What Is a Relay? Types and Selection Guide
A relay is a switch that opens and closes the contacts of a separate circuit with a low-power control signal. Types, coil voltage and contact selection are covered here.
What a relay is and where it is used
A relay is an electrically operated switch that opens and closes the contacts of a separate circuit using a low-power control signal. In an electromechanical relay, when the coil is energized the armature is pulled in and the connected contacts change position; when the coil is released, a spring returns the armature to its original position. A solid state relay (SSR) does the same job with a semiconductor and no moving parts.
In automation, a relay is chosen for three jobs: isolating a PLC output from a larger load, multiplying one signal across several contacts, and electrically isolating the control circuit from the load circuit. The products on this page are relays that switch signals on panels and PCBs. For function relays such as time, photocell or safety relays, see the Control Relays category.
Types of relays
- General-Purpose Relays: miniature general-purpose relays, including PCB types.
- Industrial Relays: industrial relays with slim and standard housings.
- PLC PCB Relays: PLC interface relays and optocouplers.
- Solid State Relays (SSR): single-phase SSRs, with DC or AC input and output options.
- Step and Impulse Relays: impulse relays and their accessories.
- Relay Boards and Relay Sockets and Accessories: sockets, jumpers, separators and SSR heat sinks.
What to look at when choosing a relay
- Coil (input) voltage: It must match the control voltage, such as 24 V DC, 48 V DC or 230 V AC. For an SSR, the input range (for example 3-32 V DC) must suit the control output.
- Load type: Is AC or DC being switched, and is the load resistive or inductive, such as a motor or a coil? The contact current and voltage values are read from the manufacturer data according to this load.
- Contact and pole arrangement: NO, NC or changeover contact and the number of poles are determined by the circuit.
- Relay type: An SSR has no mechanical wear; however, its output element heats up, and a heat sink or current derating may be needed depending on the load current. For an electromechanical relay, the contact life and load category are checked from the manufacturer data.
- Mounting: Is it socketed, soldered to a PCB, or mounted on a panel or DIN rail? Sockets and accessories are specific to the relay family; they are matched by relay code.
Difference between a relay and a contactor
A relay and a contactor work in a similar way. A contactor is built for higher current ratings than a standard relay and typically switches loads such as motors and lighting; a relay switches signal and auxiliary circuits. A thermal overload relay, which provides motor protection, is not an automation relay.
Brands in this category
- Finder and Molwex: miniature general-purpose, PCB and socketed PLC relays.
- Schneider Electric: RXM miniature relays and RXG interface relays.
- ABB: impulse relays and their accessories.
- Siemens: coupling relays and spare relays.
- Isıso, Enda, Soljex and Tense: SSRs; Isıso also lists SSR heat sinks.
Order notes
Products are sold per piece. Some products are listed as "made to order"; for these, the delivery time is confirmed before the order. You can search for the relay you want by brand code or stock code.
Automation relay comparison table
| Relay Type | Contact Configuration | Switching Feature | Application Area | Product Group |
|---|---|---|---|---|
| Slim interface relay | 1 CO (6 A) | 6.2 mm width with DIN-rail socket | PLC digital output isolation and high-density panel wiring | Slim Relays |
| Plug-in industrial relay | 2 CO / 3 CO / 4 CO (7 A – 16 A) | Lockable test button, LED, and mechanical flag indicator | Control logic, interlocking, and multi-contact signal multiplication | Industrial Relays |
| Solid state relay (SSR) | Semiconductor (Triac / MOSFET) | Silent, arc-free, high-frequency switching | PID heater control and rapidly cycling resistive/inductive loads | Solid State Relays |
Frequently asked questions
Is a relay the same as a contactor?
Their operating principles are similar, but their areas of use differ. A contactor is built for higher current than a standard relay and typically switches motor and lighting loads; a relay switches signal and auxiliary circuits. When selecting, the contact current, voltage and load category should be compared according to the manufacturer data.
What is the difference between an electromechanical relay and a solid state relay (SSR)?
In an electromechanical relay, the coil pulls in an armature and the contacts move mechanically. An SSR has no mechanical contact; the load is switched by a semiconductor such as a thyristor or TRIAC, and the input is usually isolated from the load by an optocoupler. Heat is generated at the SSR output; the need for a heat sink and current derating should be assessed according to the manufacturer data.
How is the relay coil voltage selected?
The coil voltage must match the type and value of the control voltage driving the relay. For example, a relay with a 24 V DC coil is looked for with a 24 V DC PLC output, and a relay with a 230 V AC coil with 230 V AC control. The coil type and operating voltage range should be confirmed according to the project and the manufacturer data.
What do NO, NC and changeover relay contacts mean?
An NO (normally open) contact closes when the coil is energized, and an NC (normally closed) contact opens when the coil is energized. In a changeover contact, the common terminal is connected to the NC terminal when the coil is deenergized and to the NO terminal when it is energized. The number of poles indicates the number of independent contact circuits switched at the same time.
What are a relay socket and a jumper bar used for?
A relay is mounted with a socket; the socket allows the relay to be plugged in and removed, and the mounting style (DIN rail, panel) is one of the selection criteria it determines. Accessories such as jumpers and separators are used in the terminal wiring layout of the socket. Sockets and accessories are specific to the relay family; suitability should be confirmed from the manufacturer data.