Relays are switches that open and close circuits electromechanically or electronically. Relays are either electromechanical relays or solid-state relays. In electromechanical relays (EMR), contacts are opened or closed by a magnetic force. With solid-state relays (SSR), there are no contacts and switching is totally electronic.

Relays control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams show, when a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is Normally Closed (NC), there is a closed contact when the relay is not energized. In either case, applying electrical current to the contacts will change their state.
Relays are generally used to switch smaller currents in a control circuit and do not usually control power consuming devices except for small motors and Solenoids that draw low amps. Nonetheless, relays can "control" larger voltages and amperes by having an amplifying effect because a small voltage applied to a relays coil can result in a large voltage being switched by the contacts.
Protective relays can prevent equipment damage by detecting electrical abnormalities, including over current, undercurrent, overloads and reverse currents. In addition, relays are also widely used to switch starting coils, heating elements, pilot lights and audible alarms.


Basic parts and functions of electromechanical relays include:

  1. Frame: Heavy-duty frame that contains and supports the parts of the relay.
  2. Coil: Wire is wound around a metal core. The coil of wire causes an electromagnetic field.
  3. Armature: A relays moving part. The armature opens and closes the contacts. An attached spring returns the armature to its original position.
  4. Contacts: The conducting part of the switch that makes (closes) or breaks (opens) a circuit.

    How Relays Work: Relay Diagrams

    Relays involve two circuits: the energizing circuit and the contact circuit. The coil is on the energizing side; and the relays contacts are on the contact side. When a relays coil is energized, current flow through the coil creates a magnetic field. Whether in a DC unit where the polarity is fixed, or in an AC unit where the polarity changes 120 times per second, the basic function remains the same: the magnetic coil attracts a ferrous plate, which is part of the armature. One end of the armature is attached to the metal frame, which is formed so that the armature can pivot, while the other end opens and closes the contacts.

    Types of Relays:

    Electromechanical Relays.

    1.General Purpose Relays are electromechanical switches, usually operated by a magnetic coil. General purpose relays operate with AC or DC current, at common voltages such as 12V, 24V, 48V, 120V and 230V, and they can control currents ranging from 2A-30A. These relays are economical, easy to replace and allow a wide range of switch configuration.

    2.Machine Control Relays are also operated by a magnetic coil. They are heavy-duty relays used to control starters and other industrial components. Although they are more expensive than general purpose relays, they are generally more durable. The biggest advantage of machine control relays over general purpose relays is the expandable functionality of Machine Control Relays by the adding of accessories.

    3.Reed Relays are a small, compact, fast operating switch design with one contact, which is NO. Reed Relays are hermetically sealed in a glass envelope, which makes the contacts unaffected by contaminants, fumes or humidity, allows reliable switching, and gives contacts a higher life expectancy.


    Solid State Relays, are capable of switching high voltages up to 600 VACrms. These relays are designed to switch various loads such as heating elements, motors, and transformers.

    Solid state relays consist of an input circuit, a control circuit and an output circuit.The Input Circuit is the portion of a relays frame to which the control component is connected. The input circuit performs the same function as the coil of electromechanical relays. The circuit is activated when a voltage higher than the relays specified Pickup Voltage is applied to the relays input. The input circuit is deactivated when the voltage applied is less than the specified minimum Dropout voltage of the relay. The voltage range of 3 VDC to 32 VDC, commonly used with most solid-state relays, makes it useful for most electronic circuits. The Control Circuit is the part of the relay that determines when the output component is energized or de-energized.

    A Relay Diagram of a Solid State Relay Circuit


    1.Zero-Switching Relays - relays turns ON the load when the control (minimum operating) voltage is applied and the voltage of the load is close to zero. Zero-Switching relays turn OFF the load when the control voltage is removed and the current in the load is close to zero. Zero-Switching relays are the most widely used.

    2.Instant ON Relays - turns ON the load immediately when the pickup voltage is present. Instant ON Relays allow the load to be turned ON at any point in it's up and down wave.

    3.Peak Switching Relays - turns ON the load when the control voltage is present, and the voltage of the load is at its peak. Peak Switching relays turn OFF when the control voltage is removed and the current in the load is close to zero.

    4.Analog Switching Relays - has an infinite number of possible output voltages within the relays rated range. Analog switching relays have a built in synchronizing circuit that controls the amount of output voltage as a function of the input voltage. This allows a Ramp-Up function of time to be on the load. Analog Switching relays turn OFF when the control voltage is removed and current in the load is near zero.