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Thread: Power supply grounding

  1. #1
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    Power supply grounding

    Looking for some advice here please.

    My car's head unit has two large brackets holding it to the dash. Because these brackets are attached to the chassis of the head unit they are grounded.

    There's space below the head unit where I'll be attaching the power supply, screwing it directly to one of these brackets. Since the PSU chassis and the PSU negative connections are both grounded, will it try to find a path to ground through the head unit's power connection?

    I was planning on running the + and - to the PSU from the fuse block in the trunk and grounding it at the same location as the amp in the trunk.

    So in essence, the PSU will be grounded back in the trunk and also via the head unit's chassis. Will these two grounds of different lenghts be a problem? Should I try to electrically isolate the PSU's metal chassis from the headunit?

    Thanks.

  2. #2
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    Quote Originally Posted by sebberry View Post
    Looking for some advice here please.

    My car's head unit has two large brackets holding it to the dash. Because these brackets are attached to the chassis of the head unit they are grounded.

    There's space below the head unit where I'll be attaching the power supply, screwing it directly to one of these brackets. Since the PSU chassis and the PSU negative connections are both grounded, will it try to find a path to ground through the head unit's power connection?

    I was planning on running the + and - to the PSU from the fuse block in the trunk and grounding it at the same location as the amp in the trunk.

    So in essence, the PSU will be grounded back in the trunk and also via the head unit's chassis. Will these two grounds of different lenghts be a problem? Should I try to electrically isolate the PSU's metal chassis from the headunit?

    Thanks.
    Don't worry about it at all. Just ground it separately and be happy.

    Yes it will ground via both points. However electrons will always take the path of least resistance, so a dedicated wire to the battery (best) would be less resistance then through the HU chassis and into a bolt, through the car chassis and all its spot weld points and then to the battery. Grounding it back to the amp will probably be worse, so it might not use that path.
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    Usually vehicle chassis are a lower resistance than cables.

    But if in doubt, I usually suggest using both. (That should not set up a ground loop...)

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    Quote Originally Posted by OldSpark View Post
    Usually vehicle chassis are a lower resistance than cables.

    But if in doubt, I usually suggest using both. (That should not set up a ground loop...)
    It depends greatly on the chassis and especially how it was constructed. Although for ground separation, nothing beats grounding directly to a battery only. Those pesky low frequency noises get eaten there.
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    Let's get into an "absolute ground" argument! (ie, alternator hence alternator case for those too pedantic for the engine block, or battery -ve; it depends what is supplying the power - the battery or alternator and your priority, though they should only differ by fractions of a Volt anyhow!)

    The above is NOT to imply an argument - or wish for an argument - with you! It's more of ***** relating to elsewhere (& not mp3car!).

    In fact you write as I do - both my audio (a standard 4 x 45W HU) and aux battery have their grounds cabled to the battery -ve. (The aux battery mainly because I was using telco fig-8 (2-wire) cable, and the isolation relay is mounted on the side of the main battery.)


    But no argument from me - the "quietest" electrical connection is back to the battery -ve. That's usually because the battery is a big capacitor - it is the battery that filters (out) the alternator AC ripple as well as other noise.
    (The exception is where ground-loops occur due to high-current voltage drops along distribution cables, hence the need for a more local single grounding point. But that is more for big audio systems (many tens of Amps) and where audio-noise susceptible signal grounds are involved. That is generally irrelevant to PCs etc except where they too have similar analog ground issues (sensors, audio, etc).


    My original reply was a reflection of some trivial else stupid unqualified claims I have seen elsewhere.
    But these days - with lighter panels, composite materials etc - the dedicated wire being less resistance than the car chassis is becoming more and more relevant and true.
    (I know cables have a lower resistance than fiberglass boat hulls.... LOL!)

    Thanks Toaster! As usual, you post great advice.

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    Quote Originally Posted by OldSpark View Post
    Let's get into an "absolute ground" argument! (ie, alternator hence alternator case for those too pedantic for the engine block, or battery -ve; it depends what is supplying the power - the battery or alternator and your priority, though they should only differ by fractions of a Volt anyhow!)
    One of my cars has no alternator. Quietest 12v signal in a car ever. No PC in there yet.

    Quote Originally Posted by OldSpark View Post
    The above is NOT to imply an argument - or wish for an argument - with you! It's more of ***** relating to elsewhere (& not mp3car!).
    Awww, wheres the fun in that? Nanny nanny boo boo, stick your head in doo doo. (as Tosh would say).

    In fact you write as I do - both my audio (a standard 4 x 45W HU) and aux battery have their grounds cabled to the battery -ve. (The aux battery mainly because I was using telco fig-8 (2-wire) cable, and the isolation relay is mounted on the side of the main battery.)


    But no argument from me - the "quietest" electrical connection is back to the battery -ve. That's usually because the battery is a big capacitor - it is the battery that filters (out) the alternator AC ripple as well as other noise.
    (The exception is where ground-loops occur due to high-current voltage drops along distribution cables, hence the need for a more local single grounding point. But that is more for big audio systems (many tens of Amps) and where audio-noise susceptible signal grounds are involved. That is generally irrelevant to PCs etc except where they too have similar analog ground issues (sensors, audio, etc).


    My original reply was a reflection of some trivial else stupid unqualified claims I have seen elsewhere.
    But these days - with lighter panels, composite materials etc - the dedicated wire being less resistance than the car chassis is becoming more and more relevant and true.
    (I know cables have a lower resistance than fiberglass boat hulls.... LOL!)
    All very true. Only addition would be that the battery while like a capacitor is like a really crappy high ESR capacitor. So unlike a ceramic or something that takes away all the high frequencies, it will eat all the low frequencies. For car audio, that is a great thing. If you are driving through a giant EMF it won't help though.

    Thanks Toaster! As usual, you post great advice.
    Thankyou, same to you.
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    Agreed about smaller ceramics or greencaps etc. I should have added that....
    Not that batteries are necessarily high ESR compared top BIG caps (1F etc) - many batts have lower ESR, but who'd use BIG caps for noise...? Caps for ripple or dips maybe (generally by the less informed), otherwise for AGM protection fro BIG thumps (eg - amps in the kW range).

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