Whether they are electromechanical or solid state, relays, solenoids, and contactors are all examples of switches. However, they all have differences and unique characteristics that make them better suited to different applications. This blog will cover all three and discuss certain considerations to make when deciding which device is best for you.
Relays are the most common types of electromechanical switches. Their primary use is to allow a low power signal to control a high-powered circuit, though they can also be used to allow multiple circuits to be controlled by a lone signal. Relays are available in a broad range of designs, including electromagnetic relays and solid-state relays. Electromagnetic relays use magnets to physically open and close a switch and regulate signals, current, or voltage, and solid-state relays use semiconductors to do the same. Solid state relays have no moving parts and therefore are more reliable and long-lasting. Additionally, unlike electromagnetic relays, solid-state relays are not subjected to electrical arcs that can cause internal wear.
Solenoids are a type of relay designed to remotely switch heavier currents than normal relays. Unlike smaller electromechanical relays, solenoids use a coil to generate a magnetic field when electricity passes through, thereby allowing it to open or close the circuit. Solenoid and relay are terms that are often used interchangeably, but actually refer to different things. Solenoid generally refers to a ‘metal can’ type relay, while relay refers to the standard ‘cube’ type relay. Common applications for solenoids include vehicle starters, winches, snowplows, and electrical motors. Solenoids’ main advantage is their ability to use a low input to generate a larger output through the coil, therefore reducing strain on the battery.
A contactor is a relay used when a circuit has to support an even heavier load than solenoids can handle. Featuring a range of voltage ratings from 12V DC to 1200V DC, contactors are a safe, lightweight, and economical solution for high-voltage DC power systems. Their applications include industrial electric motors in heavy trucks and equipment, buses, emergency vehicles, electric/hybrid vehicles, boats, light rail, mining, and any other system that requires more power than a standard relay or solenoid can handle. Contactors usually feature an integrated coil economizer to reduce the power needed to hold the contacts closed, which in turn increases the system’s flexibility and reliability. In most cases, contactors are available with optional auxiliary contacts.
Next, when deciding whether a relay, solenoid, or contactor is best for your given applications, there are three things to consider: current & form factor, ambient environment, and continuous vs. intermittent rating.
Current & Form Factor
In terms of current capacity, relays are the lowest, solenoids are in the middle, and contactors are the highest. In general, the price of each will match its current capacity. It is also important to note the form factor of each. The larger a carrying capacity a device has, the bigger it will be, so it is important to consider the space available.
It is also critical to consider the environment in which the device will be used. The device may need extra protection from humidity, submersion, dust, vibration, or a number of other factors which should be understood. Furthermore, the operating temperature will also affect the device and should be noted.
Continuous vs. Intermittent Rating
Finally, solenoids and contactors are rated for either continuous or intermittent use. Intermittent refers to applications that operate in quick bursts followed by a long rest period, such as a starter switch. Adversely, devices with a continuous rating can support applications requiring constant run time. This rating is a crucial consideration.
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