Circuits / Transistors
NPN transistor switch
A small base current switches a larger collector current (press S1)
Tap a part to see its voltages and currents. It is a model, not a measurement.
Predict, measure, explain
- Predict Before pressing: how much current will flow through the LED?
- Measure Press S1 and tap the LED. Then change R1 to 10 kΩ.
- Explain
Show the answerHide the answer
About 9.8 mA, set by R2, not by the transistor. With 10 kΩ the base gets ten times less current and the LED barely changes (9.7 mA): the transistor is still fully on.
What do you need?
- Push buttonSearch on Amazon
- Resistor 1 kΩSearch on Amazon
- Resistor 330 ΩSearch on Amazon
- Bipolar transistor 2N2222ASearch on Amazon
- Red LEDSearch on Amazon
- Jumper wires × 4Search on Amazon
- BreadboardSearch on Amazon
How does an NPN transistor switch work?
A small current into the base of an NPN transistor lets a much larger current flow from collector to emitter. Here, pressing S1 sends about 4.3 mA from the 5 V rail through R1 (1 kΩ) into the base of a 2N2222A. The transistor conducts and the LED lights through R2 (330 Ω). The LED current, about 9.8 mA, is set by R2, not by the transistor: once it is fully on, the transistor behaves like a closed switch. That is why raising R1 to 10 kΩ, which gives the base ten times less current (0.43 mA), barely changes the LED (9.7 mA). Release S1 and the base current stops, the transistor turns off and the LED goes dark. The same idea lets a microcontroller pin switch a load that needs more current than the pin can give.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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