OT: For You Electrical Engineers - Fuses

I_Automation

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In my car I have 2 cigarette lighter outlets.
Dashboard outlet has a 25A fuse in the engine compartment.
Cargo area outlet has a 30A fuse next to it, probably 6 inches of wire between them.


Using one of those small 12V air compressors, rated for 150PSI not 250PSI, I can plug into the dashboard outlet and run on the 25A fuse until the compressor gets so hot I thing it's going to melt the aluminum housing.


However if I plug into the 30A cargo area outlet it will blow the 30A fuse within 5 seconds. Anything else I plug into the back works fine. The plug on the compressor isn't loose or damaged.



How could a compressor (BTW - looks like 18AWG wire) run on 25A but repeatedly blow 30A? Plus, I did try a 25A in the back too, blew that.



Really missing those 1969 carbureted, no electronics power houses.
 
Is there anything else connected to that 30A fuse? Maybe something else is drawing power at the same time causing the overload?
 
Is there anything else connected to that 30A fuse? Maybe something else is drawing power at the same time causing the overload?


The fuse is right next to the outlet and I see, inside the jack cover it's mounted in, the wire goes fuse to outlet, nothing else electrical around.




Check the Physical socket, it maybe damaged and has a short


The outlet looks good as new, nothing loose, bent or damaged, no signs of arcing inside, can't even push the center part over to contact the housing and short it out with a plastic screwdriver.




See if you can measure the voltage while it's stopped and then while running. Could be a longer distance from the source causing voltage drop.


I haven't checked the voltage but it's about a 12 foot run of 12AWG to the power block at the engine, the ground lead is less than a foot long and grounded with a internal star lockwasher with the paint removed, and all wire connections are clean and tight.
 
How confident are you that the 25A fuse is not, for example, a nail in a 25A fuse holder?
 
How confident are you that the 25A fuse is not, for example, a nail in a 25A fuse holder?

Because when I got the car I had to get in the fuse box and move that fuse. The fuse has 3 holes, the center feeds the outlet. The left hole is Accessory power with the key (that I think is the stupidest idea an auto engineer thought of) and the right hole is constant battery power, which is where I moved the fuse to. EDIT: This is the other stupid idea of the miniature blade fuses, not the nice ABC glass ones I still have hundreds of and could have thrown a few in the car instead of going any buying $30.00 of spare fuses.

As far as a nail maybe I'll treat the rear outlet to an upgrade and see what starts melting.
 
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One of the biggest problems with cars is that the ground (negative) is connected to the body in many places, this can cause high resistance & overheating of the cable & all sorts of faults, I do remember a friend of mine kept blowing fuses on the main beam lights, it turned out to be a bad common return, the bulb common was connected to the body & not wired back, it was assumed that the bond between the body panels was causing the problem the positive wire to the light was getting a little warm, the auto electrician just ran a wire back from both main beams to the main common terminal.
 
One of the biggest problems with cars is that the ground (negative) is connected to the body in many places, this can cause high resistance & overheating of the cable & all sorts of faults, I do remember a friend of mine kept blowing fuses on the main beam lights, it turned out to be a bad common return, the bulb common was connected to the body & not wired back, it was assumed that the bond between the body panels was causing the problem the positive wire to the light was getting a little warm, the auto electrician just ran a wire back from both main beams to the main common terminal.


YES TO THAT!


Years back I had a F***'d up car(last one I will ever own from that scam company) that had an aluminum engine block.


I remember having to buy a set of jumper cables, cutting them in half and crimping terminals on the new ends, bolting them to a good frame ground point and putting the clamps on:

Negative battery terminal (charging problem)

Alternator housing

Starter motor mount bolt (wouldn't turn the motor over without this)

Engine block as close to the ignition coil as possible (weak spark)


All because current couldn't flow through the mated parts of the aluminum engine.
 
Because when I got the car I had to get in the fuse box and move that fuse. The fuse has 3 holes, the center feeds the outlet. The left hole is Accessory power with the key (that I think is the stupidest idea an auto engineer thought of) and the right hole is constant battery power, which is where I moved the fuse to. EDIT: This is the other stupid idea of the miniature blade fuses, not the nice ABC glass ones I still have hundreds of and could have thrown a few in the car instead of going any buying $30.00 of spare fuses.

As far as a nail maybe I'll treat the rear outlet to an upgrade and see what starts melting.


Which auto fuse has three pins and two of them are outputs? :confused:
 
Which auto fuse has three pins and two of them are outputs? :confused:


It has the mini blade fuses. in the fuse box under the hood almost all fuses have 2 connections to plug the fuses in.


The fuse for the dashboard cigarette lighter however has 3 connections, and you choose to have the outlet on with the key or constant battery power by moving the fuse to the correct 2 slots. (Seems to only appear on Jeep SUV's as far as I've seen)



Similar to a jumper for SLC5/05 Flash program or PC motherboard password bypass with 3 pins and a 2 connection jumper.




EDIT: This is the layout in the fuse box. All M# fuses are the same physical size, but the area for M7 (Dashboard cig. lighter) is longer with 3 connections for the 2 connection fuse.

Capture.PNG
 
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Could it be that you are using the wrong type of fuse for the 30A slot? Should it not be a slow blow time delayed fuse?
 
It is hard to say why fuse blow. You should meter amps wit AC/DC meter from both after socket and how much current goes trought fuse.
Add some 10-20cm cables to fuse, then meter how much current actually is. Fuse can be for several devices.
 
See if you can measure the voltage while it's stopped and then while running. Could be a longer distance from the source causing voltage drop.

This may be a design flaw. 30A can use a #10 or #8 wire. With a voltage sag, as mentioned above, current rises to compensate thus blowing your fuse.
I had a similar issue with my SUV. Wires were not gaged for the actual current, so the wires acted like a choke. Popped the fusible link - which I bypassed with a fuse sized for the wire. In the end, not a useful port.
Also - (as mentioned) if the ground connection is bad, the power will constantly surge. Voltage - no voltage - as the compressor tries to run. Fuse will eventually go. The ground has to be good all the way back to the battery. Not sure of the age of you vehicle. Maybe consider a new 'good' chassis ground.
If you're up to it, replace the wires? Both ways with #8.
... you're getting into car audio power wire range....
 

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