4.1 – Super-Capacitors + Diodes + Resistors

Several hundreds of amperes in a kid’s toy, what could possibly go wrong?

Even if it’s just a few dozens of amps current when accidentally short circuiting a super capacitor, it’s still a little dangerous. Nevertheless, it seemed to be a reasonable idea at the time.

In my idealized world, a super-capacitor should provide enough capacitance to run the machine long enough to safely shutdown the whole system. As soon as the input was disconnected, the Pi was supposed to be running for a few seconds from the power supplied by the capacitor. As the main source is detached physically by a switch, no leakage current could occur afterwards. We’d be save even if the battery was empty.

There are some commercial products with cool features. Have a look at juice4halt as an example. The whole circuit is probably save to use and would meet the requirement and even more perfectly. But it’s just too expensive and too complicated for our purpose.

After some research, I found a simple and promising circuit from a hacker space in Frankfurt, Germany. The Raspi EDLC UPS only consists of a few components, so I gave it a try.

On the breadboard and initial PCB design everything worked out fine. The charging current was within the battery’s limit (~1.6A) and the Pi ran for about a minute without any additional power supply. I used two of these 10F 2.7V super-capacitors.

The first issue with the UPS occurred right after I fixed a (what I thought unrelated) bug in v0.2. That was only the start of a series of super-capacitor problems. They somehow even influenced some digital signals. It was a pain in the ass to debug the setup and I gave up. You need resistors to bleed out the capacitors, so that the Pi starts without using the reset button after a reasonable time. The Pi cannot shutdown itself without additional components. Also, in retrospect, super-capacitors are pretty dangerous. The currents flowing under normal conditions are already pretty high , not even talking about possible failures. For a simple UPS setup, I would probably recommend going that way. For Marta, it has too many disadvantages.

Long (really, way too long) story short, this part of the PCB had so many negative side effects on other components, that I eventually decided to remove the whole UPS from PCB v.0.4. Instead, with a heavy heart, because of its price and my incapability of solving the problems, I used the OnOff SHIM add-on board.