Picking the motor is the easy half. Finding one that produces the torque you need is a catalog lookup; the electronics that make it produce that torque on command, repeatably, without cooking itself or corrupting the rest of the board, is where motion projects spend their schedule. A driver sized on continuous current alone survives a bench test and fails the first time the mechanism jams.

This article covers the electronics layer: driver selection, feedback devices, current sensing, and the protection and noise work that separates a demo from a product. Which motor type to use is an upstream decision laid out in stepper versus servo versus brushed DC, and the torque number itself comes from calculating the motor torque your product needs.

Sizing the Driver: Three Currents, Not One

Every driver datasheet leads with a continuous current rating, the least useful of the three numbers you need.

  • Continuous current is what the motor draws doing its normal job. Size the driver at 1.5 to 2 times this, because the rating assumes a thermal environment you will not reproduce.
  • Stall current is supply voltage over winding resistance, often eight to fifteen times running current. A 12 V motor with 1.5 ohms pulls 8 A stalled, and the driver sees that on every startup transient and every hard stop.
  • Peak current during acceleration sits in between and lasts long enough to matter thermally.

The practical rule: pick a driver whose continuous rating covers running current with margin and whose peak rating covers stall, then verify the current limit engages before stall current arrives.

Integrated Driver or Discrete Bridge

Below about 5 A and 40 V, an integrated H-bridge or three-phase module is the right answer: fewer parts, built-in protection, a layout you cannot get badly wrong. Above that you move to discrete MOSFETs with a separate gate driver and take on gate loop layout, dead time, shoot-through prevention, and bootstrap supply design — roughly triple the electronics effort, worth doing only when the integrated part genuinely cannot carry the load.

Either way, compute conduction loss at your actual current. A bridge with 60 milliohms per FET carrying 6 A dissipates about 4.3 W across two conducting devices, which is a real heat sink and a real thermal problem, the one covered in thermal management in electronic products.

Feedback: What You Need to Know and How Precisely

Start from the requirement, not the sensor catalog. Do you need to know that the motor is turning, how fast, or exactly where the output shaft is?

No feedback is legitimate for a fan, a pump, or a vibration motor. Open-loop steppers also count — they hold position by construction and lose it silently when overloaded, which is why they usually get a stall detection scheme.

Hall sensors in a BLDC motor are commutation sensors first, telling the driver which phase to energize. They also give coarse speed and position — a 7 pole-pair motor produces 42 hall edges per revolution, about 8.6 degrees of resolution. Good enough for speed control, not for positioning.

Incremental encoders output quadrature A and B channels, typically 100 to 4,096 counts per revolution before the 4x decode, giving real speed and relative position for $3 to $30. Position is lost at power-off, so the machine needs a homing routine against a limit switch on every startup. Absolute encoders report shaft angle over SPI or SSI at 12 to 14 bits and remember it through a power cycle; magnetic versions are now cheap enough for consumer products and eliminate homing entirely.

Sensorless control infers rotor position from back-EMF or inductance variation. It removes wires and cost and works well at speed; it is weak at startup and very low speed, where there is no back-EMF to measure. Fans, pumps, and power tools use it routinely; anything that must produce controlled torque from standstill should not. Where the load moves in a straight line, the feedback often belongs on the load rather than the motor, which overlaps with choosing a linear actuator.

Current Sensing Earns Its Place Three Times Over

Measuring motor current gives you torque, protection, and diagnostics from one circuit. Torque in a DC or BLDC motor is proportional to current, so a current loop inside a speed loop is how you get smooth, controlled force. Protection comes from acting on an overcurrent within microseconds instead of waiting for a thermal fuse. Diagnostics come from the signature: a mechanism that drew 0.9 A and now draws 1.4 A for the same motion is wearing out, which supports the monitoring in turning product data into failure warnings.

The common topologies:

  • Low-side shunt. A 5 to 50 milliohm resistor between bridge and ground, read by a standard op-amp. Cheapest, PWM-synchronous because it only sees current when the low-side FET conducts, and blind to a short to ground.
  • In-line phase shunt with a current-sense amplifier sees true phase current continuously, which good field-oriented control requires.
  • Hall-effect sensors. Galvanically isolated and lossless, and the natural choice above roughly 30 A or at mains voltage.

Whichever you use, place the shunt with a proper Kelvin connection, route the sense traces as a tight differential pair, and filter before the ADC. A poorly routed shunt reads ground bounce from the very switching you are trying to measure.

Protection and the Noise a Motor Makes

Build in overcurrent shutdown with a latch and controlled retry, thermal shutdown on the driver, undervoltage lockout so a sagging battery does not brown out the MCU mid-commutation, and a defined stop behavior on watchdog reset. Add bulk capacitance close to the bridge — roughly 100 microfarads per amp of peak current — so the switching current loop stays local instead of circulating through the board.

Then treat the motor as the noise source it is. Brushed motors arc at the commutator and emit broadband RF, so add capacitors across the terminals and from each terminal to the case. Keep the power stage physically separate from analog and RF sections, use a single-point connection between power and signal ground, twist or shield motor leads, and route encoder signals away from phase wires. This work decides whether the product passes emissions on the first attempt, with the budget consequence spelled out in EMC testing cost.

Building the Motion Stack

Projects House designs motor control from the mechanism outward: torque and duty-cycle requirements, driver and feedback selection, current sensing and protection, and control loops that hold up when the load is not what the spec said. Describe your mechanism through the contact form.