PSU: 20 pin from 24 pin derate ?

Many ATX type PSUs have an AT connector that is a 24 pin connector, or are quoted as being "20/24 pin" or "20 + 4 pin". This is main connestor, not the square 2x2 "P4" connector that plugs into the mainboard and powers the CPU (I believe). My PC motherboard has a 20 pin socket for the main connector, and the question is; is power supply/individual current ratings derated from the specification if I only use (the apppropriate) 20 pins of the 24 available ? (the extra four pins are GND, +3.3, +5, +12 [not in that order])

I ask this because I've been thinking about getting a new, cheap power supply for my PC in order to give me some spare "headroom" (I only want a cheap one because my PC is worth virtually $0, it's acquired the status of a bit of a project [plus I know my way around it a bit]).

I've several specifications for it to be to be useful to me, but I'd like some help on one particular point, which is independent of the price of the PSU (??) (see above).


Dumb_Question
25.August.2012
Compaq Presario S5160UK DT261A under XP/SP3
Processor - Celeron 2.7 GHz
Motherboard - MSI MS-6577 v2.1
RAM - 1GB + 512MB (1GB +1GB max) DDR PC2700
PSU - Octigen 300W model 10270PSOTG ('upgraded' from original Bestec 250W PSU [in 2011?])
Nvidia GeForce 6200 graphics card in AGP slot.

 

No the rating's still stand irrelevant of the wire's the power is drawn from.

Looking at your specs I wouldn't even upgrade your power supply unless your experiencing other symptoms such as your power supply overheating or your planning on adding a ridiculous number of ancillary components.

 

the 20+4 design was mada so the new power supply with the 24 pin can fit in the old mobos (and even soem new which still come with 20 pin power supply)
you simply take off the extra new 4 pins and you connect the 20 pin part.
if you have a new mobo with the 24 pin you connect the 4 pin to the 20 pin and you have a 24pin cable

 

Thanks guys for your reassuring responses.

-> Rikky

I am thinking of upgrading my PSU because it has been suggested in two other places (one on MG) that my PSU is a bit low powered for the job. One even described my PU as "crap". These suggestions arose because I have reported that my PC fails to POST or display anything on the screen at all on the screen, though power is reaching the mobo, on at least 95% of start-ups and reboots (I run it all the time now). In fact the maximum currents allowed for the various voltages are lower on the 300W Octigen than on the original 250W Bestec.

Going by the ratings on the original manufacturer-supplied unit (Bestec), they would lead me to believe my PC has heavy draw on the +5V especially and also on +3.3V rail, but not so much on +12V rail, going aganst the more modern trend. My PC is pretty old though. I have used the eXtreme power rating tool (http://extreme.outervision.com/PSUEngine) to give me an idea of what power I need my PSU to deliver, and with my system (worst case) it gives a little over 300W. (I didn't use the Pro version which will enble you to get estimates of the current draws on the iindividual voltages).

That's even without the Belkin wireless network PCI card I have, that I may wish to use.


So, that's the background.

Dumb_Question
26.August.2012
Compaq Presario S5160UK DT261A under XP/SP3
Processor - Celeron 2.7 GHz
Motherboard - MSI MS-6577 v2.1
RAM - 1GB + 512MB (1GB +1GB max) DDR PC2700
PSU - Octigen 300W model 10270PSOTG ('upgraded' from original Bestec 250W PSU [in 2011?])
Nvidia GeForce 6200 graphics card in AGP slot.

 

the problem is a 300 watt power supply does not actually gives you 300 watt it consumes 300 and say with 80% for old ones that is 240 watt you actually have.
also the rating of the wattages is for all the rails.
and also every rail and line has a limit.
in your case with an old system and propably with old hardware connected to that you will draw more power to 5v and 3.3v rail
so besides the wattage you must also look how must power the psu can supply at that lines
maybe you will have to buy a big psu so it can cover you in that lines

modern pc more or less use the 12v rail and even the devices are made to use that one over the other rails.

 

-> ouranos
a preliminary spec I have in mind for a new PSU is that the connectors must be a superset of those I use at the present time ( ie exceed 20 pin main, 4 pin CPU "P4", 3 x 4 in-line Molex [HDD/CD/DVD], 1 FDD 4 pin "mini molex"); and the current ratings on the individual rails must exceed those on my present units (Bestec has the highest, 3.3V-18A|5V-25A|12V-14A|-12V-0.8A|%Vsb-2.0A (2.5A max))). I'm not sure how fual 12V rails fit in with this. Also, this rules out many of newer the psus made by reputable manufacturers, eg, Corsair. Information at this level of detail is often incomplete.

Dumb_Question
27.August.2012
Compaq Presario S5160UK DT261A under XP/SP3
Processor - Celeron 2.7 GHz
Motherboard - MSI MS-6577 v2.1
RAM - 1GB + 512MB (1GB +1GB max) DDR PC2700
PSU - Octigen 300W model 10270PSOTG ('upgraded' from original Bestec 250W PSU [in 2011?])
Nvidia GeForce 6200 graphics card in AGP slot.