AC/DC Watts

**toxsickcity** · 07-18-2005, 02:57 AM

Hi people..

I just built a Digital Current Meter that works for both the AC & DC power.

I have a system in my home.
Specs:
Pentium4 2.4ghz(512cache)
512MB DDR
40GB 7,200
450Watt cheap Power supply (possible 55-65% efficient)
CD-Rom 52x
ATI 9600 xt
asus p4pe motherboard
2x USB device

Our power here is 220V
Tested without Monitor.
0.35Amps Desktop
0.60Amps Heavy Use (Games, etc...)

am I correct to say watts is equal to: AC
Desktop 0.35 x 220 = 77watts
Heavy 0.60 x 220 = 132watts

Is this information usefull for know what dc-dc power supply i would want?

Can I subtract some of the watts due to the power supply crappy efficiency

what is average efficiency of dc-dc psu.

based on that specs of my pc.. and change to this set of parts..
Underclock P4 to lot slower speed using a piggyback cpu multiply adjuster.
small geforce 2mx not the ati.
2.5"hdd instead of the 7200 40gb
256ddr instead of 512mb
slim cdrom instead of normal drive
what would be a good dc-dc converter to get.. 90watt be ok?

lastly how do I convert the ac watts of eg: 72watts to real watts, in thery i want to know watts in 100% efficiency

**Zorak** · 07-18-2005, 02:53 PM

The wattage will remain about the same. Converting voltages is much easier with AC, since the current is alternating and you can use a transformer to do it. For DC, it's much harder to convert voltages, and less efficient.

Watts are the same regardless if it's AC or DC. Watts are always volts x amps. If you look on the back of many appliances with worldwide power supplies, you'll see something like 110VAC, 1 amp/ 220VAC, 500ma. In both cases the wattage is the same, since you doubled the volts and halved the current.

For your computer, the wattage won't change very much when you change power supplies. If it's using 77 watts normal, 132 watts under load, it's going to use about the same even (or even more, depending on the PS efficiency) with a DC power supply.

A 90 watt supply will not work. You're still going to need a much bigger one than that. A P4 2.4 ghz at full load uses about 75 watts just by itself.

**Bugbyte** · 07-18-2005, 06:00 PM

Are you measuring the amperage before or after the power supply? That is, on the AC side or the DC side? If you measured after the power was converted to DC, you should be getting a fairly true reading of the actual wattage you'll need to plan for.

From the wikipedia definition:

Watts = Volts X Amps

77 watts = .35 amps at 220 volts

77 watts = 6.41 amps at 12 volts
132 watts = 11 amps at 12 volts

which explains why you need thicker gauge wires for lower voltage applications.

**toxsickcity** · 07-19-2005, 12:12 AM

I am measure at the AC side.

I looked up my power supply model it says it has a PF of 60% and the efficient rating on this psu is 67%
so idle 77watts of AC current being use to power the psu and the computer!

Would removing the efficiency be like this
(0.35 x 220 = 77watts) = (77 x .67 = 51watts) = 51watts!

And last night I tested my AMD system with the workstation MP processor

Power was
0.25A idle
0.35A GAME
0.36A PEAK (multimeter has PEAK reader as well)

system was on for hours and peaked 0.39a

idle was 55watts with efficiency of the power supply being 68%
= 37.4 DC watts used with idle.

that reading was with a lot of plugins... 4 usb device firewire and 3.5" 7,200 and big 5 1/4" cdrw

so possible the reading will be alot lower!

also
I tested the PSU by itself by shorting the switch pins on the atx power cable
PSU used 0.09A with nothing plugged in.
0.09a was for the psu andthe psu fans!

would I remove the 0.09 as well?

ALso thanks alot for the replys...

**Bugbyte** · 07-19-2005, 12:15 AM

Rather than trying to calculate the efficiency of the supply, why don't you measure the draw directly after it is converted to DC?

**toxsickcity** · 07-19-2005, 02:59 AM

how... the only way i know is to open the circuit of every wire that goes to the motherboard/drives...

unless there is another way?

**rsd212** · 07-19-2005, 07:49 AM

Would one of those nifty field-sensing current probes work at that low of currents? You could measure all the wires for a certain rail at once using a non-intrusive probe like that. Would require you to buy a new toy though