Homebrew Grid Tie Inverter Metering

Finally, it's always good to have some idea of the actual performance of a device. So we must do some measuring. I had started to build a filter with the idea of using this in a feedback circuit that turned out to be unnecessary. For the meter it's total overkill but I left it in. The filter consists of two halves of an LM358. This device can sense at ground and output close to ground make it an ideal (and cheap) candidate for this task. The first opamp integrates the source current and provides gain. The second opamp is a Sallen-Key filter designed with an online tool (highly recommended!). Its output goes to a 1 mA meter calibrated with a adjustable resistor and protected with a diode. The meter was found in my junk box and was fitted with a custom scale (hi-res PNG). Add in two 0603-size white LEDs and I get a nice view from my bike. The needle swings back and forth as my legs push down on the pedals, typically between 50 and 60 watts. Pedaling for 15-30 minutes is a nice exercise while watching Trevor Noah. On my own power!

The remaining images shows how the GTI performs under load. The first six photos are from my scope triggered with LINE and show both transistor's drains. As the power increases from 0 to 40 to 80 W the drain current causes a step in the drain voltage as the voltage generated by the three-phase motor is switched across the transformer. Scale is 5 V/div. The two windings perform differently because of the modification I made to it with different gauge wire. I think. The next three scope screenshots are from the source current at those three power levels. Scale here is 50 mV/div. The next image is the integrated source current at 0.2 V/div. The meter is DC without any visible ripple. The final image is from my trusty Tektronix portable scope and shows the source current while cycling.