Post by thread:[PIC] Capacitor Discharge / Backup

Hi, I need to calculate how much will and application last until its shut off. The application will use a PIC16F913 with NanoWatt technology. It's a very simple circuit, 4 seven segment displays, and nothing more really, it's basically a timer, so it has a clock which I have to maintain in case the device is unplugged from the wall. I run some calculation based on the datasheet, but I would like to check. The devices consumes: - 16.33 uA Normal Operation intosc 4Mhz (averaged with the 2, 3 and 5 volt values) - .45 uA Basic Sleep Mode - 2.66 uA For timer 1 oscillator (32.768 kHz) Now, since the Timer 1 will always be on, I have 2 currents: - 18.99 uA - 3.11 uA The second current will be consumed for the time it takes to the timer 1 to overflow (1 second), the second current will be consumed on power up, and until the device is on sleep mode again, which I estimated to be 1.1 mS. Then, I calculated the average current to be 3.13 uA and using the formula Q = C*V * e^(-t/RC) calculated the time (to reach 2.2V in the cap) to be near 25 hours... Ohh... I use a supercap of 0.1F x 5.5V. I will connect it like this: VCC ---------DIODE-------------PIC | CAP | GND Any opinions? Regards, Mauricio Jancic Janso Desarrollos - Microchip Consultants Program Member spam_OUTinfoTakeThisOuTjanso.com.ar http://www.janso.com.ar (54) 11 - 4542 - 3519 {Original Message removed}

> Ohh... I use a supercap of 0.1F x 5.5V. I will connect it like this: > > VCC ---------DIODE-------------PIC > | > CAP > | > GND > > Any opinions? The cap should be on the k side of the diode, otherwise it might discharge into the PSU (view fixed font) a k VCC---->|------PIC | = C | gnd

> Ohh... I use a supercap of 0.1F x 5.5V. I will connect it like this: You looked up the leak current of your supercap? This could be around 0.01CV, which in your case would be 55uA, and would clearly outweigh any other currents you might be taking out of the capacitor. Greetings, Maarten Hofman.

>> You looked up the leak current of your supercap? This could be around 0.01CV, >> which in your case would be 55uA, and would clearly outweigh any other >> currents you might be taking out of the capacitor. Oh... I shurelly miss that... Jinx: My original drawing was as yours, but, I don't know why, the tabulation was missing on arrival... Regards, Mauricio Jancic Janso Desarrollos - Microchip Consultants Program Member .....infoKILLspam@spam@janso.com.ar http://www.janso.com.ar (54) 11 - 4542 - 3519 {Original Message removed}

Mmm I'm a bit confused... I use this cap http://www.nec-tokin.com/english/guide/super_cap/fy_e.pdf I'm actually using an FYD model of 0.1F. I don't find the self discharge current... Anyone does? I've found a parameter saying that the discharge from 5V to 4.2V took 24 hs... Is that so? Regards, Mauricio Jancic Janso Desarrollos - Microchip Consultants Program Member infoKILLspamjanso.com.ar http://www.janso.com.ar (54) 11 - 4542 - 3519

> Mmm I'm a bit confused... I use this cap > http://www.nec-tokin.com/english/guide/super_cap/fy_e.pdf I notice that those caps don't have an inrush current spec. Some others, notably the 2.5V super-caps, do. For example 120uA limit for the 0F33 and 600uA for the 10F. This implies that you need a series charge resistor and two diodes Vcc---->|----- | | R ^ | | ------- | = | gnd Just curious, are 5.5V of different construction ?