1. Electromechanical Relays
Mechatronics ECE 5320
Assignment #1
Paul Smart

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. 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

4. 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

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

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

7. 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.

8. 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

9. 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

10. 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.

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

12. 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

13. 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

14. 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)

15. 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.