Electronics

Chapter 53

What is Electronics?

• Electronics is the careful and exact control of very small electric currents.

• It is used in many small devices such as Tv’s, computers, digital watches e.c.t.

Relationship between voltage current and resistance

• The higher the resistance in a circuit, the lower the current that will flow.

• Voltage x Current = Resistance

Diodes• A diode is a component that will allow current to

flow in one direction only.

• When the positive terminal of a battery is connected to the positive end of the diode, the diode is said to be forward biased and allows current to flow.

• When the positive terminal of a battery is connected to the negative end of the diode, the diode is reverse biased and no current can flow through it.

Forward and reverse biased

• A forward biased diode conducts. The bulb lights

• A reverse biased diode does not conduct. The bulb doesn’t light.

Forward Biased

Reverse Biased

Rectifier• The diode can act as a rectifier. It can change alternating

current to direct current.

• A diode will only allow the current through it when it is flowing in a forward direction. So the output will be direct current.

Uses of a diode

• Computers TV radios need a current that flows in the same direction(direct current). Therefore they need a diode to change ac to dc.

• Diodes can protect appliances. Computers and radios can be damaged if connected the wrong way round. With a diode in the circuit, no current would flow if connected wrongly and no damage would be done.

Rectifier

The simplest kind of rectifier circuit is the half-wave rectifier. It only allows one half of an AC waveform to pass through to the load.

Light emitting diodes(LED)

• A light emitting diode (LED) is a diode that gives out light when a current flows through it.

• An LED uses a small current and a resistor must be connected in series so that it is not damaged by too large a current.

• LED’s are used as indicators on many electronic devices such as computers TV’s and radio’s. Used to display the

numbers on digital clocks and calculators.

Symbol for LED

LED

The FLAT side also indicates which leg is the negative

LED display and meters

Light dependent diodes.(LDR)

• A light dependent resistor (LDR) is a resistor whose resistance depends on light.

• When light falls on the LDR, its resistance decreases and the current in the circuit increases.

• LDR’s are used to control street lighting and used in light meters in cameras.

A circuit showing a LDR

Experiment: To test the polarity of a battery.

• Set up circuit as shown. Connect the two LED’s in the opposite direction.

• Connect the battery to be tested between A and B.

• Which LED lights?

• Which pole, A or B is positive?

• Reverse the connections?

• Which pole, A or B is negative?

Results

Conclusion:

• If the red LED comes on, then the red LED is forward biased. Thus terminal A is positive and B is negative.

• If the green LED comes on, then the green LED is forward biased. Thus terminal B is positive and A is negative.