Odd ("triangle wave") Amperage waveform from Kill A Watt

[iambujo]

Hello - I have to say that I am very impressed at the level of effort that has been put into the project. Great job sharing all this info! I have been tinkering with the idea of making my Kill A Watt into a wireless data logger for a while, and just finally opened it up.

So, in prep to build one based on the information on the tweet-a-watt page, I put an oscilloscope on pins 1 and 4 of the opamp to get a better feel for the project. The Voltage waveform is a perfect sine wave. However, the Amperage, even with a simple 50 Watt incandescent light bulb looks more like a triangle wave then a sine wave. As I add load (floor lamp with multiple bulbs), I do see the peaks go higher. But do you have any idea why the wave form is not a sine wave, but instead a triangle wave? My Kill A Watt appears to be functioning properly on the display - a 50 Watt bulb reads 57 Watts on the Kill A Watt LCD.

If this isn't a problem, I'll continue, but it just seems a bit odd. I did see the plots describing the differences in wave forms for light bulbs, dimmed light bulbs, switching power supplies, etc. I seem to never see sine waves for the current output though.

Thanks!

[adafruit]

i got a nice sine wave but remember:
GROUND in the circuit is not 'GROUND'!

so use a floating probe, math mode and/or do something stupid with your scope

[iambujo]

Ok thanks. Next question if you have a moment. Do you recall the amplitude of the opamp output pins for Voltage and Current sensing? Voltage looks good I think, but Amperage appears to be about 1/10th the amplitude (p to p) of the Voltage signal. Granted I only had 1 or 2 - 50 watt bulbs on at the time, but the signal seems so small that I don't seem to see any swing up or down when sampling on an ADC as I turn on and off bulbs.

Sorry if my questions are way off track of the purpose of this forum. I just decided that this could be a fun project to do on my own with a PIC MCU since I have all the materials already, and I am leveraging the info on your page. This is my first PIC ADC project. The firmware and breadboard design are working well for ADC of a DC 0 to 5V supply, but the small signal coming from the Current Sense pin of the opamp just seems far too "small".

Thanks for any insight you might have

[adafruit]

the killawatt can measure up to 15A
the xbee's ADC is 10 bit with a 3.3v pp signal
so, figure out the math!

[iambujo]

Ok thanks.