On 8.12.2004 at 1:09 Mike MacNamara wrote: >The latest CPUs need a lot of amps on the +12v lines, the AMD64s need at >least 18 amps barebones just to post. but 30amps will run them OK.
OK, let me sort this one out so that we can go back to QL things: Actually, that may not have anything to do with it. The motherboard components do not use 12V directly, and in some cases not at all. What this is about is the core voltage supply for the CPU. Even though this is a low 1.5 - 1.75V in most modern CPUs, the current needed is quite astounding - 50 to 60 Amps is not uncommon. In oreder to get this sort of power supply to a CPU from an external power supply, one would need wires and connectors with a cross section abut the same as those connecting a car battery to the starter - i.e. some 10mm in diameter - completely impractical. This is why the required voltage is supplied from an on-board converter. It is left to the board designer to decide wether this converter runs off the 5V line or the 12V line. Depending on that decision, running a fast CPU on a given motherboard may require the power supply to provide extra Amps on the 5V or 12V line. If your motherboard uses an ATX2 additional 12V power connector (kind of like a shorter, almost square version of the usual large ATX power connector, with only 4 wires, two black and two yellow), your power supply needs more current capacity on the 12V line. If not, then on the 5V line. It may be worth noting that the on-board converter(s) operate like true transformers - Volts out times Amps out (=power output) equals Volts in times Amps in (=power input), minus conversion loss (10-15%). What this means is that using 12V to power the converter requires less amps than using 5V. This is the reasoning behind the small extra ATX2 connector (also known as the P4 connector) - connector contacts also have current limits. Unfortunately, driving the converter off 12V also has other consequences, but this is truly well outside the scope of this message. As a rule of thumb, for the purposes of supplying power to the CPU, every 1 Amp on the 12V line counts as 2.4A on the 5V line. All of this assumes that no other components are signifficant loads on these lines - in reality, they are not large loads, but may just be what trips the current limit in the power supply. Also, AGP graphics can be a demanding load on the 3.3V line... OK, now that you know everything about choosing the PCs power supply, we can go back to QL thematics ;-) Nasta _______________________________________________ QL-Users Mailing List http://www.q-v-d.demon.co.uk/smsqe.htm
