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Topic: Measuring Mains Voltage (Read 3 times) previous topic - next topic


So, as I mentioned before, I'll bite the bullet and get a transformer, darn it.

I don't know how far you've gotten since March but I thought I'd share my thoughts on a similar project I just started if anyone is interested.

I started my power measuring project with a Kill-A-Watt, measuring the total power consumed through one of my breakers. It soon became a pain to go look at the display and punch the data into my tablet by hand. Since then I have purchased the wireless Kill-A-Watt and two of the sensors.

Through a mix-up from Amazon I ended up with a spare readout for free so I immediately took it apart to see if there were  any ideas lurking inside. In typical P3 International (the manufacturer) style it was a pretty compact design with few clues until I get my logic analyzer out of storage. It operates on the 915 MHz ISM band and the radio chip is one of those "bonded to the PCB with a glob of epoxy over it" types. Anyway, the reason I'm posting is because of the sensor.

I don't know if anyone has looked inside the normal Kill-A-Watt but if you have you know why they put it in a plastic box. There is no isolation from the mains AC at all so you don't want to go probing around inside with it plugged in unless you are really careful. (AWOL insert cautionary gasp here. :-P) Well, the wireless one isn't any different but it's a pretty nice design and for $30 might be worth cannibalizing for a project. P3 measure the current by measuring the voltage drop across a pretty hefty piece of wire, probably just a few millivolts, and they get the voltage by a divider across LN. Since it's all inside a sealed plastic box, it's safe for consumers. I've considered cutting out the parts of the circuit I don't need and using it because it is so efficient at plugging into the wall, sampling the current/voltage and then passing the AC out to a matching socket on the face to plug your load into.

Anyone interested in discussing this? Maybe start a new thread?


  For current measurement a shunt resistor can be gotten cheaply. Here is a link from a quick search. Only $2.88


Just my quick 2 cents. Someone said you cannot measure without a neutral. Not true. All up-to-code breaker boxes now have the ground and neutral bonded. So any hot to ground will give you the same results as hot to neutral. Also, "split phase 220". All house circuits have this. If you look at your panel (breaker box) there are two buses. A 110 (closer to 120-125 now days) uses one buss and a neutral. 220 (again, closer to 240-250 now) use both buses and a ground. Some devices, such as my drier, use both hots, a neutral and a ground, to have 110 available to drive internal electronics. With the neutral and ground being bonded tho, I guess the ground is just a fail-safe if the neutral fails..


Jul 25, 2011, 09:34 pm Last Edit: Jul 25, 2011, 09:53 pm by cyclegadget Reason: 1
  Per Grumpy Mike's note: Here is some info related to U.S.A. electrical terminology and wiring.

 The Neutral wire is designed to carry current any or all the time. It is call a "grounded conductor". The ground wire is for a "fail-safe", it is supposed to take any short circuits to ground to protect the person using the item. The ground wire is called a "grounding conductor". The two terms are found in a NEC code book for anyone interested.

One important note: never assume a neutral wire or neutral bus is grounded or "safe". There are a number of reasons that can make the neutral wire "hot" and therefore just as dangerous as the hot bus in your power panel.



Please don't assume that the whole world is the U.S.A. While the physics might be the same (much to the shagrin of the U.S.) the words are not.


Please don't assume that the whole world is the U.S.A. While the physics might be the same (much to the shagrin of the U.S.) the words are not.

Sorry, I wasn't thinking of the whole world picture that is the internet. I made an adjustment to the post.



cyclegadget, thanks for the link.

EmilyJane, I haven't made much progress on this yet.  I decided that I would get power from a disassembled wall wart; at U$3.00 or so I couldn't beat the price or ease of use.  I have a small CT I'm experimenting with that is actually working pretty well.  I suddenly realized that putting it around a wire in a box was not a big deal.  There were a number of problems making a hall effect current sensor work and those CTs are easily available and simple to work with.  I also realized that I didn't need much power from a transformer that was only going to be feeding a voltage divider for measuring the AC voltage.  So, a few milliamps would be fine, of course, that decision was AFTER I decided to use a repurposed wall wart for power.

Given the above, it's become a matter of having the time to slap it together, but it's summer and the yard is eating up my free time.

But, now you've got me intrigued with the Kill-a-watt all over again.  I didn't pay much attention when the wireless ones appeared since I had one of the wired ones and I was measuring power at the main for the entire house and didn't see any need.  However, I did (of course) take a kill-a-watt apart and was impressed by the fact that they simply measure the voltage across a wire to get the current level.  If you check really closely, they also use a voltage divider to sample the AC voltage.  With these two items the rest is just accumulation and calculation.

The problem is the $38 price tag for something that I will cut up and adapt.  I think a simple short extension cord with a box in the middle holding circuitry would do the job just fine and I could use my latest love, the ardweeney, to do the processing and tie the new device into my existing XBee network.

You're right, the form factor of the wireless Kill-a-watt sensor is really nice.  I could never match that cool little package.
Trying to keep my house under control http://www.desert-home.com/


Jul 25, 2011, 10:07 pm Last Edit: Jul 26, 2011, 01:31 am by EmilyJane Reason: 1

You're right, the form factor of the wireless Kill-a-watt sensor is really nice.  I could never match that cool little package.

Okay, I just wanted to point that out. I agree, the price is a little daunting. There are plastic enclosures available that have the plug incorporated anyway (ala DIY wall wart) that are pretty inexpensive that I'll probably end up using.

I'm going to start another thread because I want to look into hacking the protocol that the wireless sensor uses. They are priced pretty reasonably for my project budget and if I can use them, I want to.

Edit: The enclosure I settled on is from this series: http://www.polycase.com/ps-series

Some have a molded in place for a receptacle.

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