Relaying data from the "Kill A Watt" to a PC

[Brightan]

...although I do not like their waveforms much -- they look too distorted, especially the voltage one (http://www.ladyada.net/make/tweetawatt/software.html). The dimmer voltage is unreal -- there is no way the 120V mains sine wave can be distorted so much ! Something is wrong.

they are only sampling the A/D once every ms, that is why it is as distorted as it is

 

[vc1234]

...although I do not like their waveforms much -- they look too distorted, especially the voltage one (http://www.ladyada.net/make/tweetawatt/software.html). The dimmer voltage is unreal -- there is no way the 120V mains sine wave can be distorted so much ! Something is wrong.

they are only sampling the A/D once every ms, that is why it is as distorted as it is

Well, even so, the mains voltage cannot be so distorted by the dimmer load -- the wire resistance between the breaker box and the load is not sufficient to produce such effect, it should have been a more or less pure sine wave. Also, while looking at the XBee ADC docs I could not find any timing specification such as acquisition time/conversion time etc. Without it, it's hard to say what phase imbalance between two channels might be. The app note says that the maximum sample rate is 1KHz divided by the number of channels which implies that with two channels the minimum sample time is 2ms, not one, but the graph seems to show that it actually one millisec even with two channels.

I've attached a 17 point 1ms ADC output from my mains.

 

[drozwood90]

VC1234,

Just wanted to let you know, that WAS the link I was searching for...good find!

--Dan

 

[Brightan]

Well, even so, the mains voltage cannot be so distorted by the dimmer load -- the wire resistance between the breaker box and the load is not sufficient to produce such effect, it should have been a more or less pure sine wave.

what they are doing is putting the meter after a dimmable switch so what they are showing is the fact that the dimming switch is chopping up the AC sin wave. The fact that it looks so bad is still the fact that there is only 17 samples and it looks even worse because when the dimmer turns on, it turns on very quickly (as opposed to the "smooth" sin wave). If you compare their 2nd picture of the 40W lightbulb without any dimming, it looks about the same as your picture at 17 samples.

 

[vc1234]

what they are doing is putting the meter after a dimmable switch so what they are showing is the fact that the dimming switch is chopping up the AC sin wave.

I see. I don't understand why, though, because this kind of measurement does not take into account possible losses in the dimmer, but it explains the waveform ugliness.