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Thread: Wiring from battery and switch questions

  1. #1
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    Wiring from battery and switch questions

    I'm going to run power directly form my battery to my inverter, and I know I need a 30A inline fuse. Where can I get one? Does the Shack carry em?

    Also, I read somewhere about using the sequential door locks on factory keyless entry systems to power on a computer (http://members.aol.com/kustomz28/pro...ivebypass.html), and I was wondering if this would work. I have an AC pushbutton switch I'd like to use to turn my computer on and off from the front of the car.

    I also want to be able to power on the car by doing a double press on the unlock button in the keyfob. How could I wire it so either method would power on the computer? I'm asking because I'm wondering if the DC current will flow through the AC switch and ruin it. Also, can I just hook the 12V DC pulse to the two pins on my mobo, or am I going to need this relay: http://www.radioshack.com/product.as...5Fid=275%2D248

    Also, will the above progressive door lock method work on any factory system? (specifically my 1999.5 Pathfinder)?

    Thanks.
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  2. #2
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    To properly fuse a line you need four fues. A fuse on the + side near the battery, A fuse on the - side near the battery. A fuse on the + side after you pass through the firewall, a Fuse on the - side after you pass through the firewall.

    This way, any short is fused.

  3. #3
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    Uhh, you only need a fuse on the + line, and you certainly don't need 4. Most people just run a cable from the battery to wherever you want the power, put on an inline fuse there (you can get inline fuse holders for like $8 from most automotive parts stores, Canadian Tire I know carries them), or you can get funky looking fuse holders from most car-audio stores for a little more expensive. If you want to be paranoid, then put an in-line fuse holder right after you take the power off the battery, and then you don't need a fuse holder anywhere else. This is usually very unnecessary because as long as you attach the cable securely where it can't get frayed or cut then you don't need to worry about the cable shorting out before you get to the fuse.

    Putting fuses on the negative line is pointless, since the car uses negative earth ground. Since the ground is actually the negative connection, you cannot short anything from the negative terminal to anything except the positive terminal, and since you'll have a fuse on the + line already you don't need any more fuses.

    Putting more than one fuse on the positive line is pointless as well.


    Additionally you will need to use the relay. The computer electrical system ground is isolated from the car's ground, and the switches for power don't run off of +12V anyway (and so attaching the +12V to the computer might just fry it). You will also notice that you can't re-use pins to connect to other switches (i.e. you can't use the '-' pin on the reset switch and the '+' pin on the power switch to turn the computer on since it's not really + or -, it's just completing a connection).

    Also switches don't care about what type of current you're using. DC or AC will both work through any switch.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  4. #4
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    Most people who do this are fools. What happens if you short the positive cable anyplace before the fuse? You get a melt down

    In your config, what happens if you short a positive line with the negitive return line, You get a melt down. There should be 4 fueses
    one on each leg of the power on each side of the fire wall. Otherwise your asking for a short and/or fire.

    A single fuse inside the vehicle directly connected to the battery is asking for trouble.

    Originally posted by Telek
    Uhh, you only need a fuse on the + line, and you certainly don't need 4. Most people just run a cable from the battery to wherever you want the power, put on an inline fuse there

  5. #5
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    It's best to have a couple of fuses, but if you put your fuse right next to the battery there is little chance of a short before the fuse. Since current flows in a full circle around the circuit, one fuse should protect the inverter, right? I have never quite understood the need for so many fuses. I could be opening up a can of worms here, but can someone explain the need for 4 fuses?!?!
    Best regards,
    Matthew

  6. #6
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    If you only fuse the positive line on the inside of the vehicle then you have a direct short to ground anyplace between the battery and the inside fuse.

    If you only have fuse on at the battery on the positive side you can short the negative side to some other incorrectly fused positive line and get a dead short to the battery.

    The reason for the fuse near the device is to prevent heavy currents during a short from going through the device. If your amp is direct attach to a battery with a fuse at the amp then a short of the ground to a + line will cause a high current flow through the + of the amp through the amp and down the ground line.

    In a perfect world, you can get by with 2 fuses at the batter on both + and - lines. With the realworld and comming through a firewall, 4 fuses is the safest best.

  7. #7
    Self proclaimed spoon feeder TruckinMP3's Avatar
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    I read a different post awhile back indiacting a specific distance from the Battery (12 to 18 inches I think) for the positive line fuse.

    If 4 fuses were needed, all autos would have 4 fuse blocks from the factory. Many have 2 some have 1. I think the big auto makers are right on this one.


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  8. #8
    Self proclaimed spoon feeder TruckinMP3's Avatar
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    I read a different post awhile back indiacting a specific distance from the Battery 12 to 18 inches I think for the positive line fuse.

    If 4 fuses were needed, all autos would have 4 fuse blocks from the factory. Many have 2 some have 1. I think the big auto makers are right on this one.


    TruckinMP3
    Sorry about the dup post, must have double clicked :-(
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  9. #9
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    I agree, for factory cables that have a defined purpose, well know cable routes and million of dollars in planning to eliminate the risk then 1 fuse block inside is fine. However for everyone else 4 fuses are neeed. also keep in mind, many of the "wires" in cars are fuse wires that will break when they are shorted.

  10. #10
    FLAC mp3z24's Avatar
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    well... ask any car stereo installer about fusing and the answer you will get is...

    2 fuses

    one RIGHT after the positive battery post. that one protects agains shorting due to a wire becoming exposed due to cutting, cracking, rubbing... etc

    the second fuse goes near the device recieving the power. that is a general safety fuse for the equipment and is generally lower rating than the fuse at the battery.

    the fuse at the battery is only for preventing a major electrical disaster and a subsequent fire.

    NO ONE has ever told me or anyone to fuse the negative/ground wire. the ground wire is generally run from the equipment to the nearest solid mounting point on the frame or interior body. somewhere that you can get a nice metal to metal contact for the ground wire.... it is also usually as short as possible.

    if you run a wire back to the battery for your ground, why would you want to put something in place that will prevent your equipment from being properly grounded...

    ~mike

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