Dry Contact (Volt-Free)
A switch output that carries no voltage from its own supply and only closes and opens an external circuit. This lets the same output drive either a 24V DC coil or a 220V AC control line; electrical separation between the receiver and the machine control circuit is preserved.
A dry contact has two terminals, and neither of them is connected to the receiver's supply terminal. When the contact closes it provides neither a current source nor a voltage reference; it merely completes the circuit you bring to it. Its opposite is a wet or sourced output that presents a ready voltage at the output.
Its three practical consequences in the field are:
- Flexibility: while the receiver is supplied with 12/24V DC, the contact can switch a contactor coil running on 220V AC.
- Noise separation: galvanic separation remains between the control circuit and the receiver electronics; the coupling of noise from the motor side into the electronics is reduced.
- A point requiring care: the contact provides no protection. Fitting a separate fuse in the switched circuit and derating the contact current rating according to the load type is up to you.
You take advantage of exactly this property when connecting the receiver in parallel with an existing hard-wired push button installation: the dry contact is bridged across the same two points as the button's contact, and no other change to the installation is needed.
The context in which this term is used: Complete guide to IP protection classes