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I am a total beginner in electronics so I need your advice. Does this simple circuit provide for a safe shutdown, with two super caps of 10F?

Schematic

(Original image)

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  • \$\begingroup\$ The diode in the top left should be a Schottky, not a Zener. I would be put one resistor across both Supercaps, rather than across each one individually, so the two Supercaps act as one. \$\endgroup\$
    – tcrosley
    Commented Apr 30, 2014 at 9:15
  • \$\begingroup\$ Thanks tcrosley! I have to put as well a pull up resistor on imput1? \$\endgroup\$
    – user3206599
    Commented Apr 30, 2014 at 9:20
  • \$\begingroup\$ If you are referring to the two lines going to the GPIO block, then no pullups are needed since you are coming off of voltage dividers. \$\endgroup\$
    – tcrosley
    Commented Apr 30, 2014 at 12:52
  • \$\begingroup\$ Could you take a look on the resistors of the NPN. Are the values correct? \$\endgroup\$
    – user3206599
    Commented Apr 30, 2014 at 14:29
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    \$\begingroup\$ You might have thought of this already, but you could simply use a read-only filesystem on the Pi, then you don't have to care about any shutdown conditions. \$\endgroup\$
    – Tom L.
    Commented May 31, 2014 at 16:38

2 Answers 2

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I think this is bad idea, because:

  1. You will be unable to use all capacitor energy. Capacitors will discharge from 5 to 4.5V or maybe 4V and your Pi will reset. In best case - 20% of capacity will be used.

  2. Even if you change your circuit by adding some DC/DC converter - 10F 5V may be not enough. Raspberry Pi (model B) needs at least 2W. Thats 400mA at 5V.

You should consider conventional rechargeable or alkaline batteries. There are many designs with LiIon 18650 cells. One cell is 8-11Wh / watt hours. That means 3-4 hours on single cell with average efficiency DC/DC converter.

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    \$\begingroup\$ Hi kamil, thank you for your help. Please look in the top right of image. There is a step up converter (2.0v to 5.0v) that will provide a stable 5v meanwhile the shutdown process finish. With 5F in theory give about 20sec of time for the shutdown. I made another diagram without tiny85 but using the logic gate. I will upload it when finished. \$\endgroup\$
    – user3206599
    Commented May 1, 2014 at 10:57
  • \$\begingroup\$ Good to know that you are able to calculate theory :) But I dont know why you stuck with these capacitors. Easier to charge? With that step-up you can use 2 Eneloop AAA batteries and they fit Pi case (i tested it). Some people say tha AAA sanyo eneloop can give 10A for a short time without exploding :) \$\endgroup\$
    – Kamil
    Commented May 1, 2014 at 11:30
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    \$\begingroup\$ I'm building a machine, space is not a problem but the component durable yes. I can't change eventually the battery after only one or two years.... So i decided to use capacitor. Do you think that it can't works in this way? \$\endgroup\$
    – user3206599
    Commented May 1, 2014 at 11:39
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If you were starting over it might be worth googling "supervisory circuit" or chip, which would get you something like this: http://www.maximintegrated.com/en/products/power/supervisors-voltage-monitors-sequencers/MAX818L.html

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