Energy Meter Design

[hisham]

Hello,

I was checking the attached TI energy meter reference design, and from the schematic the following are observed:

1- AC neutral point is connected to AGND

2- AGND is connected to DGND using the net Tie (NT2) as shown in the schematic.

3- 0V of the AC/DC Converter is connected to DGND using jumper J5 as shown in the schematic.

4- The above means that AC neutral point is also connected to the 0V of the AC/DC converter & to DGND.

The question is if i connect the neutral directly to the 0V of AC/DC converter is it going to have damage on the PCB? Please explain how this work?

Regards,
Hisham

[JS]

I can't see the pdf but if there are no low impedance closed loops it won't blow anything as all ends referenced to neutral. Neutral is not the same as earth so not safe to interface to humans or other electronics as it can drift from earth potential with very low imepance and induce very high currents, then it would be a problem.

As said not safe to interface directly to almost anything so any interface should be isolated, like wireless, optocoupled or something like that.

JS

[hisham]

I attached extract from the schematic again in this response (you may need to check in your downloads folder)

The point in energy meter you need reference point to measure the voltage, so the reference here will be the AGND, which in turns is connected to the 0V of the AC/DC converter so I am not sure if this will cause a damage for the components or not.

Regards,
Hisham

[JS]

I don't know what U4 is, if it's just a diode bridge you have a problem, but I can't see how that will result in 3.3V either.

JS

JS

[coppice]

U4 is an isolated switch mode power supply module. Why would there be a problem connecting neutral to its 0V output?

[hisham]

Hello Coppice,

Maybe there will be high potential difference between the neutral point & the 0V output of the AC/DC converter considering the voltage at the neutral point is 220VAC with respect to the line voltage.

The main thing is when the line voltage gets into the MCU, the line voltage should be in reference to the ground point, so there should be connection between the neutral and the ground.

[coppice]

Hello Coppice,

Maybe there will be high potential difference between the neutral point & the 0V output of the AC/DC converter considering the voltage at the neutral point is 220VAC with respect to the line voltage.

The difference between the neutral voltage and the 0V voltage will be small, being only the drop through the wiring - only millivolts, even with a high current load. Even if the live and neutral connections were reversed, the isolated supply would mean this is not an issue. If you use J5 to disconnect the EVM's build in modular power supply, and use an external power source, you need to take great care about isolation.

The main thing is when the line voltage gets into the MCU, the line voltage should be in reference to the ground point, so there should be connection between the neutral and the ground.

The incoming neutral connects directly to AGND, which in turn connects to DGND. Since the ADCs are fully differential there is no particular reason why the line voltage needs to reference to the ground point. The difference between them just needs to be kept within the range which the ADC can handle. In this EVM design the line voltage will, in fact, be measured with reference to ground, since the incoming neutral is tied directly to the MCU's grounds.