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How many watts needed in a power-supply?
The other day, I built a new machine. I happened to notice that the case (a mid-sized/standard-size case) came with a 450-watt power supply. (Probably a 'cheap' one.) When I looked on the shelf (idly curious), most all the separate replacement power-supplies were mostly in the 360 watt range. Lastly, the other day, I took a 'tech exam', and one of the questions listed an exact config (e.g. P4, certain make/model of disk, etc, etc) and then asked as a multiple choice, which size power-supply should you select (when building that system). Ok. So, now I'm quite curious! I carry 'spare' replacement 360-watt power supplies around as a backup repair technician. (I've installed 2 or 3 of these 360-watt supplies into mid-sized machines that had bad power-supplies and they 'work'.) My questions a (1) Am I relatively safe using these 360-watt supplies (or should be learning how to 'calculate' a more precise replacement supply for each individual machine? (2) Can someone point me to a formula/website or whatever that gives guidelines on choosing wattage for a power supply? TIA... Dave |
#2
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On Thu, 7 Apr 2005 11:48:59 -0400, "David Cook"
wrote: (1) Am I relatively safe using these 360-watt supplies (or should be learning how to 'calculate' a more precise replacement supply for each individual machine? (2) Can someone point me to a formula/website or whatever that gives guidelines on choosing wattage for a power supply? TIA... Ive seen a few but I saw this link to one that was pretty easy. Some of them can be sort of vague about some components http://www.jscustompcs.com/power_supply/ Mine came out close to 360-370 so its close to my 400 watt PS. My 12 volt rail is kind of low too 11.65-11.71. And Im suddenly plagued with random reboots every 1-2 days. The intervals are long enough that Ive done memtest 3 times already for a day or overnight but I dont really feel like doing it for several days. Im hoping its a bios problem as all my problems happened after upgrading to the latest. I thought they were driver problems and they still might be but Ive changed them and the problem is still there though its lessened. Anyway Ive seen emachines with 2000 athlon XP , dvd burner and cd drive and a 80 meg HD - ECS built in video, sound --- one of those dinky microatx cheapie boards with a 250 watt. Many emachines and super cheapo small bargain systems with everhthing builtin seem to come with low powered PSes. Around here I think the standard now at stores is 400watts. They may be crappy PSes but they tend to all be 400 watts nowadays and the lesser ones are AT PSes for older systems. The few exceptions are a few at Compusa , a low end Maddog 350 and I think they have an Antec for 380 watts or so. People tend to like that round number 400 watts I think so its generally 400 or over. |
#3
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On Thu, 7 Apr 2005 11:48:59 -0400, "David Cook"
wrote: The other day, I built a new machine. I happened to notice that the case (a mid-sized/standard-size case) came with a 450-watt power supply. (Probably a 'cheap' one.) When I looked on the shelf (idly curious), most all the separate replacement power-supplies were mostly in the 360 watt range. If only wattage ratings on generics were accurate these figures could be used in some useful way... since generics typically aren't accurately rated, you may as well ignore the numbers except "maybe" to compare to another unit in the same manufacturer's model line... and sadly enough, even then some may merely have a larger fuse. Lastly, the other day, I took a 'tech exam', and one of the questions listed an exact config (e.g. P4, certain make/model of disk, etc, etc) and then asked as a multiple choice, which size power-supply should you select (when building that system). I would seriously question the competence of whoever wrote the exam. Multiple choice where only the wattage of entire unit are listed is not a reasonable way to determine a right/wrong answer... the choice cannot be intelligently made without more info. The correct answer would be to write onto the test space that "It is negligent to choose any of them based only on the info provided". Ok. So, now I'm quite curious! I carry 'spare' replacement 360-watt power supplies around as a backup repair technician. (I've installed 2 or 3 of these 360-watt supplies into mid-sized machines that had bad power-supplies and they 'work'.) My questions a (1) Am I relatively safe using these 360-watt supplies (or should be learning how to 'calculate' a more precise replacement supply for each individual machine? "calculate" "precise" is not a realistic goal, since you won't even know the actual power useage of a particular component unless you measure it yourself. Manufacturers provide maximum ratings, ratings that are not observed by anyone when building because it's not possible to create a sustained maximum current condition on a majority of parts simultaneously. As important may be what I briefly mentioned above, that you must first know the true output capabilities of that "360W" power supply. 360W are somewhat rare too, are they Enlight/Sirtec/Highpower/etc? I ask because those have a power switch in the back, IIRC, and "random, most-compatible spares" shouldn't have a power switch on them as many OEM cases aren't compatible with (rear switch). (2) Can someone point me to a formula/website or whatever that gives guidelines on choosing wattage for a power supply? You can Google for 'em, and be no better off than you started. Regardless, this search does link the most popular ones: http://www.google.com/search?q=power+supply+calculator The actual needs are more of an estimation plus some reserve power, even more reserve for a larger case with more expansion capabilities. The goal is not to perfectly fit a system that uses 187.3W with a ~ 190W PSU, but to determine where the power rail distribution is such that you know the current needed on the most heavily loaded rails, which in the case of a P4 based system is usually the 12V rail, but often still a fair amount on 5V for motherboard and other uses. Contrast that with a P3, older or median aged Athlon system which needs more 5V amps. Add more amps as appropriate for a high-powered video card. So primarily you need a PSU with true rating higher than hungriest system voltage rail. Then consider the 2nd most hungry rail. Rarely are any other rail considerations necessary, except maybe with a very low-powered system that has a ton of USB devices and needs them powered in standby mode. Online calculators might give you figures like 12A 12V for CPU, drives and fans, 20A 5V for motherboard and misc... and still the primary issue is whether the PSU is accurately rated. Sadly unless one has experience with a particular PSU and where it's limitations are, the quickest thing to do is overspec the power supply... and it's not a bad idea if the alternative might be one marginal for the system, since the next-higher wattage typically costs only a little bit more. If you plan to use one line or a few models you might consider doing some output and load testing, or have someone who you feel is qualified to assess them for you. Once you have your preferred brand at a good price, stick with it else repeat whole process to qualify new/alternatives. |
#4
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" wrote in message ... On Thu, 7 Apr 2005 11:48:59 -0400, "David Cook" wrote: (1) Am I relatively safe using these 360-watt supplies (or should be learning how to 'calculate' a more precise replacement supply for each individual machine? (2) Can someone point me to a formula/website or whatever that gives guidelines on choosing wattage for a power supply? TIA... Ive seen a few but I saw this link to one that was pretty easy. Some of them can be sort of vague about some components http://www.jscustompcs.com/power_supply/ Mine came out close to 360-370 so its close to my 400 watt PS. My 12 volt rail is kind of low too 11.65-11.71. And Im suddenly plagued with random reboots every 1-2 days. The intervals are long enough that Ive done memtest 3 times already for a day or overnight but I dont really feel like doing it for several days. Im hoping its a bios problem as all my problems happened after upgrading to the latest. I thought they were driver problems and they still might be but Ive changed them and the problem is still there though its lessened. Anyway Ive seen emachines with 2000 athlon XP , dvd burner and cd drive and a 80 meg HD - ECS built in video, sound --- one of those dinky microatx cheapie boards with a 250 watt. Many emachines and super cheapo small bargain systems with everhthing builtin seem to come with low powered PSes. Around here I think the standard now at stores is 400watts. They may be crappy PSes but they tend to all be 400 watts nowadays and the lesser ones are AT PSes for older systems. The few exceptions are a few at Compusa , a low end Maddog 350 and I think they have an Antec for 380 watts or so. People tend to like that round number 400 watts I think so its generally 400 or over. Thanks, that link to the 'power supply calculator' really puts things in perspective. Based on running a few numbers, my practice of using a 360-watt supply seems to be plenty. Cheers... Dave |
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