About the concepts of dry junction and wet junction, active and passive

Reply 1# king82050 I am also a friend of Xinjiang and I would like to support you. My understanding is that dry contacts are passive. This issue has been discussed in detail in the forum before, you can search it before asking.

I have read the original post and explained it to my colleagues this way. But my colleagues said that active is an external 24V or 220V power supply, while passive is its own internal power supply, so I am a little confused.

Definition of active and passive dry contacts and wet contacts, active and passive uses: Active, passive: For AI, if the field instrument requires the DCS card to provide it with 24V power, then the instrument is a passive signal to DCS. On the contrary, if the field instrument has already provided 24V power to it elsewhere and does not require the DCS card to provide it, the field instrument is called an active signal. Active and passive signal wiring are different. For example, in HONEYWELL's TPS, AI cards 1 and 2 are connected to passive signals, and 2 and 3 are connected to active signals, that is, no external power supply is required. In CS3000, jumper settings are required. Dry and wet contacts: Dry contact seems to be a common name for switching quantities, but in fact, it is already a standard term in the field of industrial control. The dry contact of the relay means that the power supply of the contact circuit is supplied by the external circuit. The relay only provides a "node (contact)" to connect the circuit, and "nothing else is considered."” ； Wet contacts correspond to dry contacts: In addition to providing the contacts that complete the circuit, it also provides the current required on the contact circuit. Definition of dry contact: Passive switch ； Has 2 states: closed and open ； There is no polarity between the 2 contacts and they can be interchanged ； Common dry contact signals include: 1. Various switches such as: Limit switch, travel switch, foot switch, rotary switch, temperature switch, liquid level switch, etc. ； 2. Various buttons ； 3. The output of various sensors, such as: Sensors in environmental power monitoring: Water immersion sensors, fire alarm sensors, glass breakage, vibration, smoke and condensation sensors ； 4. Output of relay and reed switch ； The definition of a wet contact is: active switch ； Has 2 states of power and no power ； There is polarity between the two contacts and cannot be reversed. ； Common wet contact signals include: 1. If the above dry contact signal is connected to the power supply, and then connected to the other pole of the power supply as the output, it will be a wet contact signal. ； In industrial control, the voltage range of commonly used wet contacts is DC0~30V, and the more standard one is DC24V. ； AC110~220V output can also be a wet contact, although this is less common ； 2. It is not a bad idea to use TTL level output as a wet contact. ； In general, TTL levels require buffered output, for example: 7407, 245, 244, etc., forming a loop with VCC, etc. ； 244 and 245 can also form a loop with gnd ； In order to drive the remote optocoupler. 3. Collector output and VCC of NPN transistor ； 4. Collector output and VCC of Darlington tube ； 5. Output of infrared reflection sensor and through-beam sensor ； Remark: HONEYWELL TPS System : DO's FTA only has active output 24V ； There are two types of analog signals. There are three terminals (1, 2, 3) on the FTA. Among them, 1 and 2 are active, and 2 and 3 are passive. The passive one is powered through the safety cable.