Simple comparator not working, voltage dropped

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Nov 25, 2014.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    1. I have made a simple single supply comparator, ckt is attached.
    2. Mosfet I have used in N-channel , with Vgs = 4V. (part number is different as mentioned in ckt).
    Supply voltage range is 5V - 6.5V.
    3. I have to cut the output when voltage crosses above 6.5V.
    4. I have tried three different comparator with LM311 as attached, OP07 & 741 also. All have same problem.

    5. Problem is:

    If difference at Vin- & Vin+ is high then opamp/comarator works fine i.e full swing at output, which turn off/on mosfet completely & circuit is switched off/on accordingy.

    But when voltage difference at vin- & vin+ is very low or almost equal, output voltage of opamp falls. From 6.5V it remain 4.5V or nearby. Dut to mosfet don't get on/off properly & drop across mosfet occurs.. I don 't understand why this problem.
    • ckt.pdf
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  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    Comparators often oscillate right at the threshold voltage. That may be what is happening to you. You can add a 1 meg resistor from the output to the + input to make it switch clean.
  3. Papabravo


    Feb 24, 2006
    Why no decoupling capacitors?
    Why no connections on pins 5 & 6?
    Are you sure the output is not oscillating?

    Edit: ronv beat me to it.
    Hysteresis is common on comparators for this very reason.
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    If a NFET has a published Vgs threshold of 4V, it takes a Vgs of about 6 to10V to actually turn it on. You would have to use an NFET with an Vgs(th) of less than 2V to make this work. I would recommend replacing the NFet with an NPN Darlington...

    This circuit has no hysteresis, so will rapidly switch on/off as the battery voltage decreases.

    You might be interested in following this thread.