Surprise!!! P4 system actually get power from 12V power line.

I'm surprised that P4 mobo and AMD Barton mobo actually get the power source from 12V ATX cable line. It's not from usual 5V nor 3.3V. So, when you add more fans HD, DVD, the whole 12V might suffer and might impair CPU stability at the end. To add the matter even worse, current design of ATX PSU has more power (ampere) on 3.3V and 5V lines. While the 12V line remain powerless and it's not regulated very clean (10% regulation, versus 3.3V and 5V at 5%)

Read here:

http://www.firingsquad.com/guides/power_supply/page2.asp

see the box:

http://www.overclockers.com/articles686/index04.asp


Source...................... Amps.................. Watts
12v P4 Conn............. 3.35 amps.......... 40.2watts
12v ATX Conn........... 1.10 amps.......... 13.2 watts
3.3 Volt Line.............. 8.02 amps.......... 24.5 watts
5 Volt Line................. 0.83 amps.......... 4.2 watts
12 Volt Disk............... 0.27 amps.......... 3.2 watts
5 Volt Disk................. 0.93 amps.......... 4.7 watts


+12V Current in amp................ CPU type
5.742188 amp.......................... Thunderbird 1GHz
6.289063.................................. Thunderbird 1.1GHz
6.466667.................................. Thoroughbred 2100+
6.55.......................................... Thoroughbred 2200+
6.872396................................... Palomino 1800+
7.115625.................................. Thoroughbred 2400+
7.115625.................................. Thoroughbred 2600+
7.291667................................... Palomino 2000+
7.492188................................... Palomino 2100+
8.145........................................ Northwood 2GHz
8.505521.................................. Northwood 2.4GHz
8.53526.................................... Northwood 2.2GHz
9.292188.................................. Northwood 2.6 GHz
9.707813................................... Northwood 3.06GHz
10.1175..................................... Northwood 2.8GHz

Man.... I hope this is wrong.

I know my NF7-S draws from the 12v line, so did the P4 motherboards.

but the older AMD boards drew from the 5v line.

it's not that shocking is it?

Believe it or not, it is actually a good thing that newer mobo's use the +12V rail for the CPU voltage. The reason has to do with power consumption and voltage drop. Also, it takes a lot less Amps on the +12V rail to provide the same amount of "power" as it does on the +5V rail or the +3.3V rail.

For example, if your overclocked CPU required 100 watts, then you would need the following amps on each rail:
+3.3 V = 30.3 amps <== WOW
+5 V = 20.0 amps
+12V = 8.3 amps <== a lot less

Which do you think is easier to supply (30 amps on the 3.3v rail or 8.3 amps on the 12v rail)??? Remember that each wire in your computer has a maximum amps that it can carry.

As for current power supplies, let's take a look at the Antec Truepower 430 watt power supply. This is a very typical sized power supply for most newer computers. Here are the Specs for this power supply.

3.3V ...... 28A ...... +/- 3% regulation
5V ......... 36A ...... +/- 3% regulation
12V ....... 20A ...... +/- 3% regulation

The first thing you will notice is that all 3 rails have the same voltage regulation, so voltage regulation is not really an issue (or problem) with most newer power supplies. The next thing you will notice is that the +3.3V rail is NOT rated for the 30 amps required by the CPU in the example above, so it cannot be used to supply the CPU power. What might not be obvious is the fact that the 12V rail can supply more power than the 5V rail. The 12V rail can supply 240 watts of power (12*20), while the 5V rail can only supply 180 watts of power (5*36).

Welcome to the year 2000.

^^^^ um, it is actualyl 2003

larva said:

Welcome to the year 2000.

Click to expand...

yes!

the atx-p4 connector has 2 x yellow wire. yellow = 12v

If I add capacitor maybe 1500uF 16V to the back of mobo where the 12V ATX connector present, will it help to stabilize the fluctuation. My 12V rail is not steady and I check my PSU's datasheet, the 12V rail is regulated 10% ( ohh my god ).

"Welcome to year 2000"

what do you mean? you mean the current mobo design is more economical?

I mean that when the P4 was introduced aroung the year 2000, the ATX spec was modified to include the 4-pin aux 12V line because of the fact that all P4s do indeed derive Vcore from the 12V line. It's the nature of the beast, and it isn't exactly a new development. What is new is AthlonXPs migrating to a 12V based architecture, but there never was any doubt about a P4.

Edward2 said:

<snip> lots of useful info</snip>

Click to expand...

thanks for that post.

i have always wondered why it is that they use the 12v rail instead of a lower one to power a cpu that only required less than 2v.

now i know.