Master the Basics: How to Use a MOSFET As a Switch

MOSFETs are widely used to build electronic switches due to their high input impedance and ability to control high-power loads. Unlike BJTs, a MOSFET’s gate draws almost no current, making it ideal for use with low-power control devices like microcontrollers.

Why Use a MOSFET as a Switch?

• Requires very little current to activate
• Can handle higher voltages and currents than typical BJTs
• Well suited for switching motors, lights, and other loads

Low-Side N-Channel MOSFET Switch

To create a low-side switch using an N-channel enhancement mode MOSFET:

• Connect the source to ground
• Connect the drain to the low side of the load (e.g., motor)
• Connect the high side of the load to V+ (e.g., 12V power supply)

To control the gate:

• Apply a positive control voltage (HIGH) to turn ON the switch
• Drop the control voltage to 0V (LOW) to turn OFF the switch

Tip: Always add a pull-down resistor (e.g., 10kΩ) between the gate and source to ensure the gate discharges fully when the switch turns off.

Key Differences from BJT Switching

• No current-limiting resistor needed on the gate
• Use a pull-down resistor instead of a series base resistor
• Still include a flyback diode across inductive loads like motors to suppress voltage spikes

Microcontroller Compatibility

• Arduino outputs 5V digital signals
• Raspberry Pi outputs 3.3V digital signals

Not all MOSFETs will turn on fully at these voltages. Many require 10–15V at the gate. For microcontroller projects, choose a logic-level MOSFET that can switch fully on at 3.3V–5V gate voltage.

Recommended MOSFET: RFP30N06LE

A popular logic-level N-channel MOSFET for hobbyists:

• Gate threshold voltage: Low enough for 3.3V–5V logic
• Drain-source resistance (RDS(on)): < 47 mΩ (low loss)
• Max drain-source voltage (VDS): 60V
• Max continuous current: 30A

If switching more than ~1A, use a heat sink to prevent overheating.

Key Takeways

Using a MOSFET as a switch is efficient, scalable, and microcontroller-friendly. With the right component (like a logic-level N-channel MOSFET), you can build reliable and responsive switches for controlling everything from LEDs to motors. Just remember:

• Ensure VGS threshold is compatible with your control logic
• Add a pull-down resistor to gate
• Use a flyback diode for inductive loads
• Include a heat sink if switching higher currents

Mastering MOSFET switching unlocks a wide range of practical applications in hobbyist and professional electronics