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GCSE     Switch     BJT     Comparator     Fuse     Logic     Microswitch     MOSFET     Pull Up or Down     RCD     Reed     >Relay<     Tilt     Touch    

Switch Relay


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  GCSE    Switches  0 of 23    Question 260    Components 
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Deadline not set1 of 23    Question 700    Motors 
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Deadline not set4 of 23    Question 562    Relay 
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Deadline not set9 of 23    Question 1076    Relay 
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Deadline not set10 of 23    Question 265    Switch Types 
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Deadline not set13 of 23    Question 274    Switches 
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Deadline not set17 of 23    Question 1389    Switches 
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Deadline not set18 of 23    Question 1390    Switches 
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Deadline not set19 of 23    Question 1391    Switches 
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Deadline not set20 of 23    Question 1392    Switches 
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Deadline not set21 of 23    Question 1393    Switches 
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Deadline not set22 of 23    Question 1394    Switches 
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Deadline not set23 of 23    Question 1395    Switches 
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Compare Switch Types

Double Pole Double Throw Electromagnetic Relay

Electromagnetically Operated Relay Switch

A small current in the relay electromagnet coil turns on a much larger current (or perhaps a very high voltage). When a current passes through the coil, the soft iron core becomes strongly magnetised. This attracts the soft iron rocker which tilts on its hinge. The rocker presses the left hand springy contact. This makes the relay contacts touch and the circuit turns on. Large versions of this arrangement are used to control the national power distribution grid. A smaller version turns on the starter motor in a car engine. Before the telephone system went digital, the phone network relied on electromechanical switches only a little more complex than this.

Simple Relay Diagram

Mechanical Structure of a Relay

Light Sensor and Relay

Light Operated Relay Switch

The circuit above will turn the relay coil on when the light level increases. This can be used to turn on or off room or street lights.

The diode is needed because sudden changes of current in the relay coil would create a voltage pulse big enough to destroy the driver transistor/s. The diode provides a circuit where the coil current can die away slowly without creating a high voltage pulse. The high voltage pulse is caused by "Back EMF".

This is not a good solution in practice because there will be a critical light level at which the relay current causes the relay to operate in an unpredictable way. Not quite on or not quite off. The worst case would cause the lights to turn on and off rapidly. When the lights came on, the light level would increase causing them to turn off again but then the light level would be too low so they would turn back on etc etc.

A better solution would include a Schmitt trigger. This circuit has a property called hysteresis which solves the problem.

NO, NC and COM

Relay Puzzles

In each of the circuits below, what happens when you briefly press the push button switch? Try this with a real-life relay.

Relay puzzles Relay puzzles
Relay puzzles Relay puzzles

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