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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.

What do you need?

How does a MOSFET work as a switch?

A MOSFET is controlled by the voltage on its gate, not by a current: the gate is insulated, so once it is charged it takes almost nothing. Here, pressing S1 puts about 4.9 V on the gate of an IRLZ44N through R1 (220 Ω). The MOSFET turns on between drain and source and the LED lights at about 9.8 mA, set by R3 (330 Ω). The only steady current on the gate side, 0.49 mA, goes through R2, not into the gate. R2 (10 kΩ) is the pull-down: when you release S1 it drains the gate's charge to ground, so the MOSFET turns off instead of staying on with a floating gate. Even with R1 at 10 kΩ, the gate gets 2.5 V and the LED stays at 9.8 mA, because the IRLZ44N is a logic-level part, made to switch fully from a 3.3 V or 5 V pin.

Values from the Protoboard ISO simulation: a model, not a bench measurement.