Week 6 · Drivers & Motors

Session 26: Driving loads with transistors and MOSFETs

Learning outcomes

Explain why an Uno pin cannot drive a motor, relay or fan
Wire a logic-level MOSFET low-side switch with gate resistors and a flyback diode
Measure VDS and capture the turn-off spike on the scope
Choose between an NPN transistor and a MOSFET for a load

Uno, logic-level N-MOSFET (for example IRLZ44N), 220 Ω and 10 kΩ resistors, 1N4007 diode
Small DC motor or fan and a separate supply
Optional: BC337 NPN transistor and 1 kΩ base resistor
Multimeter and two-channel oscilloscope

Students read these on their phones before class. They are listed on the student Before class page.

0:0010 min
Recap
Task A reflections
0:1020 min
Demo
From transistor theory to an Uno pin
Build on what students already know about transistors: 5 V gate drive, logic-level parts, common GND, and why inductive loads need a diode.
0:3040 min
Build
MOSFET fan driver
Students build the lesson circuit and run the worked sketch's PWM ramp, checking common GND and diode direction before power.
1:1035 min
Measure
Verify the switch
Measure VDS at full power, then capture gate and drain on the scope with and without the flyback diode and record the spike height.
1:4515 min
Wrap-up
NPN or MOSFET?

Each student's fan ramps up under PWM through a MOSFET with a correctly oriented diode, and they have recorded VDS and a turn-off capture.

Drive a relay coil with a BC337 and compare the base current with the MOSFET gate current.

Rebuild from the lesson's wiring list step by step; a partner checks the diode and GND before power.

Pre-reading for S27.

Students already know the transistor theory - keep the demo short and spend the time measuring.
Remove the flyback diode only briefly, only with a small motor, and only for the scope capture.