What’s the difference between a relay and a contactor? When should you use a solenoid? And how do they relate to automation? These are just a few questions you might be asking yourself if you’re just getting started with robots, SCADA systems, or electronics in general.
While relays, solenoids, and contactors are all switches, they each bring distinct qualities to the table. Let’s examine the key differences between these devices and when they should be used. This will help you decide which one will work for you, depending on the type of application.
A Look At Relays
A relay is an electric switch that is used to send low power signals to high power circuits. Imagine that you have a large machine that requires extensive power and needs to be controlled from a substantial distance. One option is to find a very thick, very expensive cable that attaches to the machine and runs along the ground to an equally large switch. While the large cable and bulky switch will be able to handle the current of whatever is plugged in, it won’t be the most efficient choice.
Another option would be to employ an electromagnet to control the switch. The great thing about this scenario is that, while the switch needs to be able to handle the current of whatever mechanism it’s connected to, it doesn’t require that much force to turn off and on. So while you could use a massive switch connected to bulky wire, you could save space and money by employing electromagnetic power.
That’s where relays come in. Relays enable low-powered circuits to connect to high-powered circuits by employing the use of magnets to regulate signals, current, or voltage. A common example of a relay that you might use every day is an ignition switch in your car. The ignition switch turns on the motor in your car, which requires a ton of power. But the ignition switch that controls it is about the size of a loonie!
In a relay, a magnetic wire wraps around a bolt, typically iron, and in doing so, electric current can run through the bolt causing it to become magnetic. When the bolt is magnetic, it mimics a switch and can activate the mechanism it’s attached to, such as a motor.
You can also have “solid-state” relays, which use semiconductors to control power flow. The difference between solid-state relays and electromagnetic relays is that the former doesn’t use moving parts. Without the use of moving parts, the switch can technically be considered more reliable since there’s less concern about wear and tear.
How Contactors Differ
Contactors are very similar to relays, if not almost identical. Their most obvious difference is size and load capacity. Contactors are better for larger jobs that require lots of voltage. A good rule of thumb is: 10 amps or less, you should use a relay. Anything above that, you should use a contactor.
Another important difference between relays and contactors is that relays can be in two different states, Normally Closed and Normally Open. These technical terms refer to how electricity flows through a circuit. When “open,” the circuit is broken and electricity cannot flow through it. When “closed,” the electricity can flow. Contactors are almost always designed to operate with normally open contacts.
Lastly, Let’s Solenoids
If contactors and relays are siblings, solenoids are their cousins. Solenoids look similar to relays in that they have a coil wrapped around a bolt, most likely iron. However, they function quite differently from a relay.
Relays move contacts to turn on and off currents. Solenoids push and pull things, using the force to move in a linear fashion. Solenoids consist of a wound coil with magnetic material and a plunger that retracts and extends. Usually, the end of the plunger is attached to a valve or a switch. For example, you can set up a linear actuator to a solenoid, and when it’s turned on, the actuator can extend. Think about how in a factory, you might need to clamp or push something heavy.
A good way to differentiate between relays and solenoids is by remembering this: An electromechanical relay may use a solenoid as part of its design to complete a function that it cannot do on its own. A solenoid itself is an electromagnetic actuator, a self-contained actuator. It’s best used for tasks that require a quick and powerful linear motion.
When all three switches are used together for a single purpose, you can automate tasks very efficiently! For example, imagine a circuit board that controls an industrial HVAC fan in a factory. To initiate the process, you flip a switch (a relay) which then turns on a contactor. The contactor, since it can do more heavy lifting, turns on the fan. The relays can also send a message to the solenoids to open up or close certain valves, controlling the airflow. Relays can also be outfitted with timers and sensors, so they can tell the actuators to move at certain times of the day. This is just one example of how these incredible switches can work together within automation.
Relays vs. Solenoids vs. Contactors. Take Your Pick
Relays, solenoids, and contactors are sometimes used interchangeably, but they’re not the same thing. While they have similarities, and they are all in fact switched, they’re used in different applications and possess distinct qualities. Depending on the current carrying capacity needed and your overall design, one might suit your needs much better than another.
As an Automation Technician, you’ll need to understand the differences between these switches and how they’re implemented into different applications. It’s also important to see how these switches operate in real-life and how they can be used together to improve an automated experience. One of the best ways to gain this experience is to sign up for an Automation Technician Training program. Reach out to a college today to gain more information!
