Electromechanical Relays

1
Electromechanical Relays

• ELET 1133

2
Electromechanical Relay Parts
3
Three Parts
Armature Part that moves.
Contacts Connected to power external circuit.
Coil electromagnet
4
1. Apply Electric Current
Armature
Contacts
I
I
5
2. Magnetic Field Created
Armature
Contacts
I
Magnetic Field
I
6
3. Armature Attracted to Coil
Armature
Contacts
I
I
7
4. Contacts Touch.
Armature
Contacts
I
I
8
5. Current Through Contacts.
Armature
Contacts
I
I
I
I
9
External Load Circuit Connected
I
I
White Lamp
Control Circuit
Load Circuit
10
Key Idea

• Energize the coil and
• the contacts move.

11
Electromechanical Relay Advantages

• Isolation of control and load.
• Control is low power circuit.
• Load is high power circuit.
• Remote location of controls.
• Such as a computer network.
• Easily add safety devices to control.
• Can see and hear if closed or open.

12
Electromechanical Relay Disadvantages

• Coil requires significant power to operate and
  keep energized.
• Contacts move relatively slow.
• Contacts can bounce.
• Arcing as contacts near each other.
• Hard to reconfigure.
• Can be expensive.

13
Relay As 2 State Device

• Relay coil can be de-energized (normal).
• Relay coil can be energized.

14
Transistor as Relay

• For this class, a transistor is equivalent to
  the relay shown.

Collector
Collector
Base
R
Base
Emitter
Emitter
15
Normal

• De-energized state.
• Out of the box.
• No power is applied.
• Caution it could be that the usual operation
  of a device has a relay energized 99 of the
  time. Most of the time the contacts are not in
  the normal position.
• Example break the circuit to sound alarm.
  Otherwise relay is held in.

16
Normally Open

• Contacts are separated when relay coil is
  de-energized.
• Will not pass current.
• Open circuit.
• Abbreviated NO.

17
Normally Closed

• Contacts are touching when relay coil is
  de-energized.
• Current may pass through load circuit even
  though no current passes through the coil.
• Abbreviated NC.

18
Double Throw Schematic
Normally Open
Normally Closed
Physical Construction
common
NO NC
C
Shown is a single pole, double throw.
SPDT Parts kit has DPDT or two of these.
19
Simulation
Multisim Relay Example N.O. Relay File
20
Relays

• Contacts rated for less than 20 amps.
• Many different contact configurations
• spst.
• spdt.
• dpst.
• dpdt.
• Can be NO or NC.
• Remember normal position is de-energized
  position.

21
Choosing a relay

• Coil rating
• What voltage and current are required to
  energize the coil?
• Contact rating
• What voltage and current can the contacts
  withstand.
• Voltage limit due to arcing.
• Current limit due to heating.