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DESCRIPTION
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A relay is essentially a switch operated by an electro-magnet and is very useful for switching on high current devices.
There is no electrical connection between the switch contacts and the coil, therefore a low voltage circuit (and coil) can switch a larger voltage or high current device.
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CIRCUIT DIAGRAM
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Since a relay has an electromagnet, it produces EMF which can damage any electronic devices driving it. You must use a silicon diode D1 (1N4001 recommended) to absorb the EMF, connected in reverse bias. (in reverse).
The switch contacts are called changeover contacts and are labelled as:
CM = Common
NO = Normally open
NC = Normally closed.
OPERATION.
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Relay with Single Pole
Changeover contacts
Relay with Dual Pole
Changeover contacts
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Coil is not energised, CM connects to NC
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Coil is energised, CM connects to NO
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There are many different types of relays but essentially they are all electrically operated switches.
Some have 2 sets of contacts acting as individual switches that switch at the same time but are totally independent electrically.
Other types have 3 or even 4 sets of contacts (poles)
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RELAY DRIVE SUGGESTIONS
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A suitable driver for relays is a single transistor driver.
Usually the CM and the NO contacts are incorporated into another circuit to switch on a device of higher current or voltage. Below is an example of a 6v circuit controlling a 12volt lamp.
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| Written by Phil Townshend - 2008 |
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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