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Thread: Laptop DC car charger noise

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    Laptop DC car charger noise

    Hey guys,

    My Carpc is running flawless, but last thing I need to figure out is my laptop charger is adding noise to the liliput screen. Without the charger the screen is crystal clear. The noise is minimal when the charger is plugged in but enough to really make it a pain when watching movies. Any tips?

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    MySQL Error soundman98's Avatar
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    i'm first again

    what model are we talking about?

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    Laptop is an Acer Aspire 5050. Ebay DC/DC charger 19VDC 3.42A 65W. Has a cigarette lighter socket and barrel connector for the laptop. In line is a small box containing the converter. I have heard of using capacitors to clean it but can't find any details.

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    MySQL Error soundman98's Avatar
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    the easiest method would be something like a 33uF electrolytic capacitor across the output terminals-- similar to the voltage regulator data sheet:
    http://sharp-world.com/products/devi...pq05rd21_e.pdf

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    Ok soundman, thanks again. Getting late by ill give this a try tomorrow after work!

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    Cracked open the brick on the charger. It was glued shut. Tried a bunch of caps and no luck not even a bit. With it open I could hear little blips here and there that also appear on the screen on top of the other noise. Don't think its possible to clean this output. Guess I need to look into a new way of charging it. Sucks cause its the second charger I have bought. I know Carnetix makes a 19v charger but at 100 dollars its a bit pricey and how would I know if it would be a clean output until I tried it in the system.

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    Could the blips be internal arcing? Maybe conductive dust or gunk - especially across its transformer coil or switching transistors/FETs?

    Else a dry joint or cracked track or component/conductor?

    Else a component breaking down - typically caps?


    Otherwise soundman's suggestion should have worked. (Though IMO that was intended as an external addition, but your approach seems to have detected the actual cause/fault. Well done!) (Warning: Kiddies - do not try this at home... LOL!)

    If that cap didn't work, I'd suggest others - especially some small ~0.1uF (0.01uF etc) greencap or similar across the output (to suppress high-frequency noise), as well as also trying the input in case the noise is also injected back through the 12V supply.

    In any case, the caps as close to the converter as possible is best - that should prevent the output or input leads acting like antennas (inductive coupling etc).
    And you have made that possible thanks to your reckless DIY "force it apart" approach (that most of us use!!!!). (Did I say well done? (You naughty boy!))

    But "successful caps" should be determined by convenient "end of lead" testing. That should reduce the noise. Then they can be moved into the converter.


    But like I said, occasional blips or crackling suggests some breakdown - especially when irregular.

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    OldSpark, Haha thanks for that reply.

    Its a cheap supply from ebay. The blips are maybe every 5 seconds, pretty consistent. But even if I eliminated the blips I would still have the problem with the noise. I'll get my volt meter out next time and measure each cap I try to see the results. Right now I debating whether or not to buy a new supply. But really not looking forward to spending over $100 for a Carnetix. I see lots of people using these DC-DC laptop chargers and not reporting any problems. Maybe it's just my luck I got a poor made one.

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    one thing i forgot to add-- connect that capacitor between the positive output wire, and the negative input wire-- that should send any noise to the car ground instead of the pc..

    capacitors work in the way that at certain frequencies, they act as a short circuit, and at other frequencies, they act like a disconnected wire, and block the connection.

    oldspark will be best to confirm this as i am a little rusty with freq/capacitance correlations, but i believe the closer you get to 0uF, the higher the frequency the capacitor will send to ground. (not real, but for a idea-- 33uF would send 1,300 hz freq to ground, and a 0.33uF would send 13,000 freq to ground, etc, etc...)

    though, as you are suspecting, it might be time to step up to the a 'real' psu...
    Last edited by soundman98; 08-28-2011 at 12:35 AM.

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    I reckon no - connect output cap across output (and in put across input) because the cap acts as a short (for the noise) across whatever circuit.
    Cap-connecting output to input (ground) is not the same, and can introduce undesirable cross connections etc, though most dc-dc converters (for 12V etc) have a common ground - ie, output ground is the same as input ground - it is not "isolated".
    And it has to do with being across the output "impedance"...


    Although the "send any noise to the car ground instead of the pc" sounds logical, it's not how it works. (Though I envisage that in some cases, connecting the "right sized" cap that way might have a better effect, but that would be a random outcome unless you knew the converter circuit etc.)

    Such connections are probably(?) more likely to set up undesirable effects - like ringing etc.
    Though as I wrote, in this case, input & output grounds are usually the same.

    But I too am not fully aware of all converters circuits etc, and though not "rusty", I am not an expert (IMO). (But I like the suggestion - it may be worthy of further consideration or investigation....)

    If Mickz could comment....

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