1. Electronic Control Systems Week 4 – Relays and Solenoids
EET273
Electronic Control Systems
Week 4 – Relays and Solenoids

2. Relays and Solenoids
Reading: Ch. 10:1, 10:3; 11

3. Transducers A Transducer Is: Some examples:
A device that converts energy from one form to another
Some examples:
Mechanical:
Solenoid
Relay coil
Actuators
Mechanical sensors
Electro-acoustical:
Microphone
Loudspeaker

4. Relays A relay is: An electrical switch actuated by a magnetic coil
Contains an electromagnet in close proximity to a set of switching contacts
Can contain a single set of switching contacts, or several, including NC or NO contacts
“Normal” status is when coil is not energized

5. “Ice Cube” relay DPDT Relay Clear plastic for viewing contacts
Eight pins – 3 for each switch, and 2 for the coil
Relay base is often called an “octal base” due to having 8 contacts

6. Relay contacts
Switching is done by a moving metal “leaf” that moves between 2 contact points

7. Relay Circuits Ladder Logic
Coils and switches shown as separate symbols, but named similarly
Since a single relay can have many switches in parallel, a set of switches may be named CR1-1, CR1-2, etc.

8. Relay Circuits

9. Relay Circuits
Use red arrows to represent a closed circuit (and thus the flow of current)
Use red X’s to show that a switch is closed, and therefore no current flowing in that leg

10. Relays vs. Transistors
Why use a relay to do switching? Why not a transistor?
Pros:
Typically allow higher currents
No voltage drop between switching contacts
Very low resistance between closed switching contacts
Very high resistance between open switching contacts
Simplicity – can switch AC or DC, at various voltage levels, with no need for biasing circuitry
Cons:
Relays are typically slower than transistors
Mechanical – wears out over time
Cost – modern transistors are a fraction of the cost of most relays

11. On-delay & Off-delay Relays
On delay relays: delay occurs when coil is energized, no delay when coil is de-energized
Off delay relays: delay occurs when coil is de-energized, no delay when coil is energized
Arrow in symbol represents when delay occurs
Up: energized
Down: de-energized
Can be either NO or NC

12. On-delay & Off-delay Relays
Normally open, timed-close Normally open, timed-open

13. Relays – other considerations
Remember, a relay coil is just an inductor, everything you know about inductors applies!
Relay coils store energy in a magnetic field, and when switched off, this field collapses, creating a “kickback voltage”
You can reduce the effects of this “kickback voltage” through the use of diode in parallel with the relay coil

14. Solenoids
Solenoid:
A coil of wire designed to produce a magnetic field
Differs from a inductor in it’s application – an inductor’s job is to create inductance, the magnetic field is just a byproduct
Magnetic field is used to attract a moveable ferrous armature
A transducer – converts electrical energy to mechanical energy

15. Solenoid Valve – Fluid Valve
2-way valves
One path for fluid to flow
Analogous to a SPST switch – fluid is either on or off
Can be normally closed or open
Return spring side – normal state
Solenoid side – actuated state
In fluid valves, “closed” refers to a valve that is NOT passing fluid, and “open” is one that allows flow, this is the opposite from how open and closed electrical switches allow current to flow

16. Solenoid Valve 2-way bidirectional valves
Some valves have a “preferred direction” of flow
Bidirectional valves have no preferred directional of flow

17. Solenoid Valves – 3 way 3-way valves
Two paths for a common path to flow to
One path is normal open, other is normally closed
Analogous to a Form C or SPDT electrical switch
Remember, “OPEN” and “CLOSED” mean the opposite from what they mean in electrical circuits!

18. Thermocouples
When two dissimilar metals are combined, they induce a voltage that is proportional to their temperature
Different types of metal combinations will produce different voltage levels
This voltage is typically small and requires a buffer amplifier to be used effectively

19. Lab 3 – Basic Relays Using an “ice-cube” DPDT relay
Reading a relay wiring diagram
Taking voltage/current measurements on a relay
Using a timer relay
Creating an ON delay and an OFF delay

20. Lab 4/5 – H-bridge
Used to switch power connections on a DC motor to allow the motor to spin either direction