Änderungen
Aus Hackerspace Ffm
/* Motivation */
'''So no worry of proper shutdown anymore - just unplug it or operate the main power switch and the Raspberry will detect it and shutdowns nicelywhile the supercapacitors provide the power for this time.'''
== Circuit ==
[[Datei:Raspi_USV.png|UPS circuit for Raspberry using Supercapacitors]]
The powersupply goes in at the left side, the diodes prevent that current will be flowing back to the power supply and the resistors R5/R6 limit the current at startup rushing into the Supercaps (otherwise the diodes or the power supply might die). The diodes should be schottky-diodes with a high current and low V_foreward rating, so many types will do. SR430 SR340 was choosen only because it has been found in the drawer. R5/R6 could also anything between 0.5R and 2R but high wattage types should be choosen - especially at startup they might get hot.
The supercaps C1/C2 must be connected in series and the resistors R1/R2 are used that the voltage over the caps will be distributed evenly (balancing), otherwise the caps may degrade quickly. The resistors also discharge the caps slowly when the circuit is not powered (but within a quite long time) that is also good to lenghten the live of the supercaps.
== Result ==
It works pretty well but those big caps are expensive if you have to buy them. It could be done much better and with much smaller supercaps if a step-up-converter or even better a step-up-and-down-converter is used to generate the 5V or better directly the . The Raspberry uses a very unefficient linear regulator for 3.3V by default - by changing that to a switched-one the holdup time can be increased also. As most things on the Raspberry only needs (avoiding will operate on 3.3V supply, it does not matter much that power-wasting linear regulatorthe 5V is just 4.5V or even lower as it will be the case with this very simple circuit. But beware, devices on USB port (especially WLAN adapters)may really want nearly full 5V for proper operation...
