A dry contact is a simple electrical interface that does not carry voltage or current but is used to control the flow of electricity in various applications, serving as a switch or a relay.
A dry contact is generally called a “dry contact closure” or ‘the potential free contact.’ These terms are used in electrical and electronic systems.
What are Potential-free contact and Dry contacts?
A potential free contact is another term for the same concept. In potential free contact or dry contact, there is no voltage or potential on the contact. It means that it is electrically isolated from the rest of the circuit. When potential free contact closes, it completes a circuit without any voltage or potential.
A dry contact, upon closing, allows electrical current to flow through it. In other words, dry contact describes a pair of electrical terminals that do not supply electrical voltage or current but are instead used to control the flow of electrical current between other devices.
Before going into the details of dry contact, let us briefly understand wet contact.
Dry Contact – on terminals X and Y, no voltage will be measured in the open and closed states of the relay.
Wet Contact – between terminals A and B, two voltages will be measured, 0 and V, in the open and closed states of the relay contacts.
Generally, the controller or PLC has a wet contact with the same voltage level as that PLC supply. The external loads that are controlled through PLC are not driven by the PLC wet contact, and dry contacts are generated with the use of the relays. The relay contacts act as dry contacts. The dry contacts supply voltage is not taken from PLC; it is taken from the other source, and thus, the PLC output gets electrically isolated from the actual load.
Characteristics of Dry Contacts
- No Voltage or Current. Dry contacts do not carry any voltage or current, unlike typical electrical connections in a power outlet or a battery. They are electrically isolated from the circuit.
- Dry contacts are helpful in control functions, and they are primarily used to control the operation of other devices, such as switches (limit switches, proximity switches), relays, or electronic circuits. When the dry contact is closed (connected), the current flows, activating or deactivating the controlled device.
- Dry contacts have many common uses. Dry contacts are commonly found in applications where low-voltage or signal-level control is needed, such as in home automation systems, industrial control systems, and alarm systems. They are often used for interfacing between different systems or devices.
- There are so many examples related to home automation. A classic example of a dry contact is a light switch in home automation systems. When we close a switch, it is nothing but closing a dry contact, allowing electricity to flow to the light bulb and turning it on. Similarly, the doorbell button is another example of a dry contact; pressing it closes the contact, sending a signal to ring the doorbell.
- Dry contacts are flexible because they interface between devices with different voltage levels or electrical characteristics. This makes them suitable for various applications where electrical isolation is crucial.
- Dry contacts are known for their reliability and durability. This is because there are no voltages or current fluctuations across the contacts when they open or close. This minimizes wear and tear, making them suitable for applications where durability is essential.
Examples of Dry contact applications
- Industrial automation: In industrial settings, dry contacts are commonly used in control panels to manage the operation of various machines and processes. For example, they can start or stop conveyor belts, pumps, or motors.
- HVAC Systems: In heating, ventilation, and air conditioning systems, a thermostat with dry contacts can signal the HVAC system to turn on or off.
- Security Systems: Used to trigger an alarm in the monitoring station when the door opens.
- Fire Alarm Systems
- Elevator controls
Advantage of Dry Contact
The dry contact isolates the two circuits, and the problem in one part of the circuit does not affect the other parts. If the problem is in the dry contact circuit, it does not affect the wet contact circuit and vice versa.
Where we use Dry Contacts
Dry contacts are mainly used in distribution circuits of extra-low voltage (less than 50 V AC).