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Which PSU should I get?

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Mobsteroo7_apc
Cadet
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Which PSU should I get?

This was originally posted on APC forums on 5/21/2008


Hello everybody, I'm new to the concept of a UPS. But from what I've been reading, they are very good for computers and extend the life of the parts.

What I want is something that will carry my computer through a short power outage (10 minutes at most), and provide clean power for my computer (AVR I believe it is called).

The only things I plan on plugging into the battery fed outlets are my computer and one 19" LCD monitor. My computer specs are:

Power Supply: [PC Power & Cooling S75QB 750W|http://www.newegg.com/Product/Product.aspx?Item=N82E16817703009]
Motherboard: ASUS P5E
CPU: Intel Core 2 Quad Q6600
RAM: 2GB (2x1GB) OCZ Reaper HPC Edition
Video Cards: Primary eVGA 8800GT 512MB Secondary eVGA 7950GT KO 512MB
Sound Card: Auzentech X-fi Prelude
OS: Windows XP Pro 32-bit

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BillP
Administrator Administrator
Administrator
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105

Re: Which PSU should I get?

This reply was originally posted by Angela on APC forums on 5/23/2008


Hi,

[Check out this article|http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=8883]

That is our knowledge base article on what can happen with PFC power supplies. Essentially, with this particular product and its internal design, there is a larger transfer time associated with switching from online to on battery (compared to some of our other models). Also, there was another thread on here that a particular Mac computer that a user was using started buzzing while the UPS was on battery (when the UPS outputs the step approximated sine wave). So, depending on the power supply it may be alright. I cannot say what your particular power supply will actually do but in general, but I see transfer time and in rush current cause a bigger issue than the actual sine wave.

Often people confuse the on-battery output of Back-UPS, a stepped approximation to a sine wave, with a square wave output UPS. Because the rms and peak values of the square wave output UPS are identical, the on-battery output voltage is typically high, around 140 Vrms, at the beginning of an extended outage. This is so that the output voltage will be high enough, around 97 Vrms, near the time of low battery for the load to be supported.

Obviously, any load with a resistive element will be overstressed somewhat during the time the voltage is above 132 Vrms (the limit to which most equipment is designed). In the past, many UPSs had on-battery square wave outputs because it was a relatively inexpensive method of generating the power. Many computers had no trouble running with such an output, but the overstress issue remained for some load types, so now only cheap clones are found with such outputs. Unlike a square wave output UPS, the output from a stepped sine Back-UPS has peak and rms values equivalent to the sine wave.

Hope this helps as well!

See Answer In Context

3 Replies 3
BillP
Administrator Administrator
Administrator
0 Likes
0
105

Re: Which PSU should I get?

This reply was originally posted by Angela on APC forums on 5/22/2008


Hi,

As a general rule, we usually size people going by their power supply, when in reality, even though its 750W, you may only pull say 400W idle. But if you do some in depth processing, it may jump up. Your system looks pretty high end though and I'd say the average LCD monitor is about ~50-70 watts at most.

I'd take a look at the Back UPS RS line and on this page, you can adjust the power consumption on the left hand side to see the estimated runtime.

[Click here to view the Back UPS RS product line options.|http://www.apc.com/products/family/index.cfm?id=23#anchor1]

This should get you started in the right direction. There are some LCD models and all are managable via USB with our PowerChute Personal Edition which is included. This allows for graceful shutdown upon the power going out, but its optional.

Hope this helps!

Mobsteroo7_apc
Cadet
0 Likes
0
105

Re: Which PSU should I get?

This was originally posted on APC forums on 5/23/2008


Yes that helps, thanks!

I do have another question. When doing some googling I came across some information stating that power supplies with Active PFC should have a UPS with pure sine wave output instead of approximated sine wave output. Can you tell me if this is true? I don't want to buy the wrong model.

BillP
Administrator Administrator
Administrator
0 Likes
0
106

Re: Which PSU should I get?

This reply was originally posted by Angela on APC forums on 5/23/2008


Hi,

[Check out this article|http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=8883]

That is our knowledge base article on what can happen with PFC power supplies. Essentially, with this particular product and its internal design, there is a larger transfer time associated with switching from online to on battery (compared to some of our other models). Also, there was another thread on here that a particular Mac computer that a user was using started buzzing while the UPS was on battery (when the UPS outputs the step approximated sine wave). So, depending on the power supply it may be alright. I cannot say what your particular power supply will actually do but in general, but I see transfer time and in rush current cause a bigger issue than the actual sine wave.

Often people confuse the on-battery output of Back-UPS, a stepped approximation to a sine wave, with a square wave output UPS. Because the rms and peak values of the square wave output UPS are identical, the on-battery output voltage is typically high, around 140 Vrms, at the beginning of an extended outage. This is so that the output voltage will be high enough, around 97 Vrms, near the time of low battery for the load to be supported.

Obviously, any load with a resistive element will be overstressed somewhat during the time the voltage is above 132 Vrms (the limit to which most equipment is designed). In the past, many UPSs had on-battery square wave outputs because it was a relatively inexpensive method of generating the power. Many computers had no trouble running with such an output, but the overstress issue remained for some load types, so now only cheap clones are found with such outputs. Unlike a square wave output UPS, the output from a stepped sine Back-UPS has peak and rms values equivalent to the sine wave.

Hope this helps as well!