Active and passive instruments

As for the instruments on-site, they are all active instruments; there are no passive ones. Instruments cannot function properly without a power supply. However, there is a distinction between active signals and passive signals. An active signal refers to a signal in which the instrument signal and the power supply for the instrument are in the same circuit; this is typically the case with two-wire instruments. A passive signal, on the other hand, means that the instrument signal and its power supply are separated from each other, and this applies to four-wire instruments.

The original poster got it wrong; active and passive are terms that refer to the system as a whole. If a signal requires power from outside the system, it is an active signal; if it does not need power from outside the system, it is a passive signal. Refer to the following regarding two-wire, three-wire, and four-wire instrument signals. Two-wire instruments do not have a wire for power supply; in other words, they lack an independent power source, and power must be supplied from outside. Their two wires are used to transmit both power and signals. The signals they output are called passive signals. A three-wire instrument has a positive power line and a positive signal line, while the negative power terminal and the negative signal terminal share one wire (the com terminal); in total, there are three wires. The signals they output are called active signals. A four-wire instrument, in which two wires are connected to the positive and negative terminals of the power supply, while the other two wires represent the positive and negative terminals of the output signal. The signals they output are active signals.

Hey, I think you’ve misunderstood. An active signal is one that contains voltage, while a signal without voltage is a passive signal (such as a dry contact). Two-wire instruments do not require an external power supply, as there is voltage already in the circuit; it is four-wire instruments that need an external power supply.

Take another look. You have surely done system configuration before; in such configurations, there is often a choice between active signals and passive signals. For example, with two-wire pressure transmitters, passive signals are chosen, while for four-wire mass flow meters, active signals are selected.

Learn from each other*, discuss together, and also share a summary. 1. First, clarify that the active and passive types mentioned here refer to analog input signals (AI). 2. Signals that require a card to supply power to the transmitter are passive signals ； There is no need for a card to supply power to the transmitter; a transmitter that is powered separately is an active signal. 3. The signal resulting from the superposition of a 24V power supply and a signal is a passive signal ； A signal whose power supply and signal are not superimposed is an active signal. 4. For analog modules, there is also the issue of whether the input signals are active or passive; the signals may differ, and so do the wiring connections. An active signal refers to one in which the device can generate a 4–20 MA signal without the need for an external 24VDC supply, whereas pressure transmitters and temperature transmitters produce passive signals. ”(Note: The term \"active signal\", as used here to refer to signals that can be generated by a device without the need for an external 24VDC supply, seems inappropriate in my opinion. Both active and passive devices require a power source; it’s just that the 24V power supply in this case isn’t provided by the card itself, rather than meaning that no power source is needed at all.)

Simply put, an active instrument is one that requires a power source to function, while a passive instrument does not need one.

Regarding safety barriers and cards: passive signals are those in which the signals output by field instruments already carry 24V.DC voltage (even after the signal wires are disconnected from the safety barrier or card, voltage and current can still be measured at both ends of the wires). Therefore, no additional voltage needs to be supplied to the safety barrier or card. In such cases, the system must provide a passive safety barrier or passive channel for this circuit; these instruments are usually 4-wire instruments such as electromagnetic flowmeters and mass flowmeters. For the original signal: After disconnecting the signal wire from the safety barrier or card, no voltage or current can be measured at either end of the wire; therefore, the safety barrier or card must supply voltage to it. In this case, the system must provide an active safety barrier or active channel for this circuit. Such instruments are usually 2-wire instruments, such as variable transmitters and vortex flow meters. The above are my personal views; those with different opinions may point this out for discussion