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Thread: Question about ground loop.

  1. #1
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    Question about ground loop.

    Hey forum. i am prety sure there are posts like this out there, but i dont think its bad to keep the forum active... so :

    1) if a system, lets say , 2 bats, 1 carpc, 1 amp, and one LCD, have noise problems, cause by a ground loop , then , is the ground loop located, somewhere, INSIDE that system, or can it be somewhere else in the car (say , the ignition system) , and affect the carpc system>?


    2), lets say, that you want to COMPLETLY change, everything that has to do with the system (cables, connections, etc...) in regards to the noise, then what parts would have to be changed??

  2. #2
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    actually, my system looks like that:

    Name:  system.jpg
Views: 132
Size:  59.1 KB
    info:

    the alt is prety old. even with a new voltage regulator, it still has voltage drop (not less than 13v) when it has big load on it..
    isolator is a diode type. gonna change it with relay anyway
    bat1: is connected to chasis, via 2 fat wires (that bot to the chasis)
    carpc: use the M4atx dc-dc
    the lcd is powered from the m4-atx, and has VGA signal. (15.6" )

    the signal from the carpc, to the amplifier, has to pass through an RCA ground loop isolator, or else it get much noise in the speakers (i still get noise, during pc startup, even with the isolator)
    the LCD, has noise on the image.
    both noises increase in freq, with the engine RPM...

    [Edit]: Least but not last. the LCD is a 15.6" home monitor, that i have hooked u.
    at the begining, i had a smaller monitor, with the M4-atx. no noise then
    then, i tried to install the big LCD, wich was powered by an inverter (Since ir required 220hz). that is when the noise firstly came.
    then i used this: i had an inverter, that powered both my pc (DC->AC->DC from home PSU) and the Monitor no noise that way.
    after that, i changed the LCD's power supply, with a DC converter, so now it took 12v, and 5v from the carpc PSU. still had noise with the M4-atx, but now i also have nosie with the dc-ac-dc build...
    also, in all those cases, the longer the VGA cable, the stronger the noise( no change in freq ofc).. so, i HAVE to consider, that maybe the LCD intself is the culprit...
    Last edited by settra; 11-05-2013 at 08:16 AM.

  3. #3
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    A ground loop is a loop. The loop generates the noise and anything attached to that loop can pick up that noise.

    But are you sure there are no other sources - eg, the plethora of switching circuits you have? (inverter, M4 especially; PC and its internal & external grounding, etc)

    And all the above will be worse with longer leads, poor shielding etc.


    But we have already gone thru all that....

  4. #4
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    we have gone through that millions of times... i know and i am sick about it. but i cant accept that the god damn noise is stronger than me >_>. i have also started thinking of poor grounds. that the chassis, is not a good ground at all... how would i make a perfect ground?? Bolt a wire , directly to the ALT, and pull it all the way back to the trunck??

    also, is there any change, that the alternators Rectifier, is broken, and allows AC to leak through??

  5. #5
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    Well technically, AC (ripple) does leak thru, but that's not a problem.
    However a broken rectifier creates noise.

    Broken alternator rectifiers should always be repaired.

  6. #6
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    Quote Originally Posted by OldSpark View Post
    Well technically, AC (ripple) does leak thru, but that's not a problem.
    However a broken rectifier creates noise.

    Broken alternator rectifiers should always be repaired.
    Alternators make A/C and the Rectifier makes it into DC. If any part of the rectifier goes bad and it drops out the part of the AC current it is responsible for and your straight DC will have a cyclic drop out of voltage due to the missing current. This is heard as noise in your system. Replacing it will of course bring back the straight DC power and get rid of the dropouts. Some vehicles add capacitors to try and smooth out the remaining ripples that may be in the power after the alternator but in most cars your battery is the capacitor that smooths out the remaining ripple. However the ripple due to one of the diodes being bad can not be masked with the car battery where your audio system is concerned. Not to mention you just lost anywhere from 1/4 to 1/2 the power output capacity of your alternator with a blown diode and the alternator will have to work harder to make up the difference which can make the problem more pronounced. In other words... Get it replaced...

    Ground loops in essence are due to your equipment seeing different voltage levels in different locations. This can happen due to undersized wiring or bad grounding locations. If you test your voltage at different locations in your system you are likely to see different voltages. Realize that the majority of cars use the chassis as its ground wire to save having to install nearly twice the number of wires currently required. The OEM has chosen the locations of grounds carefully in most cases due to the requirements of the power and how solid of a path back to the battery. When you look at how our cars are put together and how the panels are attached you can see where you have issues. Since most panels are tack welded and then have their seams bonded with some sort of sealing material they do not make very good ground paths. You need to find things such as the sub frame or other substantial components to connect to. One of the suggested areas to connect a ground to is the seat belt mounting point. Just be careful that any connection you use does not affect the original intent of the mount point. Otherwise if you run a cable back to the front you want to run it directly to the battery if you can. With power cables you want good quality cable that is rated for more than you will ever use. Too small of a wire can cause voltage loss issues that turn up sounding like "ground loop" issues. These grounding potential differences can be very annoying to find. You are best off talking to other people that have your vehicle that have already done the work in the past and can tell you the best ground points for your vehicle. Car audio shops are sometimes a good source as are forums for your vehicle assuming such exist.

    Also realize if you are running your ground to the chassis you are also running the power through the frame to the engine block ground strap and from the engine block to the battery through the battery to block ground cable. My experience with the ground path of all cars is limited but every vehicle I have owned has the battery grounded to the block and the block grounded to the frame. There are small ground wires going to the body from the battery but I believe those to be to help the antennae have a better ground. Check out the ground path for your vehicle and make sure that all of the cables from your battery to the body have been upgraded to handle the power required by your system as well as the rest of the car.

    Good luck.

    Oh and as a last note, in the classes we had they mentioned you could use a volt meter to try and identify good ground locations but they are only rough estimates and that nothing beats hooking your wire to the location and seeing if it works. In my vehicles I try to save the hassle and run good quality wire right to the battery.

  7. #7
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    Mainly a note re the last - a voltmeter or DMM etc cannot find a good ground location, but it can be used to confirm a good ground or location when the load is running at full load.

    Also it's only 1/6th power loss due to one diode being open (1/3 if shorted, but that is unlikely for very long). It's the increased harmonic content that makes "noise" worse, and the dc-offset can cause other issues.


    And to clarify, I consider DC mathematically - DC is the "smooth" or zero-frequency voltage and any ripple/noise is superimposed AC (that's easier for circuit understanding etc). Many consider DC to be "always on the same side of GND", hence a car system is DC irrespective of the noise or transient(AC).

  8. #8
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    Quote Originally Posted by OldSpark View Post
    ...
    I agree with what you said.. The diodes I was not 100% sure on because the bridge rectifiers we looked at were 4 Diodes. There are different designs and I won't claim to know all design configurations. All I know is that an alternator creates AC and the bridge rectifier takes the AC and combines it to make it into a pseudo DC. if we look at the unfiltered output it is more of a wave than a straight DC output. Current vehicles depend on the system battery to smooth out this ripply DC into much smoother DC.

    The two indications you have that you may have a Diode bad is the lesser power and the extra noise. Sometimes you can see your headlights flickering to the RPM of the car as well.

    Rodney

  9. #9
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    Ah - you looked at a "bridge" diode package - a 4 terminal device with 4 diodes used to "full wave rectify" a non-center-tapped transformer output.
    The equivalent full-wave rectification for a center-tapped transformer (output) requires only 2 diodes.

    Alternators have three center-tapped windings - hence "3-phase" - and thus require 3 x 2 = 6 diodes.


    Incidentally....
    The ripple factor is Vac (RMS) / Vdc and that's 0.042 (4.2%) for an unfiltered alternator (3 phase aka 6 pulse etc; and that ripple frequency is six times the base AC frequency - ie, alternator speed).

    IMO a 4% ripple or about 6% peak to peak (DC +/- 3%) is pretty good on its own especially in comparison to the other noise (spikes and transients) that get induced or injected onto vehicle's DC voltage.
    But as said, batteries filter that even further.

  10. #10
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    i would think that bolting a wire, directly to the alternator would make for a better ground. why you prefer the battery??

    p,s. i will remove my alternatort to check the diodes, but it will take some time, because the car is so god damn old, that the bolts are rotten...

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