Introduction: Battery Cut Out Switch LM431 CNC Vectric

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A long time ago I made an instructable for a battery cut off switch. Today i find it is complicated and not easy to follow for many.

As battery operated devices are becoming increasingly popular because batteries get salvaged from old laptop computers. So I came up with a more simpler idea.

A cut out switch is used to protect the battery from over discharging as it otherwise will be damaged.

This simple design uses the LM 431 chip found in old TV sets and to switch the power I use Mosfets as they don't draw as much power then a relay. This design requires a P channel mosfet.

I Robert Moller assume no responsibility or liability for any errors or omissions in the content of this Instructable. The information contained in this site is provided on an “as is” basis with no guarantees of completeness, accuracy, usefulness or timeliness. The information in this site is intended solely for the personal non-commercial use of the user who accepts full responsibility for its use. While I have taken every precaution to insure that the content of this Instructable is both current and accurate, errors can occur.

As purchase orders where small and everyone wanted to make their own I now provide you with the circuit you can cut on a CNC or order online at.

The design is made in EasyEDA , a simple free to use online program. You now can even download it to use it, but must be connected to the internet to access your files. Your files can be backed up on your PC.

The construction is simple.

Header 3 is the input and the fuse in my design is very small, you may want to use a larger resettable or other fuse.

The LM431 looks like a transistor and seems to connected in reverse, but it is like a zener diode that must be connected in reverse.

Calibration is done with R3 a 10 Kilo Ohm trim potentiometer. Header 4 is only used as an extra hole so that the CNC engraver bit gets around easy.

Hard to salvage is a P channel mosfet as out of 300 TV sets you may find 2 P channel mosfets.

In the drawing you may notice that the contributor of the mosfet has Drain, Gate and Source mixed up. My design in blue lettering is correct. I like mosfets as they can handle high voltages and lots of ampere and draw minimal power.

A reversible led can be added ( no provisions in the circuit) at the input of the circuit. I now use a (5mm Red/ Green Dual Colour LED 5013R2G6D-FPC Pk-25)

in case the polarity is wrong it lights up red and if connected right it lights up green.

A green led is used at the output with a one kilo resistor . Now in MK5 extra pins can be used to disconnect the led after calibration. The 1K draws less power( the LED will not be as bright). It is needed for calibration of the circuit . After that the LED can be disconnected to save battery power.

For full brightness of the LED use a 560 Ohm resistor at 12 volt.

A variable power supply is good to have for calibration.

I connected a LIPO battery to mine. Note ,that the cut out voltage is different to a lead acid battery.

Here the design files.


Resettable Fuse Poly Switch 1.1 A 30V Fuses Polyfuse

Step 1: Make You Circuit Board on a CNC

I use Vectric Desktop as it is an easy to learn program around $A 500.

download the Vectric CRV file .

  • profile tool path, not the engrave tool path set between 0.475 and 05mm 20 degree engraver bit.(depending on you bit)
  • holes 0.8 mm for standard holes and 1mm for headers and the Mosfet.
  • 3mm bit if the board needs to be cut out.

I use double sided tape to hold the board down and level the board first.

from eBay 10Pcs 3.175mm 0.1mm Carbide PCB Engraving CNC Router Bit grinding sculpture Tool

copper boards

EasyEDA files

For those who like to etch it I include a PDF file. Remember that you may have to mirror the file for etching if use with a laminator.

That should be enough information to reproduce this idea.

Step 2: CNC Two Boards at Ones

I continued to work on this project and made a board that has 3 additional pins and on a 70x50 board you can cut two designs at once.

The three pins are intended to be jumpers. connecting the Led or disconnecting the Led after calibration.

From there I made another board and fitted 2 battery cut off switches on one board. I thought a need a view more as each battery needs a seperate circuit.

On this last one the bit had broken off. I adjusted the height and kept going, that is why it looks a bit ruff.

The trim pot I use allows for fine tuning.

for questions email to