Current monitor to electronic circuit breaker

Discussion in 'The Projects Forum' started by hp1729, Jan 5, 2016.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    A while back I posted an incomplete idea for a current monitor circuit that output a voltage proportional to the current being monitored. Thanks to some help I have a working model of that. The next step is to add an adjustable voltage comparator on the output of that and a latch.
    Originally the idea was to have the latch kill V Ref on an LM723 or LM317 accomplishing an electronic circuit breaker. The losses were too great with either of those methods. What I need is a very low loss switch.
    In this design current is small, around 250 mA and voltage is limited to 5 Volts. I would like to keep loss across the switch below 100 mV,
    The schematic attached includes the current monitor.
    Voltage comparator? Maybe LM311 or LM393. I have those on hand. The latch? CD4027, or such.
    The switch? ??? there I need ideas.
     
  2. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Why MCP6021?
    Previously the best thing I had on hand was an AD820. Almost RRIO. Thanks to help the MCP6021 was suggested for Common Mode input range. Worked super, but was limited to about 5 Volts.
    See attached for a comparison between the AD820 and MCP6021.
     
  3. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    First draft of the partial circuit.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'd suggest a P-channel MOSFET as the switch.
     
  5. hp1729

    Thread Starter Well-Known Member

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    I am not too well stocked on MOSFETs. VP0106 has an Rds of about 8 Ohms and tolerates about 250 mA. The current is within the limits of this design but 8 ohms at even 100 mA drops 800 mV. Too much. I'll build it just to see if I have any other lessons to learn. :)
    Do I need to drive it with anything more complicated?
    I don't have any power P-MOSFETs on hand. I need an Rds of about 0.33 ohms or less. Any suggestions? TO-39 or TO-5? Something I can breadboard. 1 amp or so would even be suitable.
     
    Last edited: Jan 5, 2016
  6. Alec_t

    AAC Fanatic!

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    You can get PFETs (a 'logic-level' type will be needed) in various packages and with only a few milliOhms Rds(on). The parametric search engines on any of the major component suppliers should pull up a wide choice.
     
  7. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Re: various cases
    Yep, no problems finding them in SMT cases. :)
    I changed the design slightly and went with a low side driver, N-MOSFET. This new version works okay at 50 mA.
    The design is easily adaptable to higher currents by selecting resistors in the current monitor circuit. Or higher voltages, but not with the MCP6021.
     
    Last edited: Jan 5, 2016
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If the U6 current monitor is supposed to produce an increasing output voltage for increasing supply current, I think there is a problem. As current increases, the inv. input decreases and the opamp output increases. This moves the Q1 gate closer to its source, turning off the FET and decreasing the input to the comparator. Is this the desired operation?

    Also, why is Q1 in the current monitor circuit?

    ak
     
  9. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Oops, did I draw it wrong? What I built worked. The basic design is from the data sheet for an LM307. It is basically a variable current source. Pretty standard I thought.
    (edited ton add ...)
    Yes, I reversed the inputs to the op amp when I drew it.
    Corrected drawing attached.
     
    Last edited: Jan 6, 2016
  10. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Dang, you got me wondering which way I originally made it. The current monitor section was originally built years ago, put on a small breadboard for use and just recently modified with the MCP6021, a much better chip for this application but limited to 5 Volts.

    Attached is a collection of variations on current monitors.
     
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