LED Problems!!

Discussion in 'The Projects Forum' started by Zimma, May 9, 2011.

  1. Zimma

    Thread Starter New Member

    May 9, 2011
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    :confused:

    Hey guys im ew to this forum and a newbie with electronics, im building a, building model but want to put lights into it. However i have got ultra bright 25mA, 3 foward V white LEDS i hav a 6V power supply (batteries), i want to create a series circuit of 10 LEDs i checked the calculator and have bought 150Ω resistors, a resistor an LEDs on a single circuit (x 2) works but adding others, doesnt give me anything i have attached a image to make it ALOT more clearer. I have read you can make a series circuit and run the LEDs off a 9v supply? If i need to change the ohms or the LEDs (white led running off a 9V (has to be bright)) please could you guide me as i know the little torches can run off that type of power (with about 9 LEDs in them)(coluld you please provide a diagram)​

    Sorry for my rant and baffle i really am in need of some advice THANKYOU​
     
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  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    You can put more leds in series with one resistor on a powersupply.
    The formula needed is:
    Rled = (Vsupply - Vled) / Iled
    Where Vled is the summation of the forward voltages of the leds in series.
    Vled MUST be lower as Vsupply, otherwise the current limitting will not work.

    Bertus
     
  3. Zimma

    Thread Starter New Member

    May 9, 2011
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    Thanks so what is Iled?
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    With only a 6V supply, each set of one LED and one resistor needs to be connected across the battery.

    You cannot have more than one 3V LED in series with only 6V input, because there would be no voltage left for the ballast resistor (and note that most high-brightness white LEDs require a little over 3V).

    A 9V supply could allow two LEDs, provided that they were not too much more than 3V each. Note that only one 150Ω resistor would be required to drop the remaining 3V at 20mA.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I have tutorial on LEDs here...

    LEDs, 555s, Flashers, and Light Chasers
    Chapters 1 and the 1st half of chapter 2.

    While LEDs are current controlled, you must exceed the break over voltage point (called Vf), which is very close to a constant, before they work. Most white LEDs Vf is 3.6V. Put two of them in series and you need more than 7.2V before they can start to conduct. You also need about a volt or more over that to give a resistor something to work on, so for two in series you would need at least 8.2V.

    [​IMG]

    However, if the 6V is a hard number you can put them in parallel, with a resistor per LED. The resistor is not optional, you must have one.

    [​IMG]

    Visualize this circuit with one LED per leg and you get the idea.
     
  6. Zimma

    Thread Starter New Member

    May 9, 2011
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    I have created a parallel circuit there have i not? i have put a 150ohm resistor on each LED i do understand that it maybe is higher beacuse of the currentpassing through them, ok so i scrap 6V and go for 9V is there anyway you could provide me with a quick DUMMIES Diagram on how to connect the 10 LEDs to a single circuit, including what the cicruit has to b grounded to, as its a model of a builing there is hardly any metal besides a circuar column within the structure.

    I appreciate the response thankyou, just very technical for me lol also what resistors would i need for this type of job???
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Iled is the LED current: I is commonly used to stand for current, which sounds rather odd until you find out that it stands for Intensity, from the French "Intensité (du courant électrique)".

    Your circuit is a series circuit. A series circuit is a string like single row of beads. See this link: http://www.allaboutcircuits.com/vol_1/chpt_5/1.html

    I think you should try to get the difference between series and parallel clear in your mind, then look at Bill Marsden's tutorial.
     
  8. Zimma

    Thread Starter New Member

    May 9, 2011
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    Sory for my stupidity lol:

    so i have imlemented your forumla into (6-3.2)/25= 0.112

    So from here what should i do now?
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I am assuming that the 25mA LED rating is the maximum voltage, which should never be maintained for any duration, and that 20mA is the nominal operating current.

    [​IMG]

    This circuit assumes aprox. 16mA/LED and 2.8V/LED
    (They will light, but a bit dimmer)
    Don't know the capacity of your 6V supply, but they may dim quickly.
    The circuit consumes aprox. 80mA

    [​IMG]

    This circuit assumes aprox. 16mA/LED and 2.9V/LED
    The circuit consumes aprox. 48mA
     
  10. Zimma

    Thread Starter New Member

    May 9, 2011
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    Ok i just tried Bills method, my problem was that i did not know how to create a parrallel circuit but now i have overcome it im still using the 150ohm resistors i plugged 6 leds with resisitors in to a parallel circuit and a supply of 6V and in worked! but one did pop which was my concern it went bright then green and now is yellowy, also the LEDS are quite bright too, any suggestions with why it might have popped, dodgy soldering and over heated the resistor? also i ran them single not double or tripple (personal prefrence)
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    @ iONIc: Using almost all the supply voltage up with the LED forward voltages is bad practice, particularly with a battery driven system as proposed by the OP.

    Only a small change in the supply voltage is then needed to produce a dramatic change in the LED current. With a battery supply this leads to a short battery life, as the voltage becomes too low to drive the LEDs properly long before the batteries have given their expected ampere-hour capacity.

    @ Zimma: My guess is that you put the resistors in parallel with the LEDs, so that the LEDs were straight across the battery :eek: If so I am sorry to have to tell you that this would be expected to ruin them.

    Some basic study of current flow in a circuit may be in order here...
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Generally when a LED changes color it is being overdriven with too much current. It is why one would pop too.

    Let's go through the math. I'm assuming you have 3.6V for the Vf.

    6V (power supply) - 3.6V (Vf) = 2.4VDC

    The resistor uses Ohms Law to translate this 2.4V overhead voltage into a current.

    2.4V ÷ 150Ω = 0.016A, or 16ma. Sounds good to me.

    You can not share a resistor for two LEDs in parallel, although in series is fine.

    Like I said before, the resistor is not optional.
     
  13. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Could be a bad LED!

    Try 220 Ohm resistors. This will reduce the LED current to below 20mA and they will not be quite so bright.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    This is a confusing thread.

    In post #5 an expert shows three LEDs in series and in series with a 75 ohm resistor and powered from a 9V battery.

    In post #9 another expert shows two LEDs in series and in series with a 27 ohm resistor and powered by a 6V battery, then shows three LEDs in series and in series with a 22 ohm resistor and powered by a 9V battery.

    But the OP said that his LEDs are 3V each. Please don't confuse the poor guy.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    You mean this?

    I kinda thought I stated the difference.

    I don't really buy the 3V. Most white LEDs are 3.4V to 3.6 Vf. In any case, even at 3.0 Vf 2 LEDs in series will not work with 6V, which is the point.
     
  16. superduper

    Active Member

    Dec 5, 2010
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    Therein lies the problem. Bill recommended 3-LED's in series with a 75-ohm resistor. He chose to run them all in parallel individually with a 150 ohm resistor. Hence why they are all too bright now, and probably all on the verge of popping.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to post your schematic so we don't know what was wired wrong.
    It should have been wired like this:
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Reread the comment under the picture. Bill did not recommend 3 LEDs per chain.
     
  19. superduper

    Active Member

    Dec 5, 2010
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    Oops you're right. Looks like I didn't read anything very clearly. :D:p
     
  20. Zimma

    Thread Starter New Member

    May 9, 2011
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    Thanks audio guru, i am confused, and thankyou all for the help, i have attached how i wired up the circuit (i know its basic and silly how i dont get it),

    what does that last symbol indicate within the circuit i have wired it up just how the led calculator had shown me, nevertheless i have attached how i have wired up the circuit so whatever is wrong with it please let me know as i am a complete novice also would a higher resistance be better to use without my LEDS popping as they are getting very bright.

    BILL: the LEDS foward V is shown as 3.2 from the manufacturers list
     
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