wiring 12V socket/USB charging port
- Thread starter freedomrider
- Start date
DaveInDenver
Middle Income Semi-Redneck
Cyberman makes a valid point. I couldn't tell you what's inside the USB ports, but I assume it's supposed to be 12V on the tabs so there's a 5V regulator of some sort. If that is case I can see the logic of putting a fuse between the 12V side and the USB sockets because that would be device protection and not a wiring fuse. It's possible there is a resettable thermal fuse in there, hard to say (but I doubt it if I was betting man).
Why don't you use an inline miniature fuse to jumper the 12V side to the right? Even just a cheap ATC from Autozone would be fine. A 7.5A is probably as good as any. I would have selected 5A not knowing what's inside.
http://www.optifuse.com/inline-fuseholders-lps-01.php
dwh
Tail-End Charlie
I have no disagreement with what you wrote but will note that it's the quality of the internal circuitry in my cheapo USB port that I'm fretting over.
I wouldn't sweat it. Or I should say...I *don't* sweat it.
First of all, USB is 5v.
3a * 5v = 15w.
15 watts.
Not much.
15w / 12v = 1.25a.
What are you going to do?
Install a 2a fuse to protect the 12v side of the USB doohickey?
Anything bigger isn't going to protect it from itself anyway.
Nah.
Just protect the supply wire and the 12v receptacle with an upstream fuse, and let the USB take care of itself.
Also, USB spec includes communication, so devices, such as phones, are supposed to find out how much they can draw from the port, and limit their draw to that. That doesn't always work. A "charging only" USB cable doesn't have the pins for the comms to work. Doesn't matter, the device is just going to draw as much as it can, which is only going to be as much as the USB port allows to pass through.
I use a 12v-USB dual port adapter - 8 bucks at Autozone. Rated to put out 1500ma (1.5a @ 5v or 7.5 watts or just over half an amp at 12v). Made in China. It plugs into a cigarette lighter receptacle, but unlike most devices that plug into a lighter receptacle, the tip doesn't screw off to reveal a replaceable fuse. Why would it? Even a 1a fuse would be too big.
Most (if not all) of the USB adapters don't have replaceable fuses either. Here's some from Radio Shack:
http://www.radioshack.com/dc-12v-car-adapters#prefn1=productType&prefv1=6V or less
I've charged all sort of devices with the one I have. Phones, Verizon MiFi 4g, my buddy's tablet computers...whatever. No problems at all, other than the fact that at 1500ma it is slower at charging things.
The receptacle I plug it into is fed by a #12 wire, protected by a 20a Bussman ShortStop auto-resetting breaker.
I've also got a doohickey like this:
Again, no replaceable fuse. Makes sense. It's rated for 1000ma at 5v. So it'd need a 1a fuse to protect the 5v output. Also need a .5a fuse to protect the 12v input. Also need a .05a fuse to protect the 120v input.
Good luck finding a fuse rated at 1/20th of an amp...
The 12v in 19v out power adapter I use for charging my netbook computers does have a fuse in the plug. 10a IIRC. But I've never had to replace it. None of my 120v power bricks has a replaceable fuse.
Most lighter plug stuff is rated 10a or less. I rigged a circuit rated for 20a, so I could run two 10a devices at the same time if I wanted (or a 10 and two fives, or 4 fives, or...whatever).
Just use a #12 wire to feed that unit, with a 20a or smaller fuse to protect the wire and the 12v receptacle, and let the USB take are of itself, and let whatever you plug into the 12v socket take care of itself.
No worries.
freedomrider
Ordinary average guy
I had literally just started doing the above calculations when I saw that you posted them for me. I typically over think these things but that's part of the fun since the exercise either reinforces things I kinda know or exposes me to new knowledge.
Yeah, it does seem a reasonable expectation that sophisticated electronics' charging systems self-regulate the current they receive. I'm gonna wire the circuit as you suggest and let the chips fall where they may. I'll post an update if anything smolders, burns, or blows up.
Happy holidays!
Mike
jeffjeeptj
Adventurer
There is an old adage from a mechanical person - electical things work by smoke, so wires are filled with smoke. If you let the smoke out, the electrical stuff won't work any longer.
Actually, i consider DWH's advice very worthwhile and precise.
If in doubt about the USB item, hook it up temporarily in a safe place, like on a driveway or other non-ignitable surface, preferably outside, and validate the initial condition and capacity of the device. Make sure no gas spilled at your location recently. If you hook up your highest current draw USB device and let it charge for a period of time, you can check the temp of the USB port. If something is amiss, just disconnect the supply side of the circuit, power will stop flowing, and things will cooldown. Have a dry chemical fire extinguisher ready, if you really want to be prepared.
Actually, i consider DWH's advice very worthwhile and precise.
If in doubt about the USB item, hook it up temporarily in a safe place, like on a driveway or other non-ignitable surface, preferably outside, and validate the initial condition and capacity of the device. Make sure no gas spilled at your location recently. If you hook up your highest current draw USB device and let it charge for a period of time, you can check the temp of the USB port. If something is amiss, just disconnect the supply side of the circuit, power will stop flowing, and things will cooldown. Have a dry chemical fire extinguisher ready, if you really want to be prepared.
Last edited:
v_man
Explorer
Gotta totally agree with DWH .... the fuse is there to keep the wire safe , to keep an excessive draw off an under rated wire . I could care less if my USB port or any other 12v device gets ruined...what you don't want is a wire that snakes through half your vehicle going up in smoke , quickly followed by the rest of your rig ...
A fun , and frightening , experiment is to run too many amps through too small a wire . The sheathing and wire QUICKLY self detonate ... yea try it on your garage floor ...
That being said , you are very unlikely to pull enough amps through that USB to even get close to heating up that 10 gauge wire ... *don't* sweat it
A fun , and frightening , experiment is to run too many amps through too small a wire . The sheathing and wire QUICKLY self detonate ... yea try it on your garage floor ...
That being said , you are very unlikely to pull enough amps through that USB to even get close to heating up that 10 gauge wire ... *don't* sweat it