Transistors as Switches

Transistors let a small control signal switch a large load. An MCU GPIO pin (sourcing a few mA at 3.3V) can control a motor, relay, LED strip, or solenoid drawing amps at 12V or more.

Why It Matters

Microcontrollers cannot drive loads directly — their GPIO pins max out at 10-20mA. Every actuator, power LED, relay, and motor in an embedded system needs a transistor (or transistor-based driver IC) between the MCU and the load. Understanding switching circuits is the link between Digital Logic and the physical world.

How It Works

MOSFET Basics

A MOSFET is a voltage-controlled switch. The gate-source voltage (Vgs) controls whether current flows between drain and source.

Vgs > Vth  ->  ON  (drain-source conducts, Rds_on typically 10-200 milliohms)
Vgs < Vth  ->  OFF (drain-source open, leakage only)

Vth (threshold voltage): typically 1-4V. A “logic level” MOSFET has Vth low enough to fully turn on from a 3.3V or 5V GPIO pin. Standard MOSFETs need 10V on the gate — unusable with MCUs directly.

Gate charge (Qg): the gate is a capacitor. Switching speed depends on how fast you can charge/discharge it. Higher Qg = slower switching = more transition losses.

N-Channel Low-Side Switch

The load sits between Vcc and the drain. The MOSFET connects the load to ground.

       Vcc (12V)
        |
      [LOAD]  (motor, LED strip, relay coil)
        |
    Drain ─┐
           MOSFET
    Gate ──┤     ← MCU GPIO (3.3V or 5V)
           │
    Source ─┘
        |
       GND

MCU HIGH → MOSFET ON  → current flows → load active
MCU LOW  → MOSFET OFF → no current    → load off

Simple, reliable, works with any load voltage. The load’s ground path goes through the MOSFET.

P-Channel High-Side Switch

Source connects to Vcc. Gate must be pulled LOW (relative to source) to turn on.

       Vcc (12V)
        |
    Source ─┐
           MOSFET
    Gate ──┤     ← drive circuit (NOT direct MCU if Vcc > 5V)
           │
    Drain ──┘
        |
      [LOAD]
        |
       GND

Advantage: the load keeps its ground reference intact (important for ground-sensitive circuits). Disadvantage: gate drive is trickier when Vcc is high — you often need a level shifter or an N-channel MOSFET to pull the P-channel gate low.

Flyback Diode for Inductive Loads

Motors, relays, and solenoids are inductors. When you switch off, the collapsing magnetic field generates a voltage spike that destroys MOSFETs.

       Vcc
        |
      [RELAY]──┐
        |      [D] ← flyback diode (1N4148 or similar)
        |       |    cathode to Vcc, anode to drain
    Drain ──────┘
        |
      MOSFET
        |
       GND

Always add a flyback diode across inductive loads. A 1N4148 works for small relays; use a Schottky (1N5819) for faster clamping with motors.

H-Bridge for Motor Direction

Four transistors arranged so you can reverse current through the motor.

       Vcc            Vcc
        |               |
      [Q1 P]          [Q3 P]
        |               |
        +─── MOTOR ────+
        |               |
      [Q2 N]          [Q4 N]
        |               |
       GND             GND

Q1+Q4 ON, Q2+Q3 OFF  →  forward
Q3+Q2 ON, Q1+Q4 OFF  →  reverse
ALL OFF               →  coast
Q2+Q4 ON              →  brake (shorts motor)

In practice, use an H-bridge IC (L298N, DRV8833, TB6612FNG) rather than discrete transistors. They include protection diodes, dead-time logic, and current sensing.

Logic Level MOSFETs

Part	Type	Vds_max	Id_max	Rds_on (at Vgs=4.5V)	Vth
IRLZ44N	N-ch	55V	47A	22 milliohm	1-2V
IRL540N	N-ch	100V	36A	44 milliohm	1-2V
IRLML6344	N-ch (SMD)	30V	5A	29 milliohm	0.8V
Si2301	P-ch (SMD)	20V	2.8A	100 milliohm	0.7V

Heat Dissipation

When a MOSFET is fully on, it dissipates:

P_dissipated = Rds_on x I²

IRLZ44N switching 5A: 0.022 x 25 = 0.55W (fine without heatsink). IRLZ44N switching 20A: 0.022 x 400 = 8.8W (needs heatsink or forced air).

Switching losses add to this at high PWM frequencies — the MOSFET passes through its linear region during transitions, dissipating more power.

MOSFET vs BJT

	MOSFET	BJT
Control	Voltage (gate)	Current (base)
Input impedance	Very high (gate is capacitor)	Low (base draws current)
Switching speed	Fast (ns)	Slower (us)
Power handling	Excellent (low Rds_on)	Limited
MCU drive	Direct (logic level types)	Needs base resistor (1k-10k)
Best for	Power switching, PWM	Small signal, legacy circuits

Calculation Example

# Size a MOSFET for a 12V LED strip drawing 3A
v_load = 12.0
i_load = 3.0
 
# IRLZ44N: Rds_on = 0.022 ohm at Vgs = 5V
rds_on = 0.022
p_mosfet = rds_on * i_load**2   # 0.198W (no heatsink needed)
v_drop = rds_on * i_load        # 0.066V (negligible)
 
# BJT alternative: need base resistor
# NPN 2N2222, Ic = 3A, hfe(min) = 50
hfe = 50
ib_needed = i_load / hfe        # 60 mA (too much for most MCU pins!)
# This is why MOSFETs are preferred for power switching
 
# Gate charge time (switching speed)
qg = 48e-9          # 48 nC for IRLZ44N
i_gate_drive = 10e-3  # 10 mA from MCU pin
t_switch = qg / i_gate_drive  # 4.8 us (acceptable for low-freq PWM)

Related

• Voltage Current Resistance — Ohm’s law for sizing components
• Motors and Actuators — what transistors drive
• Digital Logic — logic gates are transistor pairs
• Power Supply Design — MOSFETs in switching regulators