AUTONATOMYKNOWLEDGE IS HORSEPOWER
COURSE 02 · SEMICONDUCTORS & ELECTRONIC SWITCHING · MINI COURSE 03
Interactive mini course 03

Field-Effect Transistors
& MOSFETs

Move from current-controlled BJTs to voltage-controlled MOSFETs. Operate an electronic load, visualize pulse-width modulation, then use circuit measurements to diagnose an automotive module driver.

10 guided lessonsN-channel + P-channelPWM workbench5 diagnostic cases
Your learning progress
Completed activities0 / 17

10 lessons + 1 workbench + 5 diagnostic cases + 1 final knowledge check. Progress is saved on this device.

Learn it

Lesson 01

Remember:
Experiment • low voltage educational model

Switch a 12 V load

Select the MOSFET arrangement. Change the gate command, load, and PWM duty cycle. Watch the resulting current, switch voltage, and conduction loss.

State

Model assumptions: enhancement-mode MOSFET, resistive load, ideal 12 V-class gate drive, illustrative threshold of 2.5 V and fully driven |VGS| ≈ 8 V. Actual MOSFET transfer curves, gate-driver limits, switching losses and thermal behavior vary by device. Do not use these values to size a real circuit.

Live circuit

N-channel low-side driver

LOADN-MOSFET+12 VGate +10 VDrainSourceBattery negative / 0 V
0 ALoad current • ON
0 VVoltage across load • ON
0 V|VDS| • ON
0 WMOSFET conduction loss • ON
0 WAvg. load power at PWM duty
0 VGate-to-source voltage

PWM: gate command vs. load current

Gate commandLoad current
Gate command Load current (illustrative resistive load)
On-state current
0%
On-state loss
0 W
Engineering note

Threshold ≠ fully on

A datasheet's VGS(th) describes a gate voltage at which a specified tiny drain current begins to flow. It does not promise low resistance or safe load switching at that gate voltage. For a real part, check its RDS(on) rating at the available gate drive, maximum VGS, current, thermal limits, body diode, and gate-charge requirements.

A MOSFET gate is insulated and draws very little steady-state DC current, but charging and discharging its gate capacitance takes current. Fast or high-power switching needs a suitable driver, protection, and thermal design.
The Challenger • technician mode

Cooling fan electronic driver

Conceptual automotive circuit: a fused 12 V supply feeds a resistive-equivalent fan load. An ECU commands an N-channel MOSFET on the ground side. Select a fault, inspect simulated voltage readings, then identify the most evidence-supported next action.

FANFETECU GATEB+FUSEDSGROUND
Safety: All values below are simulated. Do not jumper an ECU output, bypass a fuse, or connect a meter's current input across a powered circuit. Verify actual wiring and test points against service information before testing a real vehicle.

Simulated meter readings

—Load feed → ground
—Gate → source
—Drain → ground
—Source → ground
—Across load
—Load current

0 / 5 diagnostic cases completed.

Final assessment

Test your MOSFET knowledge

Answer eight questions. A score of at least 6/8 completes the knowledge check. You may retry.

AUTONATOMY · A self-contained educational prototype. Simplified numerical models and diagnostic readings are for learning, not service specifications or circuit design.