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Reading a High Voltage

Discussion in 'Electronic Basics' started by Jas, Jul 28, 2004.

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  1. Jas

    Jas Guest


    I need to input a high voltage (> 500V) into an ADC of a microcontroller.
    The microcontroller runs at 5V. To do this I was thinking of using
    resistors to divide the voltage down (potential dividers). Is this a good
    way of doing it? How do I calculate the power requirements of the resistors
    if they are being used in this way?

  2. Assuming the negative side of the 500 volts is common with the
    negative side of the PIC supply, a potential divider should work. But
    not only do you have to deal with the power wasted by these resistors
    (V^2/R) you also have to worry about the voltage rating of the
    resistor and leakage paths on the board. The PIC is pretty accurate
    with any input resistance less than about 10k, so I would start with a
    10k to ground and 5.1 meg to the 500 volts. Than keeps the heat down
    to 500^2/ 5Meg=0.05 watts. Keep in mind that unless you also have a
    precision reference for your pic to compare to this divided voltage,
    the measurement will be no more accurate than the PIC supply voltage.
  3. CFoley1064

    CFoley1064 Guest

    Subject: Re: Reading a High Voltage
    Hi, Jas. Mr. Popelish has this exactly right. I'd like to add one small piece
    of advice, if I could. If it's possible your 0-500V input voltage is going to
    change quickly, you also have to take into account the capacitive divider, too.
    All resistors have an inherent capacitance in parallel with their resistance.
    It isn't much (low pF level), but in the event of rapidly changing input
    voltage, it could result in a nanosecond excursion at the input pin of a couple
    hundred volts.

    If I had to use a VDVR to measure high voltage with a PIC, I'd use a 470 pF cap
    to GND in parallel with the 10K resistor, which should swamp out the capacitive
    VDVR. It will also slow down your response time, but will give you some
    improvement in input noise.

    High Voltage VDVR
    Voltage Input PIC Input
    R1 | |
    .-. |
    | | ---
    R2| | ---
    '-' | C1
    | |
    === ===

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Good luck
  4. And possibly replace R1 with several smaller resistors in series, to
    prevent high voltage from leaking into the PIC. I would additionally
    place protection diodes from the PIC input to ground and positive
    supply, just in case.
    o +Ub
    /\ \ ZD5V6
    / \
    ___ ____ ____ ____ |
    o---|___|--|____|--|____|--|____|--o----o-----o--o PIC input
    | | |
    4 x 1.275 M .-. | _|__
    | | --- /\
    R2| | --- / \ DUS
    '-' | C1 ----
    | | |
    === === ===

    This should limit the input voltage of the PIC to -0.7 to +5.6 V,
    probably survivable.
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