Topic 7  Output Devices

Electromagnetic relay

The relay is a switch operated by an electromagnet.  It allows the small voltage and small current produced by an electronic circuit to turn on a big current at a high voltage.  The circuit symbol for a relay is:

Notice that there are two different kinds:

•  NO – normally open.  The contacts are open until the coil of the relay is energised, whereupon they are closed to complete the outside circuit.

• NC - normally closed.  The contacts are closed until the coil of the relay is energised, whereupon they are opened to break the outside circuit, switching it off.

• Many relays have multiple contacts, half of which are NO and half NC.

Relays are electromagnetic devices which have a certain amount of inductance.  When they are turned off, the collapse of the magnetic field can produce a momentary “spike” of high reverse voltage that can wreck a transistor or integrated circuit.  Therefore a reverse biased diode is placed in parallel to short out the voltage spike, thereby protecting the circuit.

Solenoids

Solenoids are at their simplest an elongated coil of wire along a tubular former.  If we put a piece of soft iron (which loses its magnetism quickly) into the middle of the tube, we find that the iron core is attracted rapidly into the tube.

Solenoids have a number of uses, especially where a short straight-line movement is called for.  Uses include:

• Door chimes (the ding-dong type)

• Remote controlled door locking

• Point motors on a model train layout

• Motors to open valves

• Electric staple guns.

Like a relay, it is a good idea to have a reverse-biased diode in parallel to protect the electronic circuitry. 

Electric Motors

Electronic circuits are used to drive electric motors, which convert electrical energy into rotational movement.  Examples include:

• Motors in washing machines,

• Motors in tape decks or CD drives.

• Model train motors.

• Traction motors in railway locomotives.

There are several different kinds of motor:

• Permanent magnet motor

• Mains induction motor

• Synchronous motors.

• Stepper motor where very precise movements are needed.  They are controlled using pulses. 

Loudspeaker

The loudspeaker is a device that uses the motor effect.  A light coil moves forwards and backwards in a radial magnetic field in response to the variations of the signals that are fed in. They are low voltage and low resistance devices, typically 4 – 8 W, so care has to be taken with load matching in the design of audio circuits.

If you want to know how the electric motor effect works, click HERE.

Buzzers

Buzzers make an audible signal in response to a direct current signal.  They take a small current, in the order of a few milliamps and can be driven by a small signal transistor.

Try Question1

Now have a go at the Quiz.  It's dead simple!

Useful Websites

http://electronics.howstuffworks.com/motor.htm

http://electronics.howstuffworks.com/speaker2.htm

http://electronics.howstuffworks.com/relay.htm