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Thread: Why there are always '2' bulk caps in NON-PFC psus?

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    Default Why there are always '2' bulk caps in NON-PFC psus?

    Cheap psus without pfc (220v only) always have two bulk caps... but why two?
    A post from Hardwaresecrets says they are a part of voltage doubler circuit, but these psus are 230V only non-pfc - do they need voltage doubler?

    20160401091127526_5P0Z6PEE.jpg

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    As you noted, it is part of being able to run 110V or 220V; they get wired up differently depending on which.
    But a 220V only unit does not need this trick; they are probably just based on the dual voltage design.
    Last edited by ashiekh; 03-27-2019 at 02:12 PM.

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    Double voltage circuit. Old design so nobody want to change.

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    Quote Originally Posted by rgbodosk View Post
    Cheap psus without pfc (220v only) always have two bulk caps... but why two?
    A post from Hardwaresecrets says they are a part of voltage doubler circuit, but these psus are 230V only non-pfc - do they need voltage doubler?
    No, in that case the caps are in series as there are only ~200VDC models - wich is not enough for rectified 230VAC - wich comes at around 325V DC...

    Here is what that is:
    http://www.electricalbasicprojects.c...ltage-doubler/

    In 230VAC they run in series.

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    So you mean that

    1. Those are based on old voltage doubler circuits, and it can wire up differently to switch between voltages.
    2. But since this is only 220V input model, they actually do not work as voltage doubler and the caps just run in series.

    Did I get it right?

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    Yep!

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    If the PSU is 230V non-PFC model, it can have only 1 bulk cap with ~400V rated voltage, and the resistors in parallel with the bulk caps (which are necessary with 2 bulk caps in series) can be omitted.
    It's been a hard day's night and I've been working like a dog.

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    I wonder how the cost compares (recall that when one adds two equal capacitors in series the resultant capacitance is half);
    maybe two 200V capacitors don't cost much more than one 400V capacitor (of equal overall capacitance)
    Last edited by ashiekh; 03-29-2019 at 01:17 PM.

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    Then guess the reasons are lower cost(maybe) and sticking on the old design.

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