The voltage reading of the pointer multimeter is too high
Sometimes when we measure electronic circuits, due to the measurement mechanism of the multimeter, circuit structure, internal resistance and other factors related to accuracy, sometimes the measurement accuracy will decrease, the operating frequency of the circuit under test, and the operation of active devices Factors such as shape and other factors will also lead to large measurement errors.
Therefore, the measurement of some circuits is more particular about the instrument. In general, the instrument with higher internal resistance should be selected to measure the voltage of the circuit, which can reduce the shunt of the instrument to the circuit. When measuring the circuit current, try to choose an instrument with a low internal resistance, but the internal resistance of the instrument cannot be zero, so after the ammeter is connected in series, the internal resistance of the instrument will inevitably divide the voltage on the circuit, so the result is the parameters during circuit debugging and operation. Inconsistent. In order to avoid the above-mentioned errors, indirect measurement is used, the current is measured when the voltage is measured, and the voltage is measured when the current is measured. When measuring the voltage, first accurately measure the resistance value of the resistor, and then measure the current of the circuit, thus indirectly obtaining the measured voltage. When measuring the voltage when measuring the current, the resistance value of the circuit is also measured first, and then the voltage drop on the resistance can be measured to obtain a more accurate current value. Of course, these methods should also be used flexibly. It is also very important to analyze the circuit, and it is necessary to proceed from reality.
The voltage reading of the pointer multimeter is too high. You can try to open the back cover and find the calibration resistor on the meter head. It is usually an adjustable resistor connected in series with the meter head. At the same time, find a better digital multimeter and adjust it to see if it can Cannot calibrate. If the deviation is not large, it can generally be calibrated.
If the deviation is large, use a digital meter to measure the voltage dividing resistors of each gear to see if there is any burnt or variable value.
If there is none, the hairspring may be aging and boring. The balance force of the hairspring needs to be adjusted, which is the stress that the upstream and downstream springs face when the pointer returns to zero. The greater the stress, the lower the sensitivity of the meter head, but the quick and accurate zeroing, the smaller the stress, the higher the sensitivity of the meter head, but the poor reset performance.
In the situation you mentioned, if there is no sign of resistance damage, it should be considered a problem with the balance spring adjustment of the meter head.
The factors that affect the accuracy of voltage measurement readings are the voltage divider circuit and meter sensitivity. It is a rare fault that the reading is too large, usually the reading is too small, it is caused by the decrease of the sensitivity of the meter head, the attenuation of the magnetic steel, and the iron pin in the magnetic gap will cause the reading to be low and the pointer to stick. At this time, the iron pin can be cleaned with self-adhesive, and the normal reading can usually be restored.
The internal resistance of the multimeter is not the same, the allowable error is +-0.2, each manufacturer has the design ideas of each manufacturer, the pointer multimeter has basically withdrawn from the ranks of electrical maintenance, but the pointer multimeter is intuitive and convenient, but Compared with the digital meter, it is much inferior, and the accuracy rate is much lower, because each gear of the pointer multimeter is completed by the control circuit built by the resistor or capacitor, and the second, the triode, and the digital multimeter is different. It is composed of circuit and gate circuit, so the reading of the digital meter is more accurate. As for the high voltage displayed, it can also be measured by this method. For example, when measuring 220 alternating current (expressed in AC), it is more accurate to measure with AC 1000V, because Its internal resistance is relatively high and stable, and it is safer for multimeters. It is also the MF47 multimeter, but its internal resistance is different. It can be said that a manufacturer has the same design idea, but the same, and the DC gear measurement follows the above method. For example, when measuring direct current (indicated by DC) above 36V, 250V DC gear is required for the most accurate measurement. You must follow a rule when measuring, that is, when measuring a certain value, you must use a gear that is at least twice as large as the measured value. For measurement, there are several kinds of MF47 multimeters produced in Nanjing. There are Jinchuan, Kehua and other manufacturers, but the circuits are similar, and each has its own design ideas. The following is the appearance of these two meters. Let's measure it and give it a try, but we should pay attention to novice electronic enthusiasts must pay attention to safety when measuring high voltage (AC 220V).
