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

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  3. JohnInTX

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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

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    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

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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.
     
  7. Chris.09

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    Feb 23, 2017
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    Ronv,

    Thanks for the help but I am relatively new to electronics.
    Can you explain specifically how you selected each resistance value and how the TL431 and FET affect the overall circuit?

    Thanks!
     
  8. Dodgydave

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  9. crutschow

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    To add to Dd's explanation:
    The Tl431 will conduct when the voltage at its control pin equals or exceeds 2.5V, keeping the P-MOSFET gate voltage low and the transistor ON (Vgs is biased with the gate more negative than the source).
    Below this voltage the TL431 turns off, allowing the gate voltage to rise to the source voltage, turning the MOSFET OFF.

    So you select the resistor values R3 and R4 such that the voltage divider gives 2.5V at the control pin, at the battery voltage you want the MOSFET to cutoff.
     
  10. ronv

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    This may help a little:
    upload_2017-2-23_12-45-0.png
    In the circuit the TL431 really acts more like an open collector comparator than a voltage reference.
    So R2 has 2 purposes. 1 is to pull up the output when the comparator is off and to turn off the FET.
    As Dave pointed out R3 and R4 set the voltage at the reference point where it will switch.
    R7 adds hysteresis so it doesn't "chatter" around the trip point. It's like it's in parallel with R4 when the Fet is on moving the reference higher.
     
  11. A Homeschoolers Workbench

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    Jul 26, 2016
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    If your looking for the easiest this may be it.

    A.H.W.
     
  12. Chris.09

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    Feb 23, 2017
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    All,

    Thank you very much for the quick responses

    I have a couple more questions, how did you find the right value of R2 to use? On the TL431 data sheet, it shows similar applications but it has a resistor value in series with the battery as well and no R2? For example, figure 20, 21, 22

    Also, do you think I could use an LT1009 chip instead of the TL431?

    Once again, thank you!
     
  13. Chris.09

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    Feb 23, 2017
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  14. crutschow

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    Not likely.
    I don't think the LT1009 can be used as a comparator, as the TL431 can.
    Why do you want to use the LT1009 instead?
     
    Last edited: Feb 26, 2017
  15. crutschow

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    The value of R2 is more or less the maximum value that can supply the necessary 1ma or so current the TL431 requires to operate while minimizing power consumption.

    Figures 20, 21, and 22 are not "similar applications".
    They are test circuits for when it is being used for it's normal voltage regulator operation.
    It is not being used as a comparator there.
     
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