PCB design question(s)

Discussion in 'General Electronics Chat' started by DC_Kid, Feb 29, 2008.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    1. i am building a current sensing circuit. currents range from 1-6amp and the sense resistor is this funny looking "air" 0.05ohm resistor (looks like a piece of flat wire bent into a flatened letter C, is meant to be lifted off the board so it can cool better in the air). i'll be using a op-amp with gain of 20 to monitor the voltage across the resistor. the output of the op-amp is then trimmed and fed to a lm3914. the monitoring circuitry is to be built on a daughter card for modular design purposes and will be connected to main board via some mezzanine headers (like those you see for IDE cables in a PC, but these are for board-to-board applications). i am concerned about the DC insertion losses found in the two headers connecting to each other from main board to daughter card. i can compensate for these losses via output trim on the op-amp but the concern there is that the insertion losses will vary over time due to metal corrosion of the header pins, heat, and vibrations. i can opt to put the op-amp IC on the main board but that means if the daughter card is not being used i have circuitry on the main board being powered but not being used. any suggestions?

    2. can anyone suggest a PC board maker that can make 2-4 boards for prototyping purposes. i use DipTrace to output gerber files and drill schedules, just need to find a place that can run a few boards without my wallet going into the IOU state....... any suggestions?
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Use connectors with gold plated contacts, the depth/quality of plating chosen by how often you intend to plug/unplug the connectors (typically, 35u in. gold plating is a much better grade than gold flash plating).

    Use a lot of overhead for amperage rating of connector. If you were to use a single pin for 6A, then you would want at least a 10A rated connector pin. Or use multiple pins in greater abundance, such as five or more 2A pins.

    Make sure to use an equal number of signal, and signal return pins. In the above example of 2A pins, you would need 10 pins total.

    When it comes to board layout, you also need to consider the trace width and thermal relieves used within the PCB that can contribute to losses. If using multiple pins, consider a power plane with solid thermal ties within the PCB, in conjunction with a properly installed press-fit connector (this could be a little difficult for the DIY home assembler).
     
  3. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    let me clarify.

    the high currents stay on the main board. the daughter card is only the monitoring circuitry. so i either bring up the voltage that is generated across the current sense resistor from the main board (two measure points btw) and feed that to the inputs of op-amp on daughter card, or, i place op-amp down on the main board. the op-amp gain at 20 is fairly low, but then again the range of voltage i am monitoring is just 0-300mV.

    i have to check and see the what materials are being used, but the header connectors i am looking at are FCi's 69916-016LF and 66953-020LF 40 pin.
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Sorry, I misunderstood the current issue.

    If you sense the voltage with a differential amp, it shouldn't matter. The connector pins are going to be pretty close in tolerance, so just match wire or trace lengths/widths going into the diff amp reasonably well and it will be okay (and, keep the +/- wires fairly close together or twisted).

    You may want to try a differential input amp first, then feed an op amp with Av=20.

    And still, use connectors with gold contacts.
     
  5. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    the INA114 is just the ticket and is what i am using.

    thanks.
     
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