Electrical --- LED --- circuit question | FerrariChat

Electrical --- LED --- circuit question

Discussion in 'Technical Q&A' started by finnerty, Jun 18, 2014.

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  1. finnerty

    finnerty F1 World Champ

    May 18, 2004
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    #1 finnerty, Jun 18, 2014
    Last edited by a moderator: Sep 7, 2017
    Need a little help, please from the electronic / electrical gurus here.

    I have a simple, low voltage DC circuit that presently has (2) miniature incandescent lamp bulbs wired in series --- see diagram below.

    I want to replace the lamps with (2) miniature LED's, but I can't recall whether this works in series, or if they need to be wired parallel ?? Do I need to add any resistors to reproduce the load of the original lamps being replaced ??

    Also, any particular spec rating or type for the LEDs ??


    Thanks in advance.
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  2. DGS

    DGS Seven Time F1 World Champ
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    #2 DGS, Jun 18, 2014
    Last edited: Jun 19, 2014
    That depends heavily on what LEDs you're using.

    LEDs come with specified voltage drops and max current.
    Some LEDs with mounts include drop resistors in the mount.

    So you'll need the specs for the LEDs you want to use.

    A quick search found a couple of bright white LEDs, but they have a 3.0-3.4 v drop across each one, so your 6.0v supply wouldn't support a series connection, as it would need 6.4v (average) plus a current limiter resistor to limit current to the max 20ma rating.

    Found online:
    LED Resistor Calculator

    For DIY calculations, look up Kirchhoff's voltage loop law.
    Kirchhoff's circuit laws - Wikipedia, the free encyclopedia
    The sum of the voltage drop across the two LEDs plus the voltage drop across your resistor (V=I x R) equals your supply voltage.
    (Where I is the common current through the resistor and LEDs: the spec current of the LEDs)

    For example, for your incandescent lamps, you're dropping three volts across each bulb, and the current is limited by the resistance of the two bulbs and the internal resistance of the voltage supply (battery?).

    For two 1.5v red indicator LEDs at 20ma, you'd have 1.5v + 1.5v + (.02 x 150) = 6v with a 150 ohm, 1/4 watt resistor.
     
  3. RacerX_GTO

    RacerX_GTO F1 World Champ
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    Current limiting Resistor calculator for leds

    It even draws you a simple schematic in full color.


    Tip - LED's are finicky. Their world is a world of absolute. Measure your voltage to get a precise reading, then make your calculations on resistors.
    As an example, many cheap LED products fry out quickly because they are resistored for exactly, precisely 12.0 Volts. Well, that's not the case when you start the car and the alternator starts spinning and bumps the voltage up to 14.3 or 14.7 Volts. Ooops.
     

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