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Thread: PSU capacity sizing?

  1. #1
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    Default PSU capacity sizing?

    Is there a generally accepted power percentage spare to use when sizing PSUs for a task?

    As ever, space in the projector is the real commodity so saving physical size on a PSU is desirable.

    I've got 2 separate loads of max 10A and max 8A at 12V, and for redundancy am installing 2 PSUs rather than 1 big one.

    I'm not sure whether to go for the 12.5A units or the 16.5A units. 12.5A seems a bit close to 10A for my liking.

    Perhaps I go for a 12.5A for the 8A load and 16.5A for the 10A load?
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  2. #2
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    The limiting factor will be continuous current or peak current, which ever is greatest. I like to leave a headroom of 2:1 as a minimum.
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  3. #3
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    Quote Originally Posted by norty303 View Post
    Perhaps I go for a 12.5A for the 8A load and 16.5A for the 10A load?
    That would be my choice, although as Wayne mentioned you want to be sure you don't have a large peak (inrush) on startup.

    Adam

  4. #4
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    Do laser diode drivers generally have large inrush currents?
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  5. #5
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    A good driver will have inrush current limiting if it is a high power driver. many have "soft start" Also a good SMPS power supply will limit its own inrush (AC in) and not overshoot its output upon startup.
    Any big supply bypass and filter caps on the powered circuit usually are what accounts for a large inrush current as they fill, but they also protect against overshoot upon start up.
    Although i guess if it was something fancy like a MHz rate switching driver, it would have a start delay and brownout protection inherent to SMPS controller IC.
    An analog driver will use quite a bit more current than a switching driver since it burns off the difference as heat.

    If your really worried about it you could use a DC current clamp or a bench supply to see how much current your stuff draws. If you want it mil spec you would scope the current and voltage upon start up and shutdown, and during operation as part of performance testing which would be documented as part of the design documentation. Diode projectors under 10 watts don't draw alot of power, to be honest i'm tired of having like 5 different PSUs inside a projector. The real clever thing to do would be to choose a suitable multi out supply to really cut the fat.

  6. #6
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    Diode projectors under 10 watts don't draw alot of power, to be honest i'm tired of having like 5 different PSUs inside a projector. The real clever thing to do would be to choose a suitable multi out supply to really cut the fat.
    Well, these projectors will only have 12V supply for modules and electronics and the galvo supply. Its just that having 2x smaller 12V supplies might be more practical for space than 1x 30A supply, and be more cost effective and less risk of complete failure.

    Probably going with Meanwell PFC SP-150-12 and SP-200-12
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  7. #7
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    as long as the supplies are not sharing a load that one supply by it's self cant to, some better supplies have the options for sharing a load, one thing i have seen done when using two supplies is having a high current diode at the output of each supply, "OR"ing is think what that was called, that and a total fail of one supply will not effect it's partner. I have seen people not put the supply in the projector, i do this as well, it's not far away from the projector and doing this cuts down my costs.
    Last edited by Draco; 03-29-2015 at 01:57.
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