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.
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, Moveable Armature
Moveable Contact
electromagnet, moveable
contact, and stationary Stationary Contact
contact. The spring keeps Spring Electromagnet
the two contacts separated
until the electromagnet is
energized, pulling the two
contacts together.
6. Wiring Up an Electromechanical Relay
This diagram shows how to wire
an electromechanical relay. Power Supply
When the control circuit turns the Load
electromagnet on, the moveable Moveable Contact
Moveable Armature
armature is drawn towards the
electromagnet and connects the Stationary Contact
moveable contact and the Spring Electromagnet
stationary contact. This
completes the circuit and delivers
power to the load.
To Control Circuit
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
Ground
To Microcontroller
Coil
Relay
120 VAC Solenoid Valve
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.
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