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Relay - Basic Properties, Terminology and Theory

What is a Relay?
Relay is 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. 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) switched by the contacts.
Relays are widely used to switch starting coils, heating elements, pilot lights and audible alarms. In addition to home dishwashers, 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. A relay contact that is normally closed (NC), means the contact is closed when the relay is not energized. In either case, applying electrical current to the contacts changes their state.
Although relays make things happen, they can also prevent things from happening. Protective relays can alleviate equipment damage by detecting electrical abnormalities including overcurrent, undercurrent, overloads and reverse currents.
Relays are either electromechanical or 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.
General Purpose Relays
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 Relays
A magnetic coil also operates machine control relays. These heavy-duty relays are used to control starters and other industrial components. They are more expensive than general purpose relays and usually more durable.
The biggest advantage of machine control relays is the ability to expand functions with the addition of accessories. Accessories available include adding additional poles, convertible contacts, transient suppression of electrical noise, latching controls and timing attachments.
Reed Relay
The fast operating reed relay is a small, compact, switch design with one normally open (NO) contact. Because it's hermitically sealed in a glass envelope, the contacts are unaffected by contaminants, fumes or humidity. This allows for reliable switching and gives contacts high life expectancy. The contact ends, often plated with gold or another low resistance material for increased conductivity, are drawn together and closed by a magnet. Reed relays are capable of switching industrial components such as solenoids, contactors and starter motors.
It consists of two reeds. When a magnetic force is applied, usually by an electromagnet or coil, it sets up a magnetic field in which the ends of the reeds have opposite polarity. When the magnetic field is strong enough, the attracting force of opposite poles overcomes the reeds stiffness 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.
Electromechanical Relay Parts
Basic Parts of a Relay
Basic parts of a coil relay include:
  • Frame - heavy-duty contains and supports relay parts
  • Coil - wire wound around a metal core. The coiled wire causes an electromagnetic field.
  • Armature - opens and closes the contacts. An attached spring returns the armature to its original position.
  • Contacts - 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 flows through the coil creating a magnetic field. Whether in a DC unit where polarity is fixed, or an AC unit where 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 the armature can pivot while the other end opens and closes the contacts.
Contacts come in a number of different configurations, depending on the number of breaks, poles and throws that comprise the relay. Some of the more common descriptions include single pole, single throw (SPST), double pole, single throw (DPST). The terms indicate the design and function of the relay.
Break - 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 or double break. A single break (SB) contact breaks an electrical circuit in one place. A double break (DB) contact breaks it in two places. Single break contacts are normally used when switching lower power devices such as indicating light. Double break contacts are used when switching high power devices such a solenoids.
Pole -The number of completely isolated circuits that a relay 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 number of poles reach a maximum of 12, depending upon the relay design.
Throw -The number of closed contact positions per pole available on a switch. A switch with a single-throw can only control one circuit, while a double-throw contact can control two.
Relay Contact Life
The useful life of a relay depends upon its contacts. Replace contacts or the entire relay when the contacts burn out. You can estimate contact's life span. To determine the relay's life, first calculate its mechanical life. Mechanical life is the number of operations (openings and closings) a contact can perform without electrical current. The mechanical life of a relay is relatively long - up to 1,000,000 operations. Electrical life is the number of operations (openings and closing) the contacts perform with electrical current at a given current rating. Contact electrical life ratings range from 100,000 to 500,000 cycles.
Arcing is one enemy of contact life. Arcing occurs when an electric switch is opened and current discharges across the contact Arcing gap. Using an arc suppressor, a device that dissipates energy across the open contacts, minimizes arcing. Another option is using the correct contact material for the application.
Another culprit that shortens relay life is sulfidation. Sulfidation is the reaction of a metal or alloy with sulfur-containing materials, producing a sulfur compound on or beneath the surface on the metal or alloy. Normally, switching the contacts at higher voltage levels burns off this sulfur compound. Too high of a voltage can result in excessive sulfidation. The easiest way to avoid sulfidation is to provide enough current to burn off contaminants or by using contacts made of alloys. Under power and over power conditions have their own set of unique problems. In an under powered situation, the film or oxidation that forms on contacts and is removed by arcing or contacts wiping against each other during operation may not occur.
When excessive power is applied, surges cause pitting that reduces contact life. Contacts should be oversized when surges are expected.
Solid-State Relay
These relays consist of an input circuit, a control circuit and an output circuit.
Input Circuit
The input circuit is the portion of the relay that the control component is connected. The input circuit performs the same function as the coil of an electromechanical relay. The circuit is activated when a voltage higher than the relay's specified pickup voltage is applied to the relay 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.
Control Circuit
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 an electromechanical relay, the coil accomplishes this function.
Output Circuit
This is the portion of the relay that switches the load on and performs the same function as the mechanical contacts of an electromechanical relay. Solid-state relays normally have only one output contact. HID lighting is an example of this type of relay. During start-up, the solid-state relay is closed, turning on the standard fixture. As current demand for the HID fixture increases, the relay control circuit de-energizes the output circuit, shutting off the conventional fixture.
Solid State Relay Comparison
The decision to use an electromechanical or solid-state relay depends on an application's electrical requirements, cost constraints and life expectancy. Solid-state relay are popular, electromechanical relays remain common. Many of the functions performed by power-hungry, heavy-duty equipment need the switching capabilities of electromechanical relays.
Although a solid state relay (SSR) accomplishes the same result as an electromechanical relay (EMR), its physical structure and the way it functions are different. An SSR switches current using non-moving electronic devices such as silicon controlled rectifiers.
Because an SSR does not have to energize a coil or open contacts, less voltage is required to "turn" an SSR on or off. And an SSR turns on and turns off faster because there are no physical parts to move. The absence of contacts and moving parts means SSRs are not subject to arcing and do not wear out. Contacts on Electromechanical Relays (EMRs) can be replaced, whereas the entire SSR must be replaced when any part is defective.
The SSRs construction exposes it to residual electrical resistance and/or current leakage whether switches are open or closed. The small voltages drops that are created are not usually a problem; however, EMRs provide a cleaner on or off condition because of the relatively large distance between contacts, which acts as a form of "insulation".
EMR Benefits
  • Multi-throw, multi-pole contacts
  • Some handle harsh environments (e.g. reed)
  • Contacts can switch AC or DC
  • Low initial cost
  • Very low contact voltage drop - no heat sink needed
  • Very resistant to voltage transients
  • No off state leakage current through open contacts
  • Some allow for contact replacement
SSR Benefits
  • Very long life when properly applied
  • No contacts to wear out
  • No contacts arcing that generate electromagnetic interference
  • Resistant to shock and vibration - no moving parts
  • Logic compatible to programmable controllers, digital circuits and computers
  • Very fast switching capability
  • Different switching modes (zero, instant on, etc.)
Relays are available as automotive, general purpose, latching, reed, solid-state, and timing varieties. There are amps contacts to work with any application. Choose from plug-in, flange, PC mount, panel mount, dip style, bifurcated, rugged, miniature, open frame, quick connect, high sensitivity, dual coil, zero cross and random switching relays. Work with a relay specialist to determine the best product for your application.
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