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Circuits / Transistors

NPN transistor switch

A small base current switches a larger collector current (press S1)

NPN transistor switch, simulated on a breadboard

Tap a part to see its voltages and currents. It is a model, not a measurement.

Predict, measure, explain

  1. Predict Before pressing: how much current will flow through the LED?
  2. Measure Press S1 and tap the LED. Then change R1 to 10 kΩ.
  3. Explain
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    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.

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