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  • How To Distinguish Between Passive Signals And Active Signals In Industrial Control?

    active power filter

    passive filter

    The active and passive signals are relative to the current signal. If the sensor device has independent working power, the current signal sent by the sensor is an active signal;

    For example, a four-wire electromagnetic flowmeter, which is powered by AC220V, and then outputs a DC4-20ma current signal. If the device itself does not have a separate working power supply, the card and module need to provide loop power, and the output 4-20 mA current signal is a passive signal.

    Some people say that the three-wire system, or the four-wire system is an active signal, and the two-wire system is a passive signal. In fact, This requires a dialectical analysis.

    For example, the transmitter signal is transmitted to the DCS card through the secondary instrument conversion, or the PLC module is also a two-wire system. This is an active signal for DCS and PLC.

    The acquisition of active signals is very simple. First, the sensor equipment is provided with independent power sources, such as AC220V, DC24V, etc. After the power-on device is operating normally, an active signal can be obtained at the signal output of the conversion header or the secondary meter. Passive signals are generally powered by DCS cards, PLC modules, or external DC24V power supplies in the signal line loop.

    Passive signal isolator

    Let’s talk about the distinction between passive contacts and active contacts.

    Passive contacts are commonly referred to as dry contacts or passive switches, and dry contacts are an electrical switch with two states of closed and open. There is no polarity between the two contacts that can be interchanged. For example, limit switches, travel switches, and foot switches are often used. Rotate the main switch reed switch and so on. Active contacts, commonly known as wet contacts or active switches, have both power and breakpoint states. There is a polarity between the two contacts that cannot be reversed.

    Active contact power supply voltages commonly used in industrial control are generally DC24V, AC220V or AC380. For example, proximity switches, photoelectric switches, and the like. In industrial control, passive contacts are used more often than active contacts because they have no polarity limitations. Dry contacts have several advantages, such as easy dry access and good interface. Handle the number of dry contact switches and so on.


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