Relays, solenoids, and contractors are all switches—whether electro-mechanical or solid-state—but there are critical differences that make each suitable for different applications.
One of the most common electro-mechanical switches in a vehicle, the main job of a Relay Valve is to allow a low power signal (typically 40-100 amps) to control a higher-powered circuit. It can also allow multiple circuits to be controlled by one signal—for example in a police car where one switch can activate a siren and multiple warning lights at the same time.
Relays come in a host of designs, from electromagnetic relays—which use magnets to physically open and close a switch to regulate signals, current, or voltage—to solid-state, which use semiconductors to control the flow of power. Because solid-state relays have no moving parts, they are generally more reliable and have a longer service life. Unlike electromagnetic relays, solid-state relays are not subject to electrical arcs that can cause internal wear or failure.
Solenoids are a type of relay engineered to remotely switch a heavier current (typically ranging from 85-200 amps). In contrast to the smaller electromechanical cube relays, a coil is used to generate a magnetic field when electricity is passed through it, which effectively opens or closes the circuit.
The terms “solenoid” and “relay” can often be used interchangeably; however, in the automotive 77007market, the term solenoid generally refers to a "metal can" type, whereas a relay typically refers to the standard "cube" style relay.
Some common applications for solenoids include vehicle starters, winches, snowplows, and electrical motors. A primary advantage of solenoids is their ability to use a low input to generate a larger output via the coil, thus reducing strain on the battery.
The contactor is the relay to use when a circuit must support an even heavier current load (typically 100-600 amps). With voltage ratings from 12V DC up to 1200V DC, contactors are a cost-effective, safe, lightweight solution for DC high-voltage power systems.
Common applications include industrial electric motors used in heavy trucks and equipment, buses, emergency vehicles, electric/hybrid vehicles, boats, light rail, mining, and other systems that simply require too much power for a standard relay or solenoid.
Contactors typically have an integrated coil economizer to reduce the power required to hold the contacts closed, which helps increase system flexibility and reliability. They are often available with optional auxiliary contacts.
The decision to use a relay, solenoid, or contactor largely comes down to the current-carrying capacity needed, while also considering how the form factor will fit into your design footprint.
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