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. 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. > Jason, you don't happen to be in the Cleveland area? I'm having Déjà > vu here with a very > similar discussion I just had with someone from a similar sounding > project, a couple of weeks ago. > Nope, never been to Cleveland. We Steelers fans break out in a rash if we go there. ;-) thx, Jason. [1] - http://interactive-matter.org/2009/08/driving-circuits-from-a-cr2032-lithium-coin-cell/ [2] - http://interactive-matter.org/2009/08/driving-circuits-from-a-cr2032-lithium-coin-cell/#comment-2041 [3] - http://www.computer.org/portal/web/csdl/doi/10.1109/ITNG.2009.259 [4] - http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P14163CT-ND _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
