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  1. #1
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    Frown Power supplies DC

    I am having a brain freeze. If I hook up two seperate dc poer supplies one 24vdc and the other 12vdc in series won't I get 24vdc and the current capacity of both power supplies? Having aheck of a time with a 2 wire 4-20ma circuit.

  2. #2
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    Quote Originally Posted by bedlite2 View Post
    I am having a brain freeze. If I hook up two seperate dc poer supplies one 24vdc and the other 12vdc in series won't I get 24vdc and the current capacity of both power supplies? Having aheck of a time with a 2 wire 4-20ma circuit.
    They will add in series. Picture them as batteries, stringing them in series adds to the voltage. Meaning that in theory you'll have 36VDC when you go through both of them.

    However, the current will be limited by the weakest of the two power supplies in your example. I'm not sure you're going to be happy with this result.

    What is it you're actually trying to do?

  3. #3
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    I have a 2 wire 4-20ma sensor and trying to hook it up to two different displays. Each has power out and an input.

  4. #4
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    Quote Originally Posted by bedlite2 View Post
    I have a 2 wire 4-20ma sensor and trying to hook it up to two different displays. Each has power out and an input.
    What about using a 500 ohm resistor and converting the signal to 2-10V?

    I think it would be easier, providing your displays can utilize 2-10V...

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  5. #5
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    GT-

    The impedance of the inputs comes into play as the 500 ohm resistor will be in parallel with them.

  6. #6
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    Quote Originally Posted by BACnet View Post
    GT-

    The impedance of the inputs comes into play as the 500 ohm resistor will be in parallel with them.
    Could it not be converted at the sensor?

    Utilizing a single 4-20 ma sensor for multiple inputs is unreliable IMHO. Depending on the controllers in question, I would rather he use the sensor once, then make it globally available to the other controller....

    I don't know enough about the system to go further than that though..

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  7. #7
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    I assume his controllers are networked, why not have them share the sensor's data via the network? That's what we always do.

  8. #8
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    Controllers are not networked, but after further reading of the display panels book I found that I could unplug their dc source and then just use the inputs as a straight plus and minus. Source from one point and series the wires to everything I need. Worked just fine. Thanks all the the assistance.

  9. #9
    Join Date
    Jul 2008
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    4-20 mA devices can often be simply connected in series up to a maximum total resistance.

    The total loop resistance is important because the power supply driving the string of devices wont be able to supply enough current to power all the devices if the total series resistance.

    The 500ohm resistor that you may need to measure 0-10Vdc across will form part of that total resistance.

    You can use a smaller resistance or increase the DC voltage but there are practical limits and many devices also have an upper voltage limit ...often 35Vdc

    Typically a total loop resistance of up to around 800 or 900 ohms will be ok and you transmitters and displays will work fine.


    http://celesco.com/faq/420.htm
    http://www.bb-elec.com/bb-elec/liter...h/curentlp.pdf
    http://www.datel.com/data/meters/dms-an20.pdf
    http://en.wikipedia.org/wiki/Current_loop
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

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