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Measuring power usage of my PC.

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zack   11-10-2001 at 12:54:32 AM

I have a multimeter, and am curious how I would measure the power usage of my pc. The thing has an amp reading, so I think I would use that. I just don't know which wires or what points i would measure at. Thanks if can help.

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WiseGuy1   11-12-2001 at 05:11:53 AM
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This may help a little. <A HREF="http://www.formfactors.org/developer/specs/atx/atx2_03p1.pdf" target="_new">http://www.formfactors.org/developer/specs/atx/atx2_03p1.pdf</A>

It's an atx specs sheet. Pretty long and it's dated 12/98 but it may give some info as far as what pins to check.
It's in pdf format BTW.

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ejsmith2   11-12-2001 at 05:30:31 AM
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Hmmmmmmmmmmmmmmmmm.


Well, there's two ways to do it.

You could measure the total power your psu is using, which will be about 5-10% more than what is actually delivered to your computer. Psu's aren't 100% efficient.

You can measure the 12/5/3.3 rails seperately, and do the math.

Don't forget, unless you bought one of the $500 model mm's, you're capped at 10 amps. And that for only 15 seconds at a time, unless you want melted wiring. The 2A side is fused, and will detonate at 3amps (they give a little leeway).

Go down to radio shack and buy a pack of 6 alligator clips to make the job easier.

Rich is the nation that has many war heroes. Long since forgotten...

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zpyrd   11-12-2001 at 06:19:37 AM
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I advise you not to do it.
It would be ok to check the voltages on the main rails using a multimeter.
Measuring current accurately means you have to put your meter in series in the circuit to be tested. Thus you have to cut your power supply conductors to install the meter.
Now if you have a clamp on multimeter it is unlikely it will measure DC amps.
Typical clamp on meters are designed for industrial purposes. IE measuring current demand by inductive load.

High quality power supplies provide stable voltage supply at all loads. And when the load fluctuates frequently any voltage fluctuation is small and of short duration.
The best tool to measure a power supply is an oscilloscope.
The scope will provide visual measurement over time with extreme accuracy.
For me it's been 12 years since my last electronics instrumentation course but the prof I had was a drill sergeant when it came to the lab projects.
If a measurement was couple microvolts, microamps or microseconds off he would ream my a$$ in front of the class.
I had a tight a$$. I got an A.

I realize that most oscilloscopes are not affordable to individuals. But they are not to expensive to rent. Check your local Yellow Pages for electronic wholesale distributors. Chances are they rent testing equipment.


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zack   11-13-2001 at 01:34:02 AM
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heh, basically i just wanted to measure and see how much it would cost to keep the computer on 24/7. I was planning on doing it outside the case, where the ac current comes in. but oh well. It's just some el cheapo $30 multimeter that someone gave me for free cause it didn't work (fuse was blown, lol).

I've been having fun with it for a few days though. I question the accuracy of it though. I can measure some batteries, and it says the voltage is higher than what the battery says, and i measure some power supplies, some measured higher, and others were lower than what the label says. If i measure like a 30 ohm resistor, it will be exactly 30 ohms on the meter.

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zpyrd   11-13-2001 at 06:12:36 AM
- 0 +

It's all about averages and tolerances.
For instance many power supplies will only provide rated voltage when a load is applied.
Batteries are generally labled with their average voltage.
Resistors have tolerance ratings. Average tolerance is +/- 5%

<font color=blue><b><i>My Rice Krispies tell me everything I need to know!</i></b></font color=blue>

Reply to zpyrd
WiseGuy1   11-13-2001 at 06:32:24 AM
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yeah, when I worked as an electrician a few years ago I can't ever remember testing a 120 volt line and actually getting 120 volts. There's so many variables involved. Older appliances pluged into the line where the plug is turned around (on the older two prong plugs) could result in a reading of 5-7 volts even after the juice was shut off. If you have a good tester you can even get voltage readings by touching your tester leads to the ground. Long story short,zpyrd's point of averages is probably correct.


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ejsmith2   11-13-2001 at 06:53:56 AM
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120 is peak to peak.

115 is Average, 110 is ERMS.

(or vice versa, I don't have a calculator with me, and I'm not about to use msft's)

Anyway, if you're wondering how much it costs to run it 24/7/365, use the number for the full power of the psu. And the monitor/printer/whatever. That's your top number, and it's probably around $200 per year.

Now you look up the individual amp ratings for all your components, add them up. That will give you your low end number.

Average the two, and that's your final. You'd be surprised how little (5% or less) you'll be off.



Rich is the nation that has many war heroes. Long since forgotten...

Reply to ejsmith2
zpyrd   11-13-2001 at 07:32:23 AM
- 0 +

To measure AC power "effectively" it has to be convereted into a DC equivalent.

When working with AC, effective measurements are RMS values. (Root Mean Square) RMS is way of taking a sinusoidal wave and converting it to a DC signal.

For instance in North America nominal household voltage is 120Vrms.
Peak value is 120*1.414=169.68V
Peak to peak is 2(120*1.414)=339.36V

1.414 is the value of the square root of 2 which is a short cut method of determining peak values of rms signals.


<font color=blue><b><i>My Rice Krispies tell me everything I need to know!</i></b></font color=blue>

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