Where to connect the fuse and why?

Hello,

I am currently going through the conundrum of where to place a fuse in my DC battery-powered circuit to protect the circuit components and the DC battery. I’ve been googling for a definitive answer but i came across diverging opinions… Which one is correct and why?

  1. Connect the fuse to the negative terminal of the battery since it’s where the actual flow of electrons originate which is opposite to the conventional flow of current from the positive terminal.
  2. Connect the fuse to the positive terminal.
  3. Connect 2 fuses, one at the positive and one at the negative battery terminals.

Also, during my research, i came across a post that advised to connect a fuse at the positive terminal since it would protect both circuit and the battery, but if the fuse is connected to the negative battery terminal, then it only protects the battery. Is this true? It doesn’t make sense to me.

So, i can’t figure out which one is correct and why? I made a simple block diagram to illustrate my question.

So, is it position A or B? Or both? And why?

fuseschematic.png

DryRun: Hello,

I am currently going through the conundrum of where to place a fuse in my DC battery-powered circuit to protect the circuit components and the DC battery. I've been googling for an answer but i came across diverging opinions... some people are saying to connect it to the negative terminal of the battery since it's where the actual flow of electrons originate which is opposite to the conventional flow of current from the positive terminal, and other sources that i found, recommend connecting the fuse to the positive terminal. And there are some sites which also advised to connect 2 fuses, one at the positive and one at the negative. I can't figure out which one is correct and why? I made a simple block diagram to illustrate my question.

So, is it position A or B? And why?

Let's correct your assumption, first. You cannot protect electronic components with a fuse. A fuse takes heat and time to open and during that short time your electronic components are destroyed.

The fuse is there to prevent a fire if a short circuit should occur.

Some part of your circuit is considered "ground". It is a common connection for your battery and the rest to f the circuit. You would normally fuse the "non-ground" lead, or the positive lead of the battery.

Paul

Let's say you use Fuse B. If Fuse B opened then your electronics and, if connected case would/could be at V+ potential. When you or your customer or some other thing or object touches your electronic case or components a circuit could be completed and current could flow through you, your customer, fluffy or some other thing or object.

Paul_KD7HB: Let's correct your assumption, first. You cannot protect electronic components with a fuse. A fuse takes heat and time to open and during that short time your electronic components are destroyed.

The fuse is there to prevent a fire if a short circuit should occur.

Some part of your circuit is considered "ground". It is a common connection for your battery and the rest to f the circuit. You would normally fuse the "non-ground" lead, or the positive lead of the battery.

Paul

But then, without a fuse, how to protect the circuit from a short or fault in the circuit?

And why connect the fuse to the positive battery terminal? I'm trying to get a clear understanding of it, instead of just blindly following the conventions.

Idahowalker: Let's say you use Fuse B. If Fuse B opened then your electronics and, if connected case would/could be at V+ potential. When you or your customer or some other thing or object touches your electronic case or components a circuit could be completed and current could flow through you, your customer, fluffy or some other thing or object.

In that case, you're considering the conventional flow of current, but in reality, the electrons are flowing the opposite way; from ground terminal to the positive terminal. So, isn't the ground terminal the actual danger and where the fuse should be?

But then, without a fuse, how to protect the circuit from a short or fault in the circuit?

Basically you don’t need to.
Any circuit that would trigger a shutdown still takes time to do it and in that time the components gets fried.

And why connect the fuse to the positive battery terminal?

Because as Idahowalker said, you want the disconnection to occur at a point that does not leave the possibility of any other path being made to activate the circuit by bypassing the fuse. Ground is common to a lot of points where as the 5V from the battery is not, or should not be.

In that case, you’re considering the conventional flow of current, …

Just forget what the elections do, it doesn’t matter when considering a circuit you will only confuse yourself like you are doing now. What is important is the whole path not one end.

"But then, without a fuse, how to protect the circuit from a short or fault in the circuit? "

The use of printed circuit boards and proper connectors is much better insurance than a fuse. If your design has a lot of wires flopping around, then a fuse might be good insurance.

We currently have only ONE product that has a fuse, actually a self resetting circuit breaker and ithe product is part of a truck self-unloading system. None of the printed circuit board assemblies use fuses.

We used to make solar battery charge controllers and some of them were fused because of the currents involved and the units were going into motor homes where the owner was apt to be fooling with the system.

Paul

DryRun: Which one is correct and why?

Both are fine. As long as the fuse gets the job done when it is meant to burn out, then that's fine for your relatively low voltage DC case.

When it comes to AC mains ------ then fuse placement (location) will be more important ----- actually will be important.

Southpark: When it comes to AC mains ------ then fuse placement (location) will be more important ----- actually will be important.

No it's not... Fuse placement has bugger all to do with AC or DC.

You are confusing AC and an AC mains electricity ;) [edit]I'm blind, he did write mains AC :confused: [/edit]

And there the fact things are referenced to each other makes the place of the fuse more important. But that also applies for DC systems. For example, it would be stupid to fuse V- (instead of V+) in a (modern) car.

Also not, fuse placement is a two part problem. It's not only about in which line to place the fuse. But also about where in the line to place the fuse. In this case, place it near the battery. That way there is also protection (for fire that is) if something happens with the wire.

DryRun: 1. Connect the fuse to the negative terminal of the battery since it's where the actual flow of electrons originate which is opposite to the conventional flow of current from the positive terminal.

Nonsense. The electrons - or conventional current, or whatever - do not "originate" anywhere.

Electricity flows in a circuit when a circuit is made. The battery - or generator - produces a potential difference which is to say one terminal develops a potential - called a Voltage - relative to the other.

But if there is no other connection to anything external, then there is no absolute potential, nothing enters or leaves. All the electrons - as with the protons in the nuclei - are already present in the circuit.

PCB traces can act as fuses if thin enough, FR4 material is flame-retardent (FR stands for flame retardent).

Of course a burnt out PCB trace is not as easily replaced as a purpose-built fuse, but a PCB based circuit can be regarded as fused.

Fuses are normally to protect wiring from catching fire, they do not protect electronics, since electronics fails much faster than a fuse wire. Only if the power source is powerful enough are fuses needed. A small 9V battery for instance isn't powerful enough to start a fire in normal wiring.

When the shielding and cases of some equipment are connected to ground, the most likely accidental short is of some wire to ground. You normally arrange that the minimum amount of circuitry is live once the fuse has blown, so the +ve terminal of the battery only if the fuse is immediately after the +ve terminal.

If the fuse was on the -ve terminal then both battery terminals would be live once the fuse had blown. Here live means at a different potential to ground.

septillion: You are confusing AC and an AC mains electricity ;)

No I'm not. I'm not confusing AC and AC mains electricity. I specifically wrote AC mains ----- so it is safety-related. I also purposely wrote 'relatively low DC voltage'.

@Southpark, I'm sorry! I'm blind / a moron! :o :( :confused:

Septillion. No you're not. All good mate! All good. Absolutely no problem.

And why connect the fuse to the positive battery terminal? I'm trying to get a clear understanding of it, instead of just blindly following the conventions.

.....In that case, you're considering the conventional flow of current, but in reality, the electrons are flowing the opposite way; from ground terminal to the positive terminal. So, isn't the ground terminal the actual danger and where the fuse should be?

To answer your why questions:

1) current flow or electron flow is irrelevant in this case. The goal (job) of the fuse is to turn off the "fault"current.

2) Electrically it makes no difference which position the fuse is. Once the fuse opens the circuit is open.

3) Most recommend the fuse goes in the positive lead. This is based on some "typical" use cases. For instance, if in a automotive circuit. the likely hood of the fault being a short to ground, here it is best to cut the high side.

Hope this helps.

In the terms current and basic electrical circuits.... it doesn't really matter where a fuse goes as long as the fuse and all loads are in series circuit because amps are the same at any point in a series circuit.

for example: a fuse, switch(es), inductive load(s) in any combination in series.

attached is a DC “over voltage protection” circuit that I have used in the past. It works pretty well.
if the load has its own voltage regulator make sure it can handle what ever the input voltage is and for the 1 ohm resistor, you’ll have to do some math for how many watts it needs to handle. You can substitute the Zener diode for what ever value you want also.

Capture.JPG

The position of the fuse matters because it defines the circumstances from which the fuse protects.

Specifically if you put the fuse in the "ground" line between the supply and the equipment, and a short circuit occurs between the "live" at the equipment and some other part which has a separate connection to the supply ground - as many will do - then the fuse offers no protection. :astonished:

This is most obvious in a vehicle where the battery negative is tied to the chassis.

The OP doesn’t show ground.

If one were to wire a circuit with a battery, motor, switch, and fuse. In series then, I disagree agree. It does not matter the position of the fuse.

Read Paul’s answer again.

I pretty sure I understand Paul’s reasons. However, if the configuration in his example was opposite. (Positive ground if you will) then just the opposite can be said.

The point I was attempting to convey is that a fuse is a “weak link”. No matter where the weak link is placed the chain will break.

At a basic level, my example is correct. It my break convention but it still works. It is testable and provable.

The answer to the OP’s question is that placement is relative to the circuits real life engineering.

We are all right in our statements.