Thank you Olin
This is what i was looking for. I am going to the electronic parts
shop and find some IRF transistors like you suggested so see what can
they handle.

Dumitru Stama

OL> 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.

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

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

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


OL> ******************************************************************
OL> Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
OL> consultant in 2004 program year.  http://www.embedinc.com/products

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