I just built this circuit, with the intent to use it as a low-voltage cut-off switch for a single li-ion cell. It works great, but once it's triggered it needs about 7 volts to turn back on, so I guess it was intended for a 2s pack at least. Source: https://www.rcgroups.com/forums/showthread.php?725572-voltage-cut-off-switch

I was wondering whether using a Mosfet with a lower gate threshold voltage would be sufficient to reduce those 7 volts to just under 4 volts? The Mosfet I'm using is STP75NF75.

schematic diagram

  • \$\begingroup\$ What's the device with the "top view" label? \$\endgroup\$
    – Drew
    Dec 18, 2018 at 17:28
  • \$\begingroup\$ it's the TL431. The circuit uses a n.o. pushbutton to reset it when its triggered, so until it is pressed there is no current going through the fet and my voltmeter reads 0.00 volts. The 8.2k and 3.3k are the voltage divider for the desired cut-off voltage and the 10k pot will allow you to adjust that voltage to your specific needs. Ofcourse you can skip the pot and just calculate the divider for whatever cut-off you need. \$\endgroup\$ Dec 18, 2018 at 20:26
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    \$\begingroup\$ @ВеселинМилов - Hi, it seems the original source of that design is the RC Groups forum - is that correct? We encourage people to give the source of anything (photos / diagrams etc.) which they use in a question, and it's a rule to do so in answers. In this case, there's plenty of discussion from the designer about how he intended the circuit to work, which may help readers here. If the source was what I found, please add that link to the question. Thanks :-) \$\endgroup\$
    – SamGibson
    Dec 18, 2018 at 21:01
  • \$\begingroup\$ Hello and thanks for the tip. It would seem that this is the source but i found this schematic on my PC so i guess i had no idea about that. I looked through the thread but i couldnt find an answer there, so my question still stands. I could ask there but it seems to be an old thread and im not sure if anyone still follows it. \$\endgroup\$ Dec 19, 2018 at 8:04

1 Answer 1


Your Programmable Zener LM431 is not referenced to 0V = Vbat-. (and poorly drawn )

You have no comparator design or hysteresis.

Start over and define your thresholds for cut-off and turn-on or hysteresis levels and choose a comparator or better, look at BMS IC design choices.

Define all functions or conditions like what purpose your switch serves and how you protect the battery from undercharge load current and how much memory capacity ( V rise) your battery has when load is switched off.


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