Transistor voltage-follower 4wire (end of wire) sense?

Discussion in 'The Projects Forum' started by alan1975, Oct 9, 2014.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Hello,

    I'm trying Google (without result) to find some basic example of transistor voltage-follower hi-current with 4 wires (sense at end).
    It's like Kelvin measurement, but it make sure that desired voltage is applied to end of device, no matter what kind of voltage drop is found on cables.

    Like for example set 0.5V at output of follower -> push 10A -> lost 0.1V at long wires measure it by sense wires and correct value of follower to get 0.5V.

    I saw this one time on laboratory power supply made by Voltcraft.

    I'm looking for any basic circuitry to understand how this should be done - the less elements the better for understanding.

    Any hints are welcome,

    Regards
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
    It the objective is to drive the load with current, then it would make sense to drive it with a current source and not worry about sensing the current at the load.

    One example:
    [​IMG]
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    get your voltage feedback for the regulator from the sense wire, that will allow your supply to compensate for voltage drop.
     
  4. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0

    I'm pretty sure that idea of operational amplifier is good to follow voltage.
    I was using it once, but to drive current and make constant discharge-r without affect of voltage drop... but here problem it quite opposite.

    I would like to be able to set for example 0.4V using reference TL431 (2.5V) and potential divider using trimming pot to set 0.4V and drive load witch that voltage.
    Problem is to make 100% sure it will be delivered to clamps and not get lost on wires.
    You can see that lose 0.1V is 25% of value, that's why i'm interested in some kind of feedback.

    That's the problem.

    What about this circuitry:
    I7ul0.png
     
    Last edited: Oct 10, 2014
  5. #12

    Expert

    Nov 30, 2010
    16,330
    6,818
    Consider this:
     
  6. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
    Upon re-reading your original post, you indeed desire a voltage source with remote sensing, and not a current source as I misread.

    #12's circuit should serve your desire for a voltage source well.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    The problem is that the transistor adds open-loop gain in that configuration which would make the circuit unstable without added compensation, causing oscillations. If you used an N-PMOSFET in a follower configuration, it adds no open-loop gain so should be stable.
     
    Last edited: Oct 11, 2014
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    I think this is what you are looking for. I'm not at all sure it would actually work, but I think the principle is what you were looking for.
     
  9. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Today i was trying this but i have problem.

    When it's open circuit i got 5.9V when i apply load (700mA) i got voltage drop about 1V.. so i'm not sure why is that happening. I have 2 power sources one is 12V for logic and second i put around 10V only for load (bulb). (it's just a breadboard test). I checked witch scope is there any oscillations but i can't found any... maybe i got wrong resistors proportion, i put 10K maybe it's to low. Something good happening because it's floating around 5V so transistor is following that op-amp (LM324).

    Right now job keeps me from electronics hobby, anyway i hope to breadboard ronv circuit this week because it's look promising, i will follow it and check the results maybe it will stop bad luck ;) .

    I'm going post result when try
    IMG_0102.JPG IMG_0106.JPG
    Regards
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    There are a couple of problems with your circuit.
    1- I don't think you want the lower 10K resistor. +S would need to be over 10 volts to make the op amp input 5.1 volts.
    2- The BC637 cannot supply 800 ma with 5 volts dropped across it. That would be 4 watts for a 0.6 watt transistor. You need one you can put on a heat sink.
     
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