max current copper area.

Discussion in 'General Electronics Chat' started by adrenalina, Mar 21, 2012.

  1. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    Hello everybody. I have found calculators that calculate the min trace width for a certain current that goes through the trace. What I was wondering is how do you calculate how much current can circulate in a copper area. I think I can explain my question better with an image.
    I attached an image which has a copper area(black) and 2 holes. Current flows from one hole to another. How much current can travel between the 2, since the copper area dissipates heat it can't be analyzed as a trace with the calculators.
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    I don't think this question can be answered. You could ask "What is the resistance between these two points" given the thickness of copper, and from that you could figure out voltage drop and power loss for a given current and temperature.

    But if you ask "How much current can travel?" then you haven't supplied enough information. How much before the copper melts? Well then, you need to specify how much cooling there is, both through the board and into the surrounding air--and is it still air in a confined space, or moving air? Or if the design is limited by some required level of efficiency, you need to say what the level of permitted loss is.
     
  3. #12

    Expert

    Nov 30, 2010
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    Try calculating the width of trace that is required to flow the current and see if there is at least that much on the circuit board.
     
  4. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    would it work to calculate the resistance between the 2 points and the thermal resistance of the copper area, knowing it is exposed to ambient air, and then decide how much temperature rise is acceptable and with that calculate the max power dissipation and finally the current?
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    Yes, that's pretty much what I meant. And note that the resistance of the copper will change as it heats up! And it won't be the same temperature all over, but maybe you could fudge that. It's a simple situation with complicated math.
     
  6. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    thanks for the help. For the resistance between the 2 points would I need to consider the whole width of the area or just the diameter of the holes for the trace width?
    Also how do you calculate the thermal resistance?? I found that for a 1 squared inch it is 43C/W, but what if it is bigger?
     
    Last edited: Mar 21, 2012
  7. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Are you not making things overly complicated? I used to design pcb's many moons ago, granted at low currents, but if you are getting to the stage were you think the copper tracks aren't going to cope with the current then leave the board either using wires or tinned copper wire links.
     
  8. John P

    AAC Fanatic!

    Oct 14, 2008
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    I think things are (unfortunately) complicated here. It's a case of current through an undifferentiated plane, not through narrow conductors as on a normal PC board.

    Just for fun, I did a Google search on resistance between points on a plane and came up with lots of stuff, like this:
    http://groups.google.com/group/sci.physics.research/browse_thread/thread/f535cd35c6ca6de8

    Someone says "I believe the problem has been solved in closed form
    using conformal mapping. It does require knowing how to use elliptic
    integrals. A good place to start is with Smythe's Static and Dynamic
    Electricity."

    Sounds like fun.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Don't over complicate it. Your image is just a trace (or track) with 2 holes. The calculators will tell you how much current that can carry based on its width and copper thickness. Basically just treat that as a regular trace. And I do heavy(ish) copper PCB's carrying quite a bit of current (up to 100-125 Amps) all the time.
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    We must also take into account the maximum permissible conductor temperature rise. The OP may find this interesting http://www.ieca-inc.com/images/Current_Conductors.pdf
    http://people.senecac.on.ca/john.ebden/aed/IPC2152.pdf
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yes and that is usually part of the trace calculators already.
     
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