The voltage reading measured by the pointer multimeter is too high.
Sometimes, when measuring electronic circuits, various precision related indicators such as the measuring mechanism, circuit structure, internal resistance, etc. of a multimeter can lead to a decrease in measurement accuracy. Factors such as the operating frequency of the tested circuit and the working condition of active devices can also cause significant measurement errors.
Therefore, the measurement of some circuits also requires attention to the instruments. In general, when measuring the voltage of a circuit, instruments with high internal resistance should be selected to reduce the diversion of the instrument to the circuit. When measuring circuit current, try to choose instruments with lower internal resistance, but the internal resistance of the instrument cannot be zero. Therefore, after connecting the ammeter in series, the internal resistance of the instrument will inevitably divide the circuit, resulting in inconsistent parameters during circuit debugging and operation. To avoid the above-mentioned errors, indirect measurement is used, measuring current when measuring voltage and measuring voltage when measuring current. When measuring voltage, first accurately measure the resistance value of the resistor, and then measure the current of the circuit, which indirectly obtains the measured voltage. When measuring current and voltage, it is also necessary to first measure the resistance of the circuit and then measure the voltage drop across the resistor to obtain a more accurate current value. Of course, these methods should also be flexibly mastered and used, and analyzing circuits is also important, especially starting from reality.
The pointer multimeter measures a high voltage reading. You can try opening the back cover and finding the calibration resistor on the meter head, which 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 be calibrated. If the deviation is not significant, it can generally be calibrated.
If there is a large deviation, use a digital meter to measure the voltage divider resistance of each gear and see if there is any burnt out or variable value.
If none of them are available, then the hairspring may have aged and become dull. The balance force of the balance spring needs to be adjusted, which is the stress on the upper and lower balance springs when the pointer returns to zero. The higher the stress, the lower the sensitivity of the meter head, but the zeroing is fast and accurate. The lower the stress, the higher the sensitivity of the meter head, but the zeroing reset performance is poor.
If no signs of resistance damage are found in the situation you mentioned, it should be considered that it is a problem with adjusting the balance spring of the meter head.
The factors that affect the accuracy of voltage measurement readings are the voltage divider circuit and the sensitivity of the meter head. A reading that is too large is a rare fault, usually due to a decrease in sensitivity of the meter head, magnetic steel attenuation, and iron pins in the magnetic gap, which can cause the reading to be too low and the pointer to be stuck. At this point, you can use adhesive to clean the iron pin, and usually the reading can be restored to normal.
