Electromechanical Relays
Outline
Why Electromechanical Relays?Common Applications of Electromechanical
RelaysThe Principle Behind Electromechanical
RelaysWhat’s Inside an Electromechanical RelayTypical Sample ApplicationElectromechanical Relay Advantages and
Limitations Important Specifications of
Electromechanical RelaysConclusion
Why Electromechanical Relays?
Separation of AC and DC circuits
Interface between electronic control circuits and power circuits
Common Applications of Electromechanical Relays
Solenoid Activation Control
Many Automotive Applications (Electric Fuel Pump)
Motor Control
Electromechanical Relays: What’s Inside
This diagram shows the basic parts of an electromechanical relay: a spring, moveable armature, electromagnet, moveable contact, and stationary contact. The spring keeps the two contacts separated until the electromagnet is energized, pulling the two contacts together.
Moveable ArmatureMoveable Contact
ElectromagnetSpringStationary Contact
Wiring Up an Electromechanical Relay
Spring
To Control Circuit
Moveable ArmatureMoveable Contact
Load
Power Supply
Electromagnet
This diagram shows how to wire an electromechanical relay. When the control circuit turns the electromagnet on, the moveable armature is drawn towards the electromagnet and connects the moveable contact and the stationary contact. This completes the circuit and delivers power to the load.
Stationary Contact
Typical Sample Application
Suppose, there is a need to control a solenoid valve for a water drain application. Control is to be accomplished with a microcontroller. The solenoid valve requires 120 VAC to open. Assuming that a 120 VAC power supply is available, how can control of the solenoid valve be accomplished using a microcontroller that can only supply 5 VDC?
This problem is easily solved using a relay. There are many relays that are turned on and off with a 5 VDC coil. The relay provides the interface between the microcontroller and the 120 VAC power supply that is needed to open and close the valve.
Typical Sample Application
120 VAC
Ground
To Microcontroller
Solenoid Valve
Relay
Coil
Circuit for Control of a 120 VAC Solenoid Valve
Electromechanical Relay Advantages
Contacts can switch AC or DCLow initial costVery low contact voltage drop, thus no heat
sink is requiredHigh resistance to voltage transientsNo Off-State leakage current through open
contacts
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
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)
Conclusion
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.
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
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