A control relay is one of the most common electrical relays used in industrial automation, control cabinets and machine equipment. It can switch signals, isolate PLC outputs, control auxiliary circuits and connect low-power control devices with field loads.
For many applications, buyers do not search only for “electromagnetic relay“. They may search for 24V relay, 24V DC relay, industrial relay, control relay, relay socket, 8 pin relay or plug-in relay. These terms are closely related to real purchasing and maintenance needs in control panels.
This guide explains how to choose a control relay for industrial automation equipment, with practical focus on coil voltage, contact rating, relay socket, pin layout and PLC control circuits.
What Is a Control Relay?
A control relay is an electrically operated switch used in control circuits. In industrial equipment, it is often used to switch a signal or auxiliary load rather than a large power load directly.
Control relays are commonly used for:
- PLC output isolation
- Signal conversion
- Auxiliary contact expansion
- Switching indicator lamps and alarms
- Controlling solenoid valves
- Controlling contactor coils
- Motor control interlocking
- Separating control voltage from load voltage
An electromagnetic control relay uses a coil and mechanical contacts. When the coil is energized, the contacts open or close. This simple structure makes it easy to test, replace and maintain.
24V Relay and 24V DC Relay
A 24V relay is widely used in industrial automation because many PLCs, sensors and control power supplies use 24V DC. A 24V DC relay can be connected to a PLC output or automation controller when the coil current is within the controller’s output capacity.
When choosing a 24V DC relay, check:
- PLC output type: transistor, relay or source/sink output.
- Coil voltage: usually 24V DC for modern automation panels.
- Coil current: must be within the PLC output capacity.
- Contact rating: must match the external load.
- Contact form: NO, NC, SPDT, DPDT or multi-pole.
- Mounting type: socket, DIN rail, PCB or panel mounting.
The coil side and contact side are separate. A PLC may energize a 24V DC relay coil, while the relay contacts switch another control voltage or load circuit.
Industrial Relay for Automation Equipment
An industrial relay is designed for control systems, machines and electrical cabinets. Compared with a small signal relay used only on a PCB, an industrial relay often needs stronger terminals, socket mounting, clear marking and reliable contact performance.
Industrial relays are used in:
- Packaging machines
- CNC equipment
- Conveyor systems
- Pump and fan control
- HVAC control panels
- Robot auxiliary circuits
- Alarm and indicator circuits
- Solenoid valve circuits
- Motor control circuits
For OEM equipment, an industrial relay may also require specific certifications, custom labels, special terminals or stable long-term supply.
Relay Socket and Plug-in Relay
A relay socket makes wiring and maintenance easier. In many control cabinets, the relay is not soldered directly onto a board. Instead, a plug-in relay is installed into a socket, and the socket is mounted on a DIN rail or panel.
The main advantages of a relay socket include:
- Faster relay replacement
- Cleaner control cabinet wiring
- Easier troubleshooting
- Optional LED indicator or test button
- DIN rail mounting compatibility
- Reduced downtime during maintenance
For control cabinets and automation panels, a plug-in relay with a relay socket is often more practical than a relay that requires rewiring during replacement.
8 Pin Relay and 14 Pin Relay
8 pin relays and 14 pin relays are common in industrial control applications. The pin count is related to the relay contact arrangement and socket type.
An 8 pin relay is often used for DPDT contact arrangements, depending on the relay series. A 14 pin relay is commonly used when more contact poles are required, such as 4PDT configurations.
When choosing an 8 pin relay or 14 pin relay, confirm:
- Coil voltage
- Contact form
- Socket compatibility
- Wiring diagram
- Contact rating
- Physical size
- LED indicator or test button options
The relay pin layout must match the socket and wiring diagram. Do not replace a relay only by appearance.
Relay Coil Voltage Selection
Relay coil voltage selection should start from the control power supply. The relay coil must match the voltage that energizes it.
Common coil voltages include:
- 12V DC
- 24V DC
- 48V DC
- 110V AC
- 220V AC
For PLC control panels and automation equipment, 24V DC is usually preferred. For traditional control cabinets, machine retrofits or power distribution equipment, 110V AC and 220V AC relay coils may also be used.
If the coil voltage is too low, the relay may not pull in reliably. If the coil voltage is too high, the coil may overheat and the relay life may be reduced.
Relay Contact Rating Selection
The relay contacts must be rated for the load they will switch. Contact rating usually includes voltage, current and load type.
A relay that can switch a 10A resistive load may not be suitable for a 10A inductive load. Motors, solenoid valves and contactor coils can generate inrush current and back EMF, which may damage relay contacts if the relay is not properly selected.
When checking contact rating, consider:
- Load voltage
- Load current
- AC or DC load
- Resistive or inductive load
- Inrush current
- Switching frequency
- Expected electrical life
For DC loads and inductive loads, engineers should use suitable protection components such as diodes, varistors or RC snubbers when required.
Power Relay vs Control Relay
A power relay is usually selected when the circuit needs to switch a higher current load. A control relay is usually selected for signal control, auxiliary switching, PLC isolation and control cabinet logic.
The difference is not only the name. Engineers should compare:
- Contact rating
- Load type
- Electrical life
- Coil voltage
- Terminal structure
- Mounting method
- Certification requirements
For industrial applications, a control relay may switch a contactor coil or signal circuit, while a power relay may switch a higher-current load directly if the rating is suitable.
NO, NC, SPDT and DPDT Relay Contacts
The contact form determines how the relay switches the circuit.
Normally open contacts close when the coil is energized. Normally closed contacts open when the coil is energized. Changeover contacts provide both normally open and normally closed functions.
Common contact forms include:
- SPST: One simple on/off contact.
- SPDT: One changeover contact.
- DPDT: Two changeover contacts.
- 4PDT: Four changeover contacts.
For a PLC control panel, SPDT and DPDT relays are often used because they provide flexible signal conversion. Multi-pole relays are useful when several circuits need to be switched at the same time.
DIN Rail Relay for Control Cabinets
The mounting method affects wiring, maintenance and panel layout.
Plug-in relays with sockets are common in industrial control cabinets because they are easy to replace. DIN rail relay sockets are suitable for organized cabinet wiring and fast maintenance. PCB relays are used inside electronic devices and compact control modules.
Common mounting options include:
- PCB mounting
- Plug-in socket mounting
- DIN rail mounting
- Panel mounting
For control panels that require fast maintenance, plug-in relays with sockets and indicators are usually preferred.
Relay for PLC, Motor and Solenoid Valve Control
A relay for PLC control is often used to isolate the PLC output from the load. A relay for motor control circuit is often used as an auxiliary control component. In many systems, the relay does not switch the motor directly. Instead, it switches a contactor coil, interlock signal, alarm circuit or control signal related to the motor starter.
For motor control circuits, pay attention to:
- Inductive load rating
- Coil surge from contactors
- Contact protection
- Electrical life under switching load
- Control voltage compatibility
- Panel wiring and replacement method
If the relay is used to switch a contactor coil, the relay contact rating should match the contactor coil current and voltage, not only the motor power.
Solenoid valves are common loads in automation equipment, pneumatic systems and machine tools. A relay for solenoid valve control must be selected carefully because solenoid coils are inductive loads.
When switching solenoid valves, check:
- Solenoid voltage: 24V DC, 110V AC, 220V AC or other voltage.
- Coil current and inrush current.
- Relay contact rating for inductive load.
- Surge suppression method.
- Switching frequency.
- Operating temperature inside the cabinet.
For DC solenoid coils, a flyback diode can help reduce voltage spikes. For AC solenoid coils, a varistor or RC snubber may be used depending on the circuit design.
Control Relay Selection Checklist
Before confirming a relay model, check the following:
- What is the PLC output voltage?
- Is the coil AC or DC?
- What voltage and current will the contacts switch?
- Is the load resistive or inductive?
- How many contacts are required?
- Is a socket or DIN rail base needed?
- Is an LED indicator or test button required?
- What is the expected switching frequency?
- Are certifications required?
- Is a custom relay specification needed?
This checklist helps reduce selection errors and avoids future control panel maintenance problems.
FAQ
What is a control relay?
A control relay is an electrical relay used to switch signals, auxiliary circuits or control loads in industrial equipment, PLC panels and control cabinets.
What is a 24V DC relay used for?
A 24V DC relay is commonly used in automation systems, PLC output circuits, control cabinets, signal isolation and auxiliary switching applications.
What is the difference between a power relay and a control relay?
A power relay is usually used for higher current switching. A control relay is usually used for signal switching, PLC isolation, auxiliary circuits and control cabinet logic.
What is an 8 pin relay?
An 8 pin relay is a relay with eight connection pins. It is commonly used with a matching relay socket in industrial control circuits.
Why use a relay socket?
A relay socket makes installation, wiring and replacement easier. It is widely used with plug-in relays in control cabinets.
Can a control relay work with a PLC?
Yes. A control relay can work with a PLC if the coil voltage and coil current match the PLC output specification.
Can a relay control a motor?
A relay can be used in a motor control circuit, usually to control a contactor coil or auxiliary signal. Large motors should normally be switched by a contactor, not a small control relay.
How do I choose relay contact rating?
Choose relay contact rating based on load voltage, load current, AC or DC load type, resistive or inductive load and expected switching frequency.
