Current Monitoring Relays

Voltage matters. Most relays are identified using the voltage the coil is rated for. 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 (or DC). It is also important to note that AC and DC coils are different, and are NOT interchangeable.

Know the difference between Normally Open (NO) and Normally Closed (NC). Most relays have either a NO contact or a NC contact. It is most common to use the NO contact on a relay, however most circuits can function using one or the other depending on how it has been set up.

A relay is not a contactor. 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 relays don’t typically switch more than 15 amps and 120V.

A little power goes a long way. Relays switch higher voltage circuits with lower voltage control signals. This allows isolation of more sensitive control equipment from higher voltage circuits. Relays are a cheap and easy way to remotely turn and off these types of circuits.

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 they can be used. Another use for relays is to monitor the “status” of certain devices, such as a motor. To do this, the relay would be wired so that the coil becomes energized when the motor is running. By doing this, a low voltage control signal is sent back to a PLC to signal that the motor is running.