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Low value resistors

Discussion in 'General Electronics Discussion' started by Rajinder, Jun 2, 2016.

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  1. Rajinder

    Rajinder

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    9
    Jan 30, 2016
    Hi,
    I have a application in which i am switching low value resistors i. e. 0.1R, 0.2R, 0.4R etc to a unit under test.
    I have come across a problem, track resistance and cable resistance which override the resistor values.
    These are software selectable so need relays to switch them to the end circuit.
    Is there any other way I can achieve this?
    Ideally I need to keep the track lengths on the PCB the same or use them on planes so the resistance remains the same.
    Any thoughts?
    Best regards,
    Rajinder
     
  2. shrtrnd

    shrtrnd

    3,783
    499
    Jan 15, 2010
    How good is your software?
    Can you factor-in the resistance of the circuits, and then zero the value so that it reads the resistors for your test?
     
  3. Rajinder

    Rajinder

    470
    9
    Jan 30, 2016
    No, the software just selects the value required, switches the relay and that's it.
    So I need help with the hardware side.
    It's proving tricky.
     
  4. davenn

    davenn Moderator

    13,721
    1,913
    Sep 5, 2009
    OK so you need to factor in the resistance of the circuit to and from the resistance and select a suitable resistance to give the appropriate value
    and even that is going to be almost impossible because of the low values you want to use

    what is the project ..... what is it supposed to achieve ??


    Dave
     
    Last edited: Jun 3, 2016
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,547
    2,122
    Jun 21, 2012
    I used to know someone who earned a living building custom motor testing equipment. He used some really big reed relays to switch the various motor currents. He custom-built reed relays by purchasing the reed switches, which were custom-made with certain alloy contacts to his specifications. IIRC there were some pretty stiff voltage hold-off, switching time, and current requirements for both make and break operation. It cost him a fortune for these switches, but he passed the cost on to his one and only customer.

    He mounted several of these in the same coil form for multi-pole switching. And his current-carrying conductors were large to minimize ohmic losses. So maybe you can go this route. Find some big-ass reed switches, add a coil around one or several of them, and use really heavy wire to reduce series ohmic resistance.

    But as Dave asked: please tell us what you are trying to DO. Maybe we can think of other alternatives.
     
  6. Miguel Lopez

    Miguel Lopez

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    Jan 25, 2012
    Are you measuring the resistances?, a four wires connection could help, but as the other members has said, what is the purpose?
     
  7. Rajinder

    Rajinder

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    Jan 30, 2016
    Ok. The resistance values are inserted in series with a 3V supply to alarm units. This way we can introduce high impedance battery faults into alarm units.
     
  8. Rajinder

    Rajinder

    470
    9
    Jan 30, 2016
    Is a 4 wire system achievable using FETs??
     
  9. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
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