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Testing Wattage consumption of your computer equipment
Making sure you’re not overloading your UPS or misc power source
By Red Squirrel
Did you ever wonder how many watts your equipment is using? Whether it is a PC with monitor, or a whole UD cluster setup with 100's of machines this article will let you know how to easily find out. Or perhaps you just want to know how much power you would need to build one of those clusters. You can estimate yourself since some appliances such as monitors show the rating on the back but some other things such as computer power supplies don't. And just because it's a 300W power supply does not mean it uses 300W of power. It all depends on how much of that power the components are using. Watts are usually the best measurement to know because it does not change. But volts and amps do. If you plug in a vacuum cleaner in an 80volt power source it will be using way more amps then if you plug it in a 120V power source.
Watts is equal to volts times amps. So basically the higher the voltage, the fewer amps being drawed. Amps are also the actual current and what decides on the power of the circuit. What I mean by that is, if there's not enough amps, you can't run much, even if the voltage is high. A static shock can have millions of volts passing through you but yet it's not enough to power a light bulb, because there are very little amps. Amps also cause wires to heat up, the more amps, the bigger wires you need since if they are too small they will melt. This explains why power cords for large equipment are thicker, while a small lamp is just a cheap cord that you can cut with a pair of scissors, since one uses more amps but same voltage, and this results in more watts as well. Because of this heat though, there is power loss, so this is why when power is delivered to far distances, it is sent in high voltages, so they can use smaller cabling (cheaper) and there's less loss of energy from heat. So it's more efficient. But just because of what I said about the static electricity, don't think you can go climb one of those poles and get a shock that tickles.
Enough theory, let's get started.
First this is what you will need:
A multi meter: One that can measure AC amps and AC voltages and should have a high enough rating. These can be purchased at a Radio Shack or similar store for about $100 and are very useful around the house.
Crocodile wires or other wires easy to manipulate and plug.
An old power cord with both ends open to connect the crocodile clips to it easily. (or equivalent, basically any thing that can easly be plugged and unplugged from the socket)
Calculator (everyone has at least one around the house)
Safety common sense (this can be hard to find in some households)
Like mentioned earlier, Watts = Volts X Amps. So we need to know how much volts we are giving to the equipment, we know it's near 120 because that's the normal house voltage. But we want to be more exact, so set your meter to measure volts AC and
put both ends in a socket that is on the same circuit as the equipment. For safety purposes, try to find a socket that is easy to access and has plenty of room to move quickly in case something happens. But if you have everything set correctly and don't short it out somehow, you're safe. Just make sure it's set to Volts AC and if your meter has different ranges, set the right range so you don't overload it. But most good meters will auto sense the range for you.
Because my plug has too many wires near it at the PC, for this picture I'm measuring a different circuit but if your house is wired like mine there should only be 2 main circuits (two separate AC phases) so they should both be near anyway. I get the same reading here and in my room.
The voltage tends to move a bit. In my case it was around 119 and 121 so let's just say 120.
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Latest comments (newest first) |
Posted by Red Squirrel on August 08rd 2004 (18:26)
lol. I knew this article would get lot of safety concerns. 
If you use common sense, dangeraus situations can be made safe. 
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Posted by safety alert on August 08rd 2004 (17:51)
You're a fuckwit for showing current measurements using bare crocodile clips on mains-powered AC voltages.
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Posted by Red Squirrel on August 08rd 2004 (16:53)
I've been zapped many ways and I can easly say from experience the most fatal is the heart thing like mentioned. By nature some people are forced to hold on tight to whatever they are being zapped by. Thankfully for me I'm the oposite. I got zapped with who knows how many amps from arm to arm, and I felt it for like an hour. It was only 120 volts. The zap might of lasted about a milisecond or 2 but enough to make me tempoarly black out and "wake up" 5 feet behind me. I was laughing though but it would of been fatal. Stupid guy put the ground wire in the live and I was holding the "grounded" device in one arm and went to unplug it with the other. Zap! 
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Posted by masher on August 08rd 2004 (11:26)
> "Did you knew that dangerous currents are measured in tens of milliamps?"
Actually, there's two ways electricity can kill you. The first is the method used by the electric chair...enough power to to simply fry you, in which case the current level doesn't matter. Its the total wattage that counts.
The second is a current passing through the region of the heart, in which case the current level is crucial...from 50 to 200 mA, depending on a few factors. This causes the heart to fibrillate, and leads to death. Lower or even higher current levels are safe, interestingly enough, though a high enough level will cause the heart to asystole...to stop beating totally.
Its more complex than this, as AC induces fibrillation far more easily than DC...but DC induces asystole much easier than AC. And even the safe AC current level varies based on frequency, voltage, and waveform. Finally, the resistance of the human body tends to decrease somewhat the longer current passes through it...so what may start out as a safe current may quickly become a deadly one.
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Posted by Privatteer on August 08rd 2004 (02:29)
With RMS meter that could be as high as 2.1amps
5 harddrives, 1 cdrom, 1 dvd drive
gf4 4200ti, xp1700 cpu.
Cheap skyhawk 400w powersupply thats probably not very efficient.
I'ld give you figures for my p42.8 but my meter took a dive from the top of a ladder and is still getting repaired.
Instead of a load having a constant impedance drawing current in proportion to the sinusoidal voltage, electronic psu change their impedance by switching on and off near the peak of the voltage waveform.
The circuit of the power supply only draws current from the AC line during the peaks of the voltage waveform, thereby charging a capacitor to the Peak of the line voltage. The DC equipment requirements are fed from this capacitor and, as a result, the current waveform becomes distorted.
Since the amount of disortion changes between every powersupply a table would be pretty difficult to do.
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