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Thread: PSU with multi-second hold up

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    Default PSU with multi-second hold up

    I was wondering how feasible it would be to make an ATX PSU with a multi second (load dependent) hold up time. This would allow the PC to survive brief power outages without a UPS. How much would it cost and how much would it hurt efficiency (if at all)?
    Last edited by Luke M; 07-08-2018 at 02:25 PM.

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    I guess you could put some massive capacitors in there... but you'd need a much bigger NTC (or something fancier) to keep the current sane as they charged up. They wouldn't hurt efficiency much since leakage current is caps is pretty low, but they would take up a lot of space. Why not just buy a small UPS if you need this?

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    Last edited by ashiekh; 07-08-2018 at 06:41 PM.

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    A massive capacitor on the primary side between the bridge and APFC, with a soft-start circuit.

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    Quote Originally Posted by wwenze View Post
    A massive capacitor on the primary side between the bridge and APFC, with a soft-start circuit.
    Super-capacitors tend to be low voltage; mine are 2600F (2.5V) each and I run 6 in series to run a UPS expecting a 12V battery; that's almost 50,000 J capacity, which could hold up a 100W computer for several minutes. Sure beats replacing an expensive battery every few years.

    Things have improved since I built my array, and 3000F (2.7V) is now available; that is not a typo... 3000 Farads
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    Last edited by ashiekh; 07-09-2018 at 12:21 AM.

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    Quote Originally Posted by Luke M View Post
    I was wondering how feasible it would be to make an ATX PSU with a multi second (load dependent) hold up time. This would allow the PC to survive brief power outages without a UPS. How much would it cost and how much would it hurt efficiency (if at all)?
    Its not feasable as it would make ATX PSU considerabely larger and expensive.

    A 330µF/400V (for 450-550W) 400V Cap is at around 3,64€ (single Price, 2,50 at 100 and 1,89€ at 1k)
    https://eu.mouser.com/ProductDetail/...cOpWzEbPMCQ%3d

    And that's only sufficient for around 17ms


    And a 1200V/400V Cap is almost 10 Bucks, still 7,05€ at 100 and 6,09€ at 1k
    https://eu.mouser.com/ProductDetail/...2bfP0izqC94%3d


    In theory, you need 10 times the capacity for 10 times holdup time...

    So for a second or two, you'd need some serious capacitance. And that's at least very impractical.
    So you'd use an external device that uses batterys if the voltages drops.
    And here you have so called online USV that are always online and clean the voltage. So you don't need that anyway...
    Or you connect the ATX PSU to an array of battarys. And there are 36VDC or so PSU for ATX Systems...

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    Quote Originally Posted by wwenze View Post
    A massive capacitor on the primary side between the bridge and APFC, with a soft-start circuit.
    Actually, scratch this idea - APFC's boost function works based on the AC waveform it senses on the mains, if the mains voltage is 0V it wouldn't draw any current. So after the APFC it is.

    I was initially thinking of using APFC's boost converter to forcefully draw out the charges remaining in the capacitor; otherwise only a handful of the charge will be used before the voltage drops by for example 10%. I guess we can rely on the SMPS itself to provide the regulation.

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    Multi-second? No. That's a lot of power.

    Closest thing I've seen is Nipron PSUs. They come with 5/25" bay "cap packs" that can keep a 180W load stable for one second.

    Alternately, you can pair up a car battery with a DC to DC PSU and a pico-PSU.

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    Quote Originally Posted by jonnyGURU View Post
    Multi-second? No. That's a lot of power.
    Ah, thats somthing I missed.
    What happens in a fault situation?
    In that case you have to disconnect the Transformer somehow...
    Or have some kind of discharge circuit that gets activated in a fault situation...

    That adds another level of "impracticality" to this issue.


    Quote Originally Posted by jonnyGURU View Post
    Alternately, you can pair up a car battery with a DC to DC PSU and a pico-PSU.
    Why bother with the Pico and the rest, when you can have a real DC-DC PSU?
    FOr example:
    https://www.powerstream.com/dc-pc-48v-1200w.htm

    But I've seen them somewhere else too...
    There are a couple of INdustry Level 36 or 48V DC units.

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    Exploring alternatives is great, but as pointed out already, a UPS is probably the solution.

    I would get an old unit and put in a lithium battery
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    Last edited by ashiekh; 07-09-2018 at 12:23 PM.

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