[Leejohnston]

Thread 8 — Power Electronics: Motors, Drivers & High-Current Control 
How to Safely Drive Real Hardware in Embedded Systems 

Power electronics is the bridge between tiny microcontroller signals 
and the real-world devices that require serious power. 
Motors, solenoids, LEDs, pumps, heaters — none of these can be powered directly by a microcontroller pin.

This thread teaches the essential circuitry and techniques needed to control high-current loads safely and effectively.

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1. Why Power Electronics Matter

A microcontroller outputs:
• 3.3V or 5V 
• at 20–40 mA (max)

Typical loads require:
• 1–10 amps 
• 6–48 volts 
• inductive protection 
• noise handling 

Power electronics ensures:
• safety 
• isolation 
• stable control 
• long component lifespan 

Without proper driver circuits, microcontrollers burn instantly.

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2. The Most Common Power Control Devices

1. BJTs (Bipolar Junction Transistors) 
Simple switches → good for small loads < 500 mA.

2. MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors) 
Modern standard → low heat, high efficiency, logic-level compatible.

3. Relays 
Mechanical switches → great for AC mains devices.

4. SSRs (Solid State Relays) 
No moving parts, silent switching, high reliability.

5. Motor drivers (H-bridges) 
Drive motors in both directions. 
Examples: L298N, TB6612, DRV8833.

6. ESCs (Electronic Speed Controllers) 
Used for brushless motors (RC drones, robotics).

Each category solves a different real-world problem.

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3. Driving Loads with MOSFETs (The Modern Standard)

A MOSFET is a voltage-controlled switch. 
It is perfect for powering:

• high-current LEDs 
• solenoids 
• pumps 
• DC motors 
• heaters 

Basic low-side switching circuit:

MCU ----> Gate 
Load ----> Drain 
Source ---> Ground 
Diode (flyback) across inductive loads

Key requirement: 
Use a *logic-level* MOSFET (e.g., IRLZ44N) 
so it fully switches on at 3.3V or 5V.

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4. The Flyback Diode — Absolutely Essential

Inductive loads (motors, relays, solenoids) 
produce dangerous voltage spikes when turned off.

If unprotected, they destroy:
• MOSFETs 
• drivers 
• microcontrollers 

Solution: 
Place a diode (1N4148, 1N4007) across the load:

    +V ---- coil ---- MOSFET ---- GND
              |              |
              +---- diode ----+

This protects everything.

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5. Example: Controlling a DC Motor with PWM

PWM + MOSFET = smooth speed control.

Pseudo-Arduino code:

Code:

int motorPin = 9;

void setup() {
  pinMode(motorPin, OUTPUT);
}

void loop() {
  for (int speed = 0; speed <= 255; speed++) {
    analogWrite(motorPin, speed);
    delay(10);
  }
}

This ramps the motor from 0 → full speed over ~2.5 seconds.

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6. H-Bridge Control (Forward/Reverse Motors)

H-bridges allow:
• forward 
• reverse 
• braking 
• speed control 

Basic control:
• IN1 = HIGH, IN2 = LOW → Forward 
• IN1 = LOW, IN2 = HIGH → Reverse 
• both HIGH → Brake 
• PWM applied to enable pin → Speed control 

Widely used modules: 
L298N, TB6612FNG, DRV8833.

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7. ESCs for Brushless Motors

Brushless motors (BLDC) run using timed 3-phase signals. 
ESCs generate these signals using:

• PWM input 
• internal microcontrollers 
• voltage regulation 
• sensorless back-EMF detection 

Applications:
• drones 
• RC planes 
• robotics 
• electric skateboards 

Example control:
Standard 1000–2000 µs RC PWM signal.

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8. High-Current Safety Principles

When dealing with >1 amp currents:

• use thick wires 
• avoid breadboards for power circuits 
• add fuses 
• add heat sinks 
• use proper connectors 
• ensure ventilation 

Never supply motors directly from the MCU’s 5V rail. 
Use a dedicated power supply.

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9. Example Project — Fan Speed Controller

Components:
• logic-level MOSFET 
• 12V fan 
• flyback diode 
• PWM pin 

Wiring:
• Fan + → 12V 
• Fan – → MOSFET Drain 
• MOSFET Source → GND 
• MCU pin → Gate 
• Diode across fan terminals 

Allows smooth, controlled airflow at variable speeds.

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10. Recommended Next Threads

• Thread 9 — Digital Signal Processing Basics 
• Thread 10 — Build a Complete Motor Controller 
• Thread 11 — Power Supply Design & Regulation 

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End of Thread — Power Electronics: Motors & High-Current Control