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LED question

Discussion in 'Electronic Basics' started by Andrew Howard, Mar 3, 2004.

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  1. You have to be careful, because a car battery is not normally 12V but a bit
    over, like 13-14V (I think). Maybe a regulating circuit could be used?
    It should.
    I don't think so, because the wire isn't carrying much current through it.
     
  2. Ian Stirling

    Ian Stirling Guest

    Nope.
    To be extra cautious, you might add a reverse biased silicon diode across
    the LED, to protect it from any nasty spikes.
     
  3. electricked

    electricked Guest

    So the car battery produces 12 volts. Let's say I want to light up a single
    LED with the car battery. Is this possible?

    V=12V
    I=20mA
    R=?

    V=IR
    R=V/I=12/0.020=600ohms

    So if I hook up a circuit in parallel with the rest of the circuitry that
    goes off the battery, and I place a resistor of 600OHMs and a single LED,
    will it light up? Does the size of the wire I use matter?

    TIA!

    --Viktor
     

  4. You need to consider the voltage that's going to appear across the resistor.
    This is given by the battery (12V) minus the LED forward voltage (about 2V).
    So we have:

    R = (12-2)/0.02 = 500 Ohms.

    Also, as already said, car batteries are closer to 14 volts than 12, so your
    original result of 600 Ohms turns out to be correct (accidentally).

    cheers,
    Costas
     
  5. Yes, it will light. Since the wire will only be carrying 20 mA, you
    could use quite small wire - I'd suggest #22 or 24 for ease of
    handling, although they are much larger than really required.

    The 20 mA current you suggest for the LED is probably its maximum
    recommended current - I usually run LEDs around 8 - 10 mA, and find
    them bright enough.

    As others have mentioned, a "12 volt" car battery is really somewhat
    higher - up to about 14.4 volts when being charged.
     
  6. John Larkin

    John Larkin Guest

    Close enough.
    No, the current is very low here.

    Should work fine; try it.


    John
     
  7. John Larkin

    John Larkin Guest

    There's no reason to regulate the voltage for a simple resistor+LED.

    It's not. You are adding the 600 ohm resistor, so you only use about
    20 ma.

    John
     
  8. electricked

    electricked Guest

    So if I'm regulating the voltage at 12V then it will work so long as the
    voltage is constant. Got ya.

    Another question now. From my theory understanding, if I connect my LED
    circuit in parallel with the other circuits hooked to the battery, wouldn't
    that make a lot of current flow in my circuit, since the resistance of my
    circuit would be much smaller than in the rest of the circuit hooked in
    parallel, then wouldn't all the current choose to go through my little
    circuit? This doesn't make sense but that's what the theory says or maybe
    i'm not understanding it right. Someone care to explain how all this works
    and fits?

    Here's a diagram of what I'm refering to:

    -------------|---------------
    | bunch | |
    | of car | |
    | circuits | |
    [LOAD] hooked --- 12V car [MY LOAD]
    | in - battery |
    | parallel | |
    | with the | |
    | battery | |
    -----------------------------

    Since resistance in MY LOAD is less than resistance in rest of circuitry
    wouldn't that cause more current to flow through my circuit? What am
    misunderstanding here?

    --Viktor
     
  9. In a parallel circuit, the current in each branch is determined only
    by the resistance in that branch.

    If you connect your LED and 600 ohm resistor across the battery, it
    will draw 20 mA (or so), regardless of what other loads may be
    connected to the battery (except when you hit the starter - the
    starter will draw so much current that the battery voltage will drop,
    and the current in all other loads will therefore drop until you stop
    cranking.)
     
  10.  
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