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Electromechanical Relays. Mechatronics ECE 5320 Assignment #1 Paul Smart. Outline. Why Electromechanical Relays? Common Applications of Electromechanical Relays The Principle Behind Electromechanical Relays What’s Inside an Electromechanical Relay Typical Sample Application

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electromechanical relays

Electromechanical Relays

Mechatronics ECE 5320

Assignment #1

Paul Smart

  • 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
reference list
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
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
why electromechanical relays
Why Electromechanical Relays?
  • Separation of AC and DC circuits
  • Interface between electronic control circuits and power circuits
common applications of electromechanical relays
Common Applications of Electromechanical Relays
  • Solenoid Activation Control
  • Many Automotive Applications (Electric Fuel Pump)
  • Motor Control
the principle behind electromechanical relays
The Principle Behind Electromechanical Relays

A relay is similar to a switch, it is either open or closed. When the switch is open no current passes through the relay, the circuit is open, and the load that is connected to the relay receives no power. When a relay is closed, the circuit is completed and current passes through the relay and delivers power to the load.

To open and close a relay an electromagnet is used. When the coil controlling the electromagnet is given a voltage, the electromagnet causes the contacts in the relay to connect and transfer current through the relay.

electromechanical relays what s inside
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 Contact

Moveable Armature

Stationary Contact



wiring up an electromechanical relay
Wiring Up an Electromechanical Relay

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.

Power Supply


Moveable Contact

Moveable Armature

Stationary Contact



To Control Circuit

typical sample application
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 application11
Typical Sample Application


To Microcontroller



120 VAC

Solenoid Valve

Circuit for Control of a 120 VAC Solenoid Valve

electromechanical relay advantages
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
electromechanical relay limitations
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
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)

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.