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