Low voltage cut-off circuit

Discussion in 'General Electronics Chat' started by neuhusky, Dec 8, 2014.

  1. neuhusky

    Thread Starter New Member

    Dec 8, 2014
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    Hi everyone,

    I'm trying to figure out a way to cut the power to a project I'm working on when the battery voltage drops below a certain threshold. I'm using a 2S1P LiPo battery @ 7.4V so I want to protect from overdischarging. Ideally, I'd like to cut the power running to my project when the battery voltage reaches 6V, which would put each cell in the battery pack around 3V, and then I'd recharge (with a balance charger).

    At the moment, I'm trying to do it with a zener diode to set the reference voltage for an op amp to act as a comparator, then feed the output of the op amp to the gate of a logic level n-channel MOSFET. The project has a maximum current draw of just over 2A (40 RGB LEDs + 10 IR LEDs + some ICs). I looked in this thread but I'm having trouble figuring out how exactly to adapt the circuits there for my needs. The picture I attached looks like it could work, but I don't know what to modify.

    Any help on these designs, or a simpler way to do this that I'm not realizing? Thanks in advance!
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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  3. JohnInTX

    Moderator

    Jun 26, 2012
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    The circuit you posted should work if you change R2 to 20K. That would put the centered wiper of the pot at the reference voltage when the battery voltage is 6.0V (if the 'Lil Professor is still working). I didn't look in detail at ronv's circuit but it looks like its a possibility.

    Have you considered an integrated charger controller? These from Microchip are about $1.50 and integrate everything you'd need for a complete charger/monitor. They also consume much less current to operate/standby as well. The LM393 runs about 1mA Typ. by itself. Several other outfits make battery management ICs as well.
     
    Last edited: Dec 8, 2014
  4. JWHassler

    Member

    Sep 25, 2013
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    Another vote for Microchip.
    If the MCP73841 2-cell controller is as easy to use as its single-cell cousin MCP73831, it's the way to go.
     
  5. neuhusky

    Thread Starter New Member

    Dec 8, 2014
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    Thanks, that looks like it'll do what I need! I'll play around with it a little and confirm. Is there anything specific about the TL431 I should look for when ordering it?

    Thanks for the response! I looked quickly at the Microchip parts available for 2-cell packs (only 4 total options, 2 of which have minimum voltages too high for this), and unfortunately the other 2 options have the UVLO start threshold at 4.15V, which would be 2.075/cell and I believe that damages them. I think those parts are more for LiFePo4, maybe the cells are more tolerant of lower voltages? Not really sure. I'll change R2 to 20K in a simulation for the other circuit and see if that helps.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Nothing special about which TL431 you use. You do need to be a little careful with the PFET to make sure it turns on and off at the voltages you are using, but we can check it if you find one you like better than the one I posted.
     
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