The use and working principle of multimeter as voltage meter and current meter
1. Use of voltmeter
A multimeter should be connected in parallel to the circuit being tested. When measuring DC voltage, attention should be paid to the polarity of the voltage at the tested point, that is, connect the red probe to the high voltage end and the black probe to the low voltage end. If the polarity of the measured voltage is unknown, you can try using the probing method for measuring current mentioned above. If the pointer deviates to the right, you can proceed with the measurement; If the pointer deviates to the left, adjust the position of the red and black gauge pens before measuring. The internal resistance of the measuring circuit is very high, which requires a larger internal resistance of the voltmeter in order to achieve high measurement accuracy. At this point, it is necessary to use a multimeter with higher voltage sensitivity (greater internal resistance) for measurement. When measuring AC voltage, there is no need to consider polarity issues, as long as the multimeter is connected in parallel to the two ends being measured. In addition, it is generally not necessary to use a multimeter with a large range or high voltage sensitivity. The internal resistance of AC power supply is smaller than that of glue. It is worth noting that the measured AC voltage can only be a sine wave, and its frequency should be less than or equal to the allowable operating frequency of the multimeter, otherwise significant errors will occur. Do not toggle the range selection switch when measuring high voltages (such as 220V) to avoid arcing and burning out the contacts of the conversion switch. When measuring high voltage greater than or equal to 100V, safety must be taken into consideration. First, fix one probe to the common of the tested circuit, and then use the other probe to touch the other testing point. If the range is not sufficient, it is necessary to switch to another gear to measure the volume frequency level. The multimeter is only suitable for measuring the volume frequency level. If there is a DC voltage on the circuit, a 0.1uF/450V capacitor must be connected in series to block the DC before measuring. When measuring the voltage in a circuit with inductive reactance, the multimeter must be disconnected before turning off the power after measurement. Otherwise, when cutting off the power supply, due to the self inductance phenomenon of the inductive components in the circuit, high voltage may be generated, which may burn out the multimeter.
2. Use of ammeter
When a multimeter is connected in series in the tested circuit, attention should be paid to the direction of the current. Connect the red probe to the end where the current flows in and the black probe to the end where the current flows out. If you don't know the direction of the measured current, you can first connect one probe at one end of the circuit, and gently touch the other probe at the other end of the circuit. If the pointer swings to the right, it means the wiring is correct; If the pointer swings to the left (below zero), it indicates that the wiring is incorrect and the two probes of the multimeter should be swapped. When the pointer deflection angle is greater than or equal to 20% of the maximum scale, try to use a large number of range gears. Because the larger the range, the smaller the shunt resistance, and the smaller the equivalent internal resistance of the ammeter, the smaller the error introduced by the measured circuit. When measuring high currents (such as 500mA), do not toggle the range selection switch during the measurement process to avoid arcing and burning out the contacts of the conversion switch.
