Watt monitoring

Hello everyone!  I have been lurking around these forums for several months actually, and felt it was a good moment to start participating. The great work you all put into this site and product is inspiring!

I am looking at developing a watt monitoring device for my energy management solution and I came across an interesting circuit that I though you all may like if you haven't seen it before. It's by a guy called rick may and the link is here:


He based his circuit on a Stephen  Woodward and his original post is here: http://www.edn.com/design/analog/4327422/Optoisolators-compute-watts-and-volt-amperes

Basically he measures watts using a small resistance and four opto isolators. In his second circuit he has two outputs, one for va and one for watts, both as analog outputs which he calibrated for 1v = 100w and 100 va, making pf calculation very easy. It is so extremely simple that I wonder why it isn't more widely used, is it because of accuracy problems?

Your comments and opinion would be great.


MartinR's picture

Re: Watt monitoring

Interesting circuit but things have moved on a bit since 1998. You can buy a meter like that for less than £10 now - like this example.

Robert Wall's picture

Re: Watt monitoring

And not practical for anything other than a single appliance connected via a plug and socket.

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