The difference between dry contacts and wet contacts in the field of industrial control

I searched for some information. Basically as follows: Definition of a dry contact: a passive switch ； Has two states: closed and open ； There is no polarity between the two contacts, so they can be swapped ； Common dry contact signals include: 1. Various switches such as limit switches, travel switches, foot switches, rotary switches, temperature switches, level switches, etc ； 2. Various buttons ； 3. Outputs from various sensors, such as those used in environmental condition monitoring: water intrusion sensors, fire alarm sensors, glass breakage sensors, vibration sensors, smoke sensors, and condensation sensors ； 4. Outputs of relays and reed switches ； Where there are dry contacts, there are also wet contacts. The definition of a wet contact is: an active switch ； It has two states: powered and unpowered ； There is polarity between the two contacts; they cannot be connected in reverse ； Common wet contact signals include: 1. If the above dry contact signals are connected to a power supply, and the other terminal of the power supply is used as the output, then that constitutes a wet contact signal ； In industrial control, the common voltage range for wet contacts is DC0–30V, with DC24V being the more standard value ； The AC110～220V output can also be a wet contact, although this is less common ； 2. It is also acceptable to use the TTL level output as a wet contact ； Under normal circumstances, TTL levels require buffered outputs, such as 7407, 245, 244, etc., which form a circuit with VCC and other components ； 244 and 245 can also form a circuit with gnd ； To drive the optocoupler at a distance. 3. Collector output of the NPN transistor and VCC ； 4. Collector output of the Darlington transistor and VCC ； 5. Outputs of infrared reflectance sensors and through-beam sensors ； In the field of industrial control, dry contacts are used far more often than wet contacts, thanks to the advantages associated with their lack of polarity: 1. They can be connected arbitrarily, which reduces engineering costs and the requirements for skilled personnel, thereby accelerating project completion. 2. It is possible to handle a large number of dry contact switches. 3. Even if the wires connecting the dry contacts become short-circuited over time, this will not damage either the local control equipment or the equipment located at a distance. 4. They are easy to connect, and their interfaces are easy to standardize. In other words, dry contacts are passive switches ； Has two states: closed and open ； There is no polarity between the two contacts, so they can be swapped ； Wet contact: active switch ； It has two states: powered and unpowered ； There is polarity between the two contacts, and they cannot be connected in reverse order. Dry contacts are the opposite of wet contacts; a wet contact consists of a switch along with a power supply, and when it is closed, there is a certain voltage present, without the need for a detection circuit to supply current. When the dry contact is closed, there is no voltage; the current is supplied by the detection circuit. By dry contact, we usually mean a passive contact, that is, a contact that does not provide its own power supply; in such cases, the power supply comes from an external source. There are dry contacts and wet contacts, and a dry contact refers to a pure switch, just like a relay contact or a manual switch. The two states are “on” and “off”. A wet contact refers to a dry contact with the power supply connected in series. The two states are “voltage present” and “high-impedance state”. Definitions, application scenarios, and adjustment methods for dry contacts and wet contacts. Dry contact seems to be a colloquial term, but in fact, it has become a standard term in the field of industrial control. Methods for conditioning dry contacts and wet contacts: Use optical isolation via optocouplers. For connecting dry contacts to an optocoupler, the sequence is as follows: VCC → current-limiting resistor → optocoupler LED → Dry Contact 1 → Dry Contact 2 → GND. An alternative sequence is: VCC → Dry Contact 1 → Dry Contact 2 → current-limiting resistor → optocoupler LED → GND. For connecting wet contacts to an optocoupler, the sequence is: Wet Contact 1 → current-limiting resistor → optocoupler LED → Wet Contact 2. If the wet contacts are powered by a DC source, reversing the order of Wet Contact 1 and Wet Contact 2 will obviously prevent the optocoupler from functioning ； If the wet contact is connected to an AC power source, it can be reversed ； This post was last edited by shanzai on 2007-12-28 08:43]

Very comprehensive,:handshake

I didn’t quite understand it before, but now I do.:loveliness: