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help w/ low voltage cutout

Discussion in 'General Electronics Discussion' started by Derekc, May 25, 2014.

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  1. Derekc

    Derekc

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    May 25, 2014
    I need help figuring what mosfet resistor and zenor to use. I am running a dc dc step down converter off two 3.7 v lion batteries. The controle pin is normally on. It's off when a 20 k ohm pull up resistor is connected from it to positive. I would like to add a depletion transistor , a zener diode in for low voltage cutoff around 7 v but am confused on what transister to use and if I would need a resistor before the zener.
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
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    Jan 21, 2010
    That's probably not going to work.

    I'd look for a 7 volt voltage detector

    Here is a chip that would work (and look, it's designed for protecting batteries). Note that the voltage limits are adjustable and that the maximum Vcc is 7V. You would need to use a resistor and zener to ensure that this device is powered with less than 7V. It looks like it would be able to directly drive a mosfet or the on/off control for the regulator.

    You would need to be careful that the power supply for this chip didn't further flatten the battery.
     
    KrisBlueNZ likes this.
  3. Derekc

    Derekc

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    May 25, 2014
    Thank you I am determined to make this work but I'm defiantly in over my head.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
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    Jan 21, 2010
    Perhaps something simpler then.

    Here is a simple TL431 circuit that lights a couple of LEDs when the input voltage drops below a certain point.

    edit: if you connect the output of this circuit to the control input of your regulator, it might be all you need.

    In your case you'll want to remove the LEDs
     
    Last edited: May 25, 2014
  5. Derekc

    Derekc

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    0
    May 25, 2014
    That I can do. Im ordering the parts now thanks man
     
  6. Derekc

    Derekc

    4
    0
    May 25, 2014
    Got the parts hooked it up and got a huge voltage drop on the output of my regulator. The tl431 circuit has a constant 2 volt output even while battery is fully charged guess it's not enough to turn it all the way off but cause major voltage drops under load.
     
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