Reed Switch Michael Ryan ECE 5320 Mechatronics Assignment 1

Outline Reed switches are a class of proximity sensor which are used to detect the presence of a magnetic field. I will discuss the construction, operation, and applications of these sensors. Additionally I will also cover their advantages, and disadvantages.

Construction The reed switch consists of a pair of flexible reeds made of a magnetic material, and sealed in a glass tube filled with inert gas The reeds are overlapped but separated by a small gap. The contact area of each reed is plated with a noble metal, such as Rhodium or Ruthenium, to provide the switch with stable characteristics and long life,

Operation Application of a magnetic field, generated by a permanent magnet or a coil, to the reed switch causes both reeds to be magnetized. This produces an N-pole at the contact area of one reed, and an S-pole at that of the other reed, in a manner shown on the drawing (left). If the magnetic attracting force overcomes the resistive force caused by elasticity of the reed, the reeds come in contact i.e., the circuit is closed. Once the magnetic field is removed, the reeds are separated again by the effect of elasticity of the reed i.e., the circuit is opened.

Major applications Major applications of reed switches include –Position Sensing –Pulse Counting –Coil Applications –Temperature Sensing

Position Sensing When an application requires proximity sensing, end position sensing, or moving part sensing, a reed switch or a reed sensor fixed to the stationary surface and a magnet fixed to the moving surface can do the job. When the moving part is away from the stationary part, the reed sensor remains un-actuated. As the moving part comes near the stationary part, the reed sensor gets actuated. This actuating distance is highly repeatable both while approaching and while moving away.

Pulse Counting Reed switches have a very high operating frequency and this feature lends itself to high speed pulse counting applications. Mounting a magnet to a rotating wheel or an object which moves back and forth repeatedly, and a reed sensor to a stationary part will generate the pulses required. The reed sensor used for sensing the pulses can be connected to a counter.

Temperature Sensing Due to their sharp cut in, cut out bandwidth and high reliability, thermal reed switches are finding applications in place of bimetallic strips, thermistors and thermostats for over-heat protection or precise temperature switching. The cut in temperature is differentiated from the cut out temperature by an abrupt and huge change in the resistance of the reed switch contact resistance of a few milli-ohms to an open circuit resistance of a few mega-ohms. In other words, a total galvanic separation occurs. Thermal reed switches can sense subzero temperatures as low as -30°C.

Coil Applications Producing a relay using a reed switch is simple compared to conventional electromechanical relays which require many moving parts. Reed switch based reed relays, made by inserting a single or multiple reed switches into a wound bobbin, are available in very small sizes and can go on for millions of operations. Current sensors find applications in almost every electronic circuit for overload protection.

Typical configuration Wiring of a reed switch will differ based on what type of load it will be attached to. If a Capacitive load or a Inductive load are used then special care must be taken.

Resistive Load When a reed switches for a resistive load it acts as a normal switch would in such a case and no special circuitry need be used to protect the switch. However in such a case the power and current ratings of the switch must still be considered.

Capacitive Load Capacitive Loads: When using reed switches for capacitive loads such as capacitors, incandescent lamps or long cables, the contacts will be subjected to high surge current. Therefore, protective circuits such as : surge suppressors or current limiting resistors, are recommended

Inductive Load Inductive Loads: When using reed switches for inductive loads such as motors, relay coil, solenoids, etc., the contacts will be subjected to high induced voltages during opening of the contacts. Such high induced voltages may cause damage to the reed switch or significantly reduce its life. Therefore, protective circuits such as : RC, varistors or clamping diodes, are recommended

Typical Specs

Advantages Compact and Lightweight The reed switches are suitable for use as a compact and lightweight magnetically responsive switch, thereby rendering equipment smaller. Ambient Resistance Contacts of the reed switch are encapsulated in a glass tube together with inert gas which protects the reed switch from the effects of the exterior environment, for example, gas, dust, or moisture in the atmosphere. Relatively stable characteristics are ensured from low to high temperatures. The reed switches are usable over a wide variety of temperatures.

High Reliability Considerably high reliability is assured due to long typical operating lifetimes. High-speed Operation Since the reed switch operates at high speed, it is easy to interface with a transistor or an IC. Long Life A long-life reed switch without mechanical friction is implemented as a result of its simple structure. Extensive Applications When used in combination with a permanent magnet, the reed switch finds extensive application in switching and sensing.

Limitations EM Interference Because the reed switch senses magnetic fields it is sensitive to electro magnetic interference from any source which comes in proximity to the sensor. Inductive Load When an inductance load is used a back electromotive force of several hundred volts arises when the contacts are opened, which results in considerable decrease in contact life. Capacitive Load If the contacts are load are connected together thought a long wire or cable, a rush of current flows by means of stray capacitance when the contacts are closed. This can significantly influence contact life.

Mounting, Solder bending cutting The glass housing of the sensor is fragile and can break due to bending during mounting and cutting. Additionally the housing may break due to excessive heat during soldering. Shock and Vibration If a reed switch or a unit incorporating a reed switch is dropped from a height of more than 30cm, the characteristics of the reed switch may change. Avoid physical shock. Additionally the glass housing may become damaged, thus rendering the switch useless.

Reference List “Reed Switches” Aleph America Corporation, James Johnson “Reed Switches” Electronics in Meccano. 6 Jan “Types or Reed Switch Applications” Reed Relays and Electronics Idial Limited sensor.com/Applications/Reed_switch_applications_overview.htmhttp:// sensor.com/Applications/Reed_switch_applications_overview.htm Bishop, Robert. The Mechatronics Handbook. New York: CRC Press, 2002.

“Reed Switch” Wikipedia. 11 February “Reed Limit Switch” Edge “Reed Switches Applications” NEC/TOKIN Vol tokin.com/english/product/pdf_dl/ReedSwitcesApplications.pdfhttp:// tokin.com/english/product/pdf_dl/ReedSwitcesApplications.pdf