Raspberry Pi UPS Using Supercapacitors

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What happens when you want to integrate a Raspberry Pi into some kind of project that gets turned on and off with mains voltage? Do you power the Pi separately, or make a UPS for it?

[Lutz Lisseck] decided he wanted to turn his ambient-lamp (Rundbuntplasma) on and off with only the main power switch in his Hackerpsace. He could build a traditional UPS using a battery pack (it’s only 5V after all!) but decided to take it a step further. He picked up a pair of 50F supercapacitors. This way his UPS would last longer than his Pi would! The caps store just enough power that when the main supply is cut, a GPIO notices, tells the Pi, and it begins a shutdown sequence lasting about 30 seconds.

While [Lutz] is using two 2.7V supercapacitors, he mentions it would be a lot cheaper to use a step-up converter instead of putting them in series — but he had the caps on hand so decided to use both.

If you need it to last a bit longer, you could make one with rechargeable batteries…

Comments

  1. Andrew Eliason says:

    The big advantage of using supercaps is that you can recharge as fast as your components will allow. I made a little night light for my niece and had to put in a series resistance to keep it from overloading the charging wall-wort! It still charges in a few minutes.

  2. John Wells says:

    I just ordered a “UPiS Advanced” UPS for my raspberry pi for the same reason (making a wall-mounted waterproof media centre that gets switched on/off directly from the mains). A great piece of kit, but expensive. This looks like a cheaper but possibly more bulky option.

    Next time I’ll probably go this route — much more comfortable playing with these than a DIY LiPo charger.

  3. OLIMEX Ltd says:

    or use A10-OLinuXino-LIME which have build in LiPo battery charger and UPS management under linux :)

  4. MattE says:

    Hm, mount root read-only and just pull the power any time you like? A bit cheaper. Assuming a lamp doesn’t need persistent storage? :)

    • Mike Szczys says:

      In this particular case, yes. But I think this would be handy in so many other applications. I’m specifically thinking of XBMC. I have a power strip that switches off with my television. I don’t power my Pi from that strip because of power down issues but this could change that.

  5. Step-up converter would also allow to suck all juice from capacitor. As capacitor discharge curve falls quickly it goes below usable voltage of raspi, while capacitor still has charge. This is one thing where batteries are ‘better’.

  6. Lwatcdr says:

    Does the Pi have a low power sleep mode? If so how long would this run keep it alive? The idea of waiting for a boot to use a lamp seems just a bit much.

  7. Blowed Up Drivers says:

    This could be very useful for CNC situations where cheap drivers are prone to frying if they lose logic power before they lose motor power. Keeping the logic controller powered allows it to switch of the mains to the motors.

    • tekkieneet says:

      Sound like they need to have proper power sequencing circuits than a UPS and the designers at the board level are the ones to be blamed for not having that.

      i.e. something like a MOSFET to cut off the motor supply when the logic rail drops.

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