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MSP430AFE253: Voltage divider for energy measurement PCB Clearance !

CHAMKHI HAITHEM
Prodigy 110 points
Part Number: MSP430AFE253
Other Parts Discussed in Thread: TIDM-TWOPHASEMETER-I2040, EVM430-F6779

Hi,

So i'm designing my 2 phases energy meter based on this application note : 

my question is related to the PCB design and the the clearance with the 220v:

according to the Figure 3 page they used a voltage divider with R38, R41, R44 and R53 to drop down the voltage under 500v.

1/ i'm wondering why using 330k resistors (R38, R41, R44) instead of one 990k resistor ?

2/ above is a schema of the voltage divider where i add the  RMS voltage value in each resistor, if we suppose that the input voltage is 230 V RMS.


when i checked the PCB design of the energy meter in the document i noticed that the clearance between the 230v and the neutral wasn't respected.
below a screenshot of the actual voltage divider inside the PCB :




 As you can see that the clearance between the 230V and the neutral is too small, normally it should be 1.25 mm ?

thanks,

  • 0
    TI__Mastermind 49692 points

    Hello Chamkhi,

    Thank you for your post. Let me address your first question.

    For your first question, there are two reasons why three resistors were used as the upper resistance of the resistor divider instead of one.

    • First, they give flexibility to implement a very specific resistance that may not be offered by a single resistor.
    • Second, they drop the voltage by fairly equal amounts. Let's assume that one resistor gets shorted when the board components are populated. Applying 77VRMS to the board is bad but not nearly as bad as applying 230VRMS, which could happen if the single resistor shorted. Based on your labels in the image above, I suspect you already understood this, but I wanted to make sure.

    For your design, I would recommend using multiple resistors instead of a single resistor.

    Regards,

    James

    MSP Customer Applications

  • 0
    TI__Mastermind 49692 points

    CHAMKHI HAITHEM said:
     As you can see that the clearance between the 230V and the neutral is too small, normally it should be 1.25 mm ?

    In general, you are correct. Here, the clearance between 230V and Neutral is very small and may not be adequate for most applications. We use these EVMs in a very controlled environment and with test equipment that include many safety features, so this isn't an issue. Please keep in mind that these are reference designs but are not intended to be used as a final product. However, on some of our newer EVM designs, we've increased some of these clearances (like the EVM430-F6779 and TIDM-TWOPHASEMETER-I2040).

    Ultimately, I would recommend that you follow any clearance specifications (e.g. UL, IPC, etc.) that are required for your design.

    Hopefully this helps!

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Prodigy 110 points

    thanks james for your answers,

    1/ about the first question that what i though at first, but i want to make sure.

    so the more resistor i use the less risk i got when one of them got shorted.

    2/ concerning the second question, i went to the new EVM TIDM-TWOPHASEMETER-I2040 and i found that the clearance was good enough.

    actually there is no copper pour next to the voltage divider, and that what i was planning to do.

    above my design :



    thanks,

  • 0 in reply to CHAMKHI HAITHEM
    TI__Mastermind 49692 points

    Any time. I sounds like you're on the right track with your design. If you have any other questions, don't hesitate to let me know.

    Regards,

    James

    MSP Customer Applications

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