Using a MOSFET and comparator together as a switch

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Discussion Overview

The discussion revolves around the use of a MOSFET and a comparator in a solar charger circuit designed to prevent overcharging of batteries. Participants explore the components needed, their configurations, and potential issues related to the design.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about using MOSFETs and comparators, specifically seeking a comparator with an internal reference voltage of 5.5V and guidance on how to connect the components.
  • Another participant suggests using a standard voltage comparator with a separate voltage regulator and a voltage divider to achieve the desired reference voltage.
  • A third participant recommends adding hysteresis to the comparator's positive feedback to mitigate noise and prevent unwanted switching behavior.
  • Another viewpoint proposes using a voltage regulator set to the cutoff voltage to stop charging when the battery reaches a specific voltage, suggesting this could eliminate the need for a comparator in this context.

Areas of Agreement / Disagreement

Participants present multiple approaches to the problem, indicating that there is no consensus on the best method to implement the circuit. Different strategies are proposed, and the discussion remains unresolved regarding the optimal solution.

Contextual Notes

Participants do not clarify the specific requirements for the MOSFET or the implications of using different configurations, leaving some assumptions and dependencies on definitions unaddressed.

Who May Find This Useful

Individuals interested in solar charger design, circuit design involving MOSFETs and comparators, or those seeking to understand battery charging management may find this discussion relevant.

Shinnog
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Hello all, I was advised to use a MOSFET and a comparator together in a solar charger that I am building to stop the solar panel from charging the batteries when full. Seems logical, right? The only problem is I've never used MOSFETs or comparators, so I'm not really sure how to. I need a comparator with an internal reference voltage of 5.5V and a MOSFET rated for 100 ma. I looked around and found a MOSFET but I couldn't find a comparator with an internal reference voltage... does anyone know where I could find one? And once I do find one how to hook it all up?
 
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You should use a standard voltage comparator along with a separate 3-terminal voltage regulator like the Motorola MC7806 (see
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-21/DSA-405486.pdf
and use two resistors (voltage divider) to get 5.5 volts.
 
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Be sure to put a couple hundred millivolts of hysteresis into your positive feedback on the comparator, to avoid noise switching and buzzing.
 
You want to stop charging when the battery reaches a certain voltage? Just charge the battery with the output of a voltage regulator set at that cutoff voltage. It'll cut off the current when the battery voltage reaches the output voltage.
 

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