driving a DC motor

[asjad]

Hi friends,

I need a bit of help regarding driving a motor.


If I connect a PIC to the base of a NPN transistor via a resistor, and the
motor connected between +vcc and the collector
(transistor configured as a switch)

If I turn the transistor on, then current will flow causing the motor to come on.

I know how to calculate to make x amount of Ic to flow, how do I know what the voltage is being supplied to the motor??

If am using PWM on the base of the transisotr, what voltage will appear across the motor???

I hope you can understand

THANK YOU
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Best Regards

[Ttelmah]

[SherpaDoug]

First you need a catch diode across the motor with cathode to the motor +vcc terminal. Without that diode when you shut off the motor it will generate a voltage spike that can damage the transistor and maybe other things on the +vcc bus.

When the transistor is ON (saturated) the motor will see the +vcc voltage minus the saturation voltage of the transistor. The saturation voltage you get from the transistor datasheet knowing the aproximate collector current, base current and temperature. Generally for small transistors it will be 0.2V to 0.5V, 1V for darlington transistors.
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The search for better is endless. Instead simply find very good and get the job done.

[dbotkin]

I'd suggest using a MOSFET rather than a bipolar transistor for this, by the way. Especially for driving small DC motors - I often use a 2N7000, or even two of them in parallel.