Which side of a fuse is Hot vs Cold?

I know this is digging up an old thread but the fuse socket I've used read one pin as 12v and the other as 7? It's the only ignition live I can use as it's an Air con and my car doesn't have aircon. I don't want to use air bags or ABS or something important. I've put the fuse tap in with the Hot side on the 12v side. Thanks for any tips
 
That sounds right. Does the fuse socket that you have used already have a fuse in it?
 
That sounds right. Does the fuse socket that you have used already have a fuse in it?
Yes, it has one in, which is strange as it's not used, well I don't have aircon. Or actually I may have recirculation so it is used.

So will the pin reading the 12v be okay as the 'hot' one for the fuse tap?
 
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So will the pin reading the 12v be okay as the 'hot' one for the fuse tap?
Yes. But what I don't understand is why the other end of the fuse is 7V. If the existing fuse is sound, then both ends of it will read 12V.
 
Yes. But what I don't understand is why the other end of the fuse is 7V. If the existing fuse is sound, then both ends of it will read 12V.
So I read the voltage with fuse in situ with ignition off, both ends of the fuse read 0. Also tested the pins of socket with fuse out, both pins read 0. With fuse out I put ignition on and one socket pin read 12.3v and the other 7v?

It confuses me.
 
The live side of the fuse tap goes to the 12V end of the socket. The 7V is being 'fed back' from some other part of the car's electrics. Just what that is, who knows and don't worry about it. Just put the new fuse in the fuse tap outer and don't put one in the holder that's closest to the fuse board.
 
The live side of the fuse tap goes to the 12V end of the socket. The 7V is being 'fed back' from some other part of the car's electrics. Just what that is, who knows and don't worry about it. Just put the new fuse in the fuse tap outer and don't put one in the holder that's closest to the fuse board.
I have since realised that the fuse is used, the cars manual said air-con/recirculation, I thought it wouldn't be used as I don't have aircon but after thinking about it I do have a air recirculation button on my dash, although I never use it I have put the original fuse in the fuse tap closest to the fuse board.
 
That sounds right to me.
 
@flip9- Well explained :D

Some 'caveats' about fuse taps-

Never tap into a safety-critical fuse. Some examples would be "Airbags" (sometimes called SRS), "ECM" or "PCM" (car engine computer), "wipers", "ABS", or any external lighting including brake lights. While a properly installed fuse tap should not be a problem with these, strange things can happen in life and a fuse tap is adding a possible point of failure. There are almost always safer fuses you can tap to get the needed power requirements.

Among other fuses, don't tap low-rated fuses (say 5A or less) as doing so can possibly overload the car wiring.

Keep your wiring tidy and secured where no possible movement of people in the car or movement of any car part (like doors) might cause problems such as pulled or pinched wires. This includes not running wires across sharp edges.

If you make a wire-to-wire connection, either solder it or use a crimp-type connector, then use electrical tape or heat-shrink tubing to insulate the connection. Also, if you make this kind of connection on a multi-wire cable, stagger the connections so that if the entire cable pulls loose or the insulation fails it will still be impossible for those wires to short against each other.

Lastly, if you are not certain that you can do the job correctly, then enlist some help from someone who can or let a pro do it for you. Car audio shops are good with this kind of thing though any mechanic should be able to handle it too.

Phil
"Among other fuses, don't tap low-rated fuses (say 5A or less) as doing so can possibly overload the car wiring. " Would you consider a 7.5A fuse as too close to low rated? It seems I have no choice for my constant power tap except a current circuit using a 7.5A.
 
Probably OK with that. As I've said elsewhere, we're talking about a very small amount of power so it should be able to deal with it.

Phil
 
What I am saying below is relevant to a dashcam on its own, NOT necessarily one of these battery parking mode backup packs (B112 etc) which take far more current when charging than a dashcam alone. But the principles are the same.

The dashcam takes about 1/2 - 3/4A so pretty any fuse will do. The likelihood of overloading any of the car's wiring by adding that small amount of current regardless of original fuse size is tiny and really not worth considering.

The thing to make sure of is which end of the existing fuse is live and insert the addafuse so that the extra current is not drawn through the existing fuse as well as the existing current. The reason for this is that if the dashcam develops a fault, the overload current could possibly be very high and blow both fuses thus you will also loose the facility that was connected to the original fuse. This is in a diagram in post 3.
In practice however, this would be an extremely unlikely scenario if the fuse value of the dashcam fuse is much lower than the original fuse. If wired correctly, the size of the original fuse is generally pretty irrelevant.
If I were tapping into a circuit fused by 7.5A or less, I would certainly make sure I knew absolutely which end of the fuse socket was live and insert the addafuse accordingly. It would be less important if you are tapping into a fuse of 15A or greater.

The 'hot' end of the fuse socket can usually only be established by removing the fuse and testing the socket with a multimeter/voltage tester.
 
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This explains it perfectly if you put it in reversed and it blows it will also cut out the other things downstream.

I have used them in extra slots in my VW that only have clips on the battery (HOT) side and so dont even need a fuse in the bottom slot.

Just meter it out to see which side has got power in existing slots
 

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