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

[Mark Rages <markrages@xxxxxxxxx>]

On Sun, Jan 3, 2010 at 8:14 PM, Jason <geda@xxxxxxxxxxxxxx> wrote:
> Have I mentioned I feel like a bull in a china shop as I plow through
> datasheets, learning as I go?  ;-)
>
> Details are below, but the big question is, can I use a 200 mF supercap
> (EDLC) to dump 100 mA into the motor for short periods ( ~ 1 second)?
> The reason I ask is because the Panasonic datasheet, found at [4], lists
> a "Maximum Operating Current" of 10 uA...  That doesn't make sense to
> me.  Nothing I've researched today on supercaps says I can't do it.
>
> Bob Paddock wrote:
>> DJ Delorie wrote:
>>> 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.
>>
>> Better check the Gate/Source (Vgs) turn on voltage of the 7002, it is
>> not the best at 3V (It is 2.5V Max).
>> It would be better to use something like the FDV305N that will turn on at 1.5V.
>>
>
> good call.  The output high voltage of an I/O pin on the pic is Vdd -
> 0.7V, or 2.3V with a fresh battery.
>
>> Also your impedance between the motor and the coil cell are not well matched,
>> unless you can put a big low leakage, low ESR cap across the voltage point
>> going to your motor/switch combo.   What will happen is you will turn
>> on the motor,
>> the voltage will sag from the coil cell to the point that your micro
>> resets, that then
>> turns off the motor, the low ESR cap will minimize this from happening.
>>
>
> I already have a cap in place, I'll double check the value to account
> for the new load.  Thankfully, the motor will only run for a second or
> so when the device state changes (haptic notification).
>
> That cap is there for in-circuit programming.  I found this [1]
> discussion on providing higher current from a CR2032 while avoiding
> brownout. This comment [2], on the same page, goes through some math for
> calculating cap values.

A warning about lithium coin cells:  if you draw more than about 15 mA
from a 2032 at any time, the battery is permanently damaged and its
capacity is reduced.

This may or may not matter to your application.   Certainly any
calculated battery life will be wrong.

These batteries are not designed for high current loads, and the
datasheets say little about it.  Sony's datasheet, for exmaple, says
"max recommended pulse current" is 12 mA, but it doesn't describe what
badness happens if you exceed the current.


Regards,
Mark
markrages@gmail
-- 
Mark Rages, Engineer
Midwest Telecine LLC
markrages@xxxxxxxxxxxxxxxxxxx


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user