Top Five Things you MUST know about Relays

In today’s video we are going to be discussing the top 5 things you MUST know about relays. Relays are an essential tool for so many common electrical circuits that we use on a daily basis.

It is important that we have a good understanding of relays and why they are so critical, and how they can help us. The things we will talk about today should help you better understand relays, and some of things that we must know when using them.

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1. Voltage matters. When using relays, it is very important that we consider that voltage that we are using. This applies to both the coil and the contact side of the relay. Most relays are identified using the voltage the coil is rated for. So… if we have a 12v relay, this typically means that that particular relay’s coil is rated for 12V DC. The contact side of the relay will be rated for an entirely different voltage, it is very common for the contact side of the relay to be rated up to 250V AC, but it can commonly switch DC voltage as well. It is also important to note that AC and DC coils are different, and are NOT interchangeable. So… when using relays, pay attention to the voltage on both the coil.. and contact side of the relay and make sure you are supplying the correct amounts of voltage to that relay.

2. Normally Open and Normally Closed… know the difference. Most relays have either a NO contact and NC closed contact however, some relays will only have one or the other. It is important to understand the difference between the two so that the electrical circuit you are controlling functions as you intend it to. If you don’t fully understand how NO and NC contacts function within a relay, please reference our other video in which we cover this in great detail. It is most common to use the NO contact on a relay, however most circuits can function using one or the other, just depending on how it has been set up. So again, know the difference… it can help determine whether an electrical circuit will function as planned, or not.

3. A relay is not a contactor, don’t get confused. Although relays and contactors function in much the same way, they are NOT the same. Contactors are generally used in scenarios where large amounts of voltage and current need to be switched. Whereas with relays, you don’t typically switch more than 15 amps and 120V. Use the right device for the right scenarios and you will avoid problems. Think of it this way. Relays = low voltage… and Contactors=high voltage.

4. A little power goes a long way. It is important to remember why we use relays in most cases. They allow us to control or switch higher voltage circuits, with lower voltage control signals. This allows you to isolate yourself and your more sensitive control equipment from your higher voltage circuits (like maybe a motor for instance) Relays are a cheap and easy way to remotely turn and off these types of circuits. When in doubt, use a relay to switch higher voltage.

5. Relays can be used both ways. Although it is most common to see relays being used to switch higher voltage circuits, this is not the only way we can use relays. Another very common use for relays it to monitor the “status” of certain devices, such as a motor. To do this, you would wire the relay so that the coil would become energized when the motor is running. By doing this, you could send a low voltage control signal back to your PLC to inform you that the motor is in fact running. This is very helpful when we want to be able to monitor our electrical circuits, and can be used in many different types of applications. So remember, relays can be used in more than one way.

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- How to Wire a Relay:    • How to Wire a Relay  
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