6 - Output devices

2016-04-27 21:51:00 +0000, 2 years and 3 months ago

6.01 - electromagnetic relay

When current flows in the coil contacts, the soft icron core is magnetised and thus attracts the soft iron armature. This armature rocks on a pivot, and so closes the NO contacts, and opens the NC contact.
Relays are useful since they can seperate a DC circuit from a AC ciruit, since there is not physical interaction between wires.

The current needed to operate a relay is called the pull-in current, the relay armature will fall back into its original state, once the current has decreased beyond a cetain value, known as a the drop-out current. A relay tends to be slow to react, since there is mechanical movement of the armature.

The symbol for a relay is shown below:

Relays can only be operated by high voltages, therefore it is nessecary to use a driver in a typical DC circuit. When a driver in series with the relay switches off, the current inside the coil of the relay falls to 0A. This generates a large induced voltage as the result of the energy stored in the coil. This induced voltage will damage the driver being used to drive the relay. A diode is connected in parallel with the relay so that it is in reverse bias with the voltage supply. The diode offers an easy path for the induced voltage and so prevents the driver from being damaged.

A diode in reverse bias to the voltage supply must be added in parallel with the relay to prevent ‘back-EMF’, thus protecting the driver.

6.02 - electromagnetic solenoid

6.03 - buzzers

6.04 - motors

6.05 - seven segment displays

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