General Fuses and Power

Discussion in 'General Discussion' started by ArcticFire-Account Closed, Monday 17th Feb, 2014.

  1. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    I posted this in the Mazda forums but nobody seems to know but figured it's not specific to my car and is a general question so wondering if anyone here knows?


    If I connect an add-a-circuit to a fuse which is in use by the car and one which is always on (i.e. the interior lights) then I get a reading from the multimeter of 12.4V from both the fuses in the add-a-circuit.

    I then connect a 15A fuse to an unused fuse slot (not to be confused with a spare fuse) such as the one next to the power locks (30A) and I'm getting a 12.4V reading, however, if I remove the fuse and connect an add-a-circuit to this same slot instead I still get a reading through the fuse in the add-a-circuit which is for the original OEM circuit but I'm not getting a voltage through the second fuse which is for the new circuit you are adding.

    I'm wondering if it's because there isn't actually a device in the car trying to draw power from it (seeing as it's an unused slot) but there is 12.4V running through it so it shouldn't matter!

    Does anyone know why this is happening?
     
    Last edited: Wednesday 19th Feb, 2014
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  2. RobB Expert Advisor ★ ★ ★ ★ ★

    You won't measure any current if there is nothing connected to it. But you will always read a voltage i.e 12.4v.

    The circuit has to draw amps to be able to measure them.
     
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  3. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    But the device I'm connecting it to should be drawing power. Perhaps I'll revisit it when it's not raining and I'm not balancing an umbrella over an open door lol

    To clarify though, if there's 12.4V running through it all the time then it should definitely work just like one of the 12.4V ones that is always live and is actually in use by a part of the car? (i.e. the interior lights)
     
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  4. RobB Expert Advisor ★ ★ ★ ★ ★

    Providing its live all time it should be fine.
    What are you connecting to it?
    Could be the load is too small to be picked by your meter.
    You are measuring the current correctly?
    i.e in series to the circuit. As opposed to in parallel for measuring voltage.
     
  5. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    The device and the way I'm measuring must be fine because it I use the live circuit on the interior lights it all comes back fine.

    I initially used the power door locks fuse which is in use because the only other always live fuse in stuck between two add-a-circuits so I've no space to slot a third one in. But the door locks fuse is 30A and the add-a-circuits are only rated to 20A so what happened was my doors wouldn't lock either manually or with the key fob! lol No fuses blew which is kind of strange. But I stuck the OEM 30A fuse back in and everything was back to normal.

    I'll perhaps revisit this 15A unused fuse slot but another idea I have is to tap into the interior lights add-a-circuit which is currently being used for my hardwired dash cam. Basically, I'll crimp the two wires so they both run off the same add-a-circuit and hope things don't go pop! I just hope it's not too much current going through the add-a-circuit cable.

    What I'm connecting to, I'll PM - I'm sure you'll understand.

    - - - Updated - - -

    Your mailbox is full Rob
     
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  6. i-DSI Expert Advisor ★ ★ ★ ★ ★

    Belgium Aalst
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    Hi Arcticfire,
    Tension is not the same as current. There's not something like a 12,4 V current. You have a tension 'over' something (e.g. a consumer like a bulb, speaker or a switch). All these are in some way resistors.
    In general you can say you
    - will always measure the tension from the power supply over any resistor in the connected circuit.
    - will have a current flowing (Amperes) through the circuit if the switches are closed and the resistors in the circuit have a normal value (no condensator in a DC ciruit!).
     
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  7. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    Sorry, when I say current I really mean voltage, just my bad terminology. I'm not showing any voltage through the fuse in the add-a-circuit for the new device being installed, yet there is a voltage in the fuse which is to act in place of the original OEM fuse.
     
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  8. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    A quick update on this, the device I've got installed is using the cigarette fuse slot via an add-a-circuit and the device works fine when the engine is turned on, however, the cigarette lighter doesn't work!

    I tried using an always live fuse being the door locking system and I couldn't lock the car! lol Although the doors are a 30A fuse and the add-a-circuit is only rated for 20A but that doesn't explain the cigarette lighter problem.

    The add-a-circuit tests out to be ok so I'm confused as to why it's no allowing the OEM circuit to work yet allows the new device to work? Going to do some more testing but any thoughts?
     
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  9. SpeedyGee Administrator Staff Team

    England Speedy Birmingham
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    How does it connect to the cig lighter circuit (soldered, crimped etc) ?

    Is it wired in parallel to the cig lighter ?
     
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  10. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    It's done through the add-a-circuit which I'm guessing must be in parallel. The add-a-circuit has a blade fuse connector for going into the fuse slot and the other end is a crimped section for where the actual device connects to the add-a-circuit. The crimp must be good though as the device works.
     
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  11. ArcticFire-Account Closed Banned Getting Started

    Scotland Graham Scotland
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    Finally got to bottom of all this, turns out the add-a-circuit for the second device was faulty and the circuit running through the OEM section of it wouldn't work. Hooked up a new add-a-circuit and it's all good.

    Regarding the unused fuse slot, did some research and as previously suspected because there's no OEM device using that particular fuse then there's going to be no current and therefore nothing for me to tap into. It can still show voltage though - hence the confusion I initially had.

    Well, happier now anyway as I ended up fitting a genuine Power Magic Pro as the unbranded Chinese version I was doing a trial on had some intermittent issues. This allowed me to route the wires much better plus my second device (it draws less than 1A) in addition to the camera runs through the Power Magic Pro on the 'battery' circuit so it means I've got battery protection running on both devices now. Happy days :Smile:
     
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