jonnyGURU Forums
Home Site Search Reviews Articles Contest Links PSU FAQs  


Go Back   jonnyGURU Forums > Computer Hardware > PC Power Supply Discussion

PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

Reply
 
Thread Tools Display Modes
  #1  
Old 03-28-2008
jonnyGURU's Avatar
jonnyGURU jonnyGURU is offline
Site Founder
 
Join Date: Oct 2006
Location: jonnyGURU forums, of course!
Posts: 10,435
Thanks: 125
Thanked 159 Times in 107 Posts
Default How Power Supplies Are Rated

What is the "wattage" number (i.e. 650W, 800W, etc.) actually telling me?

When you see the wattage rating of a power supply, you’re seeing the total maximum output capability of that particular power supply, but a computer has multiple voltage needs, and newer computers require more of the power supply’s capability to be on the +12V DC output rail. CPU’s and GPU’s regulate their power off of the +12V DC rail. Also, all of the computer’s motors run off of +12V DC: hard drive and optical drive motors, fan motors, pumps for water-cooling, etc. It wasn't too long ago that graphics cards did not require auxiliary +12V power and CPU's use to regulate their voltage from the +5V rail. An older power supply may have a lower percentage of it's power on the +12V than a more current unit.

What is the difference between "continuous" and "peak" ratings?

Some power supply units are rated for continuous output while others are rated at peak. "Continuous" means that the power supply is rated to run at it's maximum capability for no pre-determined period of time, while "Peak" indicates that the power supply will only run at the specified wattage for a brief period of time, possibly only a few seconds or up to a minute. This number is typically about 100W more than the power supply's actual continuous rating.

How does the temperature inside of my case affect the performance of my power supply?

Power supplies can perform differently depending on the temperature at which they are operating at. When a power supply is rated for it's total output wattage, it is rated to do so at a particular temperature. Anything beyond this temperature may take away from the power supply's capability. A power supply that is rated to put out 550W at 25C or 30C (room temperature) may only be able to put out 75% of that at 40C or 50C (actual operating temperature). This difference is called the "de-rating curve". A normal operating temperature for a power supply is 40C.

Is the temperature at which MTBF is measured at an indication of what temperature the power supply's output rating is measured at?

Unfortunately, no. It's a tough race out there and there are a lot of guys rating their PSU's MTBF at room temperature, even if they rate their PSU at operating temperature. Fact of the matter is, MTBF can be a significantly, often exponentially, lower number when going from 25C to 40C. For example, one unit with an MTBF at 100,000 hours @ 25C can have an MTBF of 20,000 hours at 40C. That's a pretty big difference! So it's not unusual for a manufacturer to use the higher MTBF number at the lower temperature and, in most cases, not tell us at what temperature that MTBF is derived at. But even when they do tell us the MTBF temperature, this doesn't mean the PSU is rated at this. A PSU's output capability may not be seriously compromised by heat. If the PSU does 700W continuous @ 25C and only 600W @ 40C, the difference may not be significant enough to the manufacturer to increase their continuous wattage claims, so although they may measure MTBF at 25C., they may very well be rating the PSU at 25, 40 or even 50C. Unfortunately, it all comes down to marketing. It's easier to market a PSU that runs at what it's rated at 40C then it is to market a significantly lower MTBF at the same temperature.
__________________
We are stuck with technology when what we really want is just stuff that works. -- Douglas Adams

Last edited by jonnyGURU; 04-24-2008 at 08:00 AM.
Reply With Quote
  #2  
Old 03-29-2008
Stefan555's Avatar
Stefan555 Stefan555 is offline
Shuttle user
 
Join Date: Nov 2007
Location: Sweden
Posts: 160
Thanks: 0
Thanked 0 Times in 0 Posts
Default

In addition:
When a power supply is not rated for it's total output wattage at a particular temperature (or not mentioned), the power supply is normally considered to be rated at 25C.
Reply With Quote
  #3  
Old 12-14-2012
andy1m andy1m is offline
micro ATX User
 
Join Date: Dec 2012
Posts: 2
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Does it mean a powerful fan is always better? My old Antec run pretty quietly and not really blowing.
Reply With Quote
  #4  
Old 12-14-2012
jonnyGURU's Avatar
jonnyGURU jonnyGURU is offline
Site Founder
 
Join Date: Oct 2006
Location: jonnyGURU forums, of course!
Posts: 10,435
Thanks: 125
Thanked 159 Times in 107 Posts
Default

If the PSU is inefficient, it produces more heat. Hopefully the fan is spinning up to evacuate that heat.
__________________
We are stuck with technology when what we really want is just stuff that works. -- Douglas Adams
Reply With Quote
  #5  
Old 11-02-2013
2Bornot2B's Avatar
2Bornot2B 2Bornot2B is offline
micro ATX User
 
Join Date: Mar 2011
Posts: 13
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Am new so I had to look for what MTBF meant but in any case are we assuming that PSU's have regular or more than one "failure"??
If so what is a failure??
__________________
"The difference between genius and stupidity is - genius has it's limits"
Reply With Quote
  #6  
Old 11-02-2013
rafal_iB_PL rafal_iB_PL is offline
Flux Capacitor User
 
Join Date: Aug 2010
Posts: 1,937
Thanks: 41
Thanked 41 Times in 40 Posts
Default

MTBF is completely irrelevant for end customer.

Keyword is "mean". As in average. IE. you could meet MTBF of 100k with 50% of units being able to work 200k hours until failure and 50% of units not working right away, and you have no way to tell, which group the sample you get belongs to.

Extreme example, but certainly valid under definition of MTBF.
Reply With Quote
  #7  
Old 04-05-2014
ryanita ryanita is offline
Banned
 
Join Date: Apr 2014
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Very interesting to see how the temps actually affect things, and with this in mind, i would like to know what MTBF ...thanks in advance Jonny, you know your shit man! sorry for lang but very nice threads in your category
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
K-Mex Power Supplies Abu Som3a PC Power Supply Discussion 24 01-10-2009 03:06 PM
Gigabyte power supplies Makalu PC Power Supply Discussion 5 06-24-2007 10:07 AM
Supplies, Rated at 40c/50c, and HOT rooms!? signmeuptoo PC Power Supply Discussion 10 04-20-2007 03:29 PM
Testing Power Supplies Eoin Testing Methodology Discussion 14 02-07-2007 09:41 PM
Power Supplies by Nmedia? SKYMTL PC Power Supply Discussion 10 11-02-2006 02:54 PM


All times are GMT -4. The time now is 06:59 PM.


Powered by vBulletin® Version 3.8.1
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.