emonTx not reading 0

Any ideas why, when there is no power flowing through a test rig, the emonTX is reporting a reading?  It is running on 2xAA batteries and shows a reading that fluctuates between 7W and 18W. The only other thing plugged into the test rig is a plug in monitor to compare the readings. This shows 0.

Any ideas?





glyn.hudson's picture

Re: emonTx not reading 0

The emonTx will always show some (small) reading when no power is flowing. This is due to noise. The emonTx is not very sensitive at low powers since we aired on the side of caution and choose a burden resistor which would not let the ADC input go above the emonTx input voltage even at extreames (2.7V input and 100A's flowing). Better sensitivity at low power values could be gained by sizing a larger burden resistor. See this page for the calculation: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface

Alternatively a line of code could be added to force the output to zero when reading drops below 20W.

More accurate power readings that take into accout the RMS AC voltage and Power Factor to monitor Real Power can be had by using an AC-AC voltage adapter plugged into the emonTx to give a voltage sample. 

SomeRandomBloke's picture

Re: emonTx not reading 0

 Thanks, was going to dig out an old AC adaptor, think I had one form an old answering machine although it might not have been 9V though. Although this isnt possible when hooked up in the meter cabinet. Need to check if its possible to clamp it on in the fuse box as its a fairly recent one plus there are a couple of sockets conveniently located close by.


One time it was reading 0 when no current was flowing, other times its showing something. As the house idle usage is above 100W this shouldnt make a lot of difference.





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