Low voltage cutoff for automotive accessory use?

Thread Starter


Joined May 27, 2018

I've been working on a new project, and (as usual) am hitting a wall with the electronics portion. I've run into multiple applications where having a low voltage cutoff would be beneficial, and would prefer to build my own to buying something off the shelf. This is more of an educational/get to know it driven process than one of cost savings, availability, etc.

I'm a mechanical guy professionally, but I work in a high voltage lab, so I do have a basic understanding of electricity and what various components do. I can solder, I can use meters, etc. I've even got an old Tek scope laying around that I got for a song. That said, I have little to no formal electronics or circuits education, so anything beyond the basics and I tend to get lost pretty quickly. For instance, I know what a transistor and a diode are and what they do, even how they do it, but the second "PNP," "zener," etc. are thrown out, I'm completely and unabashedly lost. Most of what I know has been learned by blowing up components and melting down arduinos in my free time.

Now, I could easily ask a coworker (we've got plenty of graduate-level electrical guys around) to help with this little project, but I'm hoping the internet can help dumb it down for me a bit more than they tend to, and help teach me what I'm doing and why, rather than just giving me a "this will work for you" schematic.

So, specifics of the project:

Essentially vehicle battery protection to save from a dead battery if aftermarket accessories are accidentally left on while the engine is turned off. I do a lot of traveling well off the beaten path (i.e. no cell reception, and nobody's going to casually drive by should I become stranded) and a fair number of electricity-gobbling gizmos to support that. Lights, refrigerator, air compressor, winch, yada yada. I'm already redundantly prepared for dead batteries (dual battery setup in my truck, portable "jumper" pack, etc), but again - I want to learn and create at the same time. I also want to understand enough to be able to tune the circuit to various applications.

I'm after a circuit that can be powered on a 12v system, so I'm assuming voltage regulators and voltage dividers are going to be in the picture. I would like to be able to adjust my "set point" for the voltage cutoff; ideally around 12.3-12.4v cutoff, with hysteresis to prevent restoring a circuit until it reaches ~13.5v. I'm absolutely OK with swapping resistors to perform these "adjustments," so long as I understand what to change and how to calculate my value(s) for a given setpoint. There's no need for an adjustable pot or microcontroller logic here, because once it's set, it will probably never again be changed. I'll be switching some relatively high-load items, but all through relays, so power handling doesn't need to be fantastic.

The first iteration of this is to go into a pelican case where I'm mounting two air compressors (40A each). All of the adjustability is just so I can tailor it to other applications; tent heater, work lights, etc. My current off the shelf battery isolator will break the circuit between batteries and keep me on the road, but I'm hoping to add some tailored priority to which items cut out first - luxury items like reading lights go before food storage, as an example.

Can anyone point me toward a good starting point? Or if somebody's really bored, walk me through this step-by-step?

Thank you!

Also, not sure if this is better suited to the automotive forum, so if so please let me know.


Joined Mar 14, 2008
Here's the LTspice simulation of a circuit using a TL431 reference IC as a voltage trigger to turn off the relay at the desired voltage.
The TL431 switches on when its Ref input voltage is above 2.5V and off when Ref is below 2.5V.
When the TL431 is on, it turns on transistor Q1, which energizes the relay.
When the voltage falls below the TL431's trigger point it turns off Q1 and thus denergizes the relay.

The Vb voltage where V(Ref) equals 2.5V is determined by the voltage divider consisting of resistors R2 and R3.
For the values shown, that occurs at a nominal 12.40V (yellow trace) as the voltage is dropping, turning off the relay.
This value can be varied by changing R2 or R3.

It turns the relay back on at a voltage of ≈13.56V.
This value can be changed by using a different value for R4, which provides positive hysteresis feedback.

There is interaction between setting R2 and R3 for the turn-off point, and the value for R4 for it to turn back on, so if you change one, you may have to change the other also.

The relay can be an automotive type 12V SPST relay.

An questions about the circuit operation?

Last edited:

Thread Starter


Joined May 27, 2018
Wow, thanks for the prompt reply!

I'll be honest and admit I have nearly no idea what's going on there, but I'm figuring it out bit by bit through datasheets and head scratching. Give me a couple of days to figure out what specific questions I need to ask and I'm sure to be back. The shunt regulator currently has me more or less baffled, but every few minutes I have another "ahhhh! I GET IT!" moment before finding the next place to stumble. For now, you've given me a fantastic place to start discovering exactly how much I don't know, so thanks again! I'll return in a few days with more educated questions than I have right now.