My Ultimate Breadboard

Discussion in 'The Projects Forum' started by Treeman, May 27, 2014.

  1. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Recipe:

    1. Take one broken Orla DMK8 midi keyboard from your mate in Paris.
    2. Realise you'll never fix it (realisation pre AAC membership) so pick up hacksaw and cut out power supply section.
    3. Bravely and cluelessly solder 5v regulator to seemingly free posts. I figured the designer wouldn't have installed it hovering over the board, there were three pins doing nothing and amazingly after bothering to apply more brain power and check the pin out found I was correct! (I try and approach these type of issues in reverse now and check first):p
    4. Pull interesting steel stair tread from hedge, take home, hack to size and pop rivet on. Save the rest for other projects.
    5. Using as many original screws, wooden side cheeks etc from the Orla assemble into useable box; spray then smudge paint through impatience.
    6. Try to make as Gucci as possible even though you have no real idea what you need. Install neon mains indicator, Led's to indicate +12v(Red),-12v(Blue) and 5v(Green). Learn that the 5v is for the Logic chips and assume (as I type) negative supply is for op amps.
    7.Mount switchable 3.5mm and 1/4" I/O on rear panel and breadboard. Mount speaker terminals for the hell of it as well as second switch and 1/4" jack (not yet wired- I had a spare hole after no planning). I have taken a small speaker (1/4W 8Ω) from a kids toy mic and mounted it via magnetism. They seemed drawn together....groan.
    8.Start building Analog kids Tabata Timer despite lacking CD4060 because of impatience.
    9.Repair Banana connections to breadboard. Get frustrated because manufacturer has coated the pins but persevere and win through eventually.
    10. Install adjustable rubber feet taken from below school desks (retired) and save £5 at Map@~n.
    11. Wait for post man to arrive.


    Please comment with any ideas to add to this-I don't know what circuits I will be building in the future (cat scarer bumped high on list) but am already thinking LED meter or V and A meters (range req.?). I will post a hand drawn (my first) schematic and a question about the different sized caps on + - 12v circuit a bit later. My guess is they differ due to current requirements....

    p.s. yes I broke the 5v regulator and just tacked it back on for now. Ugly but working. (at least the LED lights which is my only proof...Logic may be some way off for me yet.) Soldering is still a challenge.
     
    Last edited: May 27, 2014
  2. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Hi, I have posted my schematic for the breadboard power supply.
    Its neater than my handwriting which says it all.

    I have a few questions that I hope you can answer:
    Remember it was a midi keyboard PS before misappropriation.
    1. The Mains fuse is 500mA. The 5v supply fuse is T1A 1 Amp. Why is this so? I would have intuitively thought that they may be equal or the other way around.
    2. The caps on the 12v + and -12v - are different sizes. 470uf and 100uf respectively. I assume they are for ripple smoothing. Why are they different sizes?
    3. The 5v supply has a 2200uf cap. I assume this to be something to do with the supply for logic chips needing to be ultra ripple free, stabilised as well as regulated.
    4. The 10pf caps do what? The 7805 uniquely has one before the regulator.
    5. The -12v supply has a much thinner wire than the +12v. Is this due to current demand?

    Any pointers welcome.
     
    Last edited: May 27, 2014
  3. MrChips

    Moderator

    Oct 2, 2009
    12,413
    3,353
    1) Compare power = V x I

    On the output side the supply provides 5V x 1A = 5W of power.
    (Edit: Add to that the power requirements of the +12V and -12V supplies.)

    On the input side, even if you account for 50% efficiency which is being over pessimistic, you only need 10W/220V = 50mA.

    A mains fuse of 500mA is being over cautious but quite acceptable.

    2) 3) 5) -12V supply has lighter load, takes less current.

    The ripple can be estimated using the formula:

    Vripple = I / ( 2 x f x C)

    where I is the load current in amps
    f = mains frequency (50 or 60Hz, 50Hz for UK)
    C = capacitance in Farads

    4) It is likely not 10pF. Maybe 10nF. This is to prevent high frequency oscillation of the voltage regulator.
     
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  4. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15

    And because logic needs a stabilised supply the extra 10nf cap B4 the regulator is required?
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,413
    3,353
    Not really.

    Both analog and digital circuits prefer to have stable supplies.

    All three regulators should have a 0.1μF cap as close to the output pin and GND as possible.

    Voltage regulators are feedback control systems. They can oscillate if the circuit is not designed and built properly. Always follow the manufacturer's recommendations.
     
  6. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Sorry Mr Chips I'm a little confused now. My question is about the 10nf cap B4 the 5v regulator. None of the caps in this circuit are .1uf but reading between the lines of your post I checked the datasheet and see that they perhaps should be.
    I'm clearly out of my depth here - is the original design 'wrong' in the eyes of the datasheet or designed differently to suit purpose.
    Its an Italian manufactured keyboard from I assume the late 80's or 90's.
    No info on net at all.

    Thanks for your help.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,413
    3,353
    There is a wide range between what the manufacturer recommends and what you can get away with.

    I have seen people ignore the recommendations and get away with no capacitor.

    Typical recommendations are:

    10nF or 0.01μF
    100nF or 0.1μF

    also 1μF - 2.2μF, even 100μF

    It varies from one regulator to another.

    Go with the manufacturer's recommendations. There are National Semiconductor engineers here on AAC who can tell you all you need to know about this. bounterhunter wrote the datasheets for Nat Semi.

    http://forum.allaboutcircuits.com/showthread.php?t=93717&highlight=lm2940
     
    Last edited: May 27, 2014
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  8. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Thanks again Mr Chips - didn't see the link you posted at first - must be colour blind as well as selective hearing!
    I read that inputs and output capacitors need to be in a certain range or certain chips (not you but probably my 20yr old 7805) will oscillate and be unstable.
    Thanks to Bounty Hunter too - I read your datasheets - in a few months or years I may begin to make sense of them.
    Is it me or is learning electronics a revolving door of learning - situated at the foot of Chomolungma?
     
    Last edited: May 27, 2014
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