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INA138: 250VDC Back Up Battery System Monitoring

Kevin Gregg
Prodigy 10 points
Part Number: INA138
Other Parts Discussed in Thread: TMCS1123

Tool/software:

In my application, I am measuring current of the high-side of a back up battery system (250VDC) off a current shunt. I am using a din rail mounted signal conditioner from phoenix contact to isolate from my computer side. My question is a pretty basic one: Do I need to select an isolator with an isolation rating (Vrms) that only needs to handle 1) the small mV differential voltage or 2) the higher common mode voltage of the high-side of the battery system to both input terminals of the isolator?

 

Very unseasoned engineer here so feel free to explain like I am a fifth grader Blush

  • 0
    TI__Mastermind 32395 points

    Hello Appreciated Engineer,

    I will respond soon.

    Sincerely,

    Peter

  • 0
    TI__Mastermind 32395 points

    Hello,

    Sorry for the delay and welcome to the forum.

    I admit that I am not completely following if the isolator and signal condition are different in the question.

    Signal conditioners can provide isolation inherently. So that would be my first question: what is the isolation rating of the signal conditioner?

    However, if the conditioner is outputting a bipolar voltage signal (e.g., -10V to +10V), then maybe you would want to isolate or level-shift this voltage before your computer. That's my second question: what are the output signals of conditioner? And how do you plan to digitize this for your computer gather? 

    The INA138 is a unipolar current sense amplifier, so any conditioned signal of this should be unipolar as well (e.g., 0V to 10V).

    My 4th grade signal conditioner expertise supposes that using an ADC with voltage rating greater than the voltage output range of signal conditioner would be enough. Then you could use any standard host processor to read the ADC and feedback to computer terminal. Or maybe there are relatively cheap isolators than can level shift 10V signals to more common 3.3V full-scale ADCs. You would compare this isolator to just using an attenuating op amp circuit or difference amplifier powered off a +/-10V rail provided from signal conditioner

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    Prodigy 10 points

    "So that would be my first question: what is the isolation rating of the signal conditioner?" Good question... I should have clarified that. The isolator and signal conditioner are one in the same. It conditions and isolates. The signal conditioner is the Phoenix Contact 2811284. Data sheet specifies the isolation rating of 300Vrms. It provides an additional isolation rating of 250 V AC/DC. Not sure what the difference is. This is my main concern/question: The signal conditioner is just reading the small mV signal from the shunt across its input terminals, correct? Or will the common mode voltage of 250V from the battery stress the isolator as well? Basically just wondering if the 250V will be a factor. If so, I may need to reconsider the signal conditioner I have chosen since the 250V would be close to exceeding the signal conditioner's isolation rating.

    "That's my second question: what are the output signals of conditioner?" The output signal is configurable by DIP switch to a bunch of different standard current and voltage signals.

  • 0 in reply to Kevin Gregg
    TI__Mastermind 32395 points

    Hey Kevin,

    I do not know what the signal conditioner is reading because I have no schematic information, but if input is reading a floating differential voltage that has a common-mode of 250V, then condition should have some isolation. It does not matter that the differential voltage is small, it only matter what the input's voltage is with respect to signal conditioner ground.

    The 300Vrms does seem little close to the battery voltage, although I am not familiar with these insulation standards.

    I am not sure how the INA138 is fitting in here, but if you are using the signal conditioner just for battery current, maybe you could use an isolated current sensor such as the TMCS1123 or AMC1201.

    Sincerely,

    Peter