Solenoid and Power Relays

Coil voltage and contact rating are two separate choices

A relay has two electrical sides. The coil is the control side, and its voltage and type, such as 24 V DC or 120 V AC, must match the signal that drives it. The contacts are the load side, rated in amps at a stated voltage, and they must exceed the load's current. A 4-pole contactor with a 120 V AC coil, for example, switches four separate circuits when the coil is energized.

Contact ratings depend on the load. A relay rated 12 A for resistive loads may be rated lower for motors, lamps with high inrush or inductive loads, and lower again on DC. Look for the AC and DC ratings separately and the inductive or motor rating if that applies. A contactor is built for frequent, heavy switching of motors and heaters, while a small control relay is for signals and light loads.

Bases, indicators and timer functions

Plug-in relays sit in a socket or base, and the base must match the relay's pin layout, such as 8-pin or 11-pin, and its mounting, panel or DIN rail. Relays with an LED or mechanical indicator show when they are energized, which helps fault finding, and test buttons let you operate them by hand. Pick the base and any retaining clip for the same relay family.

Timer relays add a delay function. A multifunction timer relay with a supply range such as 24 to 240 V AC/DC lets one unit serve many systems, and you choose a function such as on-delay, off-delay or interval. Check the time range, the contact type and the supply voltage, and compare the wiring diagram with the terminal markings before you connect anything.

Questions about Solenoid and Power Relays

Can a relay switch a motor?

Only if its contact rating covers the motor's running and starting current. Motors draw several times their running current at start-up, so look for a motor or horsepower rating, or use a contactor sized for the motor. A small control relay is not suitable for a large motor.

Can I use a DC coil relay on an AC signal?

No. The coil type must match the control voltage: a DC coil needs DC and an AC coil needs AC. Some relays accept both within a stated range, such as 24 to 240 V AC/DC. Check the coil voltage on the label.

What does 4-pole mean on a contactor?

It means four sets of switching contacts that operate together when the coil is energized, so one contactor can switch four separate circuits or all conductors of a three-phase and neutral supply. Check how many are normally open and whether auxiliary contacts are included.