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Signals and PLC · Glossary

What is a Dry Contact?

potential free contactSignals and PLC

A dry contact is a switch or relay contact with no voltage of its own; the receiving circuit supplies the voltage.

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Dry Contact Explained

Also called a potential free contact, it simply opens or closes. It can feed sinking or sourcing inputs and isolates the two systems.

Low current circuits may need gold plated contacts, because ordinary contacts can build up oxide and fail to conduct small currents.

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Calculators for Dry Contact

CalculatorPLC IO Count CalculatorFree online calculator with formula and worked example.

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Free downloadIO List Template in Excel with Automatic I/O Count and SparesFree download in Excel and PDF.

Frequently Asked Questions

What is a Dry Contact?

A dry contact is a switch or relay contact with no voltage of its own; the receiving circuit supplies the voltage.

What is a wet contact?

A contact that provides its own voltage to the receiving circuit.

NO or NC for alarms?

Normally closed contacts that open on alarm make a broken wire look like an alarm, which is safer.

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