In brief
A relay is an electrically operated switch in which a small control current, via a coil or electronics, switches a separate circuit.
Contents
What is Relay?
In an electromechanical relay, an energised coil attracts an armature that actuates one or more contacts. The control circuit and load circuit are galvanically isolated. Relays are used to amplify or multiply signals, to separate voltage levels or to create interlocks. Solid-state relays switch via semiconductors without moving parts; they are wear-free and silent but generate heat losses and have leakage currents.
Important types are interface relays for coupling between controller and field devices, timing relays with on-delay or off-delay, monitoring relays for voltage, phases or level, and safety relays. Key parameters are coil voltage and type of current, contact type (normally open, normally closed, changeover), number of contacts, switching current and switching voltage, contact material, mechanical and electrical service life, and design, for example plug-in with socket or for printed circuit boards.
Relays are installed in large numbers in control cabinets. Contacts wear through arcing erosion, particularly with inductive loads without suppression circuitry. Typical faults are welded or contaminated contacts and defective coils. Plug-in relays can be replaced quickly if the socket fits.
In practice: maintenance and procurement
When sourcing a replacement, coil voltage, contact configuration, switching capacity and design or socket are decisive. For applications with frequent switching, a solid-state relay may be appropriate; in that case heat dissipation and type of load must be checked.
Synonyms and abbreviations: interface relay · control relay · solid-state relay (SSR) · plug-in relay · SSR
Frequently asked questions
What is the difference between a relay and a contactor?
Relays and contactors work on the same principle but differ in switching capacity. Relays switch smaller currents, mainly in control and signal circuits. Contactors are built for large load currents such as motor power, with more robust contacts and arc quenching. In practice, relays are often used to drive the coil of a contactor from the controller.
What do normally open, normally closed and changeover mean?
A normally open contact is open in the rest state and closes when the relay picks up. A normally closed contact is closed in the rest state and opens on pick-up. A changeover contact combines both with a common terminal and switches between two contacts. The contact configuration, for example two changeover contacts, belongs in every spare parts enquiry.
What information is needed to replace a relay?
Needed are coil voltage with type of current, number and type of contacts, maximum switching current and switching voltage, design with pin assignment or socket, and additional functions such as LED, freewheeling diode or test button. For timing relays, the time range and function are added. The complete type designation from the housing makes identification easier.
When should a solid-state relay be used instead of a mechanical relay?
Solid-state relays are suitable for very frequent or fast switching, for example in heating control, because they have no contact wear and operate silently. However, they generate heat losses, often need a heat sink and have a small leakage current in the off state. For safe isolation or infrequent switching, the mechanical relay often remains the better choice.