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

[Gabriel Paubert <paubert@xxxxxxx>]

On Sun, Jan 03, 2010 at 09:14:49PM -0500, Jason 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.

It really depends on the load, the 0.7V drop being for a given load current.
The gate of a MOSFET has such a high impedance that it will end ramping up 
close to 3V eventually (but it really depends on the PIC output structure:
is pull up transistor  N or P channel?).

You also have other possibilities:
- using a P channel mosfet, changing the polarity of the output (low for motor
on and putting eventually a pull-up resistor in parallel with the gate to drive
VGS closer to zero).
- buffering the output through a 1G04 gate (but beware of ICC, although it is
typically much lower that the guaranteed ICC which increases dramatically
at high temperatures).

> 
> > 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.
> 
> 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.
> 
> Some chaotic googling found this [3], which introduced me to the idea of
> supercapacitors.  Digikey has one [4] that seems to fit the bill.  It's
> datasheet claims a "low ESR" of 200 ohms.  That seems much higher than
> the 1 or 2 ohms I've seen in other places.

Well, there is a supercap in the device that controls the heating of
my house. The device is powered by 2xAAA (or AA, can't remember, changed
tham last 3 years ago) with option of using a lithium cell (can't remember 
which model either). The supercap is used to provide power while toggling
the relay that actually switches the mains and actually enable another 
set of relays, a bit gross, but works well and was already installed when
I bought the house. 

I know it quite well, since my nephew has the same device which started to 
act up and was fixed by replacing the supercap (0.047F, so hard to read
that I ordered .47F first, too big to fit).

However, you want to have power for longer than this device. The PowerStor
series from Cooper Bussmann might fit your bill (1kHz ESR down to 0.2 Ohms), 
but they are neither cheap nor very small (especially the lowest ESR PA
series):

http://www.farnell.com/datasheets/87303.pdf

You can buy them from Newark, which has quite a lot in stock (Farnell
charges me extra to ship them to Europe, but at least I can get them).

	Regards,
	Gabriel


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