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Circuits / Transistors

MOSFET switch

The gate takes voltage, not current; the pull-down R2 discharges it when S1 is released

MOSFET 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 Press S1: how much current flows into the MOSFET's gate?
  2. Measure Press S1 and tap R1. Then change R1 to 10 kΩ.
  3. Explain
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    Almost none: the gate only needs a voltage. The 0.49 mA you see goes through R1 and R2 to ground. With 10 kΩ the gate gets 2.5 V, still enough for this logic-level MOSFET: the LED stays at 9.8 mA.

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