1. Electromagnetic Relay
AIM:
To know the function and use of an electromagnetic relay
PRIOR KNOWLEDGE:
Switches in simple circuits, diodes, transistors

2. Overview An electromagnetic relay is a type of switch.
The ‘relay’ is controlled by a low power circuit such as a logic gate or comparator. This small electrical signal controls a mechanical switch.
The mechanical switch contacts can often carry large currents and cope with high voltages. The switch is part of an entirely separate circuit and controls output transducers such as heaters, motors and other high power devices.

3. Advantages & Disadvantages
Bipolar transistors and MOSFETs can be used as switches to control output devices in the same way that a relay can be used.
Advantages of a relay:
Can control both A.C. and D.C
Control circuit and output circuit are electrically separate
Can switch very large currents and voltages
Disadvantages of a relay:
Slow compared to transistors (<50Hz)
Noisy when switching
Produces a large back e.m.f. which can damage other circuits
Switch contacts wear out

4. Circuit Symbol The Electromagnetic relay has three main parts:
The electromagnetic coil. The coil typically has a resistance of several hundred ohms and only needs a few mA to work.
The mechanical connection from the coil to the switch, this is called the armature
The actual mechanical switch or switches – commonly DPDT.
The dotted line represents the physical connection that moves the switch contacts

5. Basic Operation
The control circuit provides a small current through the coil
The coil becomes an electromagnet
The electromagnet attracts the armature
The armature moves the switch contacts
The switch completes the external circuit
The picture shows circuit board mounted relays. The white one works at 5V and the black one works at 12V. There are two contacts for the coil and six DPDT switch contacts

6. Typical Relay Circuit
The circuit shows a 555 astable controlling a PCB mounted relay. Note the protection diode.
The relay is controlling a high power bulb being driven off a 24V A.C. supply (the blue wires) that is completely separate from the astable circuit running off a 5V D.C. supply.

7. Used with a Transistor
If larger relays are used, a transistor might be needed to control the current through the relay coil. Again, a protection diode is necessary.
Note how the relay is now connected to positive.

8. Protection Diode
The relay coil produces a large back e.m.f. when the relay turns off. This large voltage can easily damage op- amps, logic gates and transistors. To protect the control circuit a diode must be used.
The diode is connected in reverse bias across the relay coil. This limits the back e.m.f. induced when the relay turns off to a safe voltage of no more than 0.7V above the supply voltage.
Go back to the previous two slides and spot the protection diode in each case.

9. Summary The electromagnetic relay, or simply the relay, is a switch.
The relay can be controlled by low power, low voltage circuits such as logic circuits, microcontrollers or comparators.
The relay can control high current, high voltage (and therefore high power) circuits such as heaters, motors and bulbs.
The relay can control both A.C. and D.C. output devices.
A protection diode is usually needed to prevent damage to the control circuit.
A transistor may be used to control larger relays.

10. Questions
Why is a relay ideal for allowing a logic circuit to control a mains operated bulb?
A relay coil works at 12V and has a resistance of 600Ω. How much current does the relay coil take?
Why does a relay usually need a protection diode?
A relay is marked as 24V, 200Ω. Why is it still suitable for use with mains voltages?
Why is a relay called a relay?
When is a transistor better than a relay?

11. Answers
Because the logic circuit works at a low D.C. voltage whereas the bulb works at 230V A.C.
I = V / R  I = 12 / 600 = 0.02A or 20mA
Relays produce a large reverse voltage when they turn off which will damage logic circuits, transistors and op-amps.
The marking refer to the specifications of the coil, not the voltage and current that can be controlled by the relay.
It relays information (to turn on or off) from the control circuit to the output device.
When high speed switching is necessary.