For the Rundbuntplasma I wanted that the main power supply (a big 20A/5V supply) could be hard disconnected (aka operating the main power switch in our hacker space) from mains and the Raspberry will still shutdown nicely automatically. The idea is to buffer the 5V for the Raspberry using two big Supercapacitors (EDLCs) with 50F each from Nesscap. The circuit was designed to be as simple as possible, the drawback is that the capacity is not used efficiently and has only enough time for a proper short shutdown (about 30 seconds). Another drawback is that the power supply comes up slowly at startup (takes about 10 seconds before Raspberry starts booting) and also powers the Raspberry for quite a long time after the shutdown so the system must be unplugged for about 10min before it will boot up again. But there main advantage over batteries is the lifetime of supercaapacitors: They can be charged and discharged completely a few 100.000 times and are quick to charge - after ~2min they carry enough charge for proper shutdown.”


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