Creating a home-made kill-a-watt?

Nazo

Nazo

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I remember some time ago I passed by mention of a project to essentially build your own home-made kill-a-watt. It seems like I remember it being a relatively simple design with a resistor placed across or something and all you had to do was measure the voltage across the resistor with a multi-meter or something like that. If there is such a simple way to do this, I'd love to give it a shot right now, but at the time I didn't want to do such a thing. Now I can't seem to find anything about it when I search the Internet though. (It might even have been here, but I'm not having much luck here either.) Maybe I'm just searching wrong? I don't want a really complicated one like that jar project or anything though. I'm imagining just taking an old power panel and modifying it to be able to take a multi-meter reading with ease or something.

Anyone have any details on this more basic design? In particular, I'd like to see how much energy my computer uses now that I have a newer videocard.
 
In THEORY this could be done with a resistor in series with one of the incoming AC lines (the hot line rather than the neutral) and measuring the AC voltage drop across it. The voltage drop across a 1 ohm resistor would equal the current in amps. Multiply the amperage by the voltage (120 for the US) and you have power. But you'll need at least a 10 watt resistor and your accuracy will only be as good as the accuracy of your resistor. A 1% tolerance 10 watt shunt resistor will cost at least a couple of bucks. Honestly, you might only save a few bucks and if you don't know what you're doing, you could easily kill yourself. If you're asking this question, you probably don't have the experience to safely do it. Just buy a Kill-a-Watt rather than build a Kill-a-Nazo.
 
In principle you can put a resistor in series with a load and measure the voltage across it to calculate current.

1 ohm is a bit high, that will give you 1 volt of drop for every amp your load draws.

This creates two problems, firstly a lot of volt drop and secondly a lot of power dropped in the resistor. at 15A a 1 ohm resistor will be burning 225W. EEEK

I'd suggest a bundle of resistors in paralell. If you get 10 5W 1 ohm resistors in paralell you should be good to go.

Remember to respect the mains. The resistors need to be enclosed in a suitable enclosure and the multimeter needs to be connected with appropriately shrouded leads.

And finally remember this will only measure current it won't tell you the difference between real and reactive power.
 
I would not do this because.
1. I do not think this will be very accurate especially for PFC power supplies.
2. This is potentially dangerous. You could electrocute yourself or start a fire.
3. The parts will cost a few dollars anyways.
 
I custom built my own home utility monitor here.

Wouldn't help for what your trying to do, but shows the basic concept of AC current measurement.
 
Ok, just to be clear here, I'm not a complete absolute amateur at electronics or something. Yes, I will admit I don't know everything about everything, but that hardly means I'm so inexperienced that working with mains power will get me killed automatically. I've been working with electronics for quite a large part of my life now and that includes mains power. (Also, I want to point out here that this method I'm looking for is FAR safer than the way I've had to do it in the past -- which is to say running the power through the current meter directly which can't be fused, so in theory it could be dangerous. Though the chances of a PC with a PSU rated at 500 watts ever using 10 amps are about the same as a meteorite suddenly striking me dead at random and even then it would just destroy the multimeter which isn't very likely to actually be dangerous to me.) Just because I don't have absolute experience in actually dealing with resistors, capacitors, inductors, and etc that I know every little caveat of actually working with them to come up with tricks like this on my own doesn't mean that if I have to ask I'm incapable of doing such simplistic projects as this in absolute safety. In fact, I'd trust my own works over that of some so called professionals who do things wire many people's houses without properly grounding, maintaining appropriate polarization, or etc... (Yeah, I've found some real surprises along the way in the past. My favorite though was an improperly sealed outside circuit connected in series with the rest of the house such that when it failed it prevented everything past it from working. Of course, the owner was just lucky that the bad seal resulted in nothing worse than simple oxidation rather than any actual shorts...)

As far as prices go, real Kill-a-Watts seem to run around $30 for the cheap low end model. I assume the $20 figure people are quoting is for some online sites like Amazon or whatever where you get sellers with products that aren't even necessarily guaranteed in any real manner since this is not the MSRP certainly. The resistor I need will probably be closer to $3. Both the $5 multimeter and old power panel are already in my possession. That's $3 versus, what? $20 at best for some unknown random online site which may not even be selling the legitimate product (and thus it may not necessarily be any safer at all)? Or worse, $30+ for the real thing. I don't NEED the real thing. I just need a simple basic test...

Oh, and yeah, just to be clear, I don't need absolute accuracy. I need a rough guide for testing a few basics like how hard my PSU may be being pushed, how much power is actually being used by an amp/speaker combo (in particular I must admit I really would like to see how much energy my new "portable" speaker really uses -- if I'm understanding correctly it may use as little as 7 watts plus whatever waste the digital amp itself produces for all that I can hear it clearly across the house...) Then too, there are other various things I like to test from time to time for various other such minor reasons. None of this is really worth the cost of a Kill-a-Watt. It is worth the cost of a cheap resistor. It can be a real pain to test using the current meter though, as I said.

Also, if I have to explain the value of doing a project I really don't think I could. Suffice it to say that sometimes doing this sort of thing has its own value even if it actually costs more and requires more effort than just going to your local store and buying the equivalent device. (I definitely feel I can say that about my so called "portable" speaker for instance. There are tons of them to buy in local stores, but none I've ever seen can compare to what I've come up with now in quality, volume, and etc plus I've learned a fair bit along the way. Nevermind the fact that the end result is still far cheaper to build than the next closest in quality that I could ever find... Plus I gave my older prototype build to my mother who just loves it, so now two people have benefited from what originally was just playing around.)

mtrupi said:
http://hardforum.com/showthread.php?t=1067898
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Yes, this is exactly what I needed -- probably the very thread I saw that time before in fact. I'm not sure why my searching didn't yield that particular thread. Thank you. I thought I remembered the method being as simple as that, but couldn't seem to find verification of this.


PS. If it makes anyone feel any better, I've been shocked before plenty of times messing around with electricity as a kid and I learned long ago what not to do. I haven't felt an electric shock beyond normal static electricity from petting a cat or whatever in a very long time and I will be keeping it that way. As far as fires go, I think you're overestimating the danger of this project by multitudes, however, if it makes you feel any better, I DO have a fire extinguisher that's pretty easy and quick to reach. (Mind you, this is not because of any theoretical accidents working with electricity. I'm a lot more worried about leaving the oven on by accident or something like that.)
 
Nazo said:
None of this is really worth the cost of a Kill-a-Watt.
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But once you have the Kill-a-watt, you can test your power supply, then drag it around to all the stuff in your house and test it also!

I've tested just about everything I own!
 
Glad to help. As for killing yourself, just be careful. It can hurt something terrible but if you don't grab on and hold your chances are pretty good.

Also, a PFC supply should help make a better measurement. The purpose is to make the current waveform more sinusoidal and in phase with the voltage.

edit: Seems to be lots of people experiencing low power measurements with a KAW attached to APFC. They speculate that transients are fooling the KAW. I haven't found anyone that can show why with a lot of certainty but I believe their results.
 
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Sure you can do it, but by the time you buy enclosures, sockets, voltmeter, resistor(s), it costs the same. If you've already got everything and you want to save a few bucks, go ahead. But if you have to ask these questions, you may not be fully qualified. If you're going to do it, use a small (0.1 or so) ohm shunt - they're designed for this kind of thing. My suggestion for a 1 ohm resistor was for relativley low power applications requiring better precision (under 200 watts or so). If this is a temporary thing, i.e. spicing into an existing cord, I'd go with the neutral line for a tad better safety. If this is a permanent enclosed thing, I'd go with the hot line to prevent lifting the voltage of the neutral above ground (which could cause a small power flow back through the ground).
I've been building electronics for 25+ years, including high voltage tube amps, and have wired several houses and I wouldn't be very comfortable doing this the cheap way. The risk/cost ratio is just too high.
BTW, do you have any details available on your portable speaker system. I'm always curious as to what others are doing.
 
longblock454 said:
But once you have the Kill-a-watt, you can test your power supply, then drag it around to all the stuff in your house and test it also!

I've tested just about everything I own!
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There really just isn't that much I need to test though. This is just something I need once in a blue moon really and I never really need accuracy. It's not even really ever very important. I guess a Kill-a-Watt can theoretically be used to make things more efficient (though often enough I don't see how it's going to tell you anything other than "turn this off to save power," lol) but overall it just doesn't really do that much. It's not as if it can actually make anything more efficient itself after all. No, I just can't justify the cost of such a thing to do nothing more than check a few things. (Besides, as I said, part of the purpose here is just the project itself.)

mtrupi said:
Glad to help. As for killing yourself, just be careful. It can hurt something terrible but if you don't grab on and hold your chances are pretty good.
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Well, also I guess it's AC that causes you to jerk rather than clamp down because when I was a kid who didn't understand how those plugs in the wall are setup, I grabbed one by both sides (and it was live of course) to pull it out. Yeah, I jerked back so quickly I suspect if anyone had been watching I would have blurred, lol. I've also accidentally fully shorted that 120V before for a nice lovely smelling little explosion (which also made me jump back quite a bit, lol.) But, no, I learned from experiences like those and won't be repeating them. Well, I was a kid then and I'm not now. I won't deny that the stuff I did then was dangerous, but now it's not actually dangerous since I know basic precautions. (Oh, and I have a proper electrical system with circuit breakers that prefer to risk being too easy to trip over being too hard.)

Fenris_Ulf said:
Sure you can do it, but by the time you buy enclosures, sockets, voltmeter, resistor(s), it costs the same.
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Not really. Check that thread linked to earlier actually. I do already have everything but the resistor as I said, but the poster of that thread managed to get everything but the multimeter for FAR less than the cost of a Kill-a-Watt, so it's not just if you already have the stuff on hand. As far as the multimeters go even though it doesn't have to be that bad. The cheap one I consider to be "extra" and therefore expendable was $5 on sale from some $7 or so on Parts-Express. The only catch to that site is they have ridiculous shipping, but whenever you need other parts if you toss it in among them it's cheap. (Which is what I did. It essentially cost $0 to ship because I got something I needed and threw it in as a bonus which didn't increase the cost of shipping at all.) It's probably possible to get the same one elsewhere for quite cheap as well. So no, I still say that it's cheaper than a Kill-a-Watt even in a worse case scenario.

I've been building electronics for 25+ years, including high voltage tube amps, and have wired several houses and I wouldn't be very comfortable doing this the cheap way.
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You do realize this isn't exactly some permanent thing? The idea is merely to get a rough idea of the ranges I'm using, nothing more. So that means a 10 minute test tops and then it's back to normal. I have no intention of leaving it on there on any sort of regular basis. Immediately after testing is complete, everything goes off and right back into the normal plugs. I don't mean to build anything permanent like this like a meter in my PC case or something, I just want to test best and worse case scenarios for various reasons.

The risk/cost ratio is just too high.
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I don't know... 0 isn't very high in my book... Each to their own I guess. But, seriously for a moment, there IS no risk if it's done right -- and I think I can do this much right thank you very much. You do realize this is nothing more than a resistor on the cable and a multimeter testing across it? Pretty much the worst case scenario assuming I at least have SOME experience dealing with electronics is a damaged multimeter. That's pretty much about it. I won't cry over the lost $5. No, it's FAR more dangerous for me to get in my car and drive to work...

BTW, do you have any details available on your portable speaker system. I'm always curious as to what others are doing.
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Nothing special really. I took a Tang Band W3-881SJ 3" "full range" driver (which uses a neodymium magnet, so that might be part of why it seems to perform so amazingly well, but I still felt like a proper tweeter handles the highs much better despite its official frequency response listing) a fairly cheap tweeter also by Tang Band, some basic first order filters that aren't even well optimized, this amp (mostly because it's small, light, provides pretty good quality considering, and so portable that it even lets you use AA batteries if you want -- which of course I don't mean to ever use much since they probably wouldn't last long at all, but it's a nice touch and I do gladly run it from the batteries for short periods of time at times and am grateful for the option,) and put it all together in a relatively small LDF enclosure (though I won't deny that it is bigger than most of the portable speakers you can buy.) I started with MDF, but really there just isn't enough energy in frequencies that would be a problem for LDF to make this necessary. I did add some damping material (also on Parts-Express they had some nice vinyl based stuff which supposedly is better than those like Dynamat at a fraction of the thickness and weight) just to be on the safe side, but I'm not sure that it even is that necessary. Of course, I threw in fiberglass stuffing. (Probably a given normally anyway, but ultimately I wasn't satisfied with the results in trying to make it a ported enclosure, so it's absolutely necessary really.) Overall it didn't really cost me an arm and a leg and is still better than any I've ever heard from any retail store. That little 3" driver really never ceases to amaze me and I've used that particular one for other things as well with great results. Between this and working with my main speaker system I've learned a fair bit and I could build a much better one if I really wanted to (but the main problems are the fact I have a very limited budget and my cutting equipment lacks enough accuracy to truly do this right. I will say one thing though, budget aside, I've gotten my money's worth and then quite a lot more out of this thing...) It also produces pretty decent bass considering its limitations. It won't shake the house, but, I'm more interested in the parts you hear than the parts that you feel generally anyway. It definitely produces enough vibrations that I can feel it on the other side of a nearby wall though, lol. (My preference tends towards something a bit closer to neutral than normal anyway, so it's ok that it's a little subdued compared to a true speaker system with subwoofers.)

Overall it won't win any awards, but it's still excellent for portability purposes where I don't need audiophile quality by any means and it's still better than those pitiful things companies are selling to people. Of all I've personally used, the only one that remotely approaches it was Yamaha's NX-A01. This is still overall much better IMO though (not to mention how much more efficient it is...) Plus it still ended up being a bit cheaper overall I think (though it was never really about being the absolute cheapest so much as just being excellent for its cost. The rest was just a project to learn, produce something useful, and etc. I think I use it every day really. It's not only good for on the go, but sits right next to my bed most of the time for listening while reading, occasionally putting on some music when going to sleep with the sleep timer of my DAP, and so on.
 
Nazo said:
Well, also I guess it's AC that causes you to jerk rather than clamp down because when I was a kid who didn't understand how those plugs in the wall are setup, I grabbed one by both sides (and it was live of course) to pull it out.
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I think this true, but I'm not sure it's an absolute certainty. I've had a similar experience to yours. No, not even a project related incident. 120VAC from my hand straight thru bare feet to damp concrete floor. I didn't remember anything other than I was on the floor and my arm hurt a lot.
 
That was my little DIY kill-a-watt way back when. Most of the caveats mentioned by others in this thread are valid--it's only valid for high-power-factor loads, and only good up to a couple hundred watts.

One thing needs to be pointed out--the resistor is in the neutral line, not the hot line. That way, even though both leads on the resistor are exposed, neither is at any significant voltage.

Given the cost of the parts to construct something similar, and the amount of time it would take to build something with any significant amount of accuracy, I have to second the recommendation to just buy a kill-a-watt. If you're worried about buyer protection in case of a bad device, then your confidence in a DIY solution must be even lower.
 
I don't know why people are determined to ignore the fact that I said that I don't need accuracy. I really don't. I swear I'm starting to wonder if you people are paid off by the company that makes the Kill-a-Watt. I've said it's not what I'm looking for, I'm aware of risks and penalties to doing it this way, so can we just drop it and leave it at that? I just need a rough idea of what sort of general area it's in. Exact numbers really don't matter. I just wanted something better than running the power through the amp meter part of the multimeter since this necessitates 1+ amps actually running through my meter at that high voltage for something like a computer. As far as costs go, as I've said already some three or four times, I have a power panel, I have a cheap multimeter that I won't cry over if it dies (btw, I also have a more expensive one,) and I have an extra mole-x connector I can spare (turns out these are exactly the right size to tightly fit the test leads. Go figure.) All I need is a resistor. If you can honestly tell me that a resistor would be more expensive than a real Kill-a-Watt we'll talk, but in the meantime, I'm seeing even the more expensive "voltage sense resistors" on places like Mouser as being less than $2 + S/H and I can get something else I needed along the way. That means $9 if I get nothing else to offset the cost of shipping. This is the absolute worst case scenario. I haven't had the chance to really look to see what I can find among my old electronics that I keep around for just this sort of thing. Find for me me a Kill-a-Watt that costs between $0 and $9 (shipped) that is guaranteed to work at least as well as this method will work and we'll talk. (Eg a used model that's sold as-is won't do. I KNOW if I buy a resistor this will work for my intended purposes, however much it may lack in accuracy. I do not know that an as-is Kill-a-Watt will. It's no good to me if it has a busted display or something for instance.)

EDIT: Actually, just out of curiosity, are we really absolutely sure the Kill-a-Watt is so accurate? Specifically, I mean in the cheaper versions that are being recommended here. It does no good if you need a $50+ device to get this accuracy level that people think I need while they are suggesting the $30 version or whatever...
 
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Nazo, Go for it. Once all cautionary stuff is out of the way I always say that. You don't need a reason any better than you just want to.
 
Lol, at least someone understands. For me it's as much just to have a "project" (however simplistic it may be) as the fact that it will have some usefulness for me.
 
Nazo said:
Lol, at least someone understands. For me it's as much just to have a "project" (however simplistic it may be) as the fact that it will have some usefulness for me.
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Fair enough. As long as you understand the risks and limitations, go ahead and do it.
 
You apparently are unaware of the true concept of risk.

The deepwater horizon rig would have had no risk if "it was done right".

Nazo said:
.....But, seriously for a moment, there IS no risk if it's done right -- and I think I can do this much right thank you very much.
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The danger in live mains voltage is the path that it takes. That's the only difference between a numb finger/hand and a stopped heart.

IMO, the benefit of the Kill-A-Watt is the fact that it's cheap, it's safe (UL rated), and it does a reasonable job particularly for the price.

It seems like others have already covered the dangers of what you're going to do so I won't continue on that, but I do want to mention that you should make sure your resistor is rated for your intended use.
 
Yeah, you can stop flogging the dead horse now. I get that electricity can do bad things. I also get that if you use appropriate practices it doesn't do bad things to you barring freak uncontrollable incidences (like say a random lightning strike from nowhere with no obvious storm or something.) I really don't understand why this is still even being discussed... By your logic, electricians everywhere are all as good as dead even though they too use appropriate practices (such as by not working with live circuits...)

I think we can let it go now.
 
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