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UPS question for 500W Active-PFC PSU

Posted on 2014-11-19
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Last Modified: 2014-11-20
So I have a 500W Active PFC Power Supply for my home PC (and it probably is not pulling anything close to that... I only have 1 graphics card and one HDD (and one SSD))

APC has a document about active PFC and power supplies in which they explain the issues with using simulated sine wave UPS's... then they say

It is important to note that not all PFC power supplies will cause the UPS overload. However, the incompatibility is most acute
in the one of the following situations:
• A large server class PFC supply (e.g. rated 500W or more) is used with a Back-UPS or Smart-UPS SC.

so 500W is the very bottom limit...  And the say...
APC recommends that a true, pure sine wave, server class UPS be used..... ...However if, a [non-pure/simulated/stepped sine wave UPS like] Smart-UPS SC or Back-UPS RS is to be used, the UPS should be sized accordingly.

So my question is... does anyone know what "sized accordingly" means?  Because its probably WAY cheaper for me to buy an overpowered simulated sine wave UPS than a properly powered pure sine wave UPS.  But I don't know how overpowered it must be...

I think the most relevant quote in terms of what might happen is...
Because of the way active PFC’s operate, they can sometimes overload the UPS with momentary high inrush current. This can occur when the UPS transfers from online to on-battery operation, creating a momentary loss of power (<8ms). The PFC supply may respond by temporarily drawing an excessive amount of current.

So the question basically becomes... how much extra wattage could a 500W PSU pull with a "momentary high inrush current" trying to compensate for a 8ms power loss?

Anyone know the answer?
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Question by:Xetroximyn
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17 Comments
 
LVL 95

Expert Comment

by:John Hurst
ID: 40454148
I prefer to have a slightly larger UPS than I need. I have an APC 750XL that puts out a pure (close) sine wave and runs my Lenovo Desktop plus Monitor and my network and handles all surges. It never overloads.
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Expert Comment

by:John Hurst
ID: 40454155
its probably WAY cheaper for me to buy an overpowered simulated sine wave UPS <-- Might be. My 750XL was not expensive and accordingly not worth going the cheaper route.
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Author Comment

by:Xetroximyn
ID: 40454156
Looks like a  APC 750XL is $700 on amazon... is that right?

The main point here is that it looks like the pure sine wave ones are like $300+ and I am hoping to spend more like $200 or less and get a simulated sine wave unit... but with enough extra power to handle the 500W active PFC PSU I have.  But not sure how overpowered it must be.  Do you have any idea about that?

Thanks!
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LVL 95

Expert Comment

by:John Hurst
ID: 40454161
I got an APC SUA750XL (which may be a bit different than today's 750XL). It was just under $500 from my computer dealer.

It would easily handle a 500W power supply. The sine wave is very good with a bit of a flat top (small harmonic content). It does all I need from it.
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Expert Comment

by:John Hurst
ID: 40454165
I am hoping to spend more like $200 or less and get a simulated sine wave unit  <-- I think these are rounded square waves with lots of harmonic content. They are hard on switching power supplies. I would not recommend them.
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Author Comment

by:Xetroximyn
ID: 40454193
Thanks... what exactly do you mean by "hard on" switching power supplies? Like it would make it die quicker?  (And I assume "Switching" = "active PFC"?)
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Expert Comment

by:John Hurst
ID: 40454702
A non-sine wave voltage has a different effective voltage rating (RMS) and can strain a switching power supply and then that will likely result in shorter life.
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LVL 92

Expert Comment

by:nobus
ID: 40455138
i always start by calculating the power i need - yours may be a lot less than 500 W, so actually, you could do with an UPS of lower wattage
http://extreme.outervision.com/psucalculatorlite.jsp
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Accepted Solution

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garycase earned 500 total points
ID: 40455939
Cyberpower has true sine wave units at a much lower price point than APC.    For your system, this would be a good choice:  http://www.newegg.com/Product/Product.aspx?Item=N82E16842102131
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Author Comment

by:Xetroximyn
ID: 40456023
Wow - thanks!  That's the price range I am looking for!

So you think cyberpower is pretty good?

So I did the calculator here http://extreme.outervision.com/psucalculatorlite.jsp

It came up with 257w min and 307w recommended PSU.  So even though I will have a 500W PSU chances are it wont pull near that much. (1SSD, 1HHD, 1 gfx card (gtx 750 ti), and a i7-2600 CPU)  

So do you think that UPS could also handle my 2 LCD monitors (17" and 22")
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LVL 70

Expert Comment

by:garycase
ID: 40456064
Yes, the Cyberpower units are very nice.   I have several UPS units ... including 3 Cyberpower units, and they've performed very nicely.

There will be no issue powering a pair of LCD displays with that unit in addition to  your PC.
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Author Comment

by:Xetroximyn
ID: 40456124
Sweet - thanks so much!   I think I will get the 1000VA version for a little extra time.  :-)

So only one last question... in terms of the simulated sine wave units being bad for PSU... is it always a simulated sine wave?  or is that just when its on battery power?  I ask because I am getting my new PSU tomorrow... but not the new UPS until next week... will it be bad to keep my new PSU plugged into a simulated sine wave UPS in the meantime?
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LVL 70

Expert Comment

by:garycase
ID: 40456174
No, it won't hurt to use your simulated sine wave unit for now.    The normal power is always directly from the A/C line -- the simulated wave is only when it's switched to the inverter (i.e. when it's running on battery power).

Never hurts to have a higher capacity unit -- gives you more runtime as well as more "headroom" in terms of capacity.    All of my units are 1500va ... my Cyberpower units are these:  http://www.newegg.com/Product/Product.aspx?Item=N82E16842102134
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Author Comment

by:Xetroximyn
ID: 40456203
sweet - thanks again!
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Expert Comment

by:John Hurst
ID: 40456219
I am glad you are happy with the Cyberpower. I lead you through a lot of the sine wave issue before that and thought that was useful advice as well because you were looking for that kind of assistance.
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Author Comment

by:Xetroximyn
ID: 40456471
Yes - thank you very much!! The info you provided was very helpful as well!  I probably should have split the points... my bad!  :-/
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Expert Comment

by:John Hurst
ID: 40456477
No problem. Something to think about next time you are asking question. Thank you very much for following up.
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