Relays, Solenoids, and Transducers: Principles and Applications

1. 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: • 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 – H-bridge • Used to switch power connections on a DC motor to allow the motor to spin either direction