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一个继电器是一种电操作开关。 许多继电器使用一个电磁铁操作开关装置机械,但也使用其他操作原则。继电器被使用在有必要控制电路由一个低功率信号(完全电隔离控制和控制电路之间),或在数个电路必须控制的一个信号。第一个继电器被用于长途电报电路,重复信号来自于一个电路和再保险传输到另一个。继电器被广泛地使用在电话交流和早期的计算机来执行逻辑操作。
一个类型的继电器,可以处理高功率要求直接控制一个电动马达或其他负载称为接触器。固态继电器电源控制电路没有移动部分,而是使用一个半导体设备来执行转换。 继电器和校准操作特征,有时多个操作线圈是用来保护电路从过载或错误;在现代电力系统这些功能是由数字仪器仍被称为“保护继电器。
基本设计和操作
简单的机电式继电器。
小“摇篮”继电器常用于电子产品。“摇篮”术语是指形状的继电器的电枢。
一个简单的电磁继电器由线圈的电线裹着一个核心软铁,铁轭,提供了一个低磁阻路径对磁通,一个可移动的铁电枢,和一个或多个组联系人(有两个在继电器如图所示)。 电枢是铰链的轭和机械地链接到一个或多个组移动联系人。它是由一个春天所持有的,以便当继电器是断开的有一个气隙磁场电路。在这种情况下,一个两组联系人在继电器上图是封闭的,另一组是开放的。其他的继电器可以或多或少组联系人取决于他们的功能。火炬传递在画中也有一个接线电枢的轭。这确保连续性的电路之间移动联系人电枢,电路轨道上的印刷电路板(PCB)通过轭,这是焊接到PCB。
当电流通过线圈生成磁场,激活了电枢,和随之而来的运动的活动联系(s)要么使或减免(根据施工)连接与一个固定的联系。如果组联系人关闭当继电器被断开的,那么运动打开联系人并将这些连接,反之亦然,如果联系人都是开着的。当电流线圈被切断时,电枢是返回的力,大约一半的磁力
类型
闭锁继电器
闭锁继电器和永久磁铁
一个闭锁继电器有两个放松状态(双稳态)。这些也被称为“冲动”、“保持”,或“呆”继电器。当电流挂断,继电器仍在其最后的状态。这是通过电磁操作一个棘轮和凸轮机构,或者通过两种对立的线圈与一个在中心弹簧或永久磁铁举行电枢和接触的位置而线圈是放松,或剩余的核心。在棘轮和凸轮例子中,第一个脉冲线圈把火炬传递和第二个脉冲把它关掉。在这两个线圈的例子,一个脉冲到一个线圈把火炬传递和一个脉冲相反的线圈把继电器掉。这种类型的继电器的优点是一个线圈消耗功率只有一瞬间,而它被互换,继电器接触保留此设置在停电。一个剩余核心闭锁继电器需要一个电流脉冲相反的极性使其改变状态。
一个步进继电器是一个专门类型的多路闭锁继电器专为早期的自动电话交流。
一个漏电断路器包含一个专门的闭锁继电器。
非常早期的计算机通常存储碎片磁闭锁继电器,如铁簧继电器或后来在1 memreed ess开关。
一些早期的计算机使用普通继电器作为一种锁——他们商店在普通线簧继电器碎片或干簧继电器被喂养一个输出回电作为输入,导致一个反馈循环或顺序电路。这样一个电封闭继电器需要持续的电力
水银湿簧继电器
参见:水银开关
一个水银湿簧继电器是一种簧片继电器,联系人是湿与水星。这种继电器是用来开关低压信号(一伏特或更少)水星降低了接触电阻和相关的电压降,低电流信号,表面污染可能会组成一个接触不良,或为高速应用水星消除接触反弹。水银湿簧继电器是位敏和必须垂直安装正常工作。由于毒性和费用的液态汞,这些继电器现在很少使用。
极化继电器
一个极化继电器把电枢的两极之间永久磁铁来提高灵敏度。极化继电器被用在20世纪中间电话交换机可以探测微弱的脉冲和正确的电报失真。波兰人在螺丝,所以一个技术员可以首先调整他们最大灵敏度,然后应用一个偏见弹簧来设置临界电流,将操作继电器。
机床继电器
机床继电器是一种标准化的工业控制机工具、转移机器,和其他时序控制。他们具有大量的联系人(有时可扩展的领域),很容易转化为常开到闭合状态,方便地更换线圈,和一个形状因子,它允许简洁地安装许多继电器在控制面板。尽管这种继电器曾经的骨干自动化等行业的汽车装配,可编程逻辑控制器(PLC)主要取代了机床继电器从时序控制应用程序。
一个继电器允许电路状态转换由电气设备:
USB继电器
一个USB继电器是一个小型的硬件设备将其他设备(家里灯、直流电机,solenodis和其他人)和关闭,使用你的电脑。它是通过usb接口连接到你的电脑。
固态接触器式继电器
一个固体接触器是一种重型固态继电器,包括必要的散热器,用于频繁的开/关周期是必需的,例如电加热器、小型电动马达,照明负载。没有转动部件磨损和没有接触反弹由于振动。他们被激活的交流或直流控制信号控制信号从可编程逻辑控制器(plc)、pc、晶体管—晶体管逻辑(TTL)的来源,或其他微处理器和单片机控制。
布赫继电器
布赫继电器是一个安全装置传感的累积气体在大充油的变形金刚,这将警报缓慢积累的气体或关闭变压器如果天然气生产迅速变压器油。
迫使引导联系人继电器
一个被迫引导联系人继电器有继电器接触,是机械地联系在一起,因此,当继电器线圈通电或断开的,所有链接的联系人一起行动。如果一组联系人在继电器变得固定化,没有其他接触相同的继电器将能够移动。联系人的功能是使被迫引导安全电路来检查状态的继电器。强迫引导联系人也被称为“积极引导联系人”、“俘虏联系人”、“锁接触”,或“安全继电器”。
过载保护继电器
电动汽车需要过载保护,以防损坏从电机过载,或保护
杆,把
电路符号的继电器。(C代表共同终端在单点钻石车削与双刀双掷类型)。
因为继电器开关,这个术语用于开关也应用于继电器;一个继电器开关一个或更多的波兰人,所有的联系人可以激励线圈抛出的三种方式:
常开(没有)接触连接电路当继电器被激活;电路断开当继电器是不活跃的。它也被称为形成一个接触或“制造”接触。没有接触也可以称之为“早”或诺伊姆这位这意味着联系密切的按钮或开关之前充分参与。
肯扬(NC)接触断开电路当继电器被激活;电路连接当继电器是不活跃的。它也被称为一个表格B接触或“打破”接触。数控联系人也可以称之为“晚打破”或NCLB,这意味着联系人保持关闭状态,直到按钮或开关完全脱离。
转换(CO),或双投(DT),接触控制两个电路:一个常开接触和一个闭合接触一个共同的终端。它也被称为一个C型接触或“转移”接触(”先断后”)。如果这种类型的接触利用一个“休息之前”功能,那么它被称为表单D接触。
下列名称中经常遇到:
——单刀单掷单刀单掷。这些有两个终端,可以连接或断开。包括两个的线圈,这样一个继电器有四个终端在总。它是模棱两可的根是否正常开启或正常关闭。术语“SPNO”和“SPNC”有时被用来解决歧义。
单点钻石车削-单刀双掷。一个常见的终端连接到其他两名。包括两个的线圈,这样一个继电器有5个航站楼在总。
双刀单掷-双极单投。这些有两对终端。相当于两个单刀单掷开关或继电器驱动通过一个单一的公司
EN 50005是在适用标准继电器终端编号,一个典型的EN 50005 -兼容的单点钻石车削继电器的终端将编号11、12、14,A1和A2为C,数控,不,和线圈连接,分别。
应用程序
继电器是用来和为:
放大一个数字信号,交换大量的力量与小运行功率。一些特殊情况:
一个电报继电器,重复一个弱信号收到在最后一天,线
控制一个高压电路和一个低电压信号,因为在某些类型的调制解调器或音频放大器,
大电流控制电路与低电流信号,就像在起动器电磁的一辆汽车,
故障检测和隔离在输电和配电线路通过打开和关闭断路器(保护继电器),
一个双刀双掷交流线圈继电器与“冰立方”包装
隔离控制电路的控制电路当两个处于不同的潜力,例如当一个电源供电的设备控制从一个低压开关。后者经常应用于控制办公室照明作为低电压电线很容易安装在分区,这可能是经常搬随着需求变化。他们也可以控制的入住率探测器来节约能源,
逻辑函数。例如,布尔和函数是意识到通过连接常开继电器接触在系列、或函数通过连接常开触点在平行。转换或表格C接触执行XOR(异或)函数。类似的功能,也没有完成的NAND使用常闭触点。梯子编程语言通常用于设计继电器逻辑。
不受欢迎的灭弧
主要文章:消弧
没有足够的接触保护,发生的电流放电原因明显退化的联系人在继电器,遭受重大和可见的损坏。 每次继电器转换要么从一个封闭到开放的状态(断弧)或从一个开放封闭状态(使电弧&反弹弧),在负载,电弧电之间出现的两个接触点(电极)的继电器。休息弧通常更有活力,因此更具有破坏性。
热能的包含在生成的电弧电非常高(成千上万的华氏度),导致金属接触表面融化,池和迁移与当前。极高温的电弧裂缝周围的气体分子臭氧创建,一氧化碳，和其他化合物。 在电弧能量慢慢破坏接触金属,导致一些材料来逃避到大气细颗粒物。这非常活动使材料接触迅速降解,导致设备故障。这种接触降解大幅限制了整体生活的继电器,射程约为10000至100000年行动,这一水平远远低于机械寿命相同的设备,可以在超过2000万个手术。
保护继电器
主要文章:保护继电器
为保护电器和输电线路机电式继电器和准确的操作特征被用来检测过载、短路,和其他错误。而许多这样的继电器仍在使用,数码设备现在提供等价的保护功能。
铁路信号
部分继电器联锁使用英国问风格微型插件继电器。
英国问风格信号继电器和基地。
铁路信号继电器是非常大的和繁琐的考虑主要是小电压(低于120 V)和电流(也许100毫安),他们开关。接触广泛分布以防止危险和短路的flashovers一辈子,可能超过五十年。BR930系列插件继电器广泛应用于铁路下面英国实践。这些都是高度为120毫米,180毫米深和56毫米宽,重约1400克,可以有多达16个单独接触,说12使和4打破联系人。
因为铁路信号电路必须高度可靠、特殊技术被用来检测和防止故障继电器系统。防止错误的提要,双开关继电器接触通常使用在两个正面和负面的一面,所以,一个电路的两个假提要是需要引起错误的信号。并不是所有的继电器电路可以证明所以有依赖施工特性,比如碳银接触抵抗雷电感应接触焊接和提供交流免疫力。
光电子的光电隔离器还用于一些实例与铁路信号,特别是在只有一个单一的接触是要切换。
美国信号继电器是起源的19英寸的机架。
历史
1835年发明的继电器是由美国科学家约瑟夫亨利为了提高他的版本的电电报,开发在1831年早些时候。
有人宣称英国发明家爱德华·戴维“当然发明了电继电器”在他c.1835电报。
一个简单的装置,我们现在称之为一个继电器,是包括在最初的1840年萨缪尔·摩尔斯电报专利的。所描述的机制作为数字放大器,重复电报信号,从而允许信号被传递到期望的。这克服了问题的有限范围的早期电报方案。
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations.
A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".
Basic design and operation
Simple electromechanical relay.
Small "cradle" relay often used in electronics. The "cradle" term refers to the shape of the relay's armature.
A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing.
When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components. Some automotive relays include a diode inside the relay case. Alternatively, a contact protection network consisting of a capacitor and resistor in series (snubber circuit) may absorb the surge. If the coil is designed to be energized with alternating current (AC), a small copper "shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase current which increases the minimum pull on the armature during the AC cycle.
A solid-state relay uses a thyristor or other solid-state switching device, activated by the control signal, to switch the controlled load, instead of a solenoid. An optocoupler (a light-emitting diode (LED) coupled with a photo transistor) can be used to isolate control and controlled circuits.
Types
Latching relay
Latching relay with permanent magnet
A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that one coil consumes power only for an instant, while it is being switched, and the relay contacts retain this setting across a power outage. A remanent core latching relay requires a current pulse of opposite polarity to make it change state.
A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges.
An earth leakage circuit breaker includes a specialized latching relay.
Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later memreed in the 1ESS switch.
Some early computers used ordinary relays as a kind of latch -- they store bits in ordinary wire spring relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or sequential circuit. Such an electrically-latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically racheting relays.
In computer memories, latching relays and other relays were replaced by delay line memory, which in turn was replaced by a series of ever-faster and ever-smaller memory technologies.
Reed relay
Main article: reed relay
A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts inside an evacuated or inert gas-filled glass tube which protects the contacts against atmospheric corrosion; the contacts are made of magnetic material that makes them move under the influence of the field of the enclosing solenoid. Reed relays can switch faster than larger relays, require very little power from the control circuit. However they have relatively low switching current and voltage ratings. Though rare, the reeds can become magnetized over time, which makes them stick 'on' even when no current is present; changing the orientation of the reeds with respect to the solenoid's magnetic field can resolve this problem.
Top, middle: reed switches, bottom: reed relay
Mercury-wetted relay
See also: mercury switch
A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) where the mercury reduces the contact resistance and associated voltage drop, for low-current signals where surface contamination may make for a poor contact, or for high-speed applications where the mercury eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are now rarely used.
Polarized relay
A polarized relay placed the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay.
Machine tool relay
A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally-open to normally-closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.
A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay could switch power at a preset time. For many years relays were the standard method of controlling industrial electronic systems. A number of relays could be used together to carry out complex functions (relay logic). The principle of relay logic is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic, which is commonly used in programmable logic controllers.
Ratchet relay
This is again a clapper type relay which does not need continuous current through its coil to retain its operation.
Contactor relay
A contactor is a very heavy-duty relay used for switching electric motors and lighting loads, although contactors are not generally called relays. Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor.[2] Such devices are often used for motor starters. A motor starter is a contactor with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use where noise is a chief concern.
Solid-state relay
Solid state relay with no moving parts
25 A or 40 A solid state contactors
A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. Every solid-state device has a small voltage drop across it. This voltage drop limits the amount of current a given SSR can handle. The minimum voltage drop for such a relay is a function of the material used to make the device. Solid-state relays rated to handle as much as 1,200 Amperes have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.
USB relay
A USB relay is a small hardware device to turn other devices(home lights, DC motors, solenodis and others) on and off, using your computer. It is connected to your computer via usb.
Solid state contactor relay
A solid state contactor is a heavy-duty solid state relay, including the necessary heat sink, used where frequent on/off cycles are required, such as with electric heaters, small electric motors, and lighting loads. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.
Buchholz relay
A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.
Forced-guided contacts relay
A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".
Overload protection relay
Electric motors need overcurrent protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in the motor windings. One type of electric motor overload protection relay is operated by a heating element in series with the electric motor. The heat generated by the motor current heats a bimetallic strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.
Pole and throw
Circuit symbols of relays. (C denotes the common terminal in SPDT and DPDT types.)
Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:
Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact. NO contacts may also be distinguished as "early-make" or NOEM, which means that the contacts close before the button or switch is fully engaged.
Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact. NC contacts may also be distinguished as "late-break" or NCLB, which means that the contacts stay closed until the button or switch is fully disengaged.
Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break
before make"). If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact.
The following designations are commonly encountered:
SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.
DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each).
DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.
The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw relay (with 12 terminals).
EN 50005 are among applicable standards for relay terminal numbering; a typical EN 50005-compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC, NO, and coil connections, respectively.
Applications
Relays are used to and for:
Amplify a digital signal, switching a large amount of power with a small operating power. Some special cases are:
A telegraph relay, repeating a weak signal received at the end of a long wire
Controlling a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers,
Controlling a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,
Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),
A DPDT AC coil relay with "ice cube" packaging
Isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors to conserve energy,
Logic functions. For example, the boolean AND function is realised by connecting normally open relay contacts in series, the OR function by connecting normally open contacts in parallel. The change-over or Form C contacts perform the XOR (exclusive or) function. Similar functions for NAND and NOR are accomplished using normally closed contacts. The Ladder programming language is often used for designing relay logic networks.
The application of Boolean Algebra to relay circuit design was formalized by Claude Shannon in A Symbolic Analysis of Relay and Switching Circuits
Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital computers. See electro-mechanical computers such as ARRA (computer), Harvard Mark II, Zuse Z2, and Zuse Z3.
Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation, they are widely used in safety-critical logic, such as the control panels of radioactive waste-handling machinery.
Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to
a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed.
Vehicle battery isolation. A 12v relay is often used to isolate any second battery in cars, 4WDs, RVs and boats.
Switching to a standby power supply.
Relay application considerations
A large relay with two coils and many sets of contacts, used in an old telephone switching system.
Several 30-contact relays in "Connector" circuits in mid 20th century 1XB switch and 5XB switch telephone exchanges; cover removed on one
Selection of an appropriate relay for a particular application requires evaluation of many different factors:
Number and type of contacts – normally open, normally closed, (double-throw)
Contact sequence – "Make before Break" or "Break before Make". For example, the old style telephone exchanges required Make-before-break so that the connection didn't get dropped while dialing the number.
Rating of contacts – small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
Voltage rating of contacts – typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15 000 V
Operating lifetime, useful life - the number of times the relay can be expected to operate reliably. There is both a mechanical life and a contact life; the contact life is naturally affected by the kind of load being switched.
Coil voltage – machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes
Coil current - including minimum current required to operate reliably and minimum current to hold. Also effects of power dissipation on coil temperature at various duty cycles.
Package/enclosure – open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant, washable for printed circuit board assembly
Operating environment - minimum and maximum operating temperatures and other environmental considerations such as effects of humidity and salt
Assembly – Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
Switching time – where high speed is required
"Dry" contacts – when switching very low level signals, special contact materials may be needed such as gold-plated contacts
Contact protection – suppress arcing in very inductive circuits
Coil protection – suppress the surge voltage produced when switching the coil current
Isolation between coil contacts
Aerospace or radiation-resistant testing, special quality assurance
Expected mechanical loads due to acceleration – some relays used in aerospace applications are designed to function in shock loads of 50 g or more
Accessories such as timers, auxiliary contacts, pilot lamps, test buttons
Regulatory approvals
Stray magnetic linkage between coils of adjacent relays on a printed circuit board.
There are many considerations involved in the correct selection of a control relay for a particular application. These considerations include factors such as speed of operation, sensitivity, and hysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 us are available. Reed relays which are actuated by low currents and switch fast are suitable for controlling small currents.
As for any switch, the current through the relay contacts (unrelated to the current through the coil) must not exceed a certain value to avoid damage. In the particular case of high-inductance circuits such as motors other issues must be addressed. When a power source is connected to an inductance, an input surge current which may be several times larger than the steady current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging spark across the separating contacts.
Consequently for relays which may be used to control inductive loads we must specify the maximum current that may flow through the relay contacts when it actuates, the make rating; the continuous rating; and the break rating. The make rating may be several times larger than the continuous rating, which is itself larger than the break rating.
Derating factors
Control relays should not be operated above rated temperature because of resulting increased degradation and fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays operating at rated load are also affected by their environment. Oil vapors may greatly decrease the contact tip life, and dust or dirt may cause the tips to burn before their normal life expectancy. Control relay life cycle varies from 50,000 to over one million cycles depending on the electrical loads of the contacts, duty cycle, application, and the extent to which the relay is derated. When a control relay is operating at its derated value, it is controlling a lower value of current than its maximum make and break ratings. This is often done to extend the operating life of the control relay. The table lists the relay derating factors for typical industrial control applications.
Undesired arcing
Main article: Arc suppression
Without adequate contact protection, the occurrence of electric current arcing causes significant degradation of the contacts in relays, which suffer significant and visible damage. Every time a relay transitions either from a closed to an open state (break arc) or from an open to a closed state (make arc & bounce arc), under load, an electrical arc can occur between the two contact points (electrodes) of the relay. The break arc is typically more energetic and thus more destructive.
The heat energy contained in the resulting electrical arc is very high (tens of thousands of degrees Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current. The extremely high temperature of the arc cracks the surrounding gas molecules creating ozone, carbon monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter. This very activity causes the material in the contacts to degrade quickly, resulting in device failure. This contact degradation drastically limits the overall life of a relay to a range of about 10,000 to 100,000 operations, a level far below the mechanical life of the same device, which can be in excess of 20 million operations.
Protective relays
Main article: protective relay
For protection of electrical apparatus and transmission lines, electromechanical relays with accurate operating characteristics were used to detect overload, short-circuits, and other faults. While many such relays remain in use, digital devices now provide equivalent protective functions.
Railway signalling
Part of a relay interlocking using UK Q-style miniature plug-in relays.
UK Q-style signalling relay and base.
Railway signalling relays are very big and cumbersome considering the mostly small voltages (less than 120 V) and currents (perhaps 100 mA) that they switch. Contacts are widely spaced to prevent dangerous flashovers and short circuits over a lifetime that may exceed fifty years. BR930 series plug-in relays are widely used on railways following British practice. These are 120 mm high, 180 mm deep and 56 mm wide and weigh about 1400 g, and can have up to 16 separate contacts, say 12 make and 4 break contacts.
Since rail signal circuits must be highly reliable, special techniques are used to detect and prevent failures in the relay system. To protect against false feeds, double switching relay contacts are often used on both the positive and negative side of a circuit, so that two false feeds are needed to cause a false signal. Not all relay circuits can be proved so there is reliance on construction features such as carbon to silver contacts to resist lightning induced contact welding and to provide AC immunity.
Opto-isolators are also used in some instances with railway signalling, especially where only a single contact is to be switched.
American signaling relays are the origin of the 19 inch rack.
History
The relay was invented in 1835 by American scientist Joseph Henry in order to improve his version of the electrical telegraph, developed earlier in 1831.
It is claimed that the English inventor Edward Davy "certainly invented the electric relay" in his electric telegraph c.1835.
A simple device, which we now call a relay, was included in the original 1840 telegraph patent of Samuel Morse. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired. This overcame the problem of limited range of earlier telegraphy schemes.