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Thread: laptop DC/DC charger noise

  1. #1
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    laptop DC/DC charger noise

    I bought a HP smart car/ac/air smart charger to power my carpc laptop. Running on laptop battery there is no noise on the car speakers, plugging the charger results in noise the resembles hard disk activity. It also affects the touchscreen display. Previously I was using a different laptop with an iGo universal DC/DC adapter and only once in a while you could hear a very faint hard disk like noise.
    I think I understand why AC to DC switching converters generate noise but I didnt expect DC to DC to generate this much noise. Im experiemnting with moving the charger around and Im going to try wrapping it in tin foil and grounding it. Why is this much noise produced and how can it be eliminated?

    I dont have much space on my car and the laptop and charger are pretty much contained in a space just behind the touchscreen (where the double-din radio used to be).

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    Did a lot of testing and it seems there's some kind of ground loop between the laptop and the AMPs. While running on laptop battery the system has no noise. As soon as the DC charger is plugged the noise starts and fluctuates as the system activity changes. For example the noise changes as the HD activity changes. Moving the cursor around on on the digital fx skin causes the buttons to highlight and you can hear it in the noise. Leaving the charger plugged in and connecting another laptop does not introduce any noise.
    This is pretty much the current setup for power:


    It seems the noise goes from the charger to the AMPs. The only analog part of the system is between the USB sound card and the AMPs so thats were Im going to do some more testing.


    Usually power line filters shield the HeadUnit for example from picking up noise from the alternator. In this case is the "HeadUnit" (laptop + charger) the one introducing the noise. Will installing a power line filter between the laptop charger and the car battery stop the charger from introducing noise to the system?

  3. #3
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    What makes you say it's a ground loop?

    Why isn't it converter noise (DC-DC SMPS) whether radiated or conducted etc?

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    Its a fairly obvious ground loop.... laptop to amp via usb->rca, amp to ground via amp ground and ground to laptop via dc-dc power supply.

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    The digital signal up to the usb sound card should be immune to any induced noise. So considering the audio source is digital up to the usb soundcard and the sound card does not have power inputs, that leaves from the crossover to the AMPs.
    Another weird symptom is that as soon as I connect the pos (+) terminal of the DC laptop charger to the car battery you get the noise. The neg (-) is not even connected and the noise starts. On top of that the noise changes as system activity changes as I explained previously.
    It must be some kind of ground loop but I cant quite understand how its happening and how to fix it. So far I think the charger is introducing noise to the car electrical system that then is picked up by the AMPs power connections. I dont think RCA loop isolators will help but maybe a line power filter between the laptop charger and the car battery could break the loop.
    Anyone has any ideas?

  6. #6
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    Quote Originally Posted by justchat_1 View Post
    Its a fairly obvious ground loop.... laptop to amp via usb->rca, amp to ground via amp ground and ground to laptop via dc-dc power supply.
    A loop is obvious, but to me a ground-loop in a typical automobile has never been obvious, hence my question.

    If it is a ground loop, the solution is simple. As with (old!!) domestic systems, break the loop, or series connect, or star connect, etc.

    But I doubt it is a ground loop issue. However if someone can enlighten me....

    (Don't you miss oldskool telecommunications converters with psophometric filtering?)

    POST EDIT:
    I see some define a ground loop as a mere difference in two ground potentials (voltages).
    I use the IMO more common "loop" definition - ie, it is a circular ground loop. The DC ground potentials may be the same, but there is a circulating induced hum. Hence the reason for my question.

    The problem to me sounds like a typical noisy converter - it has insufficient filtering (probably on its input). A choke (filter) may solve it, else a better quality converter.

  7. #7
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    Quote Originally Posted by OldSpark View Post
    The problem to me sounds like a typical noisy converter - it has insufficient filtering (probably on its input). A choke (filter) may solve it, else a better quality converter.
    I don't have any other choice to change the charger model. This laptop will only charge/power from a charger that has the infamous middle pin. There are generic AC charges with the middle pin but I couldn't find any for DC/DC. Im using an original HP charger that does AC/DC/airplane.

    While testing I connected the charger using the power filter that came with my new lilliput. For some reason the new filter box style has only 4 or 5 components including a toroid and some capacitors ( a choke filter?). The older egg shaped had a bunch of components including some ICs ( a regulator?). Connecting the charger using the "simple" filter didnt make a difference. Havent tried connecting using the older egg shaped reguator.

  8. #8
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    Alas I'm not familiar with the 3rd pin (unless it's a typical battery or temp sense feature).

    But it is a noisy converter. Whether EMI/EMC or conducted. I'd expect better from HP.
    It isn't faulty is it?

    Try either or both filters. Knowing the converter frequency might help. (They are usually supersonic.)
    If a low frequency, maybe a cap across the converter's supply.

    I wonder if a DC-AC inverter with an AC-DC charger would have less noise?

  9. #9
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    laptop DC/DC charger noise

    Many of the problems that appear out of Murphy’s box upon transforming a design from
    the mythical world of textbooks and SPICE to the real world emanate from the non-ideal
    power supply. Real power supplies can cause noise and spurious oscillations that can
    force the designer into a frustrating glitch hunt. Rules of thumb can usually be applied
    successfully to simple problems, but a little understanding and forethought will usually
    provide clean solutions to even the more obscure problems. With this paper, I hope to
    provide the understanding of some of the dynamics of power distribution.

    and decoupling.
    Power
    Supply Load Cbyp
    Power
    Supply
    Bypassing
    Decoupling Cdec Load Cbyp
    Ldec
    to sensitive

    thanks

    ________________


    Tweak Windows 7 instantly and without logging off

  10. #10
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    If you want to use your computer in the car without using up the battery you can save energy and weight by using one of these high tech switchmode DC converters (car cords). They are so small, light and reliable you might want to leave your AC adapter at home. This travel laptop adapter is a voltage-boosting DC/DC converter designed to power or charge a notebook computer from an automobile power point or cigarette lighter. They are the smallest, lightest laptop adapters we know of. They are available in 60 or 120 watt versions, as listed in the table below. The cigarette lighter power adapters are used for any electronics that operate over the 15 to 24 volts range.

    These are made by a well respected manufacturer of OEM notebook power supplies. Many of the laptop manufacturers use their AC adapters as original equipment. These DC-input adapters are made in their ISO-9002 certified factory for extra quality.

    thanks

    _____________


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