Mon 14 May 2007
I have made several recent posts regarding power measurement, so here is my first average power measurement prototype. The idea is that I am measuring the current returned from the appliance by measuring the voltage drop across a 0.1Ohm resistor in series with the common mains terminal using an INA117 diff amp. I also measure the voltage delivered by using another INA117 and a resistor bridge to drop the 120Vac down to about 4Vac. I then multiply the two signal using an AD633 four quadrant multiplier and get the instantaneous power delivered. This signal’s offset represents the average power delivered to the device. Since the AD633 also divides the output by 10, I put in an optional gains stage that either leaves it as is for high-power measurements or adds a 20x gain to the signal. Finally, I use a LTC1062 switched capacitor, fifth order, low-pass filter to get rid of the 60Hz signal and return only the DC value, or the value of the power delivered. At this point, a multi-meter can be used to check the power, however, I intend to use an analog voltage display. Right now, I am testing it with RC bridge, however, I plan to put this in an enclosure later (part 2) and provide full schematics. Comments are welcome, as usual.
( ltc1062.pdf ) ( ina117.pdf ) ( ad711.pdf ) ( ad633.pdf )
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I’ve been reading your blog for a few weeks now, and I’m very impressed with the projects you undertake and the intellectual quality of the posts you write.
I hope you do post some schematics of this project because I’ve also been thinking of doing this exact same thing. I just have a few other projects I need to finish up first.
I recently noticed in the latest Circuit Cellar magazine that Microchip has introduced new ICs that deal with power measurement. I forget the part number, but it looked like it might possibly be able to replace all the ICs you’re using with one simple chip.
Thanks for the kind remarks Scott. I hope to finalize this design later this week and then post a better writeup regarding the actual power measurement computation and the schematics. As for ICs from Microchip, it may very well be that there are complete solutions to measuring power delivered to a load from the mains. I went with the chips above only because they were readily available at work.
[...] post describes the final design of my power meter as promised in a previous post. The basic ideas are the same: measure the current and voltage then multiply them and extract the [...]
What made you decide to use a shunt instead of something like this? http://www.coilcraft.com/pdfs/senhi.pdf Or hand wound, like this? http://www.edcheung.com/automa/power.htm
I picked up a few of the commercial coils surplus and they work great. You can choose your own burden value depending on what range you need your output to be. I’m working on put them on each circuit in the breaker box with a PIC887 or similar interpreting them, then RS232 to *nix box for MRTG graphs.
Nice blog!
Hi okane, I decided on the in-line measurement because that is what I had lying around. Additionally, I figured I could get a better dynamic range with simpler amplifier electronics. The idea of continuous monitoring and tracking seems kind of nice though, maybe I can try something like that out. Thanks for the compliment!
[...] AD633 four quadrant multiplier chip from Analog Devices. I have used this device in various places (including for the power measurement circuit) and have been quite impressed with its linearity. This could be, in part, due to extensive [...]
[...] of uBlog has described a project he built to measure power, with current and voltage. Part 1 is here and part 2 with schematics is here. I recommend adding his blog to your feed reader, he posts some [...]
Hi there, sorry to sound stupid but I was wondering in how you obtained the multiplier output above 0V with the voltage and current sine wave having an almost 90 degress phase shift.
Hi lmc, the only way to get a 0V reading from the multiplier is when either sinusoids equals zero while passing through a 0 or pi phase. So the output should be non-zero for all other times. Have a look at the link below, it might clear things up.
http://microblog.routed.net/wp-content/uploads/2007/05/power-meter.pdf
From the link above I have now seen how the multiplier output was obtained. Thank you for your kind reply.