What does \"two-wire passive 4-20mA signal\" mean?
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Shouldn’t a 2-wire transmission of 4-20mA signals be active? I’m confused again!
Meters that transmit 4-20mA should be active. When calibrating instruments, some standard instruments can provide 24VDC power to 4-20mA instruments. When powered by standard instruments, 4-20mA is referred to as a passive signal ； If the instrument has its own power supply, rather than being powered by a standard instrument, then 4-20mA is referred to as an active signal.
It’s not that complicated actually; it’s simply a matter of where the power source is located. This post provides detailed answers: http://bbs.hcbbs.com/forum.php?mobile=yes&mod=viewthread&page=1&tid=1176316
In other words, it is possible to accept 4–20 mADC signals, eliminating the need to provide a 24V power supply on-site; this applies to devices such as pressure and differential pressure transmitters
The counterpart to passive signals are active signals, such as the 4–20 mA DC signals from analyzers
Clarify the previous misunderstandings
Meters that transmit 4–20mA signals (such as transmitters), which require an external power supply if they do not have their own internal power source, are referred to as passive meters.
It should be internal power supply; once the instrument is powered from the outside, it can supply power to the signals itself, without the need for an additional signal power cable
Only the AI signals of three-wire or four-wire instruments are passive signals； A two-wire meter still needs to be powered, right? Only these two wires can supply power to it, and the AI signal must be an active signal.