Understanding relays and timers

Relays are electrically operated switches that allow one electrical circuit to control one or more other circuits by opening and closing its contacts in response to energizing or de-energizing its coil. 

Relays are widely used to switch starting coils, heating elements, pilot lights and audible alarms. In addition to dishwashers, refrigerators, heating and air conditioning systems, relays control the operation of machine tools, industrial assembly lines and commercial equipment.  

Protective relays can prevent equipment damage by detecting electrical abnormalities, including overcurrent, undercurrent, overloads and reverse currents.

Relays are either electromechanical (EMR) or solid-state (SSR).  

General-purpose relays are electromechanical switches that are usually operated by a magnetic coil. They operate with AC or DC, at common voltages such as 12 V, 24 V, 48 V, 120 V and 230 V, and can control currents ranging from 2 A to 30 A.  

These relays are economical, easy to replace and allow a wide range of switch configurations. Examples include interposing (between programmable logic controllers and large load) and simple logic circuits.

Electromechanical relay components

• Frame – Contains and supports the parts of the relay
  
• Coil – Wire is wound around a metal core. The coil of wire causes an electromagnetic field
  
• Armature – The moving part of the relay that opens and closes the contacts. An attached spring returns the armature to its original position
  
• Contacts – The conducting part of the switch that makes (closes) or breaks (opens) a circuit
  

A relay involves two circuits: the energizing circuit and the contact circuit. The coil is on the energizing side and the relay contacts are on the contact side.

When a relay coil is energized, current flowing through the coil creates a magnetic field. Whether in an AC or DC unit, the basic function remains the same – the magnetic coil attracts a ferrous plate that is part of the armature.

A timing relay is a simple form of time-based control, allowing the user to open or close the contacts based on a specified timing function.

They can be designed, for example, with a set of selector switches, which can easily be set to a specific function and time, thereby reducing the number of product variations required.