When using the AC voltage range of a multimeter to measure voltage, should it be connected in series or in parallel?
When using a multimeter to measure voltage, it should be connected in parallel, and when measuring current, it should be connected in series. This is the case for both alternating current (AC) and direct current (DC). The internal resistance of the voltage range of a multimeter is close to infinity. Therefore, if the voltage range is selected and the multimeter is connected in series in the circuit, there will actually be no current in the circuit or the current will be extremely small, and thus no useful data can be measured. For the current range, the internal resistance is close to 0, so connecting it in series in the circuit will not greatly affect the operation of the subsequent electrical appliances.
However, if the current range is wrongly connected in parallel in the circuit, for example, in the situation the questioner asked about, if the voltage range is replaced with the current range and connected in parallel across the two terminals of a 220V power supply, it will be extremely dangerous. It will directly cause a short circuit, and there is a possibility of burning out the multimeter or the circuit.
Measuring with the voltage range is relatively safe. Even if it is connected in series in the circuit, it will not cause as great harm as when the current range is wrongly connected. We use a multimeter to obtain some measurement data, so we must think carefully about how to measure before taking any action.
To determine whether it should be connected in series or in parallel, we first need to understand the principle of how a multimeter measures AC voltage. The meter head of a multimeter can only pass DC voltage, that is, it can only measure DC voltage originally. Then why can it measure AC voltage? It is because there is a rectifying circuit inside the multimeter. By utilizing the unidirectional conductivity of diodes, the AC signal is converted into a DC signal, so that it can measure AC voltage. In other words, when the AC range of a multimeter is used to measure the 220V household voltage, it is a parallel circuit.
Principle of measuring AC voltage: When a multimeter measures AC voltage, because there is a half-wave rectifying circuit inside the multimeter, when measuring AC voltage, the AC voltage is rectified into a DC voltage and then passes through the two meter heads. Finally, the AC voltage is measured according to the magnitude of the DC voltage. Moreover, the AC voltage displayed on the multimeter is not the instantaneous value of the AC power, but the average value over a period of time, that is, the effective value. In fact, the internal structure of a multimeter is not that simple. The signal also needs to be amplified by an operational amplifier, sampled, and then subjected to analog-to-digital conversion, and the final result is displayed on the screen.
When measuring the 220V household voltage, just select the appropriate range of the AC range. Here, we will illustrate with a Fluke digital multimeter. Insert the black and red test leads into the corresponding positions, and directly connect them across the live wire and the neutral wire of the mains power. There is no need to distinguish which one is the live wire and which one is the neutral wire, and the voltage value can be directly read out.
