pointer does not point to 0. Turn the mechanical zero knob with a screwdriver to reset the pointer to 0.
Mechanical zeroing principle: the mechanical zeroing knob is connected with a mechanical zeroing screw, which is connected with a hairspring. Turning the mechanical zero adjustment knob is equivalent to turning the internal mechanical zero adjustment screw, thus changing the tightness of the hairspring for mechanical zero adjustment.
Ohm zero adjustment: turn the multimeter to the resistance range, and the red and black probes are shorted. If the pointer does not point to 0, turn the resistance zeroing potentiometer to return the pointer to 0.
Ohm zeroing principle: The resistance zeroing potentiometer is connected with the meter in parallel with an adjustable shunt resistor. By turning the resistance zeroing potentiometer, the resistance of the adjustable resistor is changed to zero, thus changing the current flowing through the meter.
Measuring principle of multimeter gear: DC: DC: AC: AC.
DCV: DC measuring principle of DC voltage block: divide the voltage with the series resistor in the instrument, expand the range of DC voltage block, change the resistance of series resistor in DC voltage block, and change the range of voltage block. Principle of voltage division: The measured large voltage is divided by the voltage divider and becomes the acceptable full voltage of the electric meter.
Dcma: measuring principle of DC current file: the range of DC current file is extended by shunt resistor connected in parallel with instrument, and the range is changed by changing the resistance of shunt resistor in DC current file. Shunt principle: after the measured large current is shunted to the instrument through shunt resistor, the current is within 50μA A. ..
ACV: Measuring principle of AC voltage block: The measuring range of AC voltage block is expanded by the resistance voltage division connected in series with the instrument, and the AC signal is rectified into DC signal by the half-wave rectifier circuit and flows through the instrument for measurement. Because pointer multimeter's meter is a DC ammeter, it is necessary to add a half-wave rectifier circuit on the AC voltage block as a rectifier to rectify the AC signal into a DC signal and flow it through the meter for measurement. Therefore, to measure the alternating current once, it is necessary to rectify the alternating voltage block once through the rectifier diode, so that the alternating current can be measured according to the direct current flowing through the meter.
When the alternating current is in the positive half cycle, the alternating current signal is shunted by the red contact pin and the safety tube, rectified by VD 1 rectifier diode (which is used for unidirectional conduction), and the alternating current is changed into direct current, which is measured by the power meter. When the alternating current is in the negative half cycle, the signal passes through the black contact pin, passes through the VD2 divider resistor, and returns to the red contact pin without passing through the ammeter, and the ammeter does not add reverse current. That is to say, the function of VD2 is to protect VD 1. In fact, VD 1 are two rectifier diodes in the AC voltage module. After the positive half cycle is divided by overvoltage resistor, it is rectified by VD 1 and then measured by electric meter. The function of VD2 is to protect VD 1 from the influence of excessive voltage across VD 1 tube.
Measuring principle of Ω resistance: The equivalent resistance between the internal battery and the internal resistance of multimeter lies in the external resistance to be measured, so as to measure the resistance value of the measured resistance. If the resistance value of the measured resistor is large, the current flowing through the measured resistor will be small, which means that the pointer deflection will be large, while if the resistance value of the measured resistor is small, the current flowing through the measured resistor will be large, which means that the resistance value of the measured resistor will be small.
Take domestic MF-47 as an example to explain the fault maintenance of pointer multimeter;
The fault of multimeter is divided into two aspects:
1: instrument failure (instrument mechanical failure), 2: circuit failure.
Troubleshooting of multimeter meter;
Instrument 1 Troubleshooting: When measuring all gears, the pointer does not move. Generally, the fuse tubes in this troubleshooting circuit are burnt out. Remove the positive and negative wiring of the instrument from the circuit board, use a digital multimeter to break the 200 ohm gear, connect the red probe to the positive electrode of the instrument and the black probe to the negative electrode of the instrument. If the pointer deflects, it means that the moving coil (coil) inside the instrument is not burnt out. Otherwise, if the current is applied to the meter, it means that the moving coil inside the meter is not deflected.
Maintenance of meter head failure II: The amplitude of pointer deflection is very small when measuring all gears. Generally speaking, there is no doubt that the fault must exist in the meter head, because we know that when measuring all gears, the deflection amplitude of the pointer is very small, which means that the voltage divider/shunt resistor in the internal circuit of the multimeter cannot be completely damaged (open circuit or short circuit), which means that the fault exists in the mechanical meter head, which is generally caused by the loss of excitation of the magnetic steel, that is, the permanent magnet. It is because the multimeter is placed beside the high-frequency magnetic field for a long time that the demagnetization of the internal magnetic steel (permanent magnet) of the ammeter fails. Maintenance method: magnetize the instrument with magnetizer or directly replace the instrument.
Troubleshooting of meter head 3: When measuring all gears, the deflection amplitude of pointer is close to full gear. This kind of fault is generally caused by loose spring balance spring or deformation and inelastic spring balance spring. The maintenance method can be trimming the hairspring or replacing it. Of course, if the meter accidentally falls to the ground, it is easy to drop the hairspring when measuring the meter head, so that the pointer is deformed to near full.
Meter fault maintenance 4: the pointer does not reset, that is, the pointer does not return to zero, and the mechanical zero adjustment is not reflected. Generally speaking, this failure is caused by the deformation of the hairspring or the deterioration of the elasticity of the hairspring. We know that the balance spring has two functions, 1: to generate reaction data, and 2: to reset the pointer. If the pointer is not reset, it is likely that the balance spring performance is poor or the balance spring elasticity is poor, so it should be replaced. The mechanical failure of the instrument head ends here. There are only magnetic steel (permanent magnet), moving coil (coil), spring hairspring and pointer in the mechanical aspect of the meter head. If there is a mechanical failure of the meter head, these devices include: hairspring deformation, softening, weakening of elasticity, demagnetization of magnetic steel, open circuit of moving coil and short circuit of moving coil.
Instrument maintenance usually has only a few faults. These things are damaged, and poor performance will cause inaccurate measurement.
Troubleshooting of multimeter circuit;
Circuit fault repair1:The input fuse of 250 V/0.5A is blown, and all gears cannot be used without current input. This fault is the most common fault in multimeter maintenance, such as measuring large signal with small range, measuring voltage with resistor or measuring voltage with current block. The maintenance method is simple. Disassemble the multimeter, test the input fuse and find that the input fuse has been blown, and replace it with a fuse of the same model and specification to repair Model 47. As long as the fuse is blown, the multimeter will not be energized, resulting in the failure of all gears. ) this kind of fault is very common, and of course it is easy to repair.
Circuit fault repair 2: the DC voltage block cannot be accurately measured or used, and the lowest voltage-dividing resistor in the DC voltage block (15K 2.5V block 10V block 150K) is damaged or the resistance change causes the DC voltage block to be completely unusable or cannot be accurately measured. If it is easy to burn out the voltage divider resistor when using the lowest DC voltage block to measure the high voltage by mistake, the DC voltage block will not be used at all or the measurement error is very large, because as can be seen from the schematic diagram, when measuring the DC voltage, the signals all come from the lowest DC voltage block, that is, the 15K voltage divider resistor in the 1V file. If 1V block is used to measure high voltage by mistake, it is easy to damage this 15K voltage divider resistor, and all gears of DC voltage block will be unusable. Supplement: If the R22.69K resistor connected to the DC voltage block and meter is open or its resistance changes, the DC voltage block and DC current block will also be unusable. Check it carefully.
Circuit fault repair 3: AC voltage block cannot be accurately measured or AC voltage block cannot be used. If the AC voltage block cannot be used or the measurement is inaccurate, first check whether the brush is in poor contact with the circuit board, whether the circuit board contacts are oxidized, whether the contacts fall off, etc. Because of the poor contact between the brush and the circuit board, there will be a big measurement error or a gear failure, and the poor contact between the brush and the circuit board will be eliminated. If the AC voltage block still can't be used or the measurement error is large, the second step is to check the two voltage-dividing resistors at the lowest gear of the AC voltage block, because the resistance of the voltage-dividing resistors in the three gears of AC 1000V, AC 500V and AC 250V is very large, and it is generally not easy to break. The easily broken voltage divider resistance is in the range of 50V and AC 10V, which is the lowest level of AC voltage block. Focus on checking the 160K voltage divider connected to AC 50V voltage block and the 38.3K voltage divider connected to AC 10V voltage block. If the high voltage measurement is wrong, these two voltage dividers are easy to burn out (open circuit), which will lead to the failure of all gears of the AC voltage block. Eliminate voltage divider open circuit and voltage divider resistance change. If the AC voltage block still cannot be used or accurately measured, it is necessary to check the forward and reverse resistance values of the two rectifier diodes in the half-wave rectifier circuit in the AC voltage block rectifier, which may be caused by the breakdown or open circuit of the rectifier diodes. During the inspection, it was found that the rectifier diode had breakdown or open circuit. Replace the diode of the same model to repair the fault. Generally speaking, diode breakdown in AC voltage block rectifier may lead to inaccurate measurement of AC voltage block, that is, the pointer may shake when measuring AC voltage, or the error of measuring AC voltage block is large, and an open diode will generally lead to the failure of measuring AC voltage block, so it is necessary to focus on inspection.
Circuit fault repair 4: All the resistance gears cannot be used. If the battery is replaced because the battery voltage is 9V, 1.5V, the second step is to check whether the brush is in poor contact with the circuit board. If the contact is poor, all gears of the resistance gear cannot be used. If the contact is normal, the third step is to check whether there are any faults such as oxidation virtual welding in the battery wiring and resistance file wiring, and repair them. If not, the resistor block still cannot be used. Step 4, check whether the WH 1 and 17.3K resistors connected with the X 10K block are open. If the resistance of 17.3K is open, no current will pass through the meter. Check and repair.
Circuit fault repair 5: The fault of incorrect ohmic zero adjustment of all gears in the resistance gear. Check the battery voltage first. The low battery voltage of 9V and 1.5 will cause the fault that all gears of the resistance gear cannot be zeroed correctly. If the battery voltage is still inaccurate after normal zero adjustment, the second step is to check whether the brush and the circuit board have poor oxidation contact and eliminate this situation. Check whether WH 1 (resistance zeroing potentiometer) is welded on the circuit board or repaired by oxidation inspection. If not, it is still not allowed to return to zero, the battery wiring is broken and the contact is poor. Connect the 200μA current block in series with a digital multimeter on the meter head connection, and then hit the pointer multimeter on the resistance block to make the probe short-circuit for ohmic zero adjustment, to see if the pointer can be adjusted to 46.2μA A, if it is not adjusted to the whole page, it means that there is an ohmic zero adjustment circuit in the fault. First, check the resistance of WH 1 resistance zeroing potentiometer itself. If the resistance of WH 1 increases, the fault of inaccurate zero adjustment will also occur. If WH 1 is normal, check the 20K adjustable resistor connected with WH 1, which is a shunt resistor connected in parallel with the electric meter and mainly used for zero adjustment. Generally, when 20K adjustable resistor is open or its resistance changes, all gears will not be zeroed correctly. Check and replace the 20K resistor to repair the fault. We know that the working principle of resistance zeroing circuit is that the resistance zeroing potentiometer WH 1 is connected with a 20K adjustable shunt resistor. Turning the resistance zeroing potentiometer is equivalent to changing the resistance of the adjustable resistor, thus changing the current flowing through the instrument for zeroing. If the resistance value of WH 1 resistance zeroing potentiometer or 20K adjustable resistor changes or is open, it is not controlled by the potentiometer, and the current flowing through the instrument cannot be adjusted, resulting in zero adjustment failure.
Circuit fault repair 6:DC current file 5mA current file has a fault, and the pin is full. Use the current measuring pointer of the 5mA gear to repair the fault, and all other gears are normal. Only the 5mA gear of the current DC gear has a total fault. First, check the gear switch for poor contact, contact oxidation, short circuit, etc. , found that it is not. Step 2, check the shunt resistance of the current block at the 5mA gear, and find that the open shunt resistance is replaced and the fault is repaired. This fault is caused by the excessive current flowing in the instrument due to the open circuit of shunt resistor. We know that the shunt principle is to shunt the measured large current through the shunt resistor to ensure that the current flowing into the meter is within 50 μ A. Now that the shunt resistor has been turned on, the large current can only be shunted through the shunt resistor, so the fault is caused by the open circuit of the shunt resistor.
Circuit fault repair 7: DC current block is faulty, and all gears of the current block are faulty. As long as other gears are normal, only all gears of the current block are faulty, indicating that the meter, fuse tube and circuit of other gears are good, so the first step is to check whether the gear switch is in poor contact, contact oxidation, short circuit, etc. , found no, the second step is to check whether the shunt resistor in the current block is out of welding, open circuit and so on. It is found that the shunt resistance of 50mA gear is out of welding. Check and repair the fault, because the current block of the new MF-47 pointer multimeter adopts closed-circuit shunt, that is, all shunt resistors are connected in parallel and in series in the meter. In this case, if a shunt resistor in the current loop is open or off-welded, all gears of the multimeter's current gear will be invalid.
Supplement: Troubleshoot the AC voltage range, if the AC voltage range is not used and the measurement is inaccurate, check whether the brush is in poor contact with the circuit board, and whether the contacts are short-circuited by oxidation. Eliminate and check whether the lowest gear of the AC voltage range (AC 50V 160K voltage divider, AC 10V 38.3K voltage divider) has resistance change or voltage divider open circuit fault. In order to eliminate the damage of voltage dividing resistor or the failure of AC voltage level, it is necessary to check two rectifier diodes of half-wave rectifier circuit in AC voltage level rectifier to prevent the rectifier diodes from breaking down and opening. When checking the rectifier diode, it is necessary to check the filter capacitor connected with the diode. The filter capacitor connected with the diode is an AC filter capacitor. If it is damaged, the AC voltage level will also fail. The failure of DC voltage block is generally due to the burning of the lowest voltage dividing resistor in DC voltage block, which leads to the failure of DC voltage block or poor oxidation of gear switch contact. However, measuring the wrong voltage with the current block is easy to burn out the shunt resistor and the meter head protection diode. If the meter head protection diode is burnt out, the meter head is generally not energized. Don't doubt that the rice head is damaged at this time. First, check the quality of two parallel clamping and limiting protection diodes D3 and C 1 filter capacitor connected to the meter.