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

[Jason <geda@xxxxxxxxxxxxxx>]

John,

Thanks for the quick reply.

John Luciani wrote:
>    Why wouldn't you use the transistor or the FET? An MMBT3904 or an
>    N-channel
>    FET is more suitable for this application than a 1G125. I would go
>    with the MMBT3904
>    since you have a low voltage system. I used these to drive pager
>    motors in a haptic
>    compass.

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.

>    I am not sure about the duty-cycle of the motor but have you looked at
>    the coin-cell life
>    and the voltage dip when the motor is on? Make sure your coin-cells
>    are rated for high
>    pulse currents or factor the decreased battery life into your
>    specification.

Overall "ON" battery life is pretty short, ~100 hours, and the motor
only runs for a few seconds at a time when the device is turned on or
off.  It's the shelf life I'm concerned about.  Unfortunately, physical
constraints and legacy design lock me in to a CR2032.  Sure wish I could
use a CR123...

I found this [1] transistor on digikey and then used this [2] guide to
calculate an Rb of 174ohms when Rload is 29ohms, Vs is 3V, hfemin is 30,
and Ic is 100mA.

I just can't seem to divine when my Vbe is < 0.7V (ie, transistor OFF),
what will my Ic (or Rce) be?  It's not perfect, right?

thx,

Jason.

[1] -
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MMBT3904FSCT-ND
[2] - http://www.kpsec.freeuk.com/trancirc.htm



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