Control Devices: Types, Functions, and How to Select Them

A control device is a hardware component that starts, stops, or regulates the operation of a machine, circuit, or process in response to an input signal. Common examples include switches, relays, contactors, thermostats, timers, valves, and programmable logic controllers (PLCs). Each one acts on a command from a controller or sensor to switch power, regulate a process variable, or trigger an action.

This guide defines what control devices are, separates the device from the larger control system it sits inside, details the main types with their selection specs, and names the IEC and NEMA standards that govern them.

Control device vs. control system: the key distinction

These two terms are often used interchangeably, but they are not the same thing. Getting the distinction right is the foundation of correct selection.

A control device is a single component, such as a relay or a contactor. A control system is the complete arrangement of devices, logic, and feedback that operates a process. The device is the part; the system is the whole.

Control systems come in two basic forms. An open-loop system acts on a command without checking the result, like a timer that runs a pump for a set duration. A closed-loop system uses feedback to compare the actual output against the target and correct it, like a motor drive holding a precise speed regardless of load.

A practical example ties it together. On a conveyor line, a sensor detects a part, signals a PLC, and the PLC energizes a contactor that starts the motor. The contactor is the control device. The sensor, PLC, contactor, and motor together form the control system.

Functions of control devices

Control devices do more than switch signals on and off. Their core functions span five areas that keep an automated process running safely and accurately.

• Starting or stopping operations in response to user input or an environmental trigger
• Regulating process variables such as speed, temperature, flow, or voltage
• Monitoring and maintaining stability across mechanical and electrical systems
• Coordinating responses across subsystems for cohesive performance
• Providing feedback for system optimization and error correction

The right device depends on which of these functions dominates your application. A motor starter prioritizes switching; a thermostat prioritizes regulation; a PLC coordinates all five.

Types of control devices

Control devices range from a simple manual switch to a logic-driven PLC. The table below summarizes the main types, the defining spec to select on, and where to source each at eINDUSTRIFY. The sections beneath add the detail.

Switches

A switch is the most basic control device. It opens or closes an electrical circuit, either manually or automatically in response to a physical condition. Specify a switch by its pole and throw configuration (SPST, SPDT, DPDT) and whether its contacts are normally open (NO) or normally closed (NC).

Common industrial types include toggle, push-button, limit, and float switches. Limit switches detect the position of a moving part, while float switches respond to liquid level. These handle machinery, lighting, pumps, and motor control.

Relays

A relay uses an electromagnetic coil to let a low-power control signal switch a separate, higher-power circuit. There are two contact states, normally open (NO) and normally closed (NC), and many relays use a double-throw configuration that provides both an NO and an NC output from a single contact. This isolation between control and load circuits is what makes relays essential to automation and protection.

Relays come in several forms. Electromechanical relays (EMRs) use an electromagnet to move the contacts, solid-state relays (SSRs) switch with no moving parts using semiconductors for longer life and faster switching, reed relays use contacts in a sealed glass tube, and time-delay relays add a delay before switching. Source these in the eINDUSTRIFY Control Devices category.

Contactors

A contactor is a heavy-duty relay built to switch high-power loads. The dividing line is current: contactors generally refer to devices switching more than 15 amperes or circuits rated more than a few kilowatts, and are typically used to control electric motors, lighting, heating, and capacitor banks. Unlike most relays, contactors include arc-suppression features to safely interrupt high currents.

Selecting a contactor on amperage alone is a common and costly mistake. It must be matched to its utilization category (covered in the standards section below), pole count, and coil voltage. Contactors anchor motor control centers, heavy machinery, and industrial automation panels.

Relay vs. contactor: how to choose

This is the most common point of confusion in control-device selection, so it is worth isolating. Both are electrically operated switches, but they serve different power levels and duties.

Relays are designed for lower-power control, typically milliamps to a few amps, while contactors handle higher current loads from several amps to several hundred or thousand amps for heavy-duty industrial applications such as motors and machinery. The rule of thumb: use a relay to switch a signal or a small load, and a contactor to switch a motor or a heavy load. They are not interchangeable.

Thermostats, timers, valves, and PLCs

Four more device classes round out most control panels. Thermostats open or close contacts based on a temperature setpoint, holding HVAC, refrigeration, and incubator systems in range. Timers delay or sequence operations by duration rather than sensor input, automating lighting, irrigation, and batching.

Valves and actuators regulate fluid flow by opening or closing passages, either manually or under electric or pneumatic actuation, across water treatment, oil and gas, and chemical processing. PLCs are the logic engine of industrial automation, processing multiple inputs and outputs to run programmed logic across motor starters, sensors, and drives on assembly lines, packaging, and robotics.

Sensors as control triggers

Sensors are not always control devices in the switching sense, but they drive the decisions control devices act on. They detect variables such as light, temperature, pressure, or proximity, then send that data to a controller. The controller applies logic and commands the appropriate device. Source measurement devices in Sensing Devices and Instrumentation.

Standards and ratings that govern control devices

Specifying a control device against the correct standard is what separates a reliable panel from a failure-prone one. These are the standards buyers should name on a purchase order.

The most important concept here is the utilization category. Under IEC 60947-4-1, contactors are rated by how they operate: AC-1 for non-inductive or lightly inductive loads such as resistive heaters, AC-3 for squirrel-cage motors with normal starting and stopping, and AC-4 for inching, plugging, or frequent reversing.

This matters because the same contactor carries very different ratings by category. A contactor rated 25 A at AC-1 may be rated only 9 A at AC-3, because motor loads draw 6 to 7 times their full-load current as inrush on starting, causing far greater contact wear. Using an AC-1 rated contactor to switch a motor will cause premature failure. Always select on the utilization category for your actual load, not the headline amperage. 

Two regional approaches coexist. NEMA contactors are designed to NEMA ICS 2 and classified by NEMA Size (00 through 9), intentionally overbuilt and forgiving when operating conditions are uncertain, while IEC contactors are sized precisely to the utilization category for compact, application-specific performance. For North American projects, UL approval (UL 508 / 508A) is typically mandatory and often required by code under NFPA 79 and the NEC. Where the device sits in a harsh environment, specify the appropriate NEMA or IP enclosure rating as well.

How to select the right control device

Work through these steps to specify correctly the first time.

• Define the load and duty. Identify what you are switching (signal, resistive load, or motor) and how often. Motor duty drives you to a contactor with the correct AC-3 or AC-4 rating; signal switching points to a relay.
• Match the device type to the function. Use the type table above: switch for manual control, relay for signal isolation, contactor for power, timer for time-based sequencing, PLC for multi-input logic.
• Confirm the electrical specs. Verify coil/control voltage, contact or operational voltage, pole count, and contact configuration (NO/NC).
• Apply the right standard and rating. Select contactors by IEC 60947 utilization category or NEMA size, and confirm UL listing where code requires it.
• Specify the enclosure. Match the NEMA or IP rating to the environment, from IP20 open-panel use to IP65 for wash-down or outdoor installations.

These selection criteria, responsiveness, reliability, scalability, compatibility, and durability, all flow from getting these five steps right.

Real-world applications of control systems

The same device classes appear in very different configurations across industry. Here is how they combine in practice.

• Manufacturing assembly lines. PLCs, contactors, relays, and limit switches work together to start motors, sequence stations, and maintain quality. This is the densest control-device environment, tied to the Automotive and Electronics industry hubs.
• Motor control centers. Contactors paired with overload relays that trip on sustained overcurrent and control switches start, stop, and protect industrial motors, the core of Power Generation and heavy-industry panels.
• HVAC systems. Thermostats, dampers, and contactors switching compressor and fan loads regulate airflow, temperature, and humidity, drawing on the HVAC & Refrigeration category.
• Access and building control. Keypads, electronic locks, and motion sensors managed by logic controllers govern entry to secure areas, a lighter-duty but common control-device application.

Benefits of selecting the right control device

Correct device selection pays back across operations. Properly rated contactors and relays cut unplanned downtime by avoiding the welded contacts and premature failure that come from under sizing.

Matching the utilization category to the load extends device life and protects the motors and equipment downstream. Standard-compliant, UL-listed devices keep panels code-compliant and inspection-ready. And specifying the right enclosure rating prevents environmental failures in wash-down or outdoor service.

Frequently asked questions

What is a control device?

A control device is a hardware component that starts, stops, or regulates a machine, circuit, or process in response to an input signal. Switches, relays, contactors, thermostats, timers, valves, and PLCs are all control devices. Each acts on a command to switch power or adjust a process variable.

What is the difference between a control device and a control system?

A control device is a single component, such as a relay. A control system is the complete arrangement of devices, logic, and feedback that operates a process. The device is one part of the larger system.

What is the difference between a relay and a contactor?

A relay switches low-power control and signal circuits, typically milliamps to a few amps. A contactor switches high-power loads, generally above 15 amps, such as motors and heaters, and includes arc suppression. Use a relay for signals and a contactor for power; they are not interchangeable.

What is a contactor utilization category?

Defined in IEC 60947-4-1, the utilization category describes the load and duty a contactor is rated for. AC-1 covers resistive loads, AC-3 covers normal motor starting and stopping, and AC-4 covers severe inching and reversing duty. The same contactor carries a lower amp rating in AC-3 than in AC-1.

What is the difference between open-loop and closed-loop control?

An open-loop system acts on a command without checking the result, like a timer running a pump for a set time. A closed-loop system uses feedback to compare actual output against the target and correct it, like a drive holding a motor at a precise speed.

What standards apply to control devices?

IEC 60947 governs contactors, relays, and switches globally; NEMA ICS 2 covers North American contactor sizing; and UL 508A covers industrial control panels and is often code-required. Enclosure protection is specified by NEMA or IP ratings.

Source control devices through eINDUSTRIFY

eINDUSTRIFY is a premier global B2B marketplace for industrial supplies, connecting procurement, MRO, and plant teams with vetted suppliers across the full control-device range. Every seller is vetted, so you source genuine, standard-compliant switches, relays, contactors, and controllers, not gray-market stock, with the ability to compare brands and ratings in one place.

Browse the Control Devices and Industrial Control category for switches, relays, contactors, timers, and PLCs, plus related HVAC & Refrigeration and Valves categories. For bulk orders, hard-to-find devices, or full panel sourcing, submit an RFQ and our team will match you to the right suppliers with fast price comparison. Call +1-888-774-7632 or email info@eindustrify.com to get started.

Tags: industrial control devices relay and contactor selection IEC 60947 standards motor control components control panel hardware