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DC Control systems and polarity

  1. Sep 25, 2007 #1
    Control and protection electrical circuits are one of the important portions of electrical systems designed and shown by standard symbols.
    Usually the circuit is supplied by positive and negative polarity of one battery. In almost electrical circuit diagram sheets we can see a positive line of DC polarity in top of sheet and negative line in bottom of sheet that is purposed to supply electrical equipment of circuit.

    How can you explain that polarity arrangement? Why the negative polarity is usually located in bottom and positive polarity in top of sheets?
    Is it habit activity in drawing circuits or not? If all used electrical equipments, for example auxiliary relays, open and close coil of power circuit breakers and so on could be activated by inverse polarity, would we change that mentioned norm, and use negative in top of sheets and positive polarity in bottom, without any circuit configuration changing.


    Creative thinking is enjoyable,Then think about your surrounding things and other thought products. http://electrical-riddles.com

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  3. Sep 25, 2007 #2
    i'm not aware of any standard maybe guidelines?

    you can draw anyway you want. i think most of the time people associate the bottom as gnd or return path. if you feel more comfortable with negative on top then sure, but others may be confused and so would you if you get used to that while 99.9% of everyone else does the other way
  4. Sep 25, 2007 #3


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    Staff: Mentor

    Mainly it's just the modern convention. There are some older circuit configurations where there would be some nodes that had ground near the top, but those are confusing to decipher, now that the convention you mention is the norm. Usually, the more positive supplies are near the top, and more negative supplies are at the bottom of the circuit and page. If there are split supplies (like +/-12V), and some symmetry in the circuit, ground will be near the middle of the page. It is also generally a convention that inputs come in the left side of the page, and go out the right side of the page, although this is not always possible with all circuit diagrams.
  5. Sep 29, 2007 #4
    Generally electrical equipment that used in auxiliary control systems shown in bottom of pages. Therefore equipment (relay etc.) that usually installed in field area is directly connected to one of two polarity continuously, and other polarity connected to the other terminal of equipment via auxiliary contact of controller devices that shown on top of pages transitionally.
    Continuous connection must be negative, to prevent corrosion of equipment due to electrolytic action, because they usually installed in outdoor and corrosive area.
    Of course this recommendation related to battery system grounding way.


    Creative thinking is enjoyable,Then think about your surrounding things and other thought products. http://electrical-riddles.com
  6. Sep 29, 2007 #5
    I use this ask an introductory recruiting question. I draw a transistor with biasing resistors with the same value and Vcc = 10V and Vee = -10V and I ask them what the voltage is at the base.

    So far only one person has been able to tell me by just inspection.
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