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Another LED-diodes topic

Discussion in 'LEDs and Optoelectronics' started by Bounty, Nov 25, 2013.

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  1. Bounty

    Bounty

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    Nov 25, 2013
    I am sure that you get a lot of questions about basics on LED diodes so I will first apologize and then ask.
    Okay,last year I bought some cheap Chinese LED lights for a Christmas tree. Now I would like to make something else out of them now.
    The whole device is powered by 3x1.5V batteries (4.5V) and its connected to 20 LED diodes (red,green and yellow) in parallel circuit. It has a simple switch (on/off) and it has a resistor right after the switch (brown-black-black-gold (I guess its 10 ohm,correct me if I am wrong)) and the every second (starting from first one) LED diode has an smaller resistor (brown-green-brown-gold(guess-150ohm)).
    The thing is that I don't know what LED diodes are these because they does not look like the ordinary LEDs,instead of rounded top they look kinda like this,this shape and size but these don't have the bottom wider then the rest of the LED,its all the same width like this.

    I would like to make the same circuit only with 14 LEDs on the same source (4.5V).
    Can you tell me why is there a resistor on every second LED? (it's kinda weird)
    And what type of LEDs are these? Never seen them like this.
    I look forward to your answer.
    Thank you. :)
     
    Last edited: Nov 26, 2013
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The shape of the LED lens does not affect the electrical characteristics of the diode. However, the color of the LED does relate to the forward voltage drop across the diode. To make any sense of the circuit you need to describe the schematic in terms of the LED color associated with each resistor value.
     
  3. Bounty

    Bounty

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    0
    Nov 25, 2013
    All resistors are the same,except the one right after the switch.
    This is the circuit,I hope it helps.
    I would like to Use only first 14 LEDs.
    circuit.jpg
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Bounty, that circuit has all of the green LEDs in parallel without series resistors. It is NOT a good design.

    The green LEDs *may* fail prematurely.

    Leaving out these extra resistors saves a few cents in the manufacture at the cost of reliability.
     
  5. Bounty

    Bounty

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    Nov 25, 2013
    Can you tell me what is the best simple way to make a circuit with these diodes? Do I need a resistor for every single diode?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Red LEDs have a lower forward voltage than green LEDs. Yellow LEDs have a forward voltage in between.
    In a parallel circuit the green LED would light up very dimmly (if at all) if connected in parallel to a red LED without a separate resistor for the red LED. I'm not quite sure why there is the same resistor in series with the yellow LED as with the red LED. This depends on the exact characteristics of the LEDs used.

    The currrent is distributed more or less equally among the 20 LEDs (due to the parallel circuit). For only 14 LEDs you need 14/20 times less current. which means that the resistor R1 needs to be 20/14=1.4 times higher, so 14 Ohm or 14.3 Ohm (both standard values) can be used for R1. You could even stick with the 10 Ohm resistor and the LEDs being just a bit brighter.

    You may want to study the tutorial on driving LEDs in the tutorials section of this forum for more information.
     
  7. Bounty

    Bounty

    11
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    Nov 25, 2013
    And if I use all red LEDs (14x)? What resistors would I have to use? I will read it as soon as I get time,now I have to go to collage.
    Thank you for your answers!
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In order to know what resistor to use, the Vf needs to be known.

    This is a specification that should be available, or you can measure it by placing a known constant current through the diode (or even experimentally by iterateively trying different (likely) values of resistance).

    Once you know the Vf at a particular desired If (determined by the brightness you desire) then you calculate the series resistance using the formula (V - Vf)/I.

    Say you find the Vf as being 1.83V at 10mA, and this gives the desired brightness. Also assume your batteries give you 4.5V. The resistance required woul be (4.5 - 1.83)/0.01 = 267 ohms (and you might choose 270 ohms as the closest preferred value)
     
  9. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    My wife just bought little string of LEDs that run off 4 AA cells. They have a single 18Ohm resistor, then they put a pairs of blue of yellow in series, and pairs of red and green, then then parallelled all those pairs. I have no idea why this works as well as it does, but they do seem to light up pretty evenly.

    Bob
     
  10. Bounty

    Bounty

    11
    0
    Nov 25, 2013
    Does every diode has different Vf and forward current values?
    I can see on the online calculator that red diodes are most likely to have 2.0 Vf and 20 mA forward current.
    If i type these values in the calculator I get this:
    shema.jpg
    Is this okay,I mean it will work if I am able to find 27 ohm resistor?
    What if I can't find the exact value resistor,can I use others?
    Maybe this is better solution..
    shema2.png

    Bob,I guess people can invent new weird circuits just to save money. Look at my example :S
     
  11. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    27 is a standard resistor value.

    Bob
     
  12. Bounty

    Bounty

    11
    0
    Nov 25, 2013
    Red-Violet-Black-Gold Would that be the combinations of the color?
    I will go tomorrow in the shop to get these.
    Thx for info :)
     
    Last edited: Nov 26, 2013
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Well, each diode will have its own Vf at a given current. These will be close (perhaps *very* close) between diodes of the same type, but significantly different between different colour LEDs (typically).

    The Vf figure also changes with temperature in a way that means if a LED gets more current, it's Vf at any particular current will fall. This can lead to a warm LED getting warmer, and if the conditions are right spiraling out of control in a thermal runaway

    That's a typical value. Depending on what you're doing, that might be fine to base your calculations on.

    That's not a weird circuit, that's a fairly normal one :)

    Have you read the LED tutorial? It tells you all about doing this (and a whole lot more).

    https://www.electronicspoint.com/got-question-driving-leds-t256849.html
     
  14. Bounty

    Bounty

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    0
    Nov 25, 2013
    One more question, is first scheme same as second one?
    shema.jpg
    aaaaa.jpg
     
  15. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    What effect would you expect from using a different symbol to represent the same component in a schematic? Or by different 'scheme' do you mean the common-anode versus common-cathode configuration? Seven-segment LED displays are sold in both common-cathode and common-anode form, and they are not directly interchangeable. Why not?
     
  16. Bounty

    Bounty

    11
    0
    Nov 25, 2013
    Actually couldn't figure the + and - of the source on first scheme so I made this other one witch I understand completely,so I wanted to make sure its the same thing.
    It's kinda confusing for me that its + of the source directly connected to the anode,cathode to anode,cathode to the resistor and then on the - of the source..(on the first scheme)
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,387
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    Jan 21, 2010
    The circuits are essentially the same. You have strings of 2 LEDs with a resistor.

    As long as the resistor is the correct value and the LEDs are pointing the right way (the arrow points to negative - the negative terminal of the battery). nothing else matters. You can put diode, diode, resistor, or resistor, diode, diode, or diode, resistor, diode, it's all the same.

    Your circuits are not directly comparable because the resistor values are not shown in one of them, and the voltage is different. I'm only talking about the method of connection (strings in parallel and the composition of those strings).
     
  18. Bounty

    Bounty

    11
    0
    Nov 25, 2013
    Didn't know that,thanks for the tip :D

    Yes I didn't wrote the values of resistors,but I was thinking about same ones. :S
    Voltage different? On both schemes there is 4.5V source.

    Thanks for all the answers I will let you know the results. I hope I will do it right :D
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,387
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    Jan 21, 2010
    That will teach me for trying to read values from the thumbnails :D

    You are correct. In that case the resistor values should b the same (assuming the LEDs have the same forward voltage (Vf))
     
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