What do active contacts and passive contacts of limit switches actually mean?
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When selecting a product, one often encounters active and passive types; when looking up information online, it relates to dry contacts and wet contacts, but since I’m not an electrical engineer. Could someone explain it in simple terms that I can understand? In what situations should each form be used, and can these two forms be used together? What’s the difference? I hope you can explain it
Active devices require power to function, while passive devices can operate without power. Whether explosion protection is needed depends on the environment in which the site is located, to determine whether such protection is required and what type of explosion protection should be used.
This post was last edited by ZhiGE on 2020-4-13 at 20:29. English terms for DRY CONTACT and WET CONTACT are literally translated as dry contact and wet contact. Free translation: Passive contacts, active contacts; in my opinion, the latter is easier to understand. An active contact refers to a contact to which voltage has already been applied, so no further power supply is required. A passive contact refers to a contact to which no voltage is applied, serving only as an on/off signal. For example: To display the start and stop status of the motor on the DCS screen, it is necessary to send the signals indicating the motor’s operation and shutdown to the DCS control system. This is achieved by connecting the auxiliary contacts of the relays in the electrical cabinet to the DCS – with the contacts closed to indicate operating status and open to indicate shutdown status. These contacts must be passive; a 24VDC power supply is connected to the DCS cabinet and fed to the input side of the isolation relay, with the output side connected to the system cards. The purpose is to isolate potential high voltages introduced through the relay, thereby preventing the I/O cards from being damaged.
Assume there is a signal at the site, which is led out by two wires. Looking from the wire ends toward the signal end, if it contains a power source, it is an active signal (and if it is a contact, it is a wet contact); if it does not contain a power source, it is a passive signal (and if it is a contact, it is a dry contact). All contact signals require a power supply; the difference lies in whether the power is provided internally within the signal or externally. The same is true for analog signals.
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