The role of passive and active signals in industrial control

Active signals and passive signals are relative to current signals. If the sensor device has an independent working power supply, the current signal from the sensor is an active signal; for example, a four-wire electromagnetic flowmeter is powered by AC220V separately, and then outputs a DC4-20ma current signal.


If the device itself does not have an independent working power supply, the card and module need to provide loop power, and the output 4-20mA current signal is a passive signal. Some people say that the three-wire system or the four-wire system are all active signals, and the two-wire system are all passive signals. In fact, it depends on the situation. For example, the transmitter signal is transmitted to the DCS card or PLC module through the secondary instrument conversion, which is also a two-wire system. This is an active signal for DCS and PLC.

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

Let’s talk about the distinction between passive contacts and active contacts; passive contacts are commonly known as dry contacts or passive switches. Dry contacts are electrical switches that have two states of closed and open. There is no polarity interchange between the two contacts. For example, frequently used limit switches, travel switches, and foot switches. Rotate the master switch, reed switch, etc. Active contact, commonly known as wet contact or active switch, has two states: power-on and break-point. The polarity between the two contacts cannot be reversed. The active contact power supply voltage commonly used in industrial control is generally DC24V, AC220V or AC380. For example, proximity switches, photoelectric switches, etc. In industrial control, because passive contacts have no polarity restrictions, their use range is more than that of active contacts. Dry contacts have several advantages, such as easy dry access and unified interfaces. Deal with a large number of dry contact switches, etc.

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