Which side of a fuse is Hot vs Cold?

Discussion in 'Power Magic Pro' started by Ferginator, Aug 2, 2017.

  1. Hillbilly

    Hillbilly Well-Known Member

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    Ill chime in here as I have done a few of these Firstly as far as I know the CORRECT way to insert an Addafuse is diagram A and only that way.
    Why because if the camera trips the fuse the other things originally on that fuse will still work.
    If you do it the other way and it has a higher rated fuse the camera could go on fire and burn your car to bits without tripping the high rated fuse.

    Remember the fuse is to protect the car against the appliance not the other way around.
    Secondly Blackvue cameras already have a fuse in the ciggy plug. Therefore I usually buy one of them and hook it to the wire on the addafuse.Most of the cameras run a 2 amp bullet fuse and stick a 2.5 amp in the top of the addafuse. If they arent available I use a 5 amp.
    Havent had any trouble in about 5 different makes with 5 different BV cameras.

    In my VW's which are very fussy with circuits i metered out the hot side in spare fuse sockets and worked out which were always on and which were ACC. A bit tricky as some ACC circuits stay live for up to 30 mins. So I stuck the probe in till I found the Live ones with ignition OFF and marked them. Then turned IGN on and found the new ACC on ones and marked them differently. Turned car off and and had a coffee for 45 mins and went back and remetered the ACC ones They were mostly dead and so turned IGN back on and livened them up. So then you use one of the always on and one of the ACC ones that turned off to power your Power magic or whatever.
    Crimp or solder all connections and NEVER EVER jam wires down the side of fuse legs.

    When I am extending a pair of wires I bare one wire and then leave about 30mm and bare the other one. Solder both and heatshrink them individually.

    Sometimes I also put a larger heatshrink over the whole join.
    There are apparently some addafuse which are wired differentlyas discussed ??? in this thread

    https://dashcamtalk.com/forum/threads/fuse-taps-using-the-hot-side-aka-wrong-side.16700/page-5

    However in all of my many years I have yet to come across one.
    addafuse correct.jpg cigarette-lighter-inline-socketImageMain-300.jpg
    To test one Put a fuse in the top slot and see if there is voltage at the end of the wire If so its in the right way and working properly If no voltage turn it around in the socket and test again as not all cars are wired exactly the same.

    EG power from top or left would probably be considered normal
     
  2. SawMaster

    SawMaster Well-Known Member

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    If you do it either way with an over-rated cam fuse the same thing can occur, so that is not in itself a valid reason to do it as you suggest and in fact could be an argument against it!

    In circuit "A" (our method) there is only one fuse between cam and power, so if it doesn't blow when needed you've introduced an overload directly to the car wiring.
    In circuit "B" (downstream of car fuse) if the cam fuse doesn't blow when needed, there is still the car fuse protecting the circuit and that fuse will blow before the wiring can be damaged even if that shuts down whatever part of the car was being powered on the circuit {which is also in itself not a valid point for wiring safety but does relate to car safety- a separate but still important matter).

    Thus in ultimate failure control terms , circuit "B" is best even though we all do it the other way round :rolleyes:

    My personal reasons for doing "A" instead of "B" are these:
    Best engineering practice in a power-feeding circuit is to have the fewest possible connections between source and load and to have that path be as short and as direct as is possible.
    Each connection will add resistance and will serve as an additional point where failure can occur. Each side of a fuse constitutes a connection. Going through an additional fuse adds length and resistance to the circuit ;)

    A car is a corrosive environment for wiring, even in the interior, so over time connections will corrode and add resistance to a circuit
    . Resistance becomes heat which further exacerbates corrosion and can begin melting things. I've seen perfectly functioning cars with melted fuseboxes from corrosion-induced resistance even when the properly rated factory fuse was intact, untouched, and blew at the correct point when tested on the bench :eek:

    It's also much easier to diagnose a problem when each load has it's own fuse on it's own discrete circuit. I've also seen fuses which didn't blow at well above their supposed rating, so if that happened with a cam fuse in circuit "B", the car fuse could blow instead which could be misleading when diagnosing the problem. That cannot possibly happen with circuit "A" :)

    tl,dr: In our low-current-load situation here with cams, either way is safe and will work fine. The way we all do it is wrong in theory but works better in practice. So tap the 'hot' side of the car fuse when you can and don't lose sleep worrying if you can't :cool:

    Phil
     
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  3. Gabacho

    Gabacho Well-Known Member

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  4. Hillbilly

    Hillbilly Well-Known Member

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    The Blackvues have 2 or 3 amp fuses in the ciggy plug that will blow long before any fuse in the fuse box. Thats why I wire them the way I do as that gives the protection the makers devised and surely that should be taken notice of. I never use the cowboy method of cutting the plug off and wiring direct as if you dont the camera can be removed and put in another car easily, like if you go on holiday and want to put it in a rental..
    Makes sense to me.
     
  5. SawMaster

    SawMaster Well-Known Member

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    I've got some fused ciggie plugs too. The cheapest ones sometimes give trouble but the better one's don't ;) And while leaving them runs counter to my "as direct as possible" principle, getting rid of them runs counter to even more important principles: Use it as it was designed to be used and if it ain't broke don't 'fix' it :D

    Phil
     
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  6. Gabacho

    Gabacho Well-Known Member

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    Good advice
     
  7. Ferginator

    Ferginator Member

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    Sorry there was a lot to read there. so you are saying go the original way, position A, not B?

    Also you are saying, while car is off and regardless of ACC or BAT, plug the dash cams fuse into the external slot of the fuse tap, leaving the original fuse slot open on the fuse tap and then plug it in. Then use a light meter, one ground and the other in the wire of the fuse tap and see if there is a voltage on it? If yes, it is done correctly, if not, it is done wrong? how do you test the original, is it just suppose to match Hot always goes on outside?

    also is this correct for hot and cold?
    edit.png
     
  8. SawMaster

    SawMaster Well-Known Member

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    The most widely used and generally recommended method is "A"- Just follow that and you'll be fine ;)

    The rest of what we're discussing is more theoretical and engineering practice oriented, you do not need to understand it unless it interests you. And if it's confusing you then save it for later- you've got a cam to install using circuit "A" :)

    Phil
     
  9. Hillbilly

    Hillbilly Well-Known Member

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    The other reason for not using method B is that if the original equipment blows its fuse the dash cam wont work
     
  10. Ferginator

    Ferginator Member

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    I understand SawMaster. I will stick to position A but I want to make sure I understand this correctly. I don't want to accidentally blow a fuse so I want to make sure I understand it correctly. So is the diagram I showed correct? Hot is the left that is the loader that receives that voltage that on the fuse tap, its the one furthest away from the wire? Also you are saying to plug the dash cams fuse into the external slot of the fuse tap, leaving the original fuse slot open on the fuse tap and then plug it in to the fuse box where the original fuse that i will be using. Then use a light meter, one ground and the other in the wire end of the fuse tap and see if there is a voltage on it? If yes, it is done correctly, if not, it is done wrong? That won't blow the fuse with lower voltage? Also can I do this with empty slots on the fuse box and just use a 10A fuse for the original and then the external have the fuse for the dash cam, which will be smaller amp? And if so, do i use a light meter and test the fuse socket to see of there is voltage with the car on and off to determine if its BAC, always get voltage with car on n off, or ACC, only when on? Or is there some settling I would have to turn on, on my mustang, to make the fuse sockets be ACC or BAC?

    Sorry I just really want to make sure i don't blow a fuse or blow my fuse box entirely. I like to do things myself when i work on my car then to take it in but i also dont want to mess up my mustang, she is my baby. so i want to take care of it. i have had it for almost a decade now. like 2 or 3 years left. This car has been in great condition and well taken care so I don't want to mess it up by being a dummy lol I want to make sure i do it right the first time. Today i'll be getting some crimps n light meter. any preferred suggestions?
     
  11. Hillbilly

    Hillbilly Well-Known Member

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    Might I respectfully suggest that with the lack of understanding of what we have told you, you get a friend who understands, to show you what we mean and get it right the first time. Its pretty easy really.

    Also empty slots if they have power to them only have a socket on ONE side so meter it out and see if its always on or just acc on If you dont know how to figure out which is which you shouldnt be playing with it. Get a knowlwdgeable friend to show you.
     
    Last edited: Aug 6, 2017
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  12. Rajagra

    Rajagra Well-Known Member

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    It's easier than it sounds - but then everything is easy when you already understand it. That's why it's sometimes difficult to explain things clearly to someone who hasn't learned it yet.

    Maybe it will help if you use the phrase "power source leg" or "supply leg" instead of "hot leg."
    Then imagine electricity flowing from the power source like water through pipes.
    Voltage is like the water pressure, and current (Amps) is like rate of water flow.

    If you take out a fuse, or it blows, it's like blocking the pipe completely. The pressure (voltage) might still be greater on one side, but there will no current flow.
    Think of a fuse as a valve that automatically closes if flow exceeds a certain value.

    The BAT connection is like a pipe connected directly to your pressurised water source. The ACC connection is a pipe connected to the pressurised water source via a valve that is turned on and off as the car goes into different conditions.

    If you can think in terms of this analogy you should be able to work out what is what by yourself.

    But if I'm just confusing you more, Hillbilly's advice to get help is good.
     
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  13. Ferginator

    Ferginator Member

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    So i have been trying to test with the new multimeter. when i saw a few youtube videos for checking the voltage they said to turn it to 200V like the one on the position shown. when i tested this on fuses while they were still in i got responses of 12V from some fuses with the car off, as expected. go though i have some where i got no response, some with responses from both ends and responses from only one end. and the sockets i couldnt get a response at all. i decided to try the car on and test the radio and got a response but only when i turned it to 20 instead of 200? i attached the black to the ground and the red to the fuse? Just want to confirm thats correct? also when i tested the radio fuse while i had it out and had the black on a ground i got no response from either let.
    20170807_211354.jpg
    unfortunately im the tech savviest person I know. all my other friends are good with pc stuff
     
  14. Rajagra

    Rajagra Well-Known Member

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    20V or 200V ranges can both be used since multimeters are so sensitive. Both are over 12V so won't go over range, and 12V will show on both.

    The 20V range is more sensitive than you need, it will pick up random fluctuations, some of the digits will keep changing. But if you look carefully they are after the decimal point, e.g. 0.014V This is just electrical noise, but it can trick you into thinking something is there. Leaving the meter on the 200V range should make it easier.

    Another problem with the sensitivity of these meters is they can't tell the difference between 12V straight from the battery - which can power things - and 12v that is leaking through components - which shouldn't be used for power.

    E. G. Imagine a bulb that has pin 1 connected to battery 12V, and pin 2 connected to a switch which then leads to battery 0V. If you measure the voltage at pin 2 while the switch is off, it will read 12v because voltage is going through the bulb's filament.

    Bulbs shouldn't be wired that way in a car, but electronic circuits might be.

    To get around this I nearly caved in and bought one of those car tester screwdrivers with a bulb inside and a fly lead with alligator clip. They are crude devices but they do this one job really well, better than a multimeter can.

    Sent from my SM-G903F using Tapatalk
     
  15. Ferginator

    Ferginator Member

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    I found something off that didn't make sense. so instead i used a light meter, and returned the multi meter i had for a better one as well. turns out the light meter was showing me everything correct. when i have the car off. some fuses light up (BAT) and some only light with the car on (ACC). But the one spot i was originally planning to use for ACC, the radio, doe not light up at all when the car is on or off even when i put in a new 10A. but year for some reason I can use my radio so idk what is up with that. could the fuse be backwards? I left it the same way i found it before messing with it.

    so things are starting to make sense now. i have the power magic pro and just getting everything connected. i just have to make sure i get the right leg in the fuse tap
     
  16. Ferginator

    Ferginator Member

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    I think I'm starting to get it now. the side thats the "hot" has nothing to do with the fuse itself but the fuse box instead. that was what i was having issues with was looking at the diagram you were showing i thought a fuse was like a battery in which one side + and the other is -. So doesn't matter which way i put either of the 2 fuses in to the fuse tap, other then making sure the external fuse is lower than the original. That hot leg is entirely determined by the fuse box itself that is preset which gives power to the fuse, hot, and which goes to the device. do I have that correct. Btw I found 2 of the slots where one was ACC and other was BAT and they were free so that way i dont mess with any of the fuses that are already in it.

    If i have this clearly, which please let me know i do, the fuse box determines which is the hot leg and they I need to find that out when putting fuse taps in. btw how would i determine that? Do I use the digital multimeter, set it to DC, and use the black, COM, to a ground and the red on each of the 2 holes for the fuse port to go into and which ever gets a reading is the hot leg? or just light meter it and which ever lights is the hot leg?
     
    Last edited: Aug 9, 2017
  17. Hillbilly

    Hillbilly Well-Known Member

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    If i have this clearly, which please let me know i do, the fuse box determines which is the hot leg and they I need to find that out when putting fuse taps in. btw how would i determine that? Do I use the digital multimeter, set it to DC, and use the black, COM, to a ground and the red on each of the 2 holes for the fuse port to go into and which ever gets a reading is the hot leg? or just light meter it and which ever lights is the hot leg?

    YES Finally you have got it.

    However you dont actually need a ADDAfuse if using spare fuse slots. I dont know if you can buy these in USA but thats what I used as they come ready made . Its very hard to solder a wire onto a fuse as they seem to have a treatment on the metal. I have never managed to do it satisfactorily so bought these
     

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  18. Ferginator

    Ferginator Member

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    ya after poking around i found which were the hot lets and which were BAT and which were ACC empty slots. so i think i have this down.


    I looked for those but O'reilly's didn't have them but since i already got these pig tail fuse taps with the power magic pro, could I just use them or will there be issues? does it still need a 10A for the original and 3A external even if the slot was empty or do i just use the 3A in the external? Or does it not matter either way?
     
  19. Ralph2

    Ralph2 Active Member

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    Good write-ups here on this subject and it would be desirable to have a thread devoted to this subject. One small point I would like to add to the discussion I encountered when looking for the hot side.. was that the size of the probe on most meters is too thick to fit easily into the slot the fuse fits in and thus makes it difficult to "probe" with certainty. I ended up using a paper clip and alligator clamp.. But, this adds several layers of creating a possible short.
     
  20. Ferginator

    Ferginator Member

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    Ya my multimeter was too thick to fit in the mini fuse ports but my light meter, just basically a long stick with a lightbulb was small enough at the end to fight in to see which was the one that was the hot side. I found with my trial and error I found that light meter is cheap easy way to to find which is hot and which is cold fuse slot. and multimeter is to test the fuses in the fuse box to see if it's getting power or not. The only one i found not lighting up was my radio fuse but oddly my radio works fine which I know im still learning but I think that my radio shouldnt be working if my radio fuse isn't getting a response.
     
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