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Beginner getting into electonics - help please

Discussion in 'General Electronics Discussion' started by dbov22, Aug 27, 2012.

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  1. john monks

    john monks

    693
    1
    Mar 9, 2012
    CocaCola. An extreme is the largest or smallest value possible in a function. In this case you were implying that some internal impedance of the LEDs would limit the current that will pass through the LEDs. In this case we already know what the minimum value is, 20mA as defined by db0v22. This is reasonable because if the LEDs offered no limiting at all the current would be much higher. The other extreme would be the current as if no limiting or impedance was offered by the LEDs. So I calculated both extremes knowing full well that the true current would be somewhere in between.
    I hope this clears that up.
     
  2. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Nope that does not clear it up at all...

    db0v22 stated

    He then revised to this...[​IMG]



    Yet, you keep pushing numbers that work in your favor for a particular instance, if you are going to stand on the crutch of what db0v22 said and use that at the worst case 'extreme' why do you pick and choose only select parts and not everything he said? Why are you still using 40 LEDs in your equations? It's clear that the clusters are going to vary in size by his own description, down to 10 LEDs, this is one extreme or worst case in this example by db0v22 definition... So if you want to lean on the 'extremes' crutch of what db0v22 state you have to take in all variables and possibilities that push the 'extreme' in said instance... I have been saying your math is only as good as the number you put in, when you ignore some variable and hand pick others the math is rubbish...



    Lets do your math for the real 'extreme' in this instance as stated by db0v22 using 10 LEDs ignoring all other possibilities...

    Taking the short circuit current of 7a you obtain a total battery impedance of 0.643 ohms. The LEDs running at 20ma you obtain a total LED current of 0.2A. So the battery voltage drops to 4.37 volts (4.5-0.2*0.643 ohms).
    The LED impedance will be 4.85 ohms (4.37-3.40)/0.2


    Now riddle me this, based on your math, in the 'extreme' case we have presented as outlined by db0v22...



    True or False?


    And I still don't see how these LEDs are limited to 20mA in either case, just because db0v22 says that is the value he would like to drive them at does not equate to what they will actually drive at in any particular circuit...
     
  3. john monks

    john monks

    693
    1
    Mar 9, 2012
    The example I used was with 40 LEDs in parallel powered by 3 AAA cells in series, no transformer..
    So in my example the answer is True.
     
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