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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?

  • #2
    i'm first again

    what model are we talking about?
    My OLD 2001 Mitsubishi Eclipse GT:
    "The Project That Never Ended, until it did"


    next project? subaru brz
    carpc undecided

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    • #3
      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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      • #4
        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
        My OLD 2001 Mitsubishi Eclipse GT:
        "The Project That Never Ended, until it did"


        next project? subaru brz
        carpc undecided

        Comment


        • #5
          Ok soundman, thanks again. Getting late by ill give this a try tomorrow after work!

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          • #6
            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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            • #7
              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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              • #8
                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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                • #9
                  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; 08-27-2011, 11:35 PM.
                  My OLD 2001 Mitsubishi Eclipse GT:
                  "The Project That Never Ended, until it did"


                  next project? subaru brz
                  carpc undecided

                  Comment


                  • #10
                    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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                    • #11
                      Ok hopefully Mickz will comment. Thanks so far for the help you two. If I can fix it would be great. My second option is to go with the CNX 1900. Will get into that a bit later though if this doesn't work out.

                      Comment


                      • #12
                        UPDATE: I updated the other thread and referred Mickz to here. But that assumes he's subscribed or reads that thread... /end UPDATE

                        You could PM Mickz... (IE - to reply on here; preferably not solve it in a PM-reply.)
                        Or if I get to offloading something on my mind re another thread, I may remember to let Mickz know about this thread... (That's one IF and one REMEMBER - ie, short-term probability is low; 10-year probability is good. ha-frigging-ha.)


                        And I just noted... [WARNING - here comes another ramble. Arrrgh! But hopefully informative...]
                        Originally posted by soundman98 View Post
                        ....i believe the closer you get to 0uF, the higher the frequency the capacitor will send to ground... (eg) 33uF would send 1,300 hz freq to ground, and a 0.33uF would send 13,000 freq to ground...)
                        Yes - the smaller the capacitor, the higher the frequency. But the cap value depends on the circuit impedance (resistance)....

                        It is merely the application of the time-constant formula - ie, T = RC where R = resistance, C = capacitance, and T is time in seconds (for ~67% or response, ie, ~2/3rd of rise or decay time, but forget that crap, it's just time....)
                        That's used in filters, but inverted.
                        IE frequency f = 1/T (the inverse/reciprocal of Time. For the bamboozled, it's like saying 1/4 is the reciprocal of 4. IE 1/4 is literally 1 divided by 4).

                        f = 1/(RC).

                        For a given set up, R is usually constant. IE - R is the equivalent Resistance of the power supply etc.

                        Hence f is proportional to 1/C. And f is usually referred to as the roll-off frequency - that's where the frequency (of the noise) begins to roll off and be reduced. (And there are low-pass and high-pass filters.)

                        So f ~ 1/C (where ~ in this case means proportional rather than its usual "approximately")

                        So the smaller the C, the bigger or higher the roll-off or cut-off frequency. (It's not cut-off after that, it rolls off. The higher the frequency, the more it is cut or attenuated.)

                        So smaller caps cut the higher frequencies.

                        Conversely, you need bigger caps to cut low frequencies - hence BIG caps to reduce ripple in power supplies or 12V audio systems. (It may seem strange, but it is the same thing. Ripple and surges etc are merely "noise" or frequencies injected or added to the DC supply - even if they only appear for a few cycles...)

                        So you have a BIG cap - or battery (which is like a big cap) - to smooth out 12V low-frequency ripples. (Alternator output, amplifier surges etc.)


                        Now I'd suggest a look at Wikipedia's Low-pass filter. Ignore the writing, just look at the pretty pictures - namely the second on the RHS which shows the roll-off (of a low pass filter - aka - let's kill higher frequencies!). Also, the 3rd pic on the RHS shows the RC of the filter. The R is that "equivalent resistance" of the supply that I spoke off.

                        That "cut off" frequency moves lower as as the cap size is increased. (IE - a big cap removes lower frequencies, ie, smooths DC supplies.)


                        Now those of you that are astute may rightly assume that a big cap will "kill" all high frequencies. And not that I can answer that (I thought about that a few weeks ago!), but for various reasons, that is usually not the case. Hence why most power supplies (even in amps and TVs & PCs etc) have small caps in addition to the bigger xx,000 uF used to remove the "DC ripple". The big caps are usually electrolytic caps (eg, 1,000uF etc). The smaller caps (in parallel) are typically ceramic or greencaps (eg, 0.001uF, 0.1uF etc).
                        (It has to do with capacitor construction materials and methods. Electrolytics are "slow" (ie, NOT hi-frequency) whereas ceramics etc are "fast".)

                        Hence why a few months back I suggested some small ~0.1uF caps for a similar problem, whereas someone (soundman?) later replied and suggested bigger caps instead (based on their experience).
                        IMO that meant that the noise involved was of lower frequency than I expected, or lower circuit resistances, etc, were involved).


                        So what's all the above cap crap about?
                        Well... it was intended to let soundman know that he was correct (as usual), but also explain the reason, and maybe demystify the whole thing. (And thereby also why a cap in one situation may not work in the next - ie, it's a different circuit resistance, or a different frequency.)


                        I'd like to add some extensions of this, but maybe enough is enough?

                        [ F.ex - a cap "blocks DC" - ie, it passes AC (noise). The smaller the cap, the higher frequency is passes (ie, higher roll off; coupled with the fact that bigger caps in practice don't necessarily pass high frequencies like they should in theory.
                        A high-pass filter has the cap in series with the "signal" (it blocks the low frequencies). But put that capacitor across the signal or load (as per that Wiki pic #3, or across the converter as we are doing), and those high frequencies short or get "shunted" to ground - ie. it becomes a low-pass filter (DC is not blocked and low frequencies pass through like the DC).
                        Hence the 2 basic topologies - if you want to block or prevent DC going thru, insert a (series) cap. Same for low frequencies.
                        If you want to let low frequencies thru, then you shunt or short the high frequencies to ground. (Which reminds me of soundman's cap from output +ve to input gnd - we merely have to short the crap "with itself" (if that makes sense??!).)
                        { BTW - DC is a low frequency - its frequency is 0 Hz. All noise etc is AC and has a frequency. And all waveforms (like square, triangular etc) can be represented by a collection of sinusoid (AC) signals of different frequencies (harmonics) and amplitudes. Put a 70Hz low-pass filter on a 60Hz square wave (eg, an inverters' stepped or modified sinewave) and you get a smooth 60Hz sinewave - the higher frequencies that "add" to the 60Hz "fundamental" sine-wave that makes it a stepped or square wave are removed. }

                        All that from a simple T=RC formula. Like I say, everything is the same, just different. (Viz - extended, paralleled instead of series. And if you know enough - but can reduce it back to the fundamental or basic principle it really is. (No harmonic or frequency pun was intended for that fundamental!)
                        And if all this boggles you, just realise that it did far worse to me! But involvement, desire, curiosity, questioning, and finding the right teacher (text or verbal language or pics etc) got me there. Oh - I didn't mention frustration....
                        But I did mention that enough is enough! ]
                        Last edited by OldSpark; 08-29-2011, 10:58 PM. Reason: Update:

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                        • #13
                          Just had a quick read of the thread and I agree with OldSpark, the charger is arcing or flashing over. Could be any number of reasons already mentioned. But also consider a contaminated PCB that's starting to break down

                          As far as trying to get rid of general noise (not arcing), thatís difficult unless you know the frequency and bandwidth of the noise. You can try various values of caps as Soundman suggested and sometimes you get lucky, but thatís usually for noise at relatively low RF frequencies.

                          The problem you have with caps and High Frequency noise is itís not just about the value of the cap. It depends on the construction, placement and lead length. In other words you need a cap made for RF bypassing to begin with.

                          As a side note, there are a number of reasons why a big cap will not have any effect at RF frequencies. For a start it has way too much inductance because of its physical size and construction.

                          Because of this you are effectively placing an inductor (coil/choke) in series with the cap and this blocks RF from reaching it, and it get worse as the frequency increases. Again, this is just a basic answer.

                          I really canít offer a solution that is cost effective in removing hash/noise from a low cost supply.
                          Palm sized ainol MiniPC, 8" Transreflective PRO, Win10, Reverse camera, Dual 10HZ GPS RX's for Speed Display & Sat Nav, FM-DAB & Phone Modules, iDrive interface. T-Screen HVAC control, custom microcontrollers, microcode and FE.

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                          • #14
                            Thanks for the help all of you really appreciate it. Bit the bullet and I will be in possession of Carnetix 1900 wednesday next week! Time to get this install completed! Are there car shows specifically for mp3car type vehicles?

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                            • #15
                              there's AFK fest, other then that, not really..
                              My OLD 2001 Mitsubishi Eclipse GT:
                              "The Project That Never Ended, until it did"


                              next project? subaru brz
                              carpc undecided

                              Comment

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