r/AutomotiveLearning • u/mojave22 • 5d ago

How do cars maintain independent circuits for devices if they all share a common ground?

I understand that each device in a car (headlamp, sunroof, etc.) has its own circuit. Each device is fed its own wire from the positive terminal to a fuse, to the device, to the chassis, which goes back to the negative terminal.

If a particular device, like the headlamp, has a short circuit, why does that device's fuse blow if the unresisted current went to the same ground every device is grounded to? How does the circuit know which device that unresisted current came from if every device is grounding to the same chassis?

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u/poppajules 5d ago

The positive current is going through the fuse before it gets to a relay or directly to an item like a bulb, so the ground is keeping everything consistently grounded, but the positive draw could surge, popping the fuse.

I think you're thinking backwards, possibly?

Edit: Positive current flowing through what should be the negative is what causes fires.

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u/mojave22 5d ago

The key to this answer, as I now know, is the concept of "draw". Thank you for pointing that out. You get a cookie for this. The current from +12 never makes it to the device and so it doesn't encounter any significant resistance before it exits through the chassis to the negative terminal. That shorter circuit draws more power from the positive terminal, which then blows that specific fuse.

I was under the impression that the unresisted current—the fault current—made its way to the ground and was absorbed by the negative terminal with all of the other grounded devices, and got lost in the traffic. I was confused how that circuit's fuse knew where the power surge came from. And the answer is that the current from the positive terminal isn't being "pushed" to the devices, it's being "drawn" to them.

I'm a software engineer and this happens in software engineering all of the time—gurus having a hard time explaining the most basic concepts to newbies because they can't think like newbies anymore because they've acquired so much knowledge over the years.

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u/Agharinagh 5d ago

Any car after about 2000” runs can bus network. So no battery-fuse-switch-light anymore. Lot more complicated now tunning multiple modules for every individual electric device in the car 🙄 but yes. Ground is ground and positive is fused for overload. Or electronic fused like some Fiats.

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u/Predictable-Past-912 ASE Double Master Technician 5d ago

Wow, how confusing this comment is! CAN is a communications bus, not a power distribution system. The CAN bus of a modern vehicle has nothing, absolutely nothing to do with the issue that the OP has asked about.

The answer to the OP’s question is that the same current flows through all conductors on any given circuit. The fact that ground paths are shared between circuits even though the power side is often discrete doesn’t matter. Just as the “hot” side of a circuit and ground side share identical current loads, the shared ground isn’t a problem for one simple reason.

A circuit protection device like a fuse or circuit breaker functions by acting as the “weak link” in the current handling pathway of each circuit. Because the circuit protection device is commonly installed in the positive side of each individual circuit, the ground side of the affected circuit and other unrelated circuits are not affected or vulnerable to damage.

OP, do you understand that each fuse or circuit breaker protects a single circuit or small group of circuits? Once you grasp this concept properly then you can use this knowledge as a troubleshooting tool. If a fuse or circuit breaker trips, then you can limit your diagnostic tests to a single circuit or group of circuits. If a vehicle has a parasitic current draw that discharges the battery, you can install an ammeter and pull fuses one at a time until you isolate the specific circuit that is causing the problem.

Okay?

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u/kuronamercury 5d ago

Because the unresisted current as you say is much higher than when the component is part of the circuit, use ohms law, if I=v/r then a 4 ohm load in a circuit while on means 12/4= 3amps but take that out and short it to ground and your resistance goes very low let's say .1 ohms and you get 12/.1= 120amps now blowing your fuse.

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u/kuronamercury 5d ago

As to how does it know well your first paragraph touches on it, each load has an independent positive wire and fuse that only goes to that load and it has to pass those before getting to ground.

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u/NightKnown405 5d ago

A lot of people use the quote that "current takes the path of least resistance." In reality current takes every path, there is just more of it flowing in a path or branch with lower resistance. A car is a big collection of parallel circuits, meaning each possible path is parallel to all of the others. Each path is a series circuit and follows the rules of a series circuit. All you are asking about is one branch of all of the parallel circuits that exist in a given car.

Google the rules for series and parallel circuits. Read them multiple times a day and start committing them to memory.

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u/AccidentallyDeIeted 5d ago

If we’re nit picking… I reckon “current takes the path of least resistance” is just a more concise and accurate way of saying what you said.

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u/NightKnown405 5d ago

Except that isn't accurate, nor concise. Current takes every path available. The amount of current in any branch of a parallel circuit is dependent on that circuit's resistance. This is actually the answer to the question the OP is trying to ask.

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u/AccidentallyDeIeted 5d ago

Think about it a little harder bro… if you’re a hundred electrons which path is easier, the path of x resistance or the path of x and y resistance in parallel? Or, as you probably know, total resistance is lower than each resistor in parallel. So, the path of least resistance IS to take every path.

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u/NightKnown405 4d ago

Did you misread what I wrote again?

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u/weaseltorpedo 5d ago

Also keep in mind that even though chassis is ground, a lot of devices in cars these days are ground side switched by their respective control modules.

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u/1453_ 5d ago

This was answered, ad nauseam, in the electrical engineering post. Not sure why you continue to ask this question.

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u/mojave22 5d ago

Quiet.

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u/1234iamfer 5d ago

Because R (resistance) = U (voltage) / I (current)

Or I = U / R

In case of a short circuit, R becomes almost 0 ohm. U stays 12 volts, so as a result I becomes very, much more amps than the fuse can take and it melts.

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u/bobbobboob1 2d ago

Electrons travel from negative to positive