Simplest and safest way to measure watts on mains
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Simplest and safest way to measure watts on mains

by mojo on Tue Mar 10, 2009 8:55 am

The Tweet-a-Watt has inspired me to come back to an old project I did some research on a few years ago - power monitoring with an AVR.

The main problem is how to safely measure power on the mains in a safe way. The Tweet-a-Watt idea is genius in that respect.

I was thinking of using a Hall Effect sensor. You get easy connection and isolation from mains voltages, and can use the ADC after a bit of scaling with an op-amp to measure current. The problem is that you also need to measure voltage to calculate power. There are some people doing that with sites on the web, but I had a different idea.

Maybe I'm just spouting rubbish but since the mains voltage is basically a sine wave with fixed frequency (50/60Hz), with a bit of maths you should be able to estimate power pretty accurately without actually bothering to take voltage measurements. At most all you need to know is the zero crossing point, and if you can't infer that from the current measurements then at least it's easy to measure and isolate with a simple pulse output from a transistor fed into an opto-isolator.

I'd love some feedback on this idea before I go and blow myself up :)
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Re: Simplest and safest way to measure watts on mains

by eil on Wed Mar 11, 2009 1:11 am

Tweet-a-watt does monitor power on the mains. Unless you're talking about the feed line that comes into a house? If so, I'd be keen to hear about suggestions for this as well. I'd LOVE a tweet-a-watt-like device that can measure all of the current being consumed by everything in the house.

I don't see any reason to monitor voltage/frequency separate from current since those are pretty standard and stable (unless you have a really crappy power company or street transformer). If you wanted the extra precision, though, you can just plug a tweet-a-watt into any outlet since the voltage and frequency will be identical everywhere in the house.
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Re: Simplest and safest way to measure watts on mains

by mojo on Wed Mar 11, 2009 8:23 am

eil wrote:Tweet-a-watt does monitor power on the mains. Unless you're talking about the feed line that comes into a house? If so, I'd be keen to hear about suggestions for this as well. I'd LOVE a tweet-a-watt-like device that can measure all of the current being consumed by everything in the house.


Ah, sorry, didn't make myself clear, I would be monitoring normal mains banned too. It's really just an interesting idea/project. I have already built a USB controllable mains strip with six individually switched outlets, and thought it might be interesting to add energy consumption monitoring too.

As for whole house stuff, I don't really know much about it to be honest, but I did see a web site where someone simply used a clamp meter on one line. That's what made me think of doing it this way - cheap clamp meters measure AC by simply giving an RMS readout based on a 50/60Hz sine wave.
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Re: Simplest and safest way to measure watts on mains

by franklin97355 on Wed Mar 11, 2009 12:28 pm

There are devices that clamp around wires and contain a hall effect that will measure voltage (and current with additional parts) there was an article in either Nuts and Volts or Circuit Cellar about a whole house, per circuit breaker monitoring system. Google hall effect and power monitoring for some hits.
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