How can an Op Amp voltage comparator be used as a voltage controlled switch?

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Discussion Overview

The discussion revolves around the design of a voltage-controlled switch using an operational amplifier (Op Amp) voltage comparator. Participants explore the integration of two voltage sources, one constant at 3V and another that charges up to 3.5V and discharges, with the goal of controlling the switching behavior based on the voltage levels of these sources.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes using an Op Amp voltage comparator, specifically the LM393, to create a circuit that switches between the two voltage sources based on their levels.
  • Another participant suggests that a 555 timer could be an alternative solution, noting its common use in similar applications.
  • Concerns are raised about the feasibility of using a 3V source to charge a 3.5V source, questioning how this would work without an additional power supply.
  • A later reply indicates that the lack of headroom in the Op Amp comparator due to the absence of a power supply could be problematic, recommending an analog voting circuit instead.
  • One participant suggests using MOSFETs as voltage-controlled switches, controlled by the comparator, and mentions the need for hysteresis to prevent rapid switching between states.
  • There is a question about whether an independent power supply is available for the comparator and switching circuit or if it must be powered by the 3V source.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to implement the voltage-controlled switch, with no consensus reached on the optimal solution. There are also unresolved questions regarding the power supply requirements and the feasibility of the proposed methods.

Contextual Notes

Participants highlight limitations related to the power supply configuration and the operational characteristics of the components discussed, such as the Op Amp's headroom and the need for hysteresis in the switching mechanism.

Daniel Floyd
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Hi

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.
 
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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)
 
can anybody help with this its pretty urgent ?
 
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.
 
But how could 3V source will be able to charge V2 up to 3.5V?
 
Jony130 said:
But how could 3V source will be able to charge V2 up to 3.5V?

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.
 
Daniel Floyd said:
Hi

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.
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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