What exactly do active contacts and passive contacts of a limit switch mean?
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When selecting limit switches, one often encounters the distinction between active and passive types; when searching online, this is related to dry contacts and wet contacts as well, 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 ； Mainly, it’s possible to give examples of limit switches on valves; other explanations might be hard to understand. Do passive limit switches still need to be connected to a power source?
Active types require external power supply, that is, wet contacts； Passive means it doesn’t require power supply; in other words, it’s a dry contact. Just think of it that way. There are basically two types of valve position switches nowadays. One type uses NUMUR signals to detect high and low current levels in order to determine the switch status; this type requires an isolation barrier for power supply. Another type utilizes equivalent circuits together with reed switches to achieve the same function ； Another type is the dry contact version, which only operates for on/off switching; this type can exist without an isolation barrier or power supply.
All signals require a power supply. The difference lies in whether the power supply is within the signal or outside it. In other words, when looking at the signal lead, if there is a power supply inside, it is active; if there is no power supply and external power is required, it is passive. For switch signals, active switches are called wet contacts, while passive ones are called dry contacts.
A limit switch whose output is powered is called an active or wet contact; its output does not require an external power supply, but generally a power cable is needed to feed power into such a limit switch； An output that does not carry electricity is called passive or a dry contact; such an output requires external power in order to be detected, and this power supply on the PLC or DCS is referred to as a query power source.