Electromechanical Relays


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Electromechanical relays are an excellent solution to separate electronic control circuitry and power circuitry. Electromechanical relays are not the best choice in high frequency switching applications and do have a limited life due to wear on the contacts inside the relay. When used in the a proper application, the electromechanical relay provides safe and reliable integration between power circuits and control circuits.

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Electromechanical Relays

  1. 1. Electromechanical Relays
  2. 2. OutlineWhy Electromechanical Relays?Common Applications of Electromechanical RelaysThe Principle Behind Electromechanical RelaysWhat’s Inside an Electromechanical RelayTypical Sample ApplicationElectromechanical Relay Advantages and LimitationsImportant Specifications of Electromechanical RelaysConclusion
  3. 3. Why Electromechanical Relays?Separation of AC and DC circuitsInterface between electronic control circuits and power circuits
  4. 4. Common Applications of Electromechanical RelaysSolenoid Activation ControlMany Automotive Applications (Electric Fuel Pump)Motor Control
  5. 5. Electromechanical Relays: What’s Inside This diagram shows the basic parts of an electromechanical relay: a spring, moveable armature, Moveable Armature Moveable Contact electromagnet, moveable contact, and stationary Stationary Contact contact. The spring keeps Spring Electromagnet the two contacts separated until the electromagnet is energized, pulling the two contacts together.
  6. 6. Wiring Up an Electromechanical RelayThis diagram shows how to wirean electromechanical relay. Power SupplyWhen the control circuit turns the Loadelectromagnet on, the moveable Moveable Contact Moveable Armaturearmature is drawn towards theelectromagnet and connects the Stationary Contactmoveable contact and the Spring Electromagnetstationary contact. Thiscompletes the circuit and deliverspower to the load. To Control Circuit
  7. 7. Typical Sample ApplicationSuppose, there is a need to control a solenoid valve for a water drainapplication. Control is to be accomplished with a microcontroller. Thesolenoid valve requires 120 VAC to open. Assuming that a 120 VAC powersupply is available, how can control of the solenoid valve be accomplished usinga microcontroller that can only supply 5 VDC?This problem is easily solved using a relay. There are many relays that areturned on and off with a 5 VDC coil. The relay provides the interface betweenthe microcontroller and the 120 VAC power supply that is needed to open andclose the valve.
  8. 8. Typical Sample Application Ground To Microcontroller Coil Relay120 VAC Solenoid Valve Circuit for Control of a 120 VAC Solenoid Valve
  9. 9. Electromechanical Relay AdvantagesContacts can switch AC or DCLow initial costVery low contact voltage drop, thus no heat sink is requiredHigh resistance to voltage transientsNo Off-State leakage current through open contacts
  10. 10. Electromechanical Relay LimitationsThe contacts wear and thus have limited life depending on loadsShort contact life when used for rapid switching applications or high loadsPoor performance when switching high inrush currentsPackage Size
  11. 11. Important Specifications of Electromechanical RelaysCoil Voltage – Voltage required for switchingContact Rating – How much current the relay can handleNormally Open (NO) or Normally Closed (NC)
  12. 12. ConclusionElectromechanical relays are an excellent solutionto separate electronic control circuitry and powercircuitry. Electromechanical relays are not the bestchoice in high frequency switching applicationsand do have a limited life due to wear on thecontacts inside the relay. When used in the aproper application, the electromechanical relayprovides safe and reliable integration betweenpower circuits and control circuits.
  13. 13. Reference List http://www.rowand.net/Shop/Tech/AllAboutRelays.htm http://relays.tycoelectronics.com/schrack/pdf/C0_v4bg_2.pdf http://www.cutler-hammer.eaton.com/unsecure/html/101basics/Module18/Output/ElectromechanicalRelays.html http://www.msdignition.com/pdf/8961_8960_msd_relays.pdf http://zone.ni.com/devzone/conceptd.nsf/webmain/7C83114818EAA85786256DD400569EB7?opendocument http://www.ibiblio.org/obp/electricCircuits/Digital/DIGI_5.html
  14. 14. Links To Explore Further http://www.allegromicro.com/techpub2/phoenix/relay5.htm - Solid State Relays http://www.ssousa.com/appnote040.asp - Electromechanical Relays vs. Solid State Relays