2021-04-07, 12:31 AM
(2021-04-01, 09:45 AM)SARASTRO Wrote: I place here the progress of this issue.
I got from Aliexpress a super-capacitor boost pack of 2 Farad capacity at 16V. My own conservative calculations determined a theoretical 1,6 F charging at 12V.
This pack is very compact (65 x 35 mm) and cheaper that a LiPo pack. (Less than 8 euro)
Tested feeding a 12 to 5 V buck converted, can hold the Pi alive during a complete cycle of start up and shutdown on a Pi 3B + 5" display. (Much more if the display has independent supply line, as in my 7" waterproof display)
No need of any control electronic for the charge of super capacitor bank (Has his own balancing circuit to avoid exceeding 2,7V on any condenser).
Simply; the condenser are charged in seconds when connected to the ship's battery.
The circuit to protect the Pi in case of power-off, is divided in two parts:
A battery voltage detector: with an operational amplifier (CA3130) that compares the battery voltage (divided by 2) with a reference 5,1V created with a zener diode. When system is switched off, the op-amp turns output on, and activates a reed relay (R1).
A pulse generator to shortcut GPIO3 to GND (pins 5 and 6 on Rpi): When battery supply is working, a electrolytic capacitor is charged to 12V, and can't be discharged back because there is a diode blocking reverse way. When R1 is activated, the capacitor discharges through a voltaje reducer to 5V (7805) and activates reed relay R2. This relay is ON during the lapse of time needed to discharge the capacitor from 12V to aprox. 6,5V, and returns OFF. During this time, the contacto shorts GPIO3 to GND, forcing de Pi to shutdown, while the super capacitors are maintaining the supply during the time needed.
Of course; in this project I do not need to use I2C comms. so I can use GPIO 3 for that purpose. Also, I install, in parallel, a manual Pushbutton that allows to shutdown and re-start pi without power-off.
With that solution; the 7" display can be switched on / off from a common switch jointly with all other "navigation instruments" without problems. This is a must if your co-skipper only use the Battery that disconnects service battery in the ship.
Next development can be a more complete solution for using GPIO17 and 3V3 signal to control the primary RPi (I shall start a new thread for that)
Sorry; two pictures were incomplete
Last picture
Complete diagram
Thank you so much for sharing the solution and especially the photos of the capacitor boost pack.