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Relays – How Relays Work

How Relays Work

Relays are switches that open and close circuits electromechanically or electronically. 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 overcurrent, undercurrent, overloads and reverse currents. In addition, relays are also widely used to switch starting coils, heating elements, pilot lights and audible alarms.

Shop Relays
What is a Relay?
Electromechanical Relays vs Solid State Relays

Relays are either electromechanical relays or solid-state relays (SSRs). In electromechanical relays (EMR), contacts are opened or closed by a magnetic force. With SSRs, there are no contacts and switching is totally electronic. The decision to use electromechanical or SSRs depends on the application's electrical requirements, cost constraints, and life expectancy. Although SSRs have become very popular, electromechanical relays remain common. Many of the functions performed by heavy-duty equipment need the switching capabilities of electromechanical relays. SSRs switch the current using non-moving electronic devices such as silicon-controlled rectifiers.

The differences in the two types of relays result in advantages and disadvantages with each system. Because SSRs do not have to either energize a coil or open contacts, less voltage is required to "turn" SSRs on or off. Similarly, SSRs turn on and turn off faster because there are no physical parts to move. The absence of contacts and moving parts means that SSRs are not subject to arcing and do not wear out. Conversely, the contacts on electromechanical relays can be replaced, whereas entire SSRs must be replaced when any part becomes defective. Because of the construction of SSRs, there is residual electrical resistance and/or current leakage whether switches are open or closed. The small voltage drops that are created are not usually a problem; however, electromechanical relays provide a cleaner ON or OFF condition because of the relatively large distance between contacts, which acts as insulation.

Electromechanical Relays

Basic parts and functions of electromechanical relays 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 - A relays moving part. The armature 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.
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 contacts are on the contact side. When the coil is energized, current flows through the coil creating 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. Contacts come in several different configurations, depending on the number of breaks, poles, and throws that make up the relay. For instance, relays might be described as Single-Pole, Single-Throw (SPST) or Double-Pole, Single-Throw (DPST).

These terms will give an instant indication of the design and function of different types of relays.

  • Break - This is the number of separate places or contacts that a switch uses to open or close a single electrical circuit. All contacts are either single break (SB) or double break (DB). An SB contact breaks an electrical circuit in one place, while a DB contact breaks it in two places. SB contacts are normally used when switching lower power devices such as indicating lights. DB contacts are used when switching high-power devices such as solenoids.
  • Pole - This is the number of completely isolated circuits that relays can pass through a switch. A single-pole contact (SP) can carry current through only one circuit at a time. A double-pole contact (DP) can carry current through two isolated circuits simultaneously. The maximum number of poles is 12, depending on the design of the relay.
  • Throw - This is the number of closed contact positions per pole that are available on a switch. A switch with a single throw contact can control only one circuit, while a double-throw contact can control two.
Solid State Relays (SSRs)

SSRs 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 SSRs, 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. The control circuit functions as the coupling between the input and output circuits. In electromechanical relays, the coil accomplishes this function. A relays output circuit is the portion of the relay that switches on the load and performs the same function as the mechanical contacts of electromechanical relays. SSRs, however, normally have only one output contact.

A Relay Diagram of a Solid State Relay Circuit
Types of Electromechanical Relays
General Purpose Relays

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

Machine Control Relays

Machine control relays are also operated by a magnetic coil. They are heavy-duty and are 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. A wide selection of accessories is available for machine control relays including additional poles, convertible contacts, transient suppression of electrical noise, latching control, and timing attachments.

Reed Relays

Reed relays are a small, compact, fast operating switch design with one contact, normally open (NO), and hermetically sealed in a glass envelope, which makes the contacts unaffected by contaminants, fumes, or humidity. This allows for more reliable switching and gives contacts a higher 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. They can switch industrial components such as solenoids, contactors, and starter motors and consist of two reeds. When a magnetic force is applied, such as an electromagnet or coil, it sets up a magnetic field in which the end of the reeds assumes 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.

Types of Solid State Relays
Zero-Switching Relays

Zero switching relays turn on the load when the control (minimum operating) voltage is applied, and the voltage of the load is close to zero. These 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.

Instant ON Relays

Instant ON relays turn on the load immediately when the pickup voltage is present, and they allow the load to be turned on at any point in its up and down wave.

Peak Switching Relays

Peak switching relays turn on the load when the control voltage is present, and the voltage of the load is at its peak. They turn OFF when the control voltage is removed and the current in the load is close to zero.

Analog Switching Relays

Analog switching relays have an infinite number of possible output voltages within the relays rated range. They 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 and turn off when the control voltage is removed and current in the load is near zero.

A Relays Contact Life

The mechanical life of a relay depends on its contacts. Once contacts burn out, the contacts or the entire relay must be replaced. Mechanical life is the number of operations (openings and closings) a contact can perform without an electrical current. Relatively long, a relay can perform up to 1,000,000 operations.

The electrical life of a relay is the number of operations (openings and closings) the contacts can perform with electrical current at a given current rating. Electrical life ratings range from 100,000 to 500,000 cycles.

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