Electrical Sensors

8/12/2019 Electrical Sensors

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ELECTRICALSENSORS


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DESCRIBE THE PROPERTIES OF A

SENSOR AND ITS USAGEINTRODUCTION TO SENSORS The control and regulation of industrial processes and

systems require accurate sensing and measurement. Initially all controls and sensors were mechanical . The development of electrical transducers has created a

new field of electrical and electronic sensors. A transducer is a device that converts energy from one form

to another.


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BASIC SENSOR OPERATION Mechanical sensors were developed over time and are economical

and reliable . The most common form of a mechanical sensor is thelimit switch as illustrated in Figure 1.1.

Figure 1.1: Limit Switch

The limitations of mechanical sensors

include the following:

Slow operating speed (when compared tothe speed of electricity), requirement for

physical contact with the material or item being sensed , and limited operating life .These limitations have generated a needfor other types of sensors.


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ELECTRICAL SENSORS WHICH HAVE AN ADVANTOVER MECHANICAL SENSORS INCLUDE THE

FOLLOWING:

Higher operating speed Proximity sensing (sensing without physical

contact). Long life (they have no moving parts) Reduced maintenance Smaller size Shock resistant.


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THEIR DISADVANTAGES

INCLUDE THE FOLLOWING: High cost and inability to directly control electrical

devices.

This lack of power handling capability adds to the costand complexity of a system , as it is often necessary toinstall a relay or amplifier to increase the power handlingcapability of the sensor.


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DESCRIBE SELECTION OF SENSO In selecting a sensor for a particular problem, various physical and technical factors are

considered in order to find the most suitable sensor for the problem. Below is a list ofevaluation criteria that weight differently in accordance with the task in hand.

1. Number of switching cycles per unit time2. Dependence on particular materials3. Range4. Repeating precision5. Speed of motion of the object to be

sensed6. Sensitive to external light (with optical

sensor)7. Reflections (with optical sensors)8. Sensitivity to humidity

9. Sensitive to contamination10. Sensitive to vibration11. Temperature range12. Compatibility with control and controlled

system

13. Type of energy for sensor14. Type of energy for control15. Energy level

16. Constancy of energy17. Maintenance-repair facilities18. Service life19. Costs of signal processing20. Costs of signal conversion21. Suppliers available

22. Costs of sensor23. Cost of sensor interface24. Type of output signal (digital, binary,

analogue) Piezoelectric


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VARIOUS TYPES OF SENSOR

APPLICATION IN THE INDUSTRIE


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UNDERSTANDING LIMITSWITCHES OPERATION


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DESCRIBE THE FUNCTIONS AND

PROPERTIES OF LIMIT SWITCHE A mechanically operated switch is one that is controlled automatically

by factors such as pressure, position, and temperature . Limit switches are designed to operate only when a predetermined

limit is reached , and they are usually actuated by contact with an objectsuch as a cam.

These devices take the place of human operators . They are often usedin the control circuits of machine processes to govern the starting,stopping, or reversal of motors.


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A limit switch is a snap-acting switch housed in a small enclosure In a snap-acting switch, as in a toggle switch, the actual switching of the circuit

takes place at a fixed speed no matter how quickly or slowly the activating

mechanism moves . The small size and variety of operating levers make these switches very useful as

limit switches. They can operate with very small pressures on the operating levers, which allows

a great deal of sensitivity.


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Many machine tool operations require a repeated forward andreverse action in their operation. Figure 2.3 illustrates areciprocating motion machine process that uses two limit

switches to provide automatic control of the motor. Each limitswitch (LS1 and LS2) has two sets of contact, one normally openand the other normally closed.


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UNDERSTANDINGPROXIMITY SENSOROPERATION


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DESCRIBE THE FUNCTIONS AND

PROPERTIES OF PROXIMITY SEN Proximity sensors or switches are pilot devices that detect the presence of anobject (usually called the target) without physical contact . They are solid-stateelectronic devices that are completely encapsulated to protect againstexcessive vibration, liquids, chemicals and corrosive agents found in theindustrial environment . Proximity sensors are used when:

Hostile environments demandimproved sealing properties,

preventing proper operation of

mechanical switches. Long life and reliable service arerequired.

A fast electronic control systemrequires a bounce-free input signal.

The object being detected is too small, toolightweight, or too soft to operate amechanical switch.

Rapid response and high switching ratesare required, as in counting or ejectioncontrol applications.

An object has to be sensed through non-metallic barriers such as glass, plastic, and

paper cartons.


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DESCRIBE INDUCTIVE

PROXIMITY SENSORS An inductive proximity sensor is a sensing device that isactuated by a metal object as shown in Figure 3.1 below.

A typical inductive sensor is shown in blockdiagram form in Figure 3.2 in the following page.The sensor consists of two parts: The detection coiland support circuitry.

Figure 3.2: Inductive Sensor Block Diagram


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The support circuitry uses external power to run an oscillatorcircuit inside the sensor .

The oscillator produces a high frequency sinusoidal voltage,which is applied to the detection coil .

The detection coil , which is placed directly behind the sensor face , produces an oscillating sensing field as illustrated in Figure 3.3. Note that the field is actually three dimensional and not twodimensional as shown.


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When a metal target enters the sensing field , eddy currents areinduced in the target.

These eddy currents cause a loss of power in the oscillatorcircuit , which results in a reduced oscillator voltage .

A detection circuit contained within the support circuitrysenses the oscillator's reduced voltage and switches theoutput.

The inductive proximity sensor contains no mechanical relaycontacts . It is a single integrated circuit.

This integrated circuit contains all the components necessaryfor the operation of the sensor.

Integrated circuits are capable of withstanding high shockloads and vibration. This results in the sensor having a lowfailure rate and a long life expectancy.


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The output in the sensor provided in your experiment is a DC type with a NPNnormally open (NO) output.

This circuit arrangement is illustrated in figure 3.4. When a significant amountof metal is present within the sensing range of the sensor, the transistor is turnedon. The sensor's output is connected to ground through the transistor. To turn onthe output indicator, the transistor closes the circuit between the indicator andground. This type of circuit operation is known as "current sinking .

When a device is connected between the source and the transistor, the transistorsinks" the current to ground. Often this type of sensor is connected to a

programmable controller or other logic device. This type of output produces alogic one or "true" when there is no metal present. If the programmablecontroller requires a logic zero or "false" you would need to use a sensor withan NPN normally closed (NC) output or a PNP NO output.


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Proximity Detection - The sensing distance of aninductive proximity sensor depends on various factors .Included among these factors are: target size and targetmaterial. All of these factors are inter-related. Table 2

below summarizes these parameters.


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Describe Circuit Wiring For An

Inductive Proximity Sensor Inductive proximity sensors used as switches arc wired into a controlcircuit in a manner similar to limit switches.

As illustrated in Figure 3.5, this inductive proximity sensor requiresDC power supplied to the black and red terminals.

The positive is connected to the red terminal and the negative isconnected to the black terminal.

A low current relay must be used to interface the sensor if it isnecessary to control a high current device or a device that is designedto operate from a voltage other than 24 VDC.

The output from the sensor is taken from the white and black or whiteand red terminals depending on whether the sensor has currentsourcing or current sinking respectively.


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SHIELDED INDUCTIVE PROXIMIT

SENSORS The ferrite core concentrates the radiated field in the direction ofuse. A metal ring is placed around the core to restrict the laterialradiation of the field . Shielded proximity sensors as shown inFigure 3.6 can be flush mounted in metal, but a metal-free space

is recommended above and around the sensing surface.


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SENSOR MOUNTING Since inductive proximity switches are of the non-contact type,

care must be taken when mounting to prevent false triggering.Always consult the manufacturer's data sheet for mountingconstraints before attempting to install. Figure 3.8 illustrates theminimum clearance values required when mounting in a metal

panel.


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If the application indicates that the sensor must be flush mounted,many manufacturers have shielded switches available (at aslightly higher cost) that can be mounted flush with the panel.

Figure 3.9 shows the minimum required distance within themounting of the inductive proximity sensors in adjacent positionand the opposite position.


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DESCRIBE CAPACITIVE

PROXIMITY SENSORS A capacitive proximity sensor is a sensing device that is actuated by conductive and non-conductive materials.

The operation of capacitive sensors is also based on the principle

of an oscillator . Instead of a coil, however, the active face of a capacitive sensor is

formed by two metallic electrodes rather like an openedcapacitor. See Figure 3.11. The electrodes are placed in the

feedback loop of a high-frequency oscillator that is inactive withno target present. As the target approaches the face of the sensor, it enters the

electrostatic field that is formed by the electrodes. This causes anincrease in the coupling capacitance , and the circuit begins tooscillate.


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CURRENT SOURCING OPERATIO DC proximity sensors can be either current sourcing or current

sinking devices . Current sourcing sensors use a PNP transistor toswitch load current and current sinking type sensors use a NPNtransistor . The type of transistor used is an important factor indetermining the compatibility of the sensor with control systeminputs.

The following drawing shown in Figure 3.13 shows the outputstage of a current sourcing type sensor.


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CURRENT SINKING OPERATION In a current sinking sensor, an NPN transistor is used. When the

NPN transistor switches on, conventional current flows throughthe load to the transistor. This is referred to as current sinkingsince the direction of conventional current is into the sensor. Theoutput in the sensor provided in your experiment is DC type withan NPN NO output. The circuit arrangement is shown in Figure3.14.


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SENSOR MOUNTING A minimum clearance distance must be provided when

mounting a capacitive proximity switch in a panel or above anysurface. This is to prevent the material of the panel or bracketfrom being sensed and affecting the sensor's operation. If theapplication requires flush mounting, many manufacturers

produce a shielded sensor (at a slightly higher cost) that can be

mounted flush. Figure 3.17 illustrates the minimum clearancedistances required to ensure the proper operation of thecapacitive sensor


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DESCRIBE THE FUNCTIONS ANDPROPERTIES OF PHOTOELECTRICSENSORS

A photoelectric sensor includes the following elements : anemitter (light source), a receiver to detect the emitted light , andassociated electronics that evaluate and amplify the detectedsignal causing the sensors output to change the state (high tolow or low to high).


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The photoelectric switch is constructed with a window in the casing.Also, a transistor can be coupled with the photoelectric diode to amplifythe signal to a level that is compatible with machine control circuits.This combination is called a PHOTO TRANSISTOR.

The light source originally used in photoe lectric switches was anincandescent lamp, similar to the bulbs in your home . The chiefadvantage of this type is that you can actually see the light beam,making it easy to align the source with the receiver. It also helps toidentify when the lamp is burned out . The main disadvantage of thistype is that it cannot be used in sunlight or in areas where there is a lotof external light.

Another problem with using the incandescent lamp is mechanicalvibration, which can cause the filament to break resulting in prematurefailure. The infrared light emitting diode (IRLED) is the latest method

used in photoelectric switching. It is a solid state device and itunaffected by mechanical vibrations. The life expectancy of the IRLEDis hundred of -years while the incandescent lamp is in the range of 1,000to 10,000 hours. Because of this, lamp life is life is no longer a majorsource of system failure. Since the LED has no filament to break , it can

be reliably used on application where there is heavy vibration


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The LED's life is so long that it usually will outlast thelife of the switch contacts. Therefore, manufacturers

permanently encase the LED in the photoelectric switch.This helps protect the switch and reduce cost.

The above advantages have made the modulated LED-type photoelectric switch the best choice for mostapplications. The only application it cannot approach isthe sensing of colour differences . This is because theLED only emits light of one wavelength (colour). Thisapplication still requires the use of an incandescent lightsource.

The method in which the receiver receives the light beamfrom the transmitter is called the DETECTIONMETHOD or SCANNING TECHNIQUE.


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There are three primary methods used in industry. Thethree methods are: 1) Direct, 2) Retro reflective, and 3)Proximity as shown in Figure 4.2.


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DESCRIBE THROUGH-BEAM OR DIRDETECTION METHOD

Through-beam sensors have a transmitter , which emit pulses , anda receiver . If the beam between the transmitter and the receiver isinterrupted the output of the receiver switches the state (high tolow or low to high). Advantages:

Large sensing distance is possible as emitter and receiverare kept opposite to each other.Suitable for precise detection oflarge as well as small objects.Repeatability and indexing

precision are not impaired evenif the object surface or background is reflecting.

Field Of Application: Through Beam Sensors are used for sensing semi-transparent opaque objects such asglass/plastic bottle, silver breakage detection, paper breakage detection, dooropening/closing etc.


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DESCRIBE RETRO-REFLECTIVEDETECTION METHOD

The retro-reflective method uses a reflector located in the pre-set operatingrange . The operating range is adjusted for the reflector . The pulses are bouncedoff the reflector and the echo pulses are returned to the sensor. When a target

blocks the echo pulses , the output is activated . The retro-reflective method istypically used in applications where the target is good sound absorber.

Advantages: Easy assembly compared to the

through beam type. Large active sensing range

compared to diffused beam type.

Field Of Application: This sensor can be used where it is difficult to install a through-beam sensor due to spaceconstraint. Furthermore, simple wiring makes it suitable where sensing objects are biggerin size. Thus, these sensors are used for loop control in decoiler, edge detection in paper /sheet metal etc.


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DESCRIBE DIFFUSED-REFLECTIVE O

PROXIMITY DETECTION METHOD Diffused-reflective or proximity method is the standardmode of operation . In this mode, objects travelling in anydirection into the operating range of the sound cone willcause the sensor to switch output states.

Reflectivity is affected bymaterial type, colour, andtexture . A higher degree ofreflectivity is desirable whenusing proximity detection

because it will better reflectthe beam back to the sensor .


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proximity detection is the type of reflection that is taking place. If the object issmooth and highly polished , it will reflect the beam off the same angle as thelight striking the surface . This is called SPECULAR reflection and is shown inFigure 4.7. lf the object is rough or unpolished , such as wood, the beam will bereflected off at many angles in approximately equal amounts. This is calledDIFFUSE reflection and is shown in Figure 4.8.

Advantages: CorrectionFactor For SensingDistance: Transmitter and receiverare housed in the samehousing.As the self-reflection of anobject is used for detection;dark & light marks can be

distinguished .Field Of Application:

These sensors are particularly used for position sensing and counting of non-metallic objects . It is also used for label sensing, level sensing, height sensing,

plastic film sensing, edge detection of paper or sheet metal etc.


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Typical Photoelectric Sensor

ApplicationsFeatures of this type of sensor include: Noncontact detection . Noncontact detection eliminates damage either

to the target or sensor head, ensuring long service life andmaintenance-free operation.

Detection of targets of vir tually any mater ial . Detection is based onthe quantity of light received , or the change in the quantity of lightreceived, or the change in the quantity of reflected light . This methodallows detection of targets of diverse materials such as glass metal,

plastics, wood, and liquid. L ong detecting distance . The reflective-type photoelectric sensor can

detect a distance of 1 m, and the through-beam type has a detectingdistance of 10 m.


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For example , a reflective-type photoelectric sensor used for thedetection of the presence of absence of a label is shown in Figure4.9(a). In Figure 4.9(b), through-beam photoelectric sensor heads are

positioned above and below the resistors travelling on a productionline. A variation on the line changes the quantity of the laser beam, thussignalling a defect.


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In most photoelectric sensors, a light-emitting diode (LED) is thelight-transmitting source, and a phototransistor is the receiving source(Figure 4.10). In operation, light from the LED falls on the input of

the phototransistor, and the amount of conduction through thetransistor changes. Analog outputs provide an output proportional tothe quantity of light seen by the photo detector.


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Dark Operate Dark Operate (DO) is an operating mode in which the load is

energized when light from the emitter is absent from thereceiver.


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Light Operate

Light Operate (LO) is an operating mode in which the load isenergized when light from the emitter reaches the receiver.


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Operating Modes Table 1 below shows the relationship between operating mode

and load status for through, retro-reflective and diffuse scan

sensors.


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UNDERSTANDING FIBREOPTICS SENSOR OPERATION


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Glass Fibres Optical fibre can be made of either glass or plastic. Glass optical

fibres consist of a bundle of very thin glass strands, each

typically measuring 0.051 mm (0.002 in.) dia. A flexiblestainless steel armoured sheath is usually added to protect the

bundle of cladded fibres, but for some applications a polyvinyl-chloride jacket (PVC) is used.

Glass, by nature, is very resilient, a trait that enables it to perform reliably under extreme conditions such as hightemperatures or a corrosive environment. Glass fibre bundles

can withstand operating temperatures as high as 450F asstandard product. Customers whose applications have operatingtemperatures >450F can special-order cables capable ofsurviving operating temperatures as high as 1200F.


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Plastic Fibres Plastic fibre-optic cable usually consists of a single strand typically 0.254

1.52 mm dia. These fibres are flexible, and excellent for applications thatrequire repeated flexing as well as for use in extremely tight areas. Theygenerally are sold with a cutting device that allows customers to trim to thedesired length.

If the sensor is going to be exposed to harsh chemicals, solvents, or hightemperatures, glass fibres are preferable. But plastic fibres can be sheathedwith teflon, nylon, or polypropylene for added immunity to hostileenvironments.

The degree to which light energy is attenuated as it travels through optical

fibre is influenced by three factors: the fibre material, the distance travelled inthe fibre, and the wavelength of the light. Glass fibres perform fairlyconsistently at all wavelengths. Plastic fibres, however, tend to absorb lightfrom IR LEDs. Visible LEDs, such as red, exhibit less attenuation in plasticoptical fibre and are therefore in wider use .


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Describe Sensing Modes And

Fibre-Optic Assemblies Because fibre-optic sensor systems are a derivative of photoelectric sensingtechnology, photoelectric sensing modes (diffuse reflective, through-beam,retro-reflective) are also available for fibre optics. The two types of fibre-optic assemblies that address these sensing modes are individual andbifurcated .

Fibre-optic through-beam mode, as shown in Figure 5.3, requires two cables.One is attached to the emitter of the remote sensor and is used to guide lightenergy to a sensing location. The other is attached to the receiver of theremote sensor and is used to guide light energy from the sensing location

back to the remote sensor. As with standard through-beam photoelectric

sensing, the emitter and detector cables are positioned opposite each other.Sensing is achieved when the light beam that extends from the emitter to thereceiver fibre-optic cable is interrupted.


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A bifurcated fibre-optic assembly isused for both diffuse reflective andretro-reflective sensing. In contrast toan individual cable, a bifurcated cable

combines the emitter and the receivercable assemblies into one assembly.The emitter and receiver strands arelaid side-by-side along the length ofthe cable as shown in figure 5.3 andare randomly mixed at the sensing

point, an ideal configuration forapplications that require a compactsensing tip. When an object is in frontof the sensing tip of the bifurcatedcable, light from the emitter cablereflects off the object and back into thereceiver of the remote sensor via thereceiver cable, and detection isachieved.

C l Ci i


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Control Circuits Fibre optic switches (FOS) are connected into a control circuit

similar to a limit switch except that the infrared switch requires

a power supply to operate. As illustrated in figure 5.4, the fibreoptic switch explained in the sensor kit requires powerconnected to one set of terminals. The other set of terminals arethe contacts to which a load is connected. One terminal willconduct current when it sees dark, the other when it sees light.


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Advantages Of Optical Fibres

Optical fibres offer less attenuation of transmitted signals.

Optical fibres can handle data at a wide range of transmission rates,even high-speed computer data can be handled by optical fibres. The frequency of light signal used in optical fibre transmission is

very high (10 14 Hz). Optical fibres can transmit power, but this capability is limited. The security of communication systems involving optical fibres is

much higher than for other systems. Any attempt to physicallyintercept the light travelling down an optical fibre will change thecharacteristics of the signal.

Optical fibres are small. Optical fibres weigh very little. Optical fibres are flexible.


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Disadvantages Of Optical Fibres

1. Optical fibres cannot carry electricity. This is a major drawback sincevirtually all equipment used in measurement, entertainment, andcommunications is electrical or electronic in nature.

2. Optical fibres are so small that they are difficult to handle. Coupling andsplicing fibres are more complicated than soldering copper conductors.

3. Optical fibres are vulnerable to radiation. Ionizing radiation in some partsof the spectrum can change the index of refraction of the fibres.

Having said that though, the advantages of using optical fibres far outweighthe disadvantages.


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UNDERSTANDING MAGNETICREED SWITCH OPERATION

b d h


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Describe Magnetic Reed Switch

Functions And Operation This type of switch is used to switch smallelectrical currents. The typical magnetic reedswitch is rated to switch 50 to 250 milliampereswith a maximum voltage of 230 volts. Thisswitch requires no physical connection, such as

a handle or knob to actuate it. The switchconsists of a glass tube containing two smallferromagnetic metal reeds.

These reeds form the actual switch contacts.When a significantly strong magnetic field is

brought near the reed switch, a magnetic field isinduced in the reeds.

The free ends of the reeds move because the poles of the induced magnetism obey the law ofmagnetism that unlike poles will attract eachother.

M i d i h il bl i i l i h


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Magnetic reed switches are available in typical switchconfigurations. These include SPST, SPDT, DPST, and DPDT. ASPST switch is a single pole single throw switch. A normally

open (NO) switch of this type is shown in Figure 6.2 below. Theswitch arrangement is known as a Form A configuration. This isthe most widely used configuration for the magnetic reed switch.

h f h


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Characteristics of The

Magnetic Reed Switch Being mechanical the magnetic reed switch exhibits thecharacteristics of a typical mechanical switch. The illustration inFigure 6.3 below shows the operation of the magnetic reedswitch with effect of the magnetic field on it. If the switch is a

NO switch, the reeds will produce unlike poles. When theseunlike poles become strong enough, the magnetic attraction willcause the free ends of the reeds to attract each other. This closesthe switch.

Th i f h i h i ff d b h d


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The operation of these switches is affected by the amount andtype of current passing through them. Switching of excessivecurrents will cause the reeds switch to pit and burn. Repeated

overloads will cause the reeds to stick together and the switch isruined. Figure 6.4 illustrates the derating curve and specificationsfor a typical magnetic reed switch. A higher switched voltagemeans load current must be lower.


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Electrical Sensors