Motor driver

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kinds of motor drivers

There are many kinds of motor drivers, each one specialized to drive its own type of motors:

  • servo motor controller
  • stepper motor controller
  • DC motor controller ("brushed")
  • AC motor controller ("brushless")
  • ... (todo: fill in the other kinds) ...

In all cases, we have an electric motor that has wires coming out of it. At any one instant, the motor controller connects each wire to either the Hi voltage on the + side of the battery, or to the Lo voltage on the - side of the battery, or neither. When we tell the motor controller to make the motor go "forwards" or "backwards" or "fast" or "slow", the motor controller changes which wire is connected to which end of the battery (or not connected at all). Some motor controllers switch the connections thousands of times per second in some modes.

A DC motor controller that is 'reversible' generally uses an 'H bridge'. This 'H-bridge' uses four output drivers in a configuration that resembles an H where the load is the cross bar in the middle. The lines on either side of the load (the downward strokes in the H) represent a series connection of a pull-up driver and a pull-down driver. This allows each terminal of the load to be connected to either the positive supply rail, or the negative supply rail. This allows a positive, negative or zero voltage difference across the load. This load voltage is then utilized to provide the desired control required of the motor. The various combinations can give a 'forwards' torque on a DC motor, a 'backwards' torque on the same motor, can allow the motor to free-wheel (without any applied torque) or can provide a locking of the motor such that it resists any attempt to rotate it.

A single phase AC motor is generally driven in the same way as a DC motor, however instead of operating the motor drive as a constant DC voltage (in either the 'forward' or 'reverse' direction) the AC motor is driven by an approximation to a sinewave. This approximation is created using the H bridge and driving it with a PWM input such that both the positive and negative voltage periods are the same. This is normally achieved either using a sawtooth waveform compared against a sine wave reference, or is done using a lookup table in a microcontroller.

Push Pull Transistor Circuit: one half-bridge. (Fixme: show the flyback diodes, and convert to the more common MOSFET drive transistors ... also replace the resistive "load" with a (M) motor symbol.)

A similar method is used to drive multiphase (3-phase) AC motors, however instead of just using an H bridge, only a half H bridge is used per phase (3 half-bridges). Each phases half bridge is then driven in the same manner as for the single phase motor, with a phase difference between the phases as appropriate.

Most stepper motor controllers uses 2 independent H bridges (4 half-bridges) for the 2 independent coils of a stepper motor. Each possible state (one bridge driving current one way, the other way, or free-floating) of both bridges gives 4 "full steps", 4 "half-steps" between the full steps. The "microstepping" motor controllers use PWM to gradually change in a sine-wave-like manner from adjacent full-steps and half-steps.

((fill in more details here...))

noise control

Many motors make sparks when the brushes make or break contact. This causes causes lots of electrical noise ("brush noise"). Your TV-watching neighbors won't be happy if you allow this noise to leak out.

Some people fix this by slapping a .1uF cap across the motor leads. (photo of noise-control capacitor on a Open Servo).

"Sparks emit RF energy from DC to daylight as I was once told by an EMC expert." -- HydraRaptor: "DC to daylight". More details: HydraRaptor: "GM3 motor suppressor"

current sense

Often people want to measure the current going through the motor.

See current sense for several different techniques.

tolerance against software bugs

Some motor controller circuits are such that, if the software accidentally sets the "wrong" pins hi or lo, you get a short circuit through the output drivers. This will generally cause a high current to flow, due to the low on state resistance of the output drivers, which may destroy other electronic components before finally blowing the supply fuse.

Other motor controller circuits are such that, if the software accidentally sets the "wrong" pins hi or lo, the worst that could happen is the motor spins the wrong way. These circuits are designed so that, no matter what the inputs, it is impossible to get a short circuit through the output drivers. Between "one branch on" and "the other branch on", there is a minimum "blanking time" which has "both branches off". This guarantees that we never have "both branches on" (short circuit).

Guess which type of design I prefer?

FET driver

What do you put between the CPU output pins and the 4 FETs of the H bridge?

The simplest solution is to use 2 lo-side nFETs and 2 hi-side pFETs, and use a power supply for the motor that has the same voltage as the CPU power supply, and drive the 4 FETs directly using 2 CPU output pins. One of those output pins connects to the gates of the left side and controls whether the left leg of the motor is Hi or Lo. The other output pin connects to the gates of the right side and controls whether the right leg of the motor is Hi or Lo.

But alas, that circuit won't work for any of the following situations:

  • you want to run the motor off a much higher voltage -- say 12 V.
  • you want to use 4 nFETs (because they are slightly cheaper, and it's simpler to stock one kind of FET rather than 2 kinds) rather than 2 nFETs and 2 pFETs. You need a "nFET high side driver". There are several clever circuits for generating a "Vpp" voltage that is higher than your motor power supply voltage; "Vpp" is needed to turn high-side nFETs completely on.
  • you want more isolation between the "noisy" motor power supply and the "quiet" CPU power supply.
  • you are driving a large FET with high gate capacitance, and your CPU output pins can't source or sink enough current to turn the FET on and off fast enough.
  • You want a hardware-enforced blanking time as alluded to earlier.

external links

A random collection of semi-related links in no particular order (please prune out the irrelevant ones):

  • WZMicro: has many low cost single stepper motor driver, Dual stepper motors controolers, and Microstepping motor driver at affordable price and many Stepper Motor DIY projects .
  • Avayan Electronics has many bipolar microstepping stepper motor driver boards and DC motor H bridge drive boards (up to 40A continuous). Many (all?) of them are open source. Avayan sells insanely cheap empty PCBs [1] [2].
  • AVRSTMD: AVR-Based Microstepping Bipolar Chopper Stepper Motor Driver (STMD). Based on two National Semiconductor LMD18245T 3A, 55V DMOS Full-Bridge Motor driver chips and a Atmel AVR ATMega48. Drives one stepper motor. Optically isolated so you can connect directly to your printer port. Open Source - The schematic, parts list, and software are all freely downloadable. Easily repaired -- removable screw terminals; all parts are through-hole; etc.
  • Open Source Motor Control (OSMC): The OSMC is a high-power H-bridge circuit designed to control permanent magnet DC motors. It was designed expressly as a motor control for robot combat. Supply voltage: 13V to 50V (36V max battery rating). Output Current (continuous): 160A. Uses 4 MOSFETS (IRFB3207) in each leg of the H bridge, for a total of 16. Bridge Driver: Intersil HIP4081A
  • RepRap stepper motor driver is based around the Allegro A3982 bipolar Stepper Motor Driver with Translator (up to 35 V and up to ±2 A). Like all the RepRap electronics, it is open-source and available on the Sourceforge RepRap project files. You can buy the fully assembled board from (among other places) MakerBot Industries
  • GoBox: a group designing motor driver electronics, a charge controller to optimize getting energy from a variety of energy sources (MTTP solar, wind, water, etc.), and related devices. "The designs and programs are released under a Hardware Open Source License."
  • H-Bridge by Bob Blick
  • the Open Source Motor Controller Project
  • LiniStepper $30 each; Open Source! Circuit Diagram, PCB (Board) Layout, and PIC Software all available. Nice photos of the LiniStepper at .
  • "Design of a High Current Bipolar Stepper Motor Driver"
  • H-Bridge Fundamentals An in-depth article on the design of Mosfet H-Bridges
  • Open Source circuits and information on stepper motor controllers] -- in particular, "PMinMO: Stepper Driver Information"
  • ePanorama ePanorama Motor Control page
  • "Electronic Design of DC Motor Drives" has detailed schematics and PCB layout for a system that has a PC send commands through the serial port to a Microchip PICmicro, which does PWM control of 2 H bridges. Each half-bridge uses a IRF9530N (100V 14A pfet plus flyback diode) and a IRF530 (IRF530NPBF: 100V 17A nfet plus flyback diode), driven by a small transistor inverter based on a BD135 npn, for a total of 12 discrete transistors.
  • OpenServo wiki -- developing a digital servo motor that accepts "Go to position X" commands and also more complex curves, and returns actual servo position, speed, voltage and power consumption.
  • MOSFETs and MOSFET drivers

L298 quadruple half-bridge driver, aka dual full-bridge driver: pin-compatible with SN754410 and L293 and L293D. They all require external flyback diodes when driving motors (the L293D is apparently the only one with built-in flyback diodes).

  • L297 stepper motor controller + L298 dual full-bridge driver: for driving one (4-wire) bipolar stepper motor (2 A); direction and step inputs; half-stepping; on-chip PWM chopper limits current. external diodes are required (preferably Shottky). (Both chips come in through-hole and SMT versions)
  • L298 dual full-bridge driver can also be used to drive 2 independent DC motors (2 A each); external diodes are required (preferably Shottky). L293D is similar, but only rated up to 1 A, 36 V.
  • Motor Shield - Arduino motor/stepper/servo control: has 2 connections for 5V 'RC' 'hobby' servos; and also 4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, 4.5V to 36V; each L293D can be independently programmed to either drive a unipolar or bipolar stepper motor with full, half, or microstepping -- or else drive two bi-directional DC motors -- so this board can control a maximum of 4 DC motors and 2 RC servomotors.
  • SN754410 quadruple half-bridge driver is pin-compatible with the L293. DIP package. 1 A per bridge continuous (2 A peak) with thermal shutdown protection, 4.5 V to 36 V. Requires external flyback diodes (preferably Schottky).


Using the A3977 microstepping driver chip from Allegro:


astronomy telescopes use motor drivers:


Robots use motor drivers.

self-balancing personal transportation systems

Main Article: vehicle

Self-balancing personal transportation systems use motor drivers:


(This doesn't have much to do with motor drivers -- is there a better page for electric power generation tips?)

further reading

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