What circuit is best to run LEDs from a car's fuse board?

Discussion in 'Automotive Electronics' started by skykonig, Jul 18, 2016.

  1. skykonig

    Thread Starter New Member

    Jul 16, 2016
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    Hi everyone, I plan to install some LEDs in my dads car this weekend and would like some technical help :)

    Below I have 2 circuits but not sure which one is better. I plan to connect a piggyback cable to the cars fuse box with a 1amp fuse, and wire this to some LED strips. The LED strips originally came with a cigarette lighter socket with a 1amp fuse in, but I unsoldered this and resoldered it directly to the piggyback cable (with my dads help). As it already had a 1amp fuse in should I use an inline fuse as well as the 1amp fuse in the fuse box, or is it not necessary?

    Hopefully I've explained everything well enough, if not i'm only 13 so go easy on me xD

    Thank you.
    circuit1.PNG circuit2.PNG
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    No need for 2 fuses in series.. 1 is sufficient to protect the wiring.
     
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  3. dl324

    Distinguished Member

    Mar 30, 2015
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    The second with only one fuse is sufficient. In an over current situation with two fuses of the same capacity in series, one would always blow before the other; so having two wouldn't provide more protection and is an unnecessary complication.

    However, if there are other things connected after the existing fuse, it would be better to connect the LEDs before that fuse and with the original in-line fuse.
     
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  4. skykonig

    Thread Starter New Member

    Jul 16, 2016
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    Thanks a lot guys! :)
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    You're welcome.

    You didn't show how you were going to connect the LEDs, but make sure any connected in parallel have their own current limiting resistor. Putting LEDs in parallel without their own resistors can cause a cascading failure due to current hogging.

    If they're designed to operate from 12V, they already have current limiting built-in.
     
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  6. benta

    Member

    Dec 7, 2015
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    And disconnect the ground lead from the battery before starting work. This is a general comment for all car electric projects.
     
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  7. skykonig

    Thread Starter New Member

    Jul 16, 2016
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    They're LED strips and do have little resistors in the strips so all should be good :D
    oh my gosh thank you for telling me that! I had no idea :eek:
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Another option is to just remove power to the branch you're working on; e.g. remove the 1A fuse. This will avoid the inconvenience of having to reset things that lose their settings when power is removed.

    When working on autos, you're usually told to disconnect the battery when working on just about anything. If you know what you're doing, you can use your best judgement.
     
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  9. benta

    Member

    Dec 7, 2015
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    I detect sarcasm here. Not nice.

    Many people who work on cars have a tendency to disconnect the positive/live lead, which is a bad idea.

    Sorry to try helping, I'll stay away from your threads in the future.
     
  10. skykonig

    Thread Starter New Member

    Jul 16, 2016
    12
    1
    Hey benta, I didn't mean that to sound sarcastic :/ sorry if it appeared that way.

    reading it back again it does sound a bit like sarcastic :/ oops... sorry :/
     
  11. benta

    Member

    Dec 7, 2015
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    'Nuff said. I've run into that situation myself when quick posting. Apology accepted, no bad feelings from my side.

    Benta.
     
  12. MrAl

    Well-Known Member

    Jun 17, 2014
    2,440
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    Hello there,

    Well as far as disconnecting the battery though that's really for older cars with no computer module. In many cars with a computer module that checks various sensors disconnecting the battery will clear out the monitors and reset everything to "not ready". "Not ready" means not ready for inspection. That means if their car is due for inspection they wont get through until after they perform a 'drive cycle' or drive a number of miles under the right circumstances to get the monitors to flip back to "ready". That can be one big pain in the neck and take several weeks to get right. This means it is best to avoid disconnecting the battery in modern cars. It's better to pull the fuse for that one thing if possible or find some other way.

    Even garages will not disconnect the battery anymore unless it's a definite necessity because they know it resets all the inspection monitors. Even when they change the battery for a new one they know enough to keep the system 'alive' by connecting power to the OBDII connector while they swap out the new battery for the old battery.

    Some cars are easier to get back to 'ready' than others, but it's better not to fool around with it if you dont have to. So the bottom line is, if you can find any other way then dont disconnect the battery.
     
  13. benta

    Member

    Dec 7, 2015
    101
    24
    MrAl,
    I agree with you when we're talking about a pristine car, but a vehicle subjected to:
    you have absolutely no idea what cabling has been done before. It sounds like a breadboard project to me.
    Sorry, but in that case I prefer to disconnect the battery (neutral/ground!!).

    Benta.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    Any fuse on the car fuse panel is probably rated too high for your LED application. An inline fuse is a good idea and should be rated to just handle the LED load + a small safety margin. It should be as close to the fuse board as convenient so as much of the added wire is protected as possible.

    Manufacturers like Littlefuse etc publish design tables to help calculate fuse rating - if your application has a reservoir/decoupling electrolytic, you may need to select a slow blow type.
     
  15. hp1729

    Well-Known Member

    Nov 23, 2015
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    You should have two basic circuits. One straight to the battery and powered all the time, and another only powered when the ignition switch is on.
     
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