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Thread: Wiring Help for BMW E46 Car Puter!

  1. #1
    Low Bitrate scottholmes's Avatar
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    Wiring Help for BMW E46 Car Puter!

    Hi All

    So, i started looking at doing this many years ago, 2010 according to my last post and it's still sat here waiting to be done. My plan is to replace the headunit in my 2002 E46 318se with a small PC just for something to keep me occupied. Back in 2010 when i asked a similar question things happend and i never got around to doing it, but i kept all the bits.

    I started thinking about doing it all over again as i still have the car and i hate the head unit, so i started researching all over again. Back then my main worry was the electrics, i have no confidence and worry too much so in googlig this time i came across what looks like a much nicer way of doing it, powering it all from the 17 Pin Connector that goes into what will be my old head unit. I found this diagram and have been looking at it for days, got most of it worked out apart from the Capacitor.

    I know what its there for, i can understand why but i cant see or workout (or have had no luck contacting the blog owner) is what size capacitor it is. I know this is a long shot but has anyone a clue what size it it, or if not what size would be recommended for it?

    http://aidilj.com/2011/03/e46-carputer-wiring-harness/

    Apart from the iBUS connector this is excatly how i want my setup to be, switched so i can choose not to have the pc on for short journeys.

    Ta in advance!

  2. #2
    Variable Bitrate
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    maybe 2200F or 4700

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    FLAC SNOtwistR's Avatar
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    Hi just from looking at the picture, my assumption is a 6800uF to 10000uF cap around 16-25 volt. Just my 2 cents SNO

    Here is a pic of a 10000uF beside a standard auto relay for comparison
    Name:  10000cap.jpg
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    Last edited by SNOtwistR; 01-02-2014 at 10:51 PM.

  4. #4
    Low Bitrate scottholmes's Avatar
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    Thanks to you both for the advice!

    If i was to get a specific cap what would be the issue if i got one that was larger than needed? Can it cause any problems to my kit?

  5. #5
    FLAC SNOtwistR's Avatar
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    As long as the voltage is covered I personally would go with the 25v model not 16v. We used the 10000uF 25v to get an extra 5-6 seconds of power after a button press via relay. SNO

  6. #6
    Raw Wave
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    Thanks Sno - you beat me.
    I was going to advise a 25V rating. IMO 16V is too low for automotive circuits. Besides, electrolytic caps age (dry out) and the extra voltage margin should add greatly to its life. Higher voltages are fine but they tend to get too big physically. [ Though I have seem same sized (uF) caps for different voltages being exactly the same size - I suspect a manufacturing of "one size instead of many" approach (unless it's like the old days when higher voltage or lower tolerance (variation) ratings were given to the best in a batch - a bit like CPU speed ratings...). ]

    The bigger the capacitance, the slower it discharges into a load. (To clarify, by that I/we mean it discharges at the same current (rate) into the load, but its voltage falls slower.)
    Twice the capacitance (F, uF etc) means it lasts twice as long (has double the charge).

    There should be no harm with an overrated or oversized capacitor (unless you get into the Farad(s) range whereby initial charging currents can be extremely high).
    The only critical thing is to NOT have a cap whose voltage rating is too low.
    Oh - and not to connect (polar) electrolytics with the wrong polarity. (Like underrated voltages and aging, that usually results in a bang!)


    & btw - an axial leaded electrolytic - I haven't seen those (in the shops) for years! They all seem to be radial (PCB) mount.

  7. #7
    Maximum Bitrate rray's Avatar
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    You guys think that is just a stiffening cap to prevent the computer from glitching when you turn things on in the car like electric window defroster or heated seats? It's wired in exactly like my stiffening cap according to his diagram, but mine's 1F.

  8. #8
    Raw Wave
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    That's what caps do. When parallel with a voltage source they smooth (filter) variations by charging and discharging.
    In series it's the opposite - or complement - they remove/block DC and only pass AC.
    Another way of looking at the parallel situation is that they shunt (short) AC to GND.
    Or in summary, caps block DC and only pass AC (thru them).

    Of course inductors are a capacitor's complement. In series they pass DC but resist current change so it's like a parallel cap. In parallel they only let AC pass (by...).
    In summary, inductors block AC and pass DC (thru them).
    But both are frequency dependent - low frequencies will pass thru inductors/coils better (the higher the frequency the more it is blocked) whereas with caps the higher the frequency the easier it passes thru the cap.
    But they are total complements. Upon application of power, an inductor looks like an open circuit whereas a cap looks like a short circuit (hence care is needed for big caps like 1F etc - unless they have a high ESR but that would defeat the purpose of having them wouldn't it?)


    I've always found that term "stiffening cap" silly or hilarious. It's merely a filter, or smoother. IMO the only stiffening is the effect it has on audio salespeople, and caraudio buffs that don't understand that they aren't needed. (Noting that those that do understand why they have or need cap probably would not have such primal reactions.)
    Last edited by OldSpark; 01-03-2014 at 11:34 AM.

  9. #9
    Low Bitrate scottholmes's Avatar
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    Wow, excellent help there! If not a little daunting....

    I have never liked electrics and with the Capacitor here it's something i really have never done before and i dont want to end up with indoor fireworks. I havent even yet seen one that wasnt attached to my PC motherboard let alone connecting the correct wires...

    More research i guess! Thanks again.

  10. #10
    Raw Wave
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    Ignore my 'duality' & complement ramble - that was intended more for other readers tho even then I thought I might have thrown in too much.
    Really it's just one piece of almost intuitive knowledge and the rest follows. IE - a cap blocks DC (& therefore passes AC). The rest can be derived from that.
    (Or start with an inductor aka choke or coil shorts DC since it's a mere wire, but it's coils block AC - if that's easier. But IMO knowing a cap's symbol is like its construction - namely 2 parallel plates - the cap is the easier starting point of reference. An air gap is an open circuit and therefore DC won't pass.)
    [ Whereas AC can transfer across gaps - as it does in transformers. ]

    Note that the above is "knowledge in retrospect" - ie, the logic used to remember or understand rules or behavior, tho in this case the logic isn't quite as arbitrary what I'd use to remember phone numbers or PINs etc.


    IMO your only issues are:
    1 - ensure adequate voltage rating for the cap, eg 25V or higher (that's the only fireworks issue);
    2 - the bigger the cap, the longer it lasts. Not that that is necessarily desirable - read below.


    But I assume the cap is there for some simple filtering rather than hold up time. That because that cap is also in parallel with other loads on that +12V line so it's not just holding up the PSU (M3-ATX).
    In fact for an M?-ATX I'd expect a smaller cap to help prevent Mx-ATX noise. (You can read elsewhere the problems that M series can cause.)

    And the cap might not be needed.
    The beauty is that being in parallel, it'd be easy to add in later (or change) if needed. Just allow a tap point - eg, a couple of flying leads (but well insulated of course!).


    I'd suggest research won't yield much and just try either without the cap, else whatever is cheap and convenient - eg, maybe a 4,700uF or 10,000uF as they are common sizes.
    Last edited by OldSpark; 01-03-2014 at 06:32 PM.

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