67.transistor

Author : Khairi

TRANSISTOR

Transistor is made of semiconductor material

Transistor is produced by attaching n-type semiconductor and p-type semiconductor

Transistor is used as electronic switching or current amplifier

There two types of transistor ; npn transistor and pnp transistor

Author : Khairi

TRANSISTOR

n p n attachment

n p n

p n p attachment

p n p

npn transistor

pnp transistor

PRODUCING A TRANSISTOR

Author : Khairi

TERMINALS OF TRANSISTOR

n p n

npn transistor

collector,C

base,B

emitter,E

SYMBOLC

B

E

TRANSISTOR

Author : Khairi

p n p

pnp transistor

collector,C

base,B

emitter,E

SYMBOLC

B

E

TRANSISTOR

TERMINALS OF TRANSISTOR

Author : Khairi

AMPLIFIER

Amplifier is used to amplified current

Amplifier is widely used as audio amplifier such as in radio and television

Transistor is used to build an amplifier

TRANSISTOR

Author : Khairi

Connection of npn transistor as an amplifier

RB

Base circuit

BE junctionForward biased

B

E

Collector circuitBC junction

Reverse biased

C

TRANSISTOR

C ----- +

B ----- +

- --- E --- -

Author : Khairi

RBB

E

C

When base current ,IB flows

IB

Collector current, IC flows through transistor

IC

Emitter current,IE is produced

IE

IE = IB + IC

A bigger magnitude of current produced

TRANSISTOR

Connection of npn transistor as an amplifier

Author : Khairi

There are 2 types of automatic controlled switch :

1. Light controlled automatic switch

e.g. Street lights will be switched on at night and will be switched off at daytime automatically

2. Heat controlled automatic switch

e.g. Fire alarm will sound automatically when the fire occurs

AUTOMATIC SWITCH

Author : Khairi

BASIC UNDERSTANDING OF AUTOMATIC SWITCH

Vi

R1

R2VB

VB = ViR1 + R2

R2

Value of VB (potential difference across R2) is very important in determining whether the transistoris activated(turn on) or not activated(turn off)Value of VB is depending on R2 and R1

AUTOMATIC SWITCH

Author : Khairi

VB = ViR1 + R2

R2

If R2 is high - VB becomes high - transistor is activated

AUTOMATIC SWITCH


Vi

R1

R2

VB

Author : Khairi

VB = ViR1 + R2

R2

If R2 is high - VB becomes high - transistor is activated

AUTOMATIC SWITCH


Vi

R1

R2

VB

Author : Khairi

VB = ViR1 + R2

R2

If R2 is low - VB becomes low – transistor is not activated

AUTOMATIC SWITCH


Vi

R1

R2

VB

Author : Khairi

LIGHT CONTROLLED AUTOMATIC SWITCH

LDR

Light controlled automatic switch uses light dependent resistor (LDR)

Resistance of LDR is high in the darknessand low in bright light

Dark --- R highLight ---- R low

AUTOMATIC SWITCH

Author : Khairi

LDR

Light bulb

RB

R1

R2

AUTOMATIC SWITCH


Author : Khairi

LDR

Lightbulb

RB

R1

R2

In bright light Resistance of LDR is low

Potential difference of VB is low

VB

Based current,IB is smallIB

Transistor is not activated

IC does not flow

Light bulb does not light up

VB = ViR1 + R2

R2

AUTOMATIC SWITCH


Author : Khairi

LDR

Lightbulb

RB

R1

R2

In the darkness

Resistance of LDR is high

Potential difference of VB is high

VB

Base current,IB is bigIB

Transistor is activated

IC flows through transistor

IC

Light bulb lights up

VB = ViR1 + R2

R2

AUTOMATIC SWITCH


Author : Khairi

EXERCISE

LDR Lightbulb

RB

R1

R2

Explain what happen in the darkness and bright light situation

VB = ViR1 + R2

R2

Bright light, R1 low, VB high,

IC flows through transistor and light bulb lights up

IB big, transistor is activated,

AUTOMATIC SWITCH

Author : Khairi

LDR Lightbulb

RB

R1

R2

VB = ViR1 + R2

R2

Dark, R1 high, VB low,

IC does not flow through transistor and light bulb does not light up

IB small, transistor is not activated,

AUTOMATIC SWITCH

EXERCISE

Explain what happen in the darkness and bright light situation

Author : Khairi

HEAT CONTROLLED AUTOMATIC SWITCH

thermistorHeat controlled automatic switch uses thermistor

Resistance of thermistor is low when the temperature is highResistance of thermistor is high when the temperature is low

AUTOMATIC SWITCH

Author : Khairi

thermistor

RB

R1

R2

siren

AUTOMATIC SWITCH


Author : Khairi

thermistor

RB

R1

R2

siren

VB = ViR1 + R2

R2

High temperature

Resistance of thermistor is low

R1 low

VB big

VB IB big

IB

Transistor is activated

IC flows through transistor

IC

Siren sounds

AUTOMATIC SWITCH


Author : Khairi

thermistor

RB

R1

R2

siren

VB = ViR1 + R2

R2

Low temperature

Resistance of thermistor is high

R1 high

VB small

VB IB small

IB

Transistor is not activated

IC does not flow

Siren does not sound

AUTOMATIC SWITCH


Author : Khairi

EXERCISE

thermistor

RB

R1

R2

siren

Vi = 6.0 V

10 kΩ

Siren is set to make a sound when the potential difference across R2 is 4.0 V.

Determine the resistance of thermistor that enable the siren to sound

AUTOMATIC SWITCH

Author : Khairi

thermistor

RB

R1

R2

siren

Vi = 6.0 V

10 kΩ

VB = ViR1 + R2

R2 4 = 6 [10 000/(R1 + 10 000)] 4 = 60 000/(R1 + 10 000) R1 + 10 000 = 60 000/4 R1 = 5 kΩ

AUTOMATIC SWITCH

Siren is set to make a sound when the potential difference across R2 is 4.0 V.

EXERCISE

Author : Khairi

thermistor

RB

R1

R2

siren

Vi = 6.0 V

10 kΩSiren is set to make a sound when the potential difference across R2is 2.0 V.If in a fire,the resistance of thermistor becomes 20 kΩ.Will the siren sound ?

VB = ViR1 + R2

R2

VB = 6 [10 000/(20 000 + 10 000)]

VB = 2 V VB = 2.0 VSiren sounds

AUTOMATIC SWITCH

EXERCISE

Author : Khairi

thermistor

R1

R2

siren

Vi = 6.0 V

Modify the circuit to control a fan which turn on when temperature is high.Explain how your circuit works.

AUTOMATIC SWITCH

EXERCISE

Author : Khairi

thermistorR2

R1

Vi = 6.0 V

AUTOMATIC SWITCH

EXERCISE

Thermistor is interchange with R1

Siren is replaced with relay

Relay is connected to fan and 240 V ac power supply

fan

240 Vac

Author : Khairi

thermistorR2

R1

Vi = 6.0 V

AUTOMATIC SWITCH

EXERCISE

When temperature is high,the resistance of thermistor becomes low.

Potential difference across R1 becomes high.

IC flows and relay is turned on.

fan

240 Vac

IB flows and transistor is activated.

Relay switches on the secondary circuit of 240 V ac and fan.

Author : Khairi

Which of the following is correct about the transistor ?

Electrode P Type of transistor

A. Emitter pnp

B. collector pnp

C. Emitter npn

D. Collector npn

PAST YEARS QUESTIONPAST YEARS QUESTION

Author : Khairi

In which circuit does the bulb light up ?

RulesC --- +B --- +

- --- E --- -


Author : Khairi

What change should be done to light up the bulb during the day time ?

A. Reverse the terminal of battery

B. Interchange R1 and R2

C. Replace the npn transistor with a pnp transistor

D. Replace resistor R3 with a resistor of lower resistance


Author : Khairi

Which circuit will the light-emitting diode (LED) light up when the switch is on ?

A. B. C. D.

RulesC --- +B --- +

- --- E --- -


Author : Khairi

a. State the relationship between resistance and intensity of light received by the light dependent resistor

Resistance is inversely proportional to intensity of light

b. Complete the diagram,if the motor works when there is light


Author : Khairi

c. What is the function of relay,R in the circuit ?

To switch on the secondary circuit


Author : Khairi

d. Explain how the circuit works.

1. When there is light,resistance of LDR is low

2. Base voltage become high

3. Base current flows and transistor is activated

4. Collector current flows and relay function

5. Relay switch on the secondary circuit


Author : Khairi

Modify the circuit in the diagram to enable a fan to switch on automatically when the room is hot.Explain your modification.


Author : Khairi

Modified circuit as shown in the diagram.

1. When temperature increases,the resistance of thermistor decreased.

2. Potential across R1 becomes high.

3. Base current flows and transistor is activated.

4. Collector current flows and relay switch on the secondary circuit which contain high voltage power supply and fan connected in series.


Education

67.transistor