Need to Upgrade Mobile Fusing

Discussion in 'Mobile Radio Systems' started by NO6O, Mar 31, 2011.

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  1. NO6O

    NO6O Moderator Volunteer Moderator Platinum Subscriber QRZ Page

    I agree about the PSK-31 power. However, when operating as a mobile station, I frequently have trouble with DX stations copying my signal. I sometimes raise my power, to make the contact, and it doesn't always help.

    I spoke with the N8XJK battery booster manufacturer, and was told that the booster has its own 40 amp fuse inside. Further, if I have a fuse from the booster to my radio (which I now do), I should probably not use a fuse between the battery and the booster. I'm not sure whether this is a good idea, because it's usually the best practice to have the fuses at the battery. However, if fuses ARE kept at the battery, they probably should be 40 amps, because that's what the booster can pull. This was the basis of my original question.
  2. W8JI

    W8JI Ham Member QRZ Page
    and this link files/fcs_pdfs/codesofpractice/08-05-12 fcs 1362 final.pdf

    I can't understand why people want to give advice that is potentially harmful to gear and the vehicle. If they don't understand the system, they shouldn't give any advice.

    Anyone at odds with me about negative lead fusing and negative lead termination is also at odds with the new European standard on negative leads. Europe has a directive that old installations done with the negative lead to the battery result in fire and equipment hazards. The new directive is old systems that were grounded to the battery should be changed to a chassis connection, and the negative lead fuse removed and a direct connection used from the gear to the chassis. This is for equipment with a ground path.

    Running the negative lead to the battery post for accessories or accessory devices that are grounded or have a ground path is very clearly dumb-headed engineering. As for a typical ground wire from battery to chassis acting like a fuse for amplifier or inverter loads and opening, good luck with that myth!!! Fusing current of a number 10 copper wire is 330 amperes. Most battery to chassis grounds are around number 8. I run my 3000 watt inverter through a small fuse link of number 12 wire, on the HOT lead.

    For those people interested, here are the problems with grounding to the battery post:

    1.) If the gear has a ground path from the equipment to the vehicle chassis, that ground path becomes part of the battery ground. If anything ever opens from the battery to vehicle chassis ALL of the vehicle negative lead ground currents will flow through the gear. This causes ground loops that can put noise into the equipment, or can damage the equipment or overheat wiring. The negative fuse does NOT protect the equipment hazard, unless the negative fuse is sized for the smallest conductor leaving the accessory device to the vehicle chassis. That would be key jack or microphone sized wiring in most radios, so you would need a one or two amp negative fuse.

    2.) If the battery negative should open, the equipment path becomes the full ground. Current through the device can be 100's or 1000's of amperes until the fuse blows, and the path can be through foil traces and semiconductors. A positive to chassis short can drive the negative post up above chassis even if it has a good connection.

    3.) The negative lead fuse adds voltage drop even when good, and the small but finite resistance of a good system diverts currents through the other leads from the radio that are grounded.

    By grounding to the chassis, less noise is induced in the equipment or the vehicle's system. Also any possibility of a fault powering the negative lead, or flowing through the radio's PC boards or external connections like antenna leads or speaker leads, is greatly reduced.

    So here are the rules people with common sense would follow:

    1.) If the accessory equipment is designed to be fully ground independent with no negative lead to case or external connections, it can run to the battery negative post provided that lead is fused.

    2.) If the accessory equipment has a common ground to external ports or the cabinet from the negative lead, the negative lead should be grounded to the vehicle chassis without a fuse. If it is a really heavy draw, like hundreds of amperes, it would pay to improve the negative post to chassis ground and ground up near (but not on) that ground.

    Nothing else makes sense.

    73 Tom
  3. NO6O

    NO6O Moderator Volunteer Moderator Platinum Subscriber QRZ Page

    Hi Tom,

    Thank you for posting that information, and the link to the diagrams and PDF. In the PDF, at the top of page 31, it refers to making the negative connection to the frame as close as possible to where the negative battery terminal connects to the frame. In my case, I could simply move the negative wire from the battery to the frame. However, is it best practice to keep the positive and negative wires separated, or can they still be twin lead? I didn't find anything in the PDF about that, so far.
  4. K0BG

    K0BG Ham Member QRZ Page

    Tom while I agree with your assessment, the automobile manufacturers don't. For example, here's is GM guide: Ford's, Chrysler's, and about 20 others are virtually the same, including every EU auto maker.

    I do understand the issue you present, about equipment grounds not being floated in amateur radio gear. And even the ramifications of a failure in the battery's accessory ground. I've only seen that happen twice, and both times when the starter ground failed.

    There is another issue too (I'm not sure if you're the one who brought it up), about using the chassis (seat bolt?) as a ground return. Doing so will play havoc with the vehicle's electronics. This is the main reason for the wiring guides in the first place.

    A lot of the possibilities boil down to improper maintenance, and extremely poor wiring practices. Some of the photos sent to me for inclusion in my web site's photo gallery are so poorly done, it's a wonder they haven't gone up in flames.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    This can't be emphasized enough. As a note, with modern equipment where the negative is common to its chassis, just mounting the rig to the vehicle may ensure the negative is returned to the vehicle chassis (and make any negative wiring to battery immaterial). But, when I install a rig, I don't bet on it. Just recently, I installed my TS130S into my pickup. My ground is short and to the nearest vehicle chassis location.
  6. K0BG

    K0BG Ham Member QRZ Page

    Bryan, one of the issues is voltage drop, as I mentioned before. While it might not be insignificant, there is another issue at hand.

    The various accessory grounds throughout the average, modern vehicle may indeed seem haphazard, but I can assure you they are not. Commonly supplied devices, use common grounds. This is done to reduce the possibilities of a ground loop causing problems with the myriad of on-board devices. If you impress a load through the steel body of the vehicle, it is likely to cause a differential between grounding points. You personally might not ever have a problem, but you're not exempt.

    Now that OBDII-EOBD has become the de facto standard, it is even more important that we follow the wiring recommendations of the manufactures. I won't argue about the efficacy of direct or indirect battery ground wiring. However, I will argue about using the chassis for a ground return. If you have a late model vehicle equipped with OBDII-EOBD, you might be writing error codes you don't know about until something fails under warranty.

    As I said above, to each his own. And, you take your chances.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    I wasn't looking for approval. Suffice to say that I've investigated this and my installation (for my application) is quite sound, again.

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