EmonTX v3 Voltage Readings


I've just got my emonTX v3 up and running, and notice that the voltage it is reading is 247V. I thought UK power should be 230-240V?

Does this suggest something is wrong? My setup is very standard, with an AC-AC adapter powering the unit (plus voltage sampling), CT transformer on single phase, and a single DS18B20 temperature sensor.


Robert Wall's picture

Re: EmonTX v3 Voltage Readings

The maximum UK voltage permitted is 253 V (230 V + 10%), even though the centre voltage is still likely to be 240 V. The lower limit is 216.2 V (230 V - 6%). All the components in the voltage measurement chain (the adapter, and in your emonTx V3 the divider resistors, the ADC and the ADC reference voltage - the 3.3 V supply) have a tolerance band and it could be that in your case they have added together all in one direction. Do you have an accurate measure of your voltage? If you have measured it, what is the tolerance band of your meter?

You could read the Building Blocks article on the accuracy of the inputs. Although that was written for the emonTx V2, most of it remains accurate. The biggest change (from memory, the only significant change) is the 3.3 V supply (1% tolerance) is now used instead of the internal reference for the ADC.

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