Glossary · Electrical engineering and control panels
Relay output
Also known as: Relay contact output
German: Relaisausgang
In industrial control, a relay output is a digital output channel in which a relay contact switches the load circuit, providing galvanic isolation and allowing AC or DC loads of different voltages.
- Control panels
In one sentence
A relay output switches the load with a relay contact, providing isolation and handling AC or DC loads of different voltages.
Example
A relay output of the controller switches a 230 V AC horn in the plant hall, isolated from the 24 V control circuit.
How it applies
- Engineering: Relay outputs are flexible but slower than transistor outputs and have limited electrical life. They are unsuited to high switching frequencies, such as PWM. Check contact rating for the load type, especially inductive and lamp loads.
- Maintenance: Frequent switching wears contacts; a relay output that switches many times per minute may need a solid-state alternative.
- Documentation: State contact type, rating and common terminals in the I/O documentation. Groups of relay outputs often share a common terminal, which matters for wiring different voltages.
Relay output vs. transistor output
A Transistor output switches DC loads quickly and without wear but without galvanic isolation between channels in many designs. A relay output switches AC and DC with isolation but more slowly and with mechanical wear.