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How to choose between a Solenoid, Relay, or Contactor

What is the difference between a contactor and a relay? When is it appropriate to use a solenoid in automation? What does this have to do with automation? These are some of the questions you might ask yourself if you’re just starting with robotics, SCADA systems or electronics in general.

Relays, solenoids and contactors all function as switches but each one brings its own unique qualities. Let’s look at the differences between these devices, and how they should be used. These will help you determine which one is right for you, depending upon the application.

Take a look at Relays

A relay is an electric switch used to transmit low-power signals to high-power circuits. Consider a large machine, which requires a lot of power and must be controlled from a distance. You can find a thick and expensive cable that connects to the machine, then runs along the ground to an identically large switch. Although the bulky cable and switch can handle the current from whatever device is plugged into, they won’t be the most efficient.

An electromagnet could also be used to control the switch. This scenario has the advantage that the switch must be able to handle the current from the mechanism it is connected to. However, the switch doesn’t need to exert much force to turn on and off. You could connect a huge switch to a bulky wire but you could also save money and space by using electromagnetic power.

Relays are the solution. Relays allow low-powered circuits to connect to high-powered circuits through the use of magnets. They regulate signals, current, and voltage. An ignition switch is a common relay you may use every day. The ignition switch is what turns on your car’s motor, which takes a lot of power. The ignition switch that controls the motor is only about the same size as a loonie.

A relay uses a magnet wire to wrap around a bolt. Typically, the wire is made of iron. In doing so, an electric current can flow through the bolt making it magnetic. The bolt can become magnetic and activate any mechanism attached to it, such as a motor, by acting like a switch.

There are also “solid-state” relays that use semiconductors to regulate power flow. Solid-state relays are different from electromagnetic relays in that they don’t have moving parts. The switch is technically more reliable without moving parts because there’s less wear and tear.

How contractors differ

Contactors can be almost identical to relays. The only difference between them is their size and load capacities. For large jobs that require a lot of voltage, contactors work better. Relays should be used for voltages less than 10 amps. You should use a contactor for anything above 10 amps.

Relays and contactors have another important distinction: they can be in either Normally Closed or Normally Open states. These terms describe how electricity flows through a circuit. If the circuit is “open”, electricity can’t flow through it. The electricity can flow when the circuit is “closed”. Contactors are almost always designed for operation with normally open contacts.

Let’s Solenoids!

Solenoids can be considered siblings to relays and contactors. Solenoids are similar to relays, in that they have a coil wrapped about a bolt, most probably iron. They work differently to a relay.

Relays are used to move contacts and turn on or off currents. Solenoids pull and push things using the force of gravity to cause them to move in a linear manner. Solenoids are made up of a coil of magnetic material wound around a plunger which retracts and extends. The end of the plunger is usually attached to a switch or valve. You can attach a linear actuator to a solenoid. The actuator will then extend when the solenoid is turned on. You might have to push or clamp something heavy in a factory.

It is easy to distinguish between solenoids and relays by noting that an electromechanical relay might use a solenoid to perform a function it cannot. A solenoid is an electromagnetic actuator. It’s a self-contained actuator. This is best for tasks that require fast and powerful linear motion.

You can automate tasks by using all three switches together. Imagine a circuit board controlling an HVAC fan at a factory. You flip a switch (a relay), which turns on the contactor to initiate the process. Because it is able to do more heavy lifting, the contactor turns on the fan. Relays can send messages to solenoids to control the airflow by telling them to open or close valves. You can equip relays with sensors and timers, which can be used to tell actuators when they should move. This is only one example of the amazing switches that can be used in automation.

Relays vs. Solenoids. Pick Your Favorite

Although they are often interchangeable, relays, solenoids and contactors can be used in different ways. Although they may have some similarities and can all be switched, they have different uses and distinct characteristics. One might be more suitable for your needs than the other depending on how much you need and what your overall design is.

Last word

You will need to be able to distinguish between switches and understand how they are implemented in different applications. It is important to understand how the switches work in real life and how they can be combined to enhance an automated experience. Sign up for the Automation Technician training program to get this experience. For more information, contact a college today!

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