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Basics

A brushed DC motor (e.g. a 130-size motor) has speed that mainly depends on:

  • the average voltage applied across the motor
  • the mechanical load on the motor

The essence of PWM (Pulse Width Modulation) is:

  • using a high-frequency switching square wave to change the effective (average) voltage seen by the motor

For example (assuming a 5 V supply):

PWM duty Equivalent voltage Speed
0% 0 V stopped
25% ≈1.25 V very slow
50% ≈2.5 V medium
100% 5 V full speed

Note: due to rotor inertia and the motor's inductance, the motor won't follow the PWM pulses as on/off flashes — it rotates smoothly.

Warning: Do NOT connect a PWM pin directly to the motor

Very important:

❌ Do NOT connect an MCU / Arduino / ESP PWM output pin directly to a 130 motor.

Reason:

  • A 130 motor can draw a start (stall) current of around 1 A to 2 A.
  • A PWM pin can only source/sink a few milliamps.
  • Connecting directly will likely burn the IO pin or cause the MCU to reset.

Recommended Solutions

Solution 1 — PWM + N-channel MOSFET (most recommended)

Schematic (conceptual):

MCU PWM --- R_gate (100 Ω ~ 1 kΩ) --- Gate (N-MOSFET)

Drain --- Motor --- +V

Source --- GND

Motor: reverse (flyback) diode across motor terminals (cathode to +V, anode to MOSFET drain)

Key points:

  • Use a logic-level N-channel MOSFET (fully enhances at logic-level gate voltage): examples — AO3400, IRLZ44N, IRLZ34N.
  • Add a flyback diode across the motor to clamp inductive spikes: examples — 1N5819, SS14 (Schottky), FR107.
  • Place a small gate resistor (100 Ω ~ 1 kΩ) between the MCU PWM pin and the MOSFET gate to reduce ringing and limit gate charge current.
  • PWM frequency: typically 1 kHz ~ 20 kHz for small DC motors.

Notes on diode orientation: the diode should be placed reverse-biased across the motor in normal operation (cathode to +V, anode to the MOSFET side) so it conducts when the MOSFET switches off.

Solution 2 — PWM + Motor Driver IC (easy, supports direction)

Suitable for forward/reverse control plus speed control.

Common driver ICs and notes:

  • L298N: old design, significant power loss (drop), but widely used.
  • TB6612FNG: very suitable for 130 motors (low loss, easy to use).
  • DRV8833: efficient for low-voltage motors.

Control method:

  • PWM -> EN (enable)
  • IN1 / IN2 -> direction control

Solution 3 — Relay (NOT suitable for PWM)

  • Relays can only switch on/off and are not suitable for high-frequency PWM.
  • Using relays for PWM will cause contact chattering and quickly wear or burn the contacts.

Summary

  • Use a MOSFET or motor driver IC between the MCU PWM pin and the motor.
  • Always include a flyback diode across the motor.
  • Choose a logic-level MOSFET and appropriate PWM frequency (1 kHz–20 kHz).
  • Avoid driving motors directly from MCU IO pins or using relays for PWM.

PWM frequency recommendations (important)

Frequency Result
< 200 Hz Motor will produce audible humming
500 Hz – 5 kHz Usable
5 kHz – 20 kHz ✅ Best (inaudible)
> 30 kHz MOSFET heating increases

Common problems and explanations

Q: When I change PWM a little, the motor speed suddenly jumps?

Possible causes:

  • Motor starting voltage threshold (there is a minimum voltage needed to overcome static friction)
  • Duty cycle too low to overcome friction or load
  • MOSFET is not a logic-level type and doesn't fully turn on at the MCU's gate voltage

Solutions:

  • Ensure a minimum PWM duty (e.g., >= 20%) for reliable startup
  • Implement a soft-start by ramping PWM gradually
  • Use a proper logic-level MOSFET or a suitable motor driver IC

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