Dumitru Stama wrote:
> I want to hook up a motor and control its motion using one output pin
> of a PIC microcontroller. I just want to try to spin it without using
> PWM or any other fancy stuff.
> Since i want a simple design i am thinking about using a Darlington
> transistor that can handle about 1A of current.

A darlington wouldn't be my first choice since it has such a high on
voltage.  A simple NPN with sufficent gate drive or a N channel FET would be
good enough.  What is the maximum power supply voltage on the other side of
the motor?  If it's low enough, a "logic" level FET would be fine.  As long
as you can guarantee 20V won't be exceeded, IRFU3706 would work very well
with the gate tied directly to the PIC output.  It can handle a lot more
than the 4A motor stall current, and its on resistance is so low that it can
take that indefinitely and will barely get noticeably warm since it won't
even dissipate 100mW.

> Now, my question is : what if the motor will get stuck somehow and it
> will suck up to 3, 4A ? What will happen in this case ? Will the
> Darlington survive ?

Think about it.  A darlington will have 900mV accross it roughly, probably
more at 4A.  Even so, 4A * 900mV = 3.6W.  That will require some sort of
heat sinking, plus it eats up a volt or so of the motor's drive.

You can also do this with a plain old NPN power transistor.  This might be a
good choice if the motor supply is too high for a logic level FET and you
don't want to create a separate 12V supply just for the FET gate drive.
Let's say you can get a minimum guaranteed gain of 15 at 4A.  That means the
base current will have to be 267mA.  If your 5V supply can handle that, all
you need is another NPN was emitter follower and resistor from its output to
the base of the power transistor.  Let's say the emitter follower drops
700mV and the base of the power transistor needs another 700mV.  That leaves
5V - 700mV - 700mV = 3.6 volts drop between the two.  3.6V / 267mA = 13.5
ohms, so 10ohms is a good value.  A 10ohm resistor here will dissipate about
1.3W, so a standard 2W resistor will do fine.


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