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Op Amp (voltage Comparator)

  1. Mar 16, 2013 #1

    I am trying to create a circuit which i can use as a voltage controlled switch.

    I have 2 voltage inputs,

    ** A constant 3V source V1,

    ** A source which charges up to 3.5V and then slowly discharges V2.

    I would like to amalgamate both sources so as when V2 discharges below 3V the V1 will be used allowing V2 to charge back up to 3.5V e.t.c

    I have done some research and i believe that an Op Amp voltage comparator can be used for this and believe that i can implement the LM393 but am struggling to design a circuit that can do this operation.

    Can any help and explain how this can be achieved.
  2. jcsd
  3. Mar 16, 2013 #2
    forgot to mention this must be low power my V2 supply is 50mA and 3.5V Maximum (switches on and off thus super capacitor charging circuit)
  4. Mar 18, 2013 #3
    can anybody help with this its pretty urgent ?
  5. Mar 18, 2013 #4
    You could probably use a 555 timer for this. It has the basic functionality you want. There are a MILLION circuits on the web using this part. It's a classic analog circuit.
  6. Mar 18, 2013 #5
    But how could 3V source will be able to charge V2 up to 3.5V?
  7. Mar 18, 2013 #6
    Oh you don't have a power supply? I thought V1 and V2 were inputs. Hmm.... this is unusual requirement... I think I get it now.

    The problem with using an op amp-based voltage comparator is it won't have any headroom because you don't have a power supply. You need an analog voting circuit where the higher voltage of the two gets passed to the output. Try googling that I guess.
  8. Mar 19, 2013 #7


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    Staff: Mentor

    A MOSFET from each supply could be used as a pair of voltage-controlled switches, their control signal coming from the comparator. Include an inverter so one MOSFET is OFF whilever the other is ON.

    The comparator should have some hysteresis (implemented using positive feedback) so it switches when V2 falls below 3V but doesn't switch over again until V2 has risen to almost 3.5V

    Do you have an independent power supply to power the comparator and the switching circuit, or will this need to be powered by the fixed 3V source?
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