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Thread: Laptop DC car charger noise

  1. #11
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    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.

  2. #12
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    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...]
    Quote 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 at 10:58 PM. Reason: Update:

  3. #13
    Maximum Bitrate Mickz's Avatar
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    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.
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  4. #14
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    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?

  5. #15
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    there's AFK fest, other then that, not really..

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