A question from a beginner about circuits
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Today I saw a coworker testing a combustible gas detector; it has three terminals: power +, power -, and one for the signal wire (during testing, only the power + and - terminals are connected, while the signal terminal is not connected). To measure the output current, one probe of a multimeter is placed on the negative power terminal and the other probe on the signal terminal, and this allows the current to be displayed. I’m a bit puzzled – isn’t this method similar to measuring voltage? Moreover, since the signal wire isn’t connected, isn’t that meant to prevent the formation of a closed circuit? Yet it’s still possible to measure the current in this way
This method of measurement is commonly used in three-wire or four-wire instruments that operate with a power supply alone, such as three-wire alarms and four-wire flow meters. The signal line is used to transmit the current from the transmitter to the DCS system at a remote location. . Even without connecting this wire, the actual output current of the transmitter still exists objectively at the signal terminal and the power terminal. Here it should be noted that the \"power terminal\" mentioned by the original poster is not a separate power source. Rather, both the common signal line and the power line use this as their common terminal. .
Short circuit – a signal loop has already been formed at the signal terminal. . . . Furthermore, this type of measurement can only determine whether the transmitter itself is functioning properly. .
Is this a circuit? How was that formed? It’s not obvious.............
Please look on the second floor. . Read it a few more times. Be sure to understand that this alarm device is three-wire, which means the power supply is separate. . . The signals are separate. . . It needs to be distinguished from ordinary two-wire instruments. .
When you use a multimeter to measure, a circuit isn’t formed
The multimeter measures the current value; it is connected in series within the signal circuit, so this constitutes the circuit. It's very clear.