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Thread: Wiring from battery and switch questions

  1. #11
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    My background is RF and not AF installations. Power is always 2 wire. 1 hot, 1 ground from the battery. Going to the frame for power ground is asking for trouble. Might get away with it in audio installations, but in high power moble transmitters its going to get you.

    With that I guess Ill go. 2 fuses on all wires comming directly from the battery. If you dont run both wires back then your covered. If you do your covered.

  2. #12
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    Ohgary, calm down. You don't need to be so obnoxious.

    What happens if you short the positive cable anyplace before the fuse? You get a melt down
    Which is why I gave 2 propositions: Either put it close to the battery (thus making it difficult to short) or make sure that your cable is securely attached and won't be shorted out and then put it by your device that you're powering.

    what happens if you short a positive line with the negitive return line
    Not if it's after the fuse. See response #1

    A single fuse inside the vehicle directly connected to the battery is asking for trouble
    Funny that. I've used that setup for 4 years now and never had an issue. Also strange how my buddy who works as an engineer at GM agreed with my installation and the way that I did it. I just put a 30A fuse right next to the battery and then ran the cable to the trunk to my amp. If there is any shorts at all, the 30A fuse will blow. I modified this setup to put a second fuse in the trunk so that I could power my inverter for my carputer as well, but really that wasn't necessary.

    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
    No, if the line is shorted BEFORE the fuse near the amp, then you have a problem with the battery and wire, but any device attached to that line will be unaffected (current takes path of least resistance). If the fuse is near the battery and the wire is shorted anywhere between the fuse and the amp, then the fuse will blow and there is no problem.

    There should be 4 fueses [sic] one on each leg of the power on each side of the fire wall
    I think your response is the best response to what you said there:

    "most people who do this are fools".

    Fusing the negative line is *pointless*. Unless you have done a lot of crappy wiring in your car then you're not going to find any incorrectly fused +ve lines. Additionally most "ground" connections are done nearby the device to the vehicle chassis, so they're not run by any rogue +ve lines anyway. For example I have a screw in my trunk that is into the chassis and that's my ground connection.

    Even if, for some unknown reason, you want to run a second wire directly to the -ve terminal of the battery, since the entire chassis of the car is ALSO THE -VE TERMINAL, the only way that you can cause a short is to have that cable itself short out another +ve cable before a fuse. If you're going to run a second -ve cable to the battery, you're most likely going to do that away from any +ve leads anyway. But this is a dead point because nobody runs a 2nd -ve line straight to the battery.

    So my opinion remains, you only need one fuse. If you want to be super careful, put 2 fuses, one near the battery and another near the device that you're powering. Note that almost all electrical wiring in the car is single fused, and

    many of the "wires" in cars are fuse wires that will break when they are shorted
    no they're not. They're 14-16 guage wires which will melt when there is a short simply due to the current that passes through them. Unfortunately these melts can occur anywhere in the line which makes them a real PITA to replace, thus why fuses are used.
    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.

  3. #13
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    Originally posted by Ohgary
    My background is RF and not AF installations. Power is always 2 wire. 1 hot, 1 ground from the battery. Going to the frame for power ground is asking for trouble. Might get away with it in audio installations, but in high power moble transmitters its going to get you.
    Why is using frame for -ve a problem?

    I look under my hood, and the -ve terminal of the battery has 1 connection: to the engine block (some I've seen run to the frame near the battery instead). There is a thick wire from the +ve terminal to the starter motor, which is essentially using the chassis for ground. Similarly everything else in the car is grounded to the chassis, nothing else runs to the -ve terminal of the battery.
    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. #14
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    I am not being obnoxious, I am answering the question.

    Now back to the issue, If you always use the chassie for ground this the entire issue of fusing the ground is eliminiated. If however you do run a wire to the battery +ve or -ve then it should be fused.

    There are a number of unfused +ve connections under the hood, Heck even the new GMC trucks are using -ve switched head lights, and the headlamp circuit have +ve active even when the headlight is off. Now ould you prefer to replace a fuse or a headlight fused link? The fuse is cheap and easy to install.

    As for making sure cables are tight to solve the problem. Fuses would be need anywhere if there were never problems. There are however going to be problems and therefore fuses.

    As for your GMC engineer freiend. He might want to check GMC's service bulletin http://service.gm.com/techlineinfo/radio.html

    Note the fueses on both +ve and -ve, also note they done use chassie for -ve. Also check out Icom install document http://www.icomamerica.com/support/d...c-706mkiig.pdf Not both + and - lines are fused.


    As for short before the fuse taking the path of least resistance, 1 your thinking only of dead shorts, not equipment malfunctions and routed shorts. With 2way gear you also have a second ground source via the antenna connections. So a +ve Sort on the -ve power lead of a radio will run through the radio to -ve (ground) of the antenna.

  5. #15
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    OK guys, enough of the flames. Can anyone answer the question?

    BTW: I got a 30A inline fuse to put near the + terminal, and my inverter has a 30A fuse built in.
    Silver 1999 Nissan Pathfinder!
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  6. #16
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    Sorry.

    Most people here seem to think that 2 fuses (at most) is fine. The fuse that's inside the amp or inverter doesn't help in this case because you're worried about shorting the wire before it gets into the amp.

    Ohgary seems to think that you need 4, but seems to have concieded that 2 can work as well, so it looks like 2 is a good bet.

    I'd just use the holder that you have near the battery terminal and if you want to, go out and get another one to put near the inverter. You can start right now by using just the one, and when you get time (if you want) you can get another.

    Hope that helps.
    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.

  7. #17
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    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.a...%5Fid=275%2D248
    What about that? And will that method of getting an AUX out on my keyless entry work with the stock system on a 1999.5 Nissan Pathfinder? Or does it work on onl a certain brand of car?
    Silver 1999 Nissan Pathfinder!
    Completion: [*********-] 90%
    Everything working! Mobile MP3s ROCK!
    LEFT TO DO: Improve power on circuit, fix slow boot time

    - 550mhz PIII on Abit BH6
    - 128MB RAM
    - 40x4 Backlit Character LCD
    - 17 key numeric keypad - Repainted buttons
    - 2.1GB laptop drive for OS
    - 4.3GB drive for MP3s

    http://www.mp3car.com/usersites/bombboyer/ Redesigned, new pics!

  8. #18
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    Also switches don't care about what type of current you're using. DC or AC will both work through any switch.
    Then why do they sell AC switches and DC switches?
    Silver 1999 Nissan Pathfinder!
    Completion: [*********-] 90%
    Everything working! Mobile MP3s ROCK!
    LEFT TO DO: Improve power on circuit, fix slow boot time

    - 550mhz PIII on Abit BH6
    - 128MB RAM
    - 40x4 Backlit Character LCD
    - 17 key numeric keypad - Repainted buttons
    - 2.1GB laptop drive for OS
    - 4.3GB drive for MP3s

    http://www.mp3car.com/usersites/bombboyer/ Redesigned, new pics!

  9. #19
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    Originally posted by bombboyer
    Then why do they sell AC switches and DC switches?
    most likely because you tend not to have 120VDC and you tend not to have 5VAC.

    Thus "DC switches" are really just switches designed for low-power, and "AC switches" are really just switches designed for high-power. All a switch does is close a contact, it's very simple. Unless you're talking about some funky superswitch, that would be my guess.
    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.

  10. #20
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    Your local RS might have a 30amp fuse holder but I doubt it. You can look at stereo stores or best buy.

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