Fuse blowing to auto/air adapter

frontier · 03-31-2009, 01:13 PM

Hi all. I have a 12 gauge line with a fuse about 6" from battery under hood (7.5a) that runs to the back of the car (trunk) and wires in to a relay. The relay is triggered by a fused line from a cig lighter power line in car that only comes on with ignition.

So, I have this all in my trunk.. and I'll plug in my auto/air adapter and it'll light up - when I turn on the laptop the autoair adapter makes some squeeling (ive noticed they do this, my other ones for other brands make some sort of noise always)- but after a minute the lights go out and it's dead. The 7.5 a fuse under the hood keeps blowing.

Now, I've checked my power feed and it's good. The line is good, didn't get damaged running it all the way back there. No other fuses blow, the relay still works and everything's fine if I keep changing the fuse :P

Is 7.5a not high enough, or is there a problem with the autoair adapter? It worked fine upfront charging the laptop for over an hour so i'm kinda stumped. Was so close too! dangit.

billmee · 03-31-2009, 10:01 PM

When you say it works in the front did you use the same relay and 7.5 amp fuse?

I wonder if the 12awg wire and/or relay is dropping the voltage forcing the psu to draw more current and that is why you are blowing the fuse.

example:
85 watts / 12.0v = 7.1 amps
85 watts / 11.5v = 7.7 amps

Bugbyte · 03-31-2009, 10:44 PM

I have an inverter and it squeals if the voltage is too low. 12 gauge may not be big enough for that run. Voltage drops over distance and you have a few feet in there.

You can fuse it higher and see if it works. If it does, that's the problem. I'd rerun it with 8 gauge.

frontier · 04-01-2009, 05:39 PM

When I said it works up front - the car has a 12v outlet in the arm rest. Auto air adapters work in this outlet consistently, even after the surgery in which I tapped it's power line to become the 'switched power' signal to the relay.

The squealing I've noticed even on my test bench from multiple autoair adapters - using an ac/dc converter. It's not too loud (the hissing,squealing) and wouldn't be heard from trunk.

The cable I ran though off a spool was cut with minimal slack, so I can't see how distance could be the culprit. Regardless, I'll try rerunning power with thicker cable and see how it goes.

thx

Bugbyte · 04-01-2009, 09:54 PM

Quote: Originally Posted by frontier

The cable I ran though off a spool was cut with minimal slack, so I can't see how distance could be the culprit. Regardless, I'll try rerunning power with thicker cable and see how it goes.

thx

It makes a huge difference over distance. I used an online voltage drop calculator and found that 8 feet of 12 gauge will drop the voltage about .2 volts. Not a lot, but you also may have more than 8 feet.

Is you alternator putting out a full 14.5 volts?

merlin12volt · 04-01-2009, 11:08 PM

What everyone has posted is true. Along with that, upgrading the fuse upfront should solve your issue as long as you have the relay wired up correctly.

Efficiency in a DC environment is always an enemy. Much more so then in AC because the voltage/current ratio is so much greater.

Example: Let VAC=voltage alternating current (House) VDC=Voltage Direct Current(Car) A=amps and W=wattage @100%efficiency.
115vacx2a=230w and 114vacx2a=228w vs. 12vdcx25a=300w and 10vdcx25=250w
Do you see how just a 2 volt drop in dc causes such a need for increase in amperage to keep up? The use of Ohms Law can be used to verify this.

That said your 12 gauge line should be fine to run the adapter. A 15' run of 12 gauge automotive grade wire should yield a maximum safe amperage of around 20 amps @ 100% efficiency. That would be the MAXIMUM rating. Running a 10 amp or 15amp would be a safe bet.

If your unit didn't come with a fuse built in (usually either on the unit or in the head of the 12 volt adapter) look on your auto/air adapter for the power consumption rating. Whatever that is (usually written in wattage) take that and divide it by 12.5volts. That is what the minimum basic voltage typically is for an uncranked battery. The result of that calculation will be the fuse rating you want to use +10%. The 10% will account for peak on demands it may see(ie voltage spike).

Hope this helps.

I have been doing custom car audio since 1996 and haven't fried anything yet.

Good luck and post some news back.

billmee · 04-03-2009, 06:44 AM

Quote: Originally Posted by merlin12volt

What everyone has posted is true. Along with that, upgrading the fuse upfront should solve your issue as long as you have the relay wired up correctly.

Efficiency in a DC environment is always an enemy. Much more so then in AC because the voltage/current ratio is so much greater.

Example: Let VAC=voltage alternating current (House) VDC=Voltage Direct Current(Car) A=amps and W=wattage @100%efficiency.
115vacx2a=230w and 114vacx2a=228w vs. 12vdcx25a=230w and 10vdcx20=200w
Do you see how just a 2 volt drop in dc causes such a need for increase in amperage to keep up? The use of Ohms Law can be used to verify this.

That said your 12 gauge line should be fine to run the adapter. A 15' run of 12 gauge automotive grade wire should yield a maximum safe amperage of around 20 amps @ 100% efficiency. That would be the MAXIMUM rating. Running a 10 amp or 15amp would be a safe bet.

If your unit didn't come with a fuse built in (usually either on the unit or in the head of the 12 volt adapter) look on your auto/air adapter for the power consumption rating. Whatever that is (usually written in wattage) take that and divide it by 12.5volts. That is what the minimum basic voltage typically is for an uncranked battery. The result of that calculation will be the fuse rating you want to use +10%. The 10% will account for peak on demands it may see(ie voltage spike).

Hope this helps.

I have been doing custom car audio since 1996 and haven't fried anything yet.

Good luck and post some news back.

Nice.

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