Electronic – Using MBED PWM for motor control

mbedtransistors

I have an mbed project with firmware that controls some PwmOut switches. All works fine firmware opens and closes switches as expected. I tested this with a LED (and resistor).

Now my goal is that instead of a light coming on, a electric brush motor (with a simple +/-) 3v-5v should run. I understand that the PwmOut from the mbed will not provide enough current to run the motor.

Will I need a transistor that connects the PwmOut port with the motor?

If yes:

  • how do I know what type I need?
  • these three legged guys – how are they hooked up?

Best Answer

You can follow the recipe at http://mbed.org/cookbook/Motor which uses the Motor interface for driving a standard DC motor with PWM and an H-Bridge.

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If, instead of using the L293D dual H bridge driver, you want to use a transistor to drive the motor. You choose the transistor mainly by looking for a switching transistor which has a current and voltage rating that exceed the current and voltage ratings of the motor (so it depends on the motor) - for high currents you may need a heatsink for the transistor.

For a bipolar junction transistor (BJT) you'll need a resistor to limit base current. Most driver circuits incorporate a diode to prevent harm to the transistor from back-EMF from the motor windings.

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The connections to a BJT are base, emitter, and collector

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You can find out which leg is which by looking at the data sheet for the specific transistor you have chosen. For example

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You can instead use a MOSFET, these can be more efficient (lower voltage drop when 'on')

Some rather good instructions can be found here

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