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How Hysteresis Works in Opamp Circuits

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    How Hysteresis Works in Opamp Circuits

    How Hysteresis Works in Opamp Circuits


    The post discusses how hysteresis resistor in opamps works and helps to create a delayed switch ON and switch OFF across its output in response to the difference between its input pins.

    enables the user to set the full charge cut off and low charge restoration of the relay apart by some voltage difference. If the hysteresis was not introduced the relay would rapidly switch ON OFF at the cut off level causing a serious issue with the system.

    The question was raised by one of the dedicated readers of this blog Mr. Mike.

    why are 4.7 zeners being used for the reference voltage? If we don’t want the 12 volts to drop below 11 for discharge,

    Is the feed back resistor going to the virtual ground point a 100K resistor? If so, why was this value chosen?

    2) Also, I apologize, I forgot to as why are there 4.7 zeners at the bases of the BC 547 transistors?

    3) Also my last question for today for this circuit. 

    The red/green indication LEDs; how do they light up? I mean the red
    LED is connected through its resistor to the top + rail, connects to
    the output of the OPAMP, then goes down in series towards the green LED.
    It would seem they would

    be both on at the same time, since they are in series, in both circuits. Does it have something to do with the

    feedback circuit and virtual ground? Oh I think I may see. So when
    the OPAMP is off, the top red LED 

    current is going through the feedback
    resistor (thus its “on”)to the virtual ground point? But how does it get 

    switched off, when the OPAMP has an output? When the OP AMP gets an
    output, I can see that going down to the green LED, but how, in that
    state, does

    the red LED then get switched off?

    Thanks again for any help!

    My Reply

    4.7 is not a fixed value it can be changed to other values also, the pin#3

    preset ultimately adjusts and calibrates the threshold as per the selected

    zener value.

    Question

    so the ref voltage is the zener is at pin 2 (top view opamp) correct?
    The 100K feedback resistor and pot are creating the hysteresis value
    (meaning, the difference between pin 2 and 3 to make the opamp swing
    high to its + rail voltage)? 

    The op amp in this config is always trying
    to make pins 2 and 3 arrive at the same value via its feedback resistor,
    correct (zero, since the feedback divider is @ 0 and pin 3 is @
    ground)?



    Ive seen this solar charger controller done without the feed back, just
    using several op amps with voltage ref pins and a pot on the other one

    just trying to understand how hysteresis is working in this case

    I dont understand the math in this circuit. Is the 100k 10k preset feedback absolutely necessary?

    In other op amp circuits, they dont use any feed back just use them in
    comparator config mode with ref 

     
    voltage at invert/non invert pin, and
    when one is exceeded, the opamp swings to its rail voltage



    What is the feed back doing? I understand the opamp gain formula, in
    this case is it 100k/10k x voltage difference of POT voltage
    (preset)value and 4.7 zener?

    or is this a Schmidt trigger type of hysteresis UTP LTP circuit

    yeah I still dont get the feed back with the 100k/10k; most opamp
    comparators Ive seen just use the opamp in saturation, could you explain
    why the feedback and gain for this? 

    Thanks
       

    Ok I goofed; the 10K preset is used to divide the voltage from the
    12volt rail, correct? So, when its preset value according to the POT
    wiper is more? than the 4.7zener, we swing the opamp high? still dont
    get the 100k feedback and why its used in a comparator circuit

    My Reply

    Please refer to the above example figure for understanding how the feedback resistor works in an Opamp circuit

    I am sure you know about how voltage dividers work?
     As soon as the full

    charge threshold is detected, as per the adjustment of pin#3 preset the
    voltage at pin#3 becomes just higher than pin#2 zener voltage, this forces the opamp
    output to swing to the supply level from its previous zero volt….meaning
    it changes from say 0 to 14V instantly.

    In this situation we can assume now that the feedback is connected between
    “positive supply” and pin#3…when this happens the feedback resistor
    starts supplying this 14V to pin#3, which means it further reinforces the
    preset voltage and adds some extra volts depending upon its resistance
    value, technically this means this feedback becomes in parallel with
    preset resistor which is set between its center arm and the positive arm.

    So suppose during the transition pin#3 was 4.8V and this switched the
    output to the supply level and allowed the supply to reach back to pin#3
    through the feedback resistor, which caused the pin#3 to a bit more higher say at
    5V….due to this pin#3 voltage will take longer to get back to below the
    4.7V zener value level because it has been raised to 5V…this is called hysteresis.

    Both LEDs will never light up because their junction is connected with
    pin#6 of the opamp which will either at 0V or the supply volt which will
    make sure that either the red LED lights up or the green, but never
    together.



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