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'[EE] DC motor connection'
2006\01\22@140342 by Dumitru Stama

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face

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.
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 ?

Dumitru Stama

2006\01\22@144609 by olin piclist

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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.


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

2006\01\22@144943 by Spehro Pefhany

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At 09:03 PM 1/22/2006 +0200, you 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.
>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 ?

I'd give that a qualified 'maybe'. If you require it to survive, you
must design it for that condition. It could be worse, BTW, something could
be forcing the motor to turn in the opposite direction.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2006\01\22@151742 by David VanHorn

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>
>
> I'd give that a qualified 'maybe'. If you require it to survive, you
> must design it for that condition. It could be worse, BTW, something could
> be forcing the motor to turn in the opposite direction.


Is it possible for something to spin the motor when it is off?
What voltages can be generated?  Will this cause a transistor failure?

Should you use another device to short the motor leads when it's off?

Isn't interfacing fun?

2006\01\22@170252 by Wouter van Ooijen

face picon face
> 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.
> 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 ?

You seriously think we can answer that without you specifying
- the voltage
- the (darlington) transistor you will be using
- the heatsingking you will use

Wouter van Ooijen

-- -------------------------------------------
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2006\01\22@172200 by William Chops Westfield

face picon face
>> what if the motor will get stuck somehow and it
>> will suck up to 3, 4A ?

Don't forget the start-up current either, which will be much
more  than the 1A run current every time you start...

My gut impression is that mechanical transients are "long" compared
to the times one may substantially abuse the spec of a transistor, but
reasonable short compared to the time for a heatsink to dissipate
the power, so you are probably ok with a transistor rated for the
stall current but heatsinking only adequate for the average power
dissipation.

BillW

2006\01\22@195049 by olin piclist

face picon face
William Chops Westfield wrote:
>> what if the motor will get stuck somehow and it
>> will suck up to 3, 4A ?
>
> Don't forget the start-up current either, which will be much
> more  than the 1A run current every time you start...

He didn't.  That is the 3-4 amps referred to above.  The startup current is
the same as the stall current assuming the motor is stationary when started.


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

2006\01\23@100038 by Bob Axtell

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Dumitru Stama wrote:

>Well, i was just asking if the Darlington would be the best sollution
>for my project. The Voltage is ~6v and without heatsinking (at start).
>I don't know exactly which Darlington i was thinking of using but it
>was one than could handle ~1A of current and max. 30v.
>I guess i will try the IRF sollution proposed by Olin and if it works
>like i want i will stick to that. Nevertheless is good to know that
>Darlingtons dissipate a lot of power and they really need heatsinking.
>I was not thinking about this when i started. I am quite new to
>electronics.
>
>  
>
What Olin is saying- and most people here will agree with him- is that
Darlingtons
are a VERY old solution to driving power loads. The losses when using
them are
incredible, about 1.8V of lost switching range, and that power turns
into HEAT..
It isn't that darlingtons cannot be used- they can- but its a solution
for a bygone
era, and it will never return. Like using leeches to treat a bad cold.

To drive power devices nowadays, we use MOSFET devices, IR makes some,
Fairchild
now makes a good assortment. These can switch  many amperes without  
measurable
heat being generated. But some care must be taken to protect these
devices from
transients (high voltage spikes), which motors are notorious for
generating..

More important for handling motors is handling rotor locks and or jams.
Power IC's are
now available that contain builtin MOSFETS, protective diodes, and
circuitry designed
to prevent damage when a rotor is locked. Some are designed in a bridge
configuration
that  allows you to reverse the motor with a minimum of fuss.

--Bob

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2006\01\23@125554 by William Chops Westfield

face picon face
On Jan 23, 2006, at 7:00 AM, Bob Axtell wrote:

> To drive power devices nowadays, we use MOSFET devices

So how come no one seems to make pin-compatible MOSFET versions
of the assorted popular darlington driver chips (ULN2xxx, LM293,
etc)?  Very annoying!

BillW

2006\01\23@163541 by Sergey Dryga

face picon face
Bob Axtell <engineer <at> cotse.net> writes:

> It isn't that darlingtons cannot be used- they can- but its a solution
> for a bygone
> era, and it will never return. Like using leeches to treat a bad cold.
>
You never know...
http://www.biopharm-leeches.com/

But, although leeches might return to mainstream, darlingtons for motor drive
will probably not.

Sergey

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