Re: gEDA-user: CR2032 with supercap, was Re: [SOT] suggestion for a SMT switch

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Mon, 2010-01-04 at 10:34 -0500, Jason wrote:
> Igor Lopez wrote:
> > Jason wrote:
> >> Assuming I need to draw 100mA for 1 sec, that's 0.100 Coulombs.  The pic
> >> has a minimum operating voltage of 2V.  Worst case, the battery is down
> >> to 2.7V, so I want no more than 0.5V drop when the motor runs.  From
> >> [2], my capacitor should be 200mF.  Yikes.
> > 
> > I am assuming we are talking about a DC motor here.
> > If the resistance of the windings are in the range of 25 ohm then you are pretty
> > well on your calculations since you will operate the motor with a
> > constant DC voltage.
> 
> Yep, 29 ohm internal resistance.  It pulls 100 mA at 3V DC even when 
> hooked up to a power supply.  I thank my lucky stars for that one. :-)

When running at a given speed, the motor current will be limited by the
back-emf generated as the motor spins.

A stalled motor may draw 100mA (stationary -> no back emf), but when
running, it will reach an equilibrium, where the speed is high enough to
produce enough back-emf to cancel (most) of the applied forward voltage.
The current draw in this condition will be what it requires to overcome
any friction, or external load on the motor.

I suspect the easiest way to get the data you need, is to measure it for
your application, with whatever load it has.

You could even be fancy.. and measure the inrush. Place a low-ohms
resistor in the ground return (to sense the current), then attach to
some kind of transistorised switch. You should be able to trigger an
oscilloscope watching the volts across that resistor from the switch
drive signal. (Hook up to a square wave generator or something).

If you need a low side switch, I'd put the resistor first to ground,
then the MOSFET, then the motor, then VCC. As long as your current sense
resistor drops only some fraction of a volt, the gate-source offset
should not effect the MOSFET switching.

Best wishes,

Peter C.



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