3.4.3Electricity
Transistors
5 min read 5 parts Lesson 11 of 16
Section 1 of 5The Transistor as a Switch
A small input voltage can control a larger current in an output circuit.
npn transistor
- Has three connectionsthe base (input), the collector and the emitter.
MOSFET
- An n-channel MOSFET has a gate (input), a drain and a source and acts in a similar way.
Switching on
- The transistor turns on when the voltage at the base reaches about 0.7 V, allowing current between collector and emitter.
Switching off
- Below that voltage the transistor stays off and no current flows in the output circuit.
Section 2 of 5Input, Process, Output
Electronic systems are described in three stages.
1
The input is a sensor such as a thermistor or LDR connected in a potential divider.
2
The sensor changes resistance with temperature or light, so the voltage at the divider output changes.
3
This voltage goes to the transistor, the process stage, which compares it with its switching value.
4
When the voltage is high enough the transistor turns on and current reaches the output device.
5
The output device, such as an LED, buzzer or motor, then operates.
Section 3 of 5Potential Divider Sensors
The position of the sensor in the divider decides when the voltage rises.
Potential Divider Sensors
Voltage share
- The component with the larger resistance receives the larger share of the supply voltage.
Thermistor on top
- As temperature rises the thermistor resistance falls, so the voltage across the lower resistor rises and the transistor switches on.
LDR on top
- In bright light the LDR resistance is low, so the lower resistor has a higher voltage and the circuit switches on.
Swapping places
- Reversing the positions of sensor and resistor makes the circuit respond when the temperature or light falls instead.
| Sensor | Condition | Sensor resistance | Voltage across lower resistor |
|---|---|---|---|
| Thermistor (top) | Hot | Low | High, transistor on |
| Thermistor (top) | Cold | High | Low, transistor off |
Section 4 of 5Worked Example
Question
- A 6.0 V supply is connected to a thermistor and a 10 kΩ resistor in series.
- The transistor input is across the 10 kΩ resistor and switches on at 0.7 V.
- The thermistor is 90 kΩ when cold and 2.0 kΩ when hot.
- Does the output operate in each case?
- Coldtotal resistance = 90 + 10 = 100 kΩ, so V across the 10 kΩ resistor = 6.0 × 10 ÷ 100 = 0.6 V.
- 0.6 V is below 0.7 V, so the transistor stays off.
- Hottotal = 12 kΩ, so V = 6.0 × 10 ÷ 12 = 5.0 V, which is above 0.7 V, so the transistor switches on.
Answer The output is off when cold (0.6 V) and on when hot (5.0 V).
Section 5 of 5Exam Checklist & Tips
3.4.3 – Key Points for the Exam
- Transistor An electronic switch controlled by a small input voltage.
- Base The input connection of an npn transistor.
- Potential divider Two resistors in series that share the supply voltage in the ratio of their resistances.
- Output device A component such as a lamp, buzzer or motor controlled by the transistor.
Common Mistakes
- Saying the transistor switches on when the sensor resistance is high without checking the circuit layout.
- Forgetting that the transistor only conducts when the input is above its switching voltage.
- Mixing up the base and the collector connections.
- Connecting the output device without a protective resistor where one is required.