Re: gEDA-user: [SOT] suggestion for a SMT switch

[Steven Michalske <smichalske@xxxxxxxxx>]

On Sat, Jan 2, 2010 at 8:02 PM, Jason <geda@xxxxxxxxxxxxxx> wrote:
> DJ Delorie wrote:
>>> I'm concerned about leakage.  This device will probably sit on a
>>> shelf for some time (with battery inserted, pic in sleep mode).  I
>>> wanted to make sure there wasn't a trickle of current through the
>>> motor.
>>
>> Perhaps you could tie 4-5 PIC pins together to get the needed current?
>>
>
> Not possible.  I have only three output pins and the other two are in
> use.  :-(
>
>> Otherwise, look for the smallest MOSFET that will handle your current,
>> as smaller size usually means smaller leakage current.  For example,
>> the Fairchild 2N7002K (SOT-23) leaks 1 uA when off (Vgs == 0) but the
>> 2N7002T (SOT-523) leaks only 1 nA when off.  The 1G125 "leaks" 10 uA
>> when on (Icc).
>>
>
> Awesome, thanks!  That was exactly what I was missing.  I'll take a look
> at the 2N7002T more carefully in the morning.
>
> thx,
>
> Jason.
>
>
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You will want to use a N-channel FET, a flyback diode across the
motor, and you will brown out your battery without bulk capacitance.

Here is a link to a cr2032 data sheet
<http://data.energizer.com/PDFs/cr2032.pdf>

I had a hell of a time driving a 50mA IR LED with these coin cells,  A
100mA motor will be tricky too.

It has a 6.8 mA pulse load with 400 Ohms series resistance at 2.7
volts.  You will need bulk capacitance and to protect the uC from
browning out.   Think of a coin cell as using a kitchen faucet to run
a fire hose.

I would thoroughly suggest that you look into the real current draw of
the motors.

How long they will pulse for etc....

Pick up a copy of the art of electronics, and some of the Forest M
Mims III books  they will have circuits for driving small motors in
them.

Best of luck.
Steve


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