Transistors (SQA National 5 Physics): Revision Note

Transistors

Electronic systems

• Electronic systems consist of three parts

  • Input

  • Process

  • Output

• Information is passed along the system by electrical signals

• Input devices detect or measure changes in the surroundings and produce an electrical signal to be processed

  • Switch - allows current to flow when closed

  • Microphone - produces a voltage when the sound level increases

  • Photovoltaic cell - produces a voltage when light intensity increases

  • Thermistor - resistance decreases when the temperature increases

  • Light-dependent resistor (LDR) - resistance decreases when the light level increases

• Output devices transform electrical energy into a useful form of energy

  • Lamp - converts electrical energy to light

  • LED - converts electrical energy to light

  • Loudspeaker - converts electrical energy to sound

  • Motor - converts electrical energy to kinetic energy

  • Relay - converts electrical energy to kinetic energy

• Process devices operate as an electronic switch between the input and output devices

  • This is the job of the transistor

What is a transistor?

• A transistor is an electronic switch

• It switches on when the input voltage reaches a certain value

• There are two types

  • npn transistor

  • MOSFET transistor

• Both types have three connections

• An npn transistor switches on at around 0.7 V

  • ON: If the input voltage across the base exceeds 0.7 V, current flows from collector to emitter

  • OFF: If the input voltage does not exceed 0.7 V, current cannot flow through the output

• A MOSFET transistor switches on at around 2.0 V

  • ON: If the input voltage across the gate exceeds 2.0 V, current flows from source to drain

  • OFF: If the input voltage does not exceed 2.0 V, current cannot flow through the output

npn and MOSFET transistor symbols

Transistor switching circuits

• A transistor switching circuit is used to represent an electronic system

• All switching circuits follow input → process → output

  • Input = a potential divider made from a sensor (e.g. LDR or thermistor) and a resistor (usually variable)

  • Process = a transistor (npn or MOSFET)

  • Output = device to be switched on or off (e.g. LED, motor, relay)

Basic set-up of a switching circuit

Light-controlled circuits

• Light-controlled circuits use an LDR and a variable resistor as the input

• Consider a circuit connected to an LED as the output:

  • When the light level decreases (gets darker), the resistance of the LDR increases, and the voltage across the LDR increases

  • With the LDR as the lower component of the divider, the voltage across the transistor increases

  • When the voltage exceeds the threshold (0.7 V for npn), the transistor conducts and current flows to the LED, switching it on

An npn transistor circuit with an LDR

• Function of the circuit: the LED switches ON when the light level gets too low

  • If the LDR and the variable resistor were swapped, it would invert the behaviour (i.e. the LED would switch OFF when the light level gets too low)

• Function of the variable resistor: controls the light level at which the transistor switches on

  • For example, increasing the resistance of the variable resistor would decrease the share of the supply voltage across the LDR

  • The LED would switch on at a lower light level (i.e. dimmer) than before, as the resistance of the LDR would need to be greater to switch on the transistor

Operation of an LDR switching on an LED

Temperature-controlled circuits

• Temperature-controlled circuits use a thermistor and a variable resistor as the input

• Consider a circuit connected to a relay and a heating circuit as the output:

  • When the temperature decreases (gets colder), the resistance of the thermistor increases, and the voltage across the thermistor increases

  • With the thermistor as the lower component of the divider, the voltage across the transistor increases

  • When the voltage exceeds the threshold (2.0 V for MOSFET), the transistor conducts and a current flows to the relay coil

  • This closes the relay switch and allows a current to flow in the heater circuit, switching the heater on

A MOSFET transistor circuit with a thermistor

• Function of the circuit: the heater switches ON when the temperature gets too low

  • If the thermistor and the variable resistor were swapped, it would invert the behaviour (i.e. with a fan instead of a heater; the fan would switch OFF when the temperature gets too low)

• Function of the variable resistor: controls the temperature at which the transistor switches on

  • For example, increasing the resistance of the variable resistor would decrease the share of the supply voltage across the thermistor

  • The heater would switch on at a lower temperature than before, as the resistance of the thermistor would need to be greater to switch on the transistor

Operation of a thermistor switching on a heater