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MOSFET switch for Battery Discharge circuit

Discussion in 'Power Electronics' started by triggerman911, Jul 6, 2015.

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  1. triggerman911


    Jul 6, 2015
    Am building a circuit designed to discharge a NiMH 12 v battery. The safe cutoff for the battery is 10 volts, my question is:

    Should I use a Mosfet as the switch on switch off component, and if so, does anyone know of a Mosfet type that will switch off at 10 volts?


    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The circuit idea per se is not bad. The MOSFET will turn off when Vgs < Vth (Vgs=Gate-source voltage, Vth=threshold voltage). You can adjust Vgs by tweaking the resistors R1 and R2.
    Unfortunately it is not as simple as your schematic for several reasons:
    • Vth of any MOSFET has comparatively high tolerances (look in any datasheet). Therefore the circuit would have to be adjusted for the actual MOSFET being used.
    • Vth is temperature dependent. You'd have to compensate this.
    • The MOSFET will not turn on/off sharply. Rather it will become less conductive with the voltage dropping towards Vth.
    • As soon as the MOSFET turns off, the battery will recover, voltage will rise and the MOSFET will turn on again.
    A reliable circuit will use a comparator circuit with Schmitt-Trigger characteristic (ask your favorite search engine) to detect the "low voltage" condition and control the gate voltage to a MOSFET to cleanly turn-off the load. The Schmitt-Trigger characteristic will prevent the circuit from turning on again when the battery recovers when the load is turned off.
    hevans1944 and Arouse1973 like this.
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