Electromechanical Relays

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.

1. Electromechanical
Relays

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. Why Electromechanical Relays?
Separation of AC and DC circuits
Interface between electronic control circuits and
power circuits

4. Common Applications of Electromechanical Relays
Solenoid Activation Control
Many Automotive Applications (Electric Fuel Pump)
Motor Control

5. 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 Armature
Moveable Contact
ElectromagnetSpring
Stationary Contact

6. Wiring Up an Electromechanical Relay
Spring
To Control Circuit
Moveable Armature
Moveable 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

7. 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.

8. Typical Sample Application
120 VAC
Ground
To Microcontroller
Solenoid Valve
Relay
Coil
Circuit for Control of a 120 VAC Solenoid Valve

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. 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. 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. 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.

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. 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