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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

Measuring amps is a bit trickier especially with mains power since you don't just shove it anywhere, but you have to break the circuit and put the amp meter in it, and be safe. So the easiest way to do this is by wiring the circuit yourself with the crocodile clips.

So turn off and unplug the device you want to test, in this case a monitor. Connect everything like shown in this picture. The arrows indicate the flow of electricity to make it easier to follow the circuit with all these wires all over. Note that this flow of electricity changes 60 times a second (50 times in some regions, the UK I believe) so this is why AC is different than DC.

Shocking!  This monitor only uses 0.5 amps!

So the white wire is plugged into the mains, and the black is the plug for the monitor. Completely ignore the ground for testing, DO NOT PLUG ANYTHING TO IT! In a permanent setup you would want to ground it, but for testing purposes it's ok to have it ungrounded for a bit, just be careful and if you feel safer ground a crocodile clip and connected to the grounding pin, but be careful since there's more chance of a wire slipping and shorting out. In the picture it's hard to see but the two wires connect to the two pins of the plug, so think of the two pins as the + and - of a device that needs power and the two wires as the + and - of a power source, since this is what it is, but with AC so the + and - order does not matter, but make sure the two wires don't contact each other since that's a short and you can blow the meter.

On the amp meter, the terminals used are the unfused 10A max and the COM. The fuse in the meter has a max of 500mA so it's best not to take a chance at blowing it, when we're measuring this much amps, and chances are you won't measure anything more then 10Amps. Never measure an entire power strip, but each item individually, to make sure of this.

When setting up the wires, make extra sure to unplug the cable that is feeding the power (the white one in this case) Once completed let part of the circuit broken like in thes picture below, plug in the cable then take both wires (the red crocodile clip and black meter cable in this case) and tick them together quickly just to be sure you won't cause some kind of short. Again, this is only to make sure you did not mess anything up such as badly setting the meter, or if some wires are touching somewhere that should not. Then hook them up together solidly and turn on the device you are measuring and you will get a reading. Lot of equipment have transformers and stuff which use up a bit of amps even when turned off, so this is why you will sometimes get a reading even when it's turned off. If you calculate all the devices in your house like this, you may be using a few amps from items just sitting there turned off!

The following pic shows the circuit unplugged and ready to go.


Take note of the reading and repeat this process for each item. So when done you should have something like this:

House voltage: 120V
PC full load: 1.2A
PC without fans, lights and UD: 0.8A
Monitor: 0.5A

Once you have all the readings, just do Volts times Amps. So...

120V X 1.2A = 144W
120V X 0.8A = 96W
120V X 0.5A = 63W

Since the first 2 readings are the same device you can decide on if you just want the average, or the most widely used. In my case I put the most widely used, since it's very rare that I turn off UD (0.2Amps) or the lights. Sometimes I may turn off a set of fans but those don't make a big difference. So I'll stick with 144W.

So now we simply add 144 + 63 which gives us 207 Watts. So now I know that my PC and monitor which are plugged into my UPS use up about 207Watts.

Also note that this is only the base of the concept and because this is AC and not DC, it get's more complicated. Amps are sometimes not in sync with the volts and unless you are using a RMS (root means square) meter, you may get inaccurate results. Some meters don't even tell you if they are RMS or not, so I'm not even sure if the one I used is or not but according to what some say these readings seem about right for the equipment used. Basically, try to not go too near your UPSes max wattage and you should be safe. Some UPSes also don't even tell you how many watts it can handle, but rather VA (volt amps) which are slightly different.

Shock hazzard Danger: The activities in this tutorial were done since I knew what I was doing and was using proper safety measures to avoid accidents. Please realize that this activity, especially with amp readings over 1 can be fatal! Be extremely careful, especially not to close any open circuits with your body as it will make the amps go through you, and possibly kill you! If you have any heart problems, this is even a bigger risk to take and should most likely be done by someone else.

Disclaimer: IceTeks, or myself may not be held responsible for any damage done during the proceeding of this article. This is for informational purposes only and should only be performed by someone who knows what they are doing!

-Red Squirrel
IceTeks Owner

27464 Hits Pages: [1] [2] 4 Comments

Latest comments (newest first)
Posted by Red Squirrel on August 08rd 2004 (19:26)
lol. I knew this article would get lot of safety concerns. em320.gif

If you use common sense, dangeraus situations can be made safe. wink.gif

Posted by Red Squirrel on August 08rd 2004 (17:53)
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! lightbulb.gif
Posted by Red Squirrel on August 08nd 2004 (18:49)
Yeah I discovered that after, so I had added it at the end that it may not be very accurate. I'm not sure if mine is RMS or not, but I doubt it since I only paid about $100 for it. Later on I'll try to figure out a better way (low cost) to measure true RMS with any meter. (maybe plug the results in some formula or something but I doubt it's that easy)

1.24 is pretty high for 240 bolts, do you have lot of high end stuff in it? Mine ran at about 1.2 (inacurate though) at 120 volts.

Posted by Red Squirrel on August 08nd 2004 (13:21)
QUOTE (Fin @ Aug 20 2004, 09:28 AM)
First you say "I knew what I was doing" and then in next sentence "Please realize that this activity, especially with amp readings over 1 can be fatal!"

There is a little conflict between what you say. Did you knew that dangerous currents are measured in tens of milliamps? Resistance of person is limitting current, so current in machine doesn't mean anything. Current depends of voltage and resistance so big voltage is the dangerous thing in these measurements.

It's possible to do something dangeraus and still know what you're doing. wink.gif

Look at those crazy stunt people like jackass. It's unsafe, but they know what they are doing so they (usually) don't get hurt.

As for the accurcy, I sort of realized that afterwards, when I did more research, but this method will still give you at least an idea of wattage. If you can afford the fancy stuff that does it right good for you, but especially here in Canada it's not obvious to get the fancy stuff, sometimes it's too expensive, sometimes you can't even get it period.

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