Less loss on current limiting on an LM723

Discussion in 'General Electronics Chat' started by hp1729, Dec 3, 2015.

  1. hp1729

    Thread Starter Well-Known Member

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    A while back there was a post suggesting an op amp circuit as a current monitor added to an LM723 to minimize the 600 mV loss across the current limiting resistor. I thought it was a good idea, but I would offer another option on where to put it in the circuit. High side current monitor and kill V Ref.
     
  2. #12

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    That schematic would make more sense if you added information about its limitations.

    For instance, the junction of R12 and the 1K resistor can not be more than 0.6 volts because the internal diode of the mosfet will short any higher voltage to ground. Then, there are no resistors in the feedback path of the third op-amp, so its output voltage can not be more than 0.6 volts which is not enough to turn on the (2) Vbe drops to cause the TIP31 to conduct.

    The LMV321 has a Vcc high limit of 5.5 volts and its maximum input range is Vcc - 0.8 volts, so its pin 3 is outside its input common mode input voltage range when connected to its own Vcc.

    Then you might explain why you would add a mosfet instead of using the transistor designated as pins 2 and 3 of what seems to be an LM723 chip (on the right side, by the third op-amp)

    I probably missed a few of the fine points, but I have started something that could help you explain why this circuit is useful.
     
  3. hp1729

    Thread Starter Well-Known Member

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    Sorry. That wasn't intended to be anything more than a concept, not a working design.
     
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  4. #12

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    That sentence alone is significant information. Thank you!
    Of course, I would be even happier if you posted a circuit that could work.
     
  5. AnalogKid

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    Q3 is upside down. Can you explain the action of U5 and Q4?

    ak
     
  6. hp1729

    Thread Starter Well-Known Member

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    "Would work" depends on what the reader needs. My intention was to offer something that could be adapted to the needs of the reader. Your comments were quite correct, of course, and yes the MOSFET was upside down, thank you all. How about this for a more working schematic? I haven't built it but it should be closer to a working design.
     
  7. #12

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    U5 is supposed to be measuring current through 0R2 by measuring the voltage drop across 0R2. If U5 had a set point, I might turn Q4 on or off at a certain current, almost all of that measured current going through Q5. Q4 controls Q3 which is supposed to dump Vref so pin5 of the third op-amp collapses when there is too much current going through 0R2. At this point the circuit becomes a constant current source. The foldback function is not implemented in this design.
     
  8. hp1729

    Thread Starter Well-Known Member

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    ??? U5 is an op amp, not a voltage comparator. The output should be an analog voltage. Out of Q4 we should get 1 V per amp through the 0.2 Ohm resistor. At about 4 Amps V Ref is killed. Constant current source??? Foldback function????

    The objective was to get rid of the 600 mV loss across the current limiting resistor normally found in the LM723 design.
     
  9. hp1729

    Thread Starter Well-Known Member

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    Perhaps an improvement to this design would be to add a flip flop between the current sensing circuit and the LM723. We would have an electronic circuit breaker of sorts.
     
  10. AnalogKid

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    Yeah, I know. I should have been more clear - I wanted the TS to explain his design so we would know how to approach some corrections.

    ak
     
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  11. #12

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    Did you just quote the paragraph wherein I did not use the word, "comparator", then argue that "comparator" is the wrong word to use here? Do you know the difference between an op-amp configured to provide the function of a comparator circuit and a comparator configured to provide the function of a comparator circuit?

    Did you use triple question marks to indicate that you don't know what a constant current source is?

    Did you use triple question marks to indicate that you don't know how a foldback function works, while presenting a demonstration about how to improve the function of an LM723?

    Did you accomplish saving the 600 mV loss by using 0.2 ohms in series with 4 amps (800 mV) for the current sensing function at 0R2?

    I'm going to take a step back for now because other people want to hear YOUR Theory of Operation instead of mine, and I agree that your point of view will be more informative. The only advice I offer is: Define your objective first, then pursue it relentlessly until you have a circuit that works.
     
  12. hp1729

    Thread Starter Well-Known Member

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    Oops, you are quite correct about the 800 mV vs 600 mV voltage drop, I used a bad example in my circuit.

    I am going to have to be more specific on my circuits from now on.
     
  13. AnalogKid

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    Not necessarily. When you are throwing out a quickie concept sketch, just flag it as such. I haven't seen Colin around here, so you're safe.

    ak
     
  14. hp1729

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    Thanks for the patience. I'm trying to build up a version to test the concept.
     
  15. ronv

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    The AD820 common mode voltage only goes to within 1 volt of Vcc so I think you will have trouble with it.
    They make a lot of high side current sense IC's. I would look at those.
    http://www.mouser.com/ds/2/405/ina168-404626.pdf
     
  16. hp1729

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    That's a definite yes on the high side current sense chips. They would be a certain improvement. For now I'm stuck working with what I have on hand.
     
  17. ronv

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    Maybe you could sense the current in the ground path then. That would let your op amps work.
     
  18. hp1729

    Thread Starter Well-Known Member

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    Ah! Good. Thank you.
     
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