In the field of industrial automation and home comfort, temperature controllers play an irreplaceable role in maintaining the desired temperature. The working principle of this controller is actually very simple: it compares the current actual temperature with the target temperature you set, and then automatically turns on or off the power of heating or cooling equipment until the two reach the same level. In this process, the role of contacts is indispensable. This article will further explore the difference between dry contacts and wet contacts in industrial temperature controllers and their specific applications.
What Is Dry Contact?
A dry contact is a passive switch with two states: closed and open. There is no polarity between the two contacts and they can be interchanged. Common dry contact signals include various switches such as outputs of limit switches, buttons, electromagnetic relays, reed switches and AC contactors, etc.
In industrial pid temperature controllers, dry contacts are usually called relays, which cannot provide voltage and current to external loads. If the heater is to be operated, an external power supply should be connected to the relay contacts.
What Is Wet Contact?
Wet contact is an active switch with two states: powered and unpowered. There is polarity between the two contacts and they cannot be reversed. Common forms of wet contacts include dry contact signals connected to the power supply, TTL level output, collector output and VCC of NPN, PNP transistors, collector output and VCC of Darlington transistor, and output of proximity sensors.
In the pid temperature controller, the SSR output is a wet contact, which can directly provide power to the solid-state relay control components. Generally, the wet contact is a 12VDC 20mA power supply, and the wet contact does not require external power supply.
Key Differences Between Dry And Wet Contacts
| Item | Dry Contacts | Wet Contacts |
|---|---|---|
|
Definition |
Passive switch with two states: closed and open |
Active switch, with two states: powered and unpowered |
|
Polarity |
No polarity, contacts are interchangeable |
Polarity, contacts cannot be interchanged |
|
Common signals |
Limit switch, travel switch, foot switch, rotary switch, temperature switch, liquid level switch, etc.; button; sensor output; relay, reed switch output |
Dry contact signal connected to power supply; TTL level output; NPN transistor collector output and VCC; Darlington tube collector output and VCC; infrared reflection sensor and through-beam sensor output |
|
Voltage range |
No voltage requirement |
Industrial control commonly uses DC0~30V, the standard is DC24V; AC110~220V output can also be wet contact |
|
Advantages |
Capable of carrying higher current; suitable for high-power devices |
More suitable for DC small loads, if dry contacts are connected to power, it can be run high-power loads as well. |
|
Security |
Since it does not carry voltage or current, the interface is highly safe and suitable for connecting sensitive components. |
Pay attention to the polarity, improper connection may damage the device |
|
Function |
It is used for signal transmission or controlling external circuits and does not directly power the load. |
It supplies power directly to the load and when closed completes the circuit and allows current to flow. |
Applications In Temperature Control Systems
Dry contacts are relay outputs in the above pid temperature controllers, and cannot directly power heaters or other intermediate devices. So if you only have intermediate relays or AC contactors, try to use these dry contacts (relay outputs). The wiring diagram is as follows for example:
Wet contacts, mainly called SSR outputs in pid temperature controllers, are outputs that can power external devices. Wet contacts are mainly used to control AC solid-state relays controlled by DC in temperature control systems. The typical wiring diagram is usually as follows:
Conclusion
In summary, dry contacts cannot directly power the load, while wet contacts can. Another important difference is that if the two contacts directly operate the load, the dry contact can operate a large load, while the wet contact can only operate a small load. As for the two contacts in the pid thermostat, the dry contact represents the relay output, and the wet contact is the solid-state relay output
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