When is a 12V circuit not an automotive circuit?

Thread Starter


Joined Nov 20, 2011
I am not an electronics engineer/expert. I have learned from this forum, but I have only recently become a member. Understanding the moderators objections to discussing automotive circuits, I would like some clarification of the policy.

I am not interested in directly modifying automotive circuitry, as it comes from the factory, but most of my current projects involve van conversions. That is, interior RV electrical systems that are separated from the automotive chassis system typically by an industrial separator relay. These RV circuits are generally powered from auxiliary SLA batteries charged from solar panels, shore AC chargers, and alternators (through said separator relays). Typical project circuits include audio amplifiers and filters, low-voltage-disconnects, charge management systems, data loggers, CAN bus communications, video, and microprocessors, all in a 12V mobile context. None of these circuits cross the separator relay threshold or modify factory engine, dash, or chassis electrical circuits, but they do monitor things like battery voltage, CAN bus data, whether the ignition is on or off, etc.

Currently, I am working on a MPPT solar panel charging circuit that is controlled by an MCU which also includes data from a sensor that monitors chassis battery voltage.

If I cannot discuss these circuits in this forum, I will respectfully move on. Please advise.


Joined Jun 7, 2009
the circuits you describe can be wholly developed independant of an 'automotive environment' and as such should be addressed as such. When it comes to tying them into your automobile is when support may fade.


Joined Jul 17, 2007
The problems arise when people want to make modifications to existing automotive systems. Discussion of repairing a malfunctioning system back to the original manufacturers' specifications is OK, but where people quickly run into trouble is when they attempt to make modifications to the original systems - and I don't mean just on the board.

Automotive environments are among the most brutal on the planet, with temperatures ranging from well below freezing to well above boiling, shock, vibration, corrosion, immersion/contamination with various fluids, and very large transients in the electrical system; the voltage might range from ~10v during starting, to transients exceeding 60v when "load dumps" occur, such as when headlamps or power accessories are turned off.

Almost anything that one would change from the original configuration could affect safety (which is an overriding concern on this forum), along with affecting emissions controls, safety lighting, etc. etc. The rules started to become rather convoluted, so rather than having what might have been a confusing set of rules, a generic prohibition against "automotive modification" topics was adopted.

As long as you're not wanting to modify/change any of the factory-installed features of a vehicle, I don't see a problem with it.


Joined Oct 2, 2009
How about just not using the words, automotive, automobile, auto, car, truck, caravan, etc. anywhere in your discussion? Just say you are powering the system off a 12V 75AH SLA battery or something like that.


Joined Jul 17, 2007
How about just not using the words, automotive, automobile, auto, car, truck, caravan, etc. anywhere in your discussion? Just say you are powering the system off a 12V 75AH SLA battery or something like that.
This is not acceptable, as by omitting the intended use of the circuit can easily result in an unsafe situation.

Keep in mind that I'm not a member of the Admin, Moderators nor owners of AAC - I just have a lot of posts, and for some reason they let me continue.

One of the reasons we no longer discuss automotive modification topics is the difficulty in designing circuits that will function reliably in such a hostile environment. It's really brutal. Just because something works in a simulation or works on a bench is NO guarantee that it will work reliably in such a hostile environment. That requires a considerable amount of testing under a very wide range of conditions; and unless you happen to work at an automotive electronics engineering facility, you most likely do not have access to such resources.

Safety is the primary overriding concern on these Forums. It is the members' responsibility to not introduce into discussions things that may cause personal injury or property damage, as per the ToS. Since the forum has only one automotive engineer that I know of (and they don't post very frequently) we simply don't have the resources to design, build and test any such mods that a member might want to try, and we certainly don't have the resources to watch over their shoulder as they build, test, and install it. Trying to advise a beginning-to-intermediate hobbyist how to build a circuit for such a use by people who are not specifically formally trained for such, is more or less like the blind leading the blind. Such mods are just not for the casual hobbyists.

Even professional automotive engineers with years of experience make mistakes that cost people's lives and their companies untold millions every day. Any time you hear of a vehicle recall being issued, it's usually due to an oversight by engineering.


Joined Mar 24, 2008
I don't know myself, I take every case on a case by case basis. I am not a gear head in the slightest, I listen to the advice of the other members.

The best advice I can give with this situation is to flag your own post, it draws immediate and direct attention. We (the moderators) have to walk a line ourselves, it isn't always obvious what the right course is.

I'll flag this thread to draw some other opinions. My personal opinion is it is probably OK


Joined Feb 19, 2009
Sorry, I respect your advice Sgt. I retract what I said. So when is a 12V circuit not an automotive circuit?
When you don't need to use automotive rated capacitors (this instance does), vibration dampers (this instance does), and other little details that rear their ugly heads, such as wires that rub on metal over a long period.

Then details such as the above can be pointed out and they aren't an issue, if they aren't pointed out, it could create a major issue. Motorcycles are the absolute worse environment, from wet and cold to extremely hot and humid in a short time. Well, maybe saltwater boats take the "Worse Environment" title, but all of them have peculiarities.

Case by case basis seems to be the best way to go. I, personally, do not want to be responsible for an RV burning down when we could have prevented it. At the same time, I also don't want to be responsible for somebody innocent getting into an accident because a modified vehicles' charging system or exterior lights failed.


Joined Jul 17, 2007
Sorry, I respect your advice Sgt. I retract what I said. So when is a 12V circuit not an automotive circuit?
Many, many applications can operate on a single or dual 12v or 15v supply. Just four applications would be:
1) Operational amplifiers used in analog circuits.
2) 4000-series CMOS logic (they have an operating range of ~3v to ~16v; some greater).
3) Timer/LED applications, such as 555/556 and various colors of LEDs.
4) Operation and control of small motors, either DC, BLDC, or stepper motors.

There are myriad other applications; this is by no means an exhaustive list. If such applications are to be just used on a bench or operated where safety is not an issue, circuit construction can even be quite haphazard and still function.

NO circuit presented on this website should be used in an application where the failure of such a circuit could endanger the safety of an individual or property.
Last edited:


Joined Jul 17, 2007
I would also say. If you are in doubt. Do not link your problem to any automative related topic.
This is again very poor advice, as omitting the intended application removes the safety aspect of the forum.

A circuit designed to flash some LEDs on something like a holiday tree or a model train set, is VERY different from designing something like a turn signal on a motorcycle. If a holiday tree or model train doesn't light up, nobody gets hurt.

If a motorcycle turn signal doesn't work, the rider(s) may very well get badly injured or killed.

It would be nice to be able to help people with such projects, if only we had the training, facilities, and the ability to directly supervise the construction/testing/installation of said projects. We don't have any of those abilities/capabilities.

If we are to be responsible and safety-conscious members adhering to the ToS, as we are obligated to do simply by posting here, we have to admit that we cannot safely allow such projects to be discussed here; and we should try to discourage people from attempting such modifications - for their own and others' safety.

There are other concerns involving legalities of modifications to systems such as lighting and engine controls. In the USA, vehicle lighting must be in compliance with Department of Transportation regulations, and various Federal and State laws (which used to have some conflicts, but are pretty much standard nowadays) - we can't hope to suggest circuits that would be compliant with the entire North American continent, much less the entire planet.

Attempting to change engine controls is not only illegal, but in many/most cases it will lead to poor fuel economy, higher emissions, and/or engine damage due to maladies such as preignition/detonation due to fuel starvation.

So, we just can't go any of these places.

Trying to help someone effect a good repair on a faulty system is very different; you're restoring the original function of the device to manufacturers' specifications instead of making changes to it.


Joined Feb 28, 2009
Wouldn't tying anything into the CAN bus present potential problems? Even an attempt at only monitoring the bus requires wiring which, under some unforeseen circumstance, could short out the bus and cause untold problems with the vehicle. If the RV system were TOTALLY independent or isolated when in motion, it would be OK, in my opinion.


Joined Sep 21, 2010
Here in Australia The RV industry when conecting systems like secondary batterys for charging from the vehicle use a voltage sensing relay. They dont sense CAN bus systems in the vehicle, posibly for safety reasons, as the CAN bus in most vehicles are used for runing the vehicle. You certainly wouldnt want to corrupt it.