A Low Battery Cut-Off Circuit

This Circuit was Designed by me in 2002 to control the
12 Volt, 5 Amp Fan on the propane furnace in my truck.

It saves killing the battery when the truck is not running.

Created Here: "Oct 24 2006"
Revised P-mos PCB Artwork: "July 13 2015"

It works VERY NICELY and also acts as an On/Off switch.

For best Stabiliy, it is recommended that these units be connected "directly to the car battery". If you don't, it can trip because of voltage drops in your cars electrical wiring, Especially as you turn things on.

The "First Schematic" shows a Mosfet Driving a "Load" by grounding it to the negative of the supply. In this Situation, the Mosfet SINKS the Negative of the load to Ground. Alternately, The "Second Schematic", created Mar 2008 uses a P-Mosfet to supply power to loads that are presently grounded.

NOTE: The 47K resistor may need to be adjusted in value to give you the appropriate range of control you require. ** This is due to Variation in these IC's.

The 5 Volt Regulator on the Circuit Boards is to Stablize the IC Supply, Resulting in a stable trip point of the IC.

Go Back to My "Projects Page"

My Assembled circuit Board, in my truck."

NOTE: When I Origionally designed this, I designed it using an N-Mosfet because I was intending to use this for Numerous Applications. Than I realized it wouldn't work on my furnace because of grounding. So I just added a relay to quickly solve the problem. "My New P-Mosfet version eliminates the need for a relay".

Click here for both "Schematics"

Click here for the two "PCB's"

Click here for Revised P-mos PCB Artwork."

Revised Picture of the New P-Mosfet Version.
By Using Two Seperate Power Leads, It Prevents Errors in the trigger points, due to voltage drops in the wires.

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