What is a Relay?

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 (24V) applied to a relay's coil can result in a large voltage (460V) being switched by the contacts.

Figure 1. General Purpose Relay

Relays are widely used to  heating elements, refrigerators, heating and air conditioning systems, relays control the operation of machine tools, industrial assembly lines and commercial equipment.

Relays are the electrical workhorses that control one electrical circuit by opening and closing Contacts in another circuit. 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.

Although relays make things happen, they can also keep things from happening. Protective relays can prevent equipment damage by detecting electrical abnormalities, including overcurrent, undercurrent, overloads and reverse currents.

Types of relayes

1-electromechanical           2- solid-state.

 In an electromechanical relay, contacts are opened or closed by a magnetic force. With a solid-state relay, there are no contacts and switching is totally electronic.

Figure 2. Solid-State Relay                         Figure 3. general purpose relay

We'll cover electromechanical relays first.

General Purpose Relay

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.

Machine Control Relay

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.

Figure 4. Machine Control Relay

The biggest advantage of machine control relays over general purpose relays is that you can easily expand the function of this type of relay by adding accessories. A wide selection of accessories are available for machine control relays, including additional poles, convertible contacts

Reed Relay

The fast operating Reed Relay is a small, compact, switch design with one contact, which is NO. The relay is hermetically sealed in a glass envelope, which makes the contacts unaffected by contaminants, fumes or humidity, allows reliable switching, and gives contacts a high life expectancy. The ends of the contact, which are often plated with gold or another low resistance material to increase conductivity, are drawn together and closed by a magnet. Reed relays are capable of switching industrial components such as solenoids, contactors and starter motors.

A reed relay consists of two reeds. When a magnetic force is applied, typically by an electromagnet or coil, it sets up a magnetic field in which the ends of the reeds assume opposite polarity. When the magnetic field is strong enough, the attracting force of the opposite poles overcomes the stiffness of the reeds and draws them together. When the magnetic force is removed, the reeds spring back to their original, open position. These relays work very quickly because of the short distance between the reeds.

Figure 56. Reed Relay

Electromechanical Relay Parts

The basic parts of a coil relay include:

●FRAME. Heavy-duty frame that 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.

Figure 57. Electromechanical Relay

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 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 (for afrequency of 60 HZ), 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.