A relay is an electrical switch that utilises an electromagnet in order to switch between the on and off positions, instead of a person having to move the switch. The relay itself does not require much power, but it can control devices that draw much more power, such as your car headlights or air conditioner. When buying a relay, determine whether you need an input or output relay as well as which of the four main types is the right choice for your needs.
When You Need an Input Relay
The task of an input relay is to read on and off information originating from limit switches and sensors. In case you require an input relay, you need to pay attention to the type of the sensor and the length of the signal wire as well as the input voltage of the relay and whether it is alternate or direct current, AC or DC. You also need to pay attention to the sensor type. The voltage level should match the control voltage.
When You Need an Output Relay
Output relays control loads, including contactors, motors, lightings, and magnetic valves. When choosing an output relay you need to consider the control voltage of the relay, which is its input voltage; the load voltage type, AC or DC, and level; and the maximum current of the load. Amongst output relays, the most common control voltage is 24 V DC. The maximum current should be high enough to be compatible with your vehicle.
Types of Relays
There are four main types of relays available. Electromechanical relays are versatile all-around solutions that fit many locations and have the widest use, but they have limitations in terms of their package size, mechanical lifetime, and switching speed. Reed relays provide better package size as well as density and speed, especially thanks to their less massive contacts, but are less robust in situations where you may encounter surge currents. Reed relays are common on multiplexer and matrix modules rather than on general purpose modules. Instead of electromechanical relays, you can rely on SSRs, solid state relays that use a photosensitive MOSFET device with an LED. The SSRs are faster, but not fully isolated between the contacts and come with higher path resistances. FET switches use CMOS transistors that drive the transistors directly and are therefore fast and cheap solutions, but they only work with low voltages, thus providing insufficient versatility.