Newb needs help to determine if a resistor is required with an LED.

Discussion in 'General Electronics Chat' started by Larry Rivard, Dec 23, 2014.

  1. Larry Rivard

    Thread Starter New Member

    Dec 23, 2014
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    Hello all. I got into soldering recently as a hobby so that I could eventually Build a mame Arcade cabinet and wire it. To practice I got some basic velleman kits to practice on. I'm now at the point where I can tin wires and solder with ease. I recently bought some components wires and battery holders to start small projects. I recently made 2 Led flashlights for my 6 and 4 year old sons. I have been looking at the instructables site and come across some great ideas and was able to make some great designs with stuff you find around the house.

    MY QUESTION IS:

    I used a single LED (specs listed below). with "2 AA" batteries which equal "3 Volts" output.
    Should I use a resistor or do I not need one. I appoligize, as I am very new to this.

    Emitting color: Blue
    Diameter: 10mm
    Lens color: Water Clear
    Wavelength: 460-465nm
    Forward voltage(V): 3.2-3.4
    Current(mA): 20
    View angle: 30
    Luminous intensity(MCD): 14,000 - 16,000


    I appoligize if this has been asked before on a similar post, but any and all help is appreciated.



     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    You should have some means of limiting the current but... 2 x AA cells are not really suitable for driving your LED properly as it requires at least 3.2V and, as you correctly stated, 2 AA cells equate to nominally only 3V. To drive the LED properly you will require 3x AA cells and a 56R resistor.
     
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  3. #12

    Expert

    Nov 30, 2010
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    LEDs act a bit like zener diodes. When you apply enough voltage to get them started, they have (almost) no ability to limit their own current. All LEDs must have a device to limit current. The easiest method is a resistor.
     
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  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    I did run 2x20W chips on VGA ooolers at approx. 60 volts, using a hacked 12/35V converter, and a laptop PSU (these have about 20V).

    With an Amp meter I did set the current to about 0.7 nominal

    Not even getting warm. It if would run away, the laptop brick would shut down.
     
  5. Larry Rivard

    Thread Starter New Member

    Dec 23, 2014
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    Do you mean a 56 Ohm resistor?
     
  6. blocco a spirale

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    Jun 18, 2008
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    Yes.
     
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  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Yesterday's thread on this perpetual topic...
     
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  8. Larry Rivard

    Thread Starter New Member

    Dec 23, 2014
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    Thank you all for your help. I found a resistance calculator online which also gives me a schematic and a layman's diagram. The site confirmed that I need a 56 ohm resistor for what I would be doing. I have just ordered them now. This is a great forum which I will be reffering to often as I move onto more complex builds. Thank you guys.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    With only 56Ω, the voltage drop across that resistor at 20mA is only 1.1V, which is almost too low. How many LEDs, what starting voltage (battery)?
     
  10. Larry Rivard

    Thread Starter New Member

    Dec 23, 2014
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    3 AA batteries and a single 10mm Ultrabrite blue LED (FV 3.4/ mA 20)
     
  11. #12

    Expert

    Nov 30, 2010
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    Better try 62 ohms to 120 ohms. There is no law that says you have to run an LED at the highest current it was designed for.
     
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  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is a discharge curve for AA batteries, at 100mA borrowed from powerstream.com.

    AA.gif

    Three new batteries: 3x1.54V = 4.62V. Three almost dead batteries: 3x1.1V = 3.3V
    The current limiting resistor has to keep the current at 20mA with new batteries, and the current drops off as the batteries voltage falls off...
     
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  13. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Obviously, with only a resistor to set the current and an unregulated supply voltage not much greater than the LED forward voltage we have to decide where the compromises will fall and everyone has an opinion.

    If an OP is struggling with the science involved in powering an LED from a battery I am reluctant to confuse matters further by introducing constant-current sources or inductive voltage boosters when all the OP is really interested in is lighting an LED without it burning out.
     
  14. Larry Rivard

    Thread Starter New Member

    Dec 23, 2014
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    Good to know. Thank you.
     
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