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What am I not getting about LED's

Discussion in 'LEDs and Optoelectronics' started by zionosis, Dec 25, 2013.

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  1. zionosis

    zionosis

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    Oct 23, 2013
    I am a noob BTW.
    Well as far as I used to know when you have a series of LED's the current stays the same throughout and the voltage drops after each LED.

    But something very strange is going on with my LED's. I have a 9V battery that according to my multimeter can put out about 85mA.

    Well my little LED's are 20mA ones I am pretty sure. Not sure about forward voltage but 3V seems to run them fine. Maybe they are 3.5 since that seems like a common standard.

    Well I have a resistor that reduces the mA to only 30 or so. Now that is more than enough for my LED's. And there is more than enough Volts for at least 2 to work properly. But that isn't what happens. I connect one LED and it runs fine. I then connect both together and they both go dim. WTF. The current drops after each LED. How is this. Not meant to happen.

    The only way I can get them both to go brighter is to use a different resistor that makes the current back up at nearly 70 instead of 30.

    But how does this make sense. If one LED runs bright at 30mA then why shouldn't 2 run bright in a series since it isn't like current drops like voltage does. But that seems to be what is going on here.

    It's annoying me very badly. This isn't even meant to happen yet it is. It's so stupid like my LED's are magically high ohm resistors.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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  3. zionosis

    zionosis

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    Oct 23, 2013
    I already knew all that in the link since I have read it all before on various other sites.

    It doesn't help. On the section about LED's in a series it says you need 25mA to power multiple 25mA LED's which is true. But for some reason it doesn't work like that for me in real life. My LED's seem to somehow divide current which makes no sense. If I put 1 on it uses all of the 30mA. Then I put 2 on and they only use 15mA and are dull. Then for the hell of it I put on 3 and they were barely lit and were using like 9mA each. WTF is this.

    BTW what is your avatar meant to be.
     
    Last edited: Dec 25, 2013
  4. BobK

    BobK

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    If it is working like that, you are not putting them in series, you are putting them in parallel. Series means connect the cathode of one to the anode of the other, and then connect the other cathode and anode just like that of a single LED.

    And by the way, if you measured the current by shorting the battery with a multimeter, something you should not do, your 9V is practically dead, it should do over 1A short circuit.

    Bob
     
  5. zionosis

    zionosis

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    Oct 23, 2013
    I know what a series and parallel circuit are for gods sake. And no it isn't in parallel. It's in a series as I have been saying. I simply connect the LED's to the other LED's. Negative to positive. Just as you would a battery when you have them in series. Maybe you guys don't believe me. Well here is a picture. I have had them like this from the start.
    [​IMG]

    Please try to be helpful instead of being condescending and telling my all this basic children information I already know.

    lol. Hope someone doesn't come on next and give and even stupider answer and ask if I am even using LED's and claim I am here trying to light mini incandescent bulbs.

    Also before anyone asks if I am even measuring mA on the multimeter and not some idiot on volts accidentally. The setting is on 200m which is nowhere near the volts part of the thing. The section I am in has the A for amps and the flat line with broken line under it for DC current. I know how to use a multimeter alright.
     
    Last edited: Dec 25, 2013
  6. KJ6EAD

    KJ6EAD

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    Well since you already know all of this I suppose it would be condescending of me to suggest that a resistor and battery do not a constant current source make. Adding a second LED in series without changing the resistance accordingly will naturally result in significantly lower current. It's a good thing that I didn't say any of that since it's covered in *steve*'s tutorial and is already common knowledge among even the newest forum members who don't need to read it. :rolleyes:
     
  7. zionosis

    zionosis

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    Oct 23, 2013
    I have done it without the resistor also and it's the same story. Does the same thing.

    Also you aren't right. By no logic if you have a resistor with a battery and a string of LED's is the current meant to divide by each LED. That is what has happened to me with or without a resistor.

    Also I am pretty sure that the purpose of the resistor is to provide a lower suitable current for the LED's. From what I can tell a resistor with a battery does provide a constant current or close to it from all the other places I have read and even tested this with the multimeter and it's true. You claim it doesn't.

    How about helping instead of saying random crap like don't use a resistor with a battery. What the F else would I use a resistor for. I sure as hell wouldn't use a resistor with my constant current LED driver I bought since it doesn't need it. You use a resistor when you want to control the current and this is used typically with crap like batteries that don't have current regulation since its just a simple battery.

    Also LED's don't have a high resistance. So you are wrong again. They should not lower the current significantly at all when you add more in. The current is meant to stay very much the same. That is my whole problem here. How about trying to help.


    Can someone on here be helpful and address the problem. Look I have a source voltage that is capable of 85mA. Then I put two LED's onto this and the mA turn to crap like as if each LED somehow drops current as they do with voltage.

    I have tried other power sources and it's the same. Even tried other LED's.

    Obviously no one on here can help since the answers range from never use a resistor with a battery to are you even in a series to linking to a page that has low level information on it because they can't answer it themselves.

    As usual I will probably just have to figure this out myself. I remember before I came on here and people claimed you can't hook different voltage LED's together and said they would not light and others would blow up which sounded stupid to me and it was since they worked just fine.

    Also had people on here saying you can't hook LED's up in parallel on here. Yet in reality you can and many manufacturers actually wire theirs up in parallel also. So fail. Doesn't seem like people know much on here.

    Also sorry but LED's don't infinitely draw current until they burn out. If you operate them at the specified voltage they only draw a certain amount of current. (A lot of people on here mustn't even know how basic electricity works) That current is the mA that comes with them in the information along with the forward voltage. Idiots on here will deny this also. People on here don't seem to know that much. Probably wouldn't even know how to make a joule thief and claim it's impossible and never use a blah blah with a blah blah. It makes you LED explode like a grenade. lol. Stupidity.
     
    Last edited: Dec 26, 2013
  8. davenn

    davenn Moderator

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    thread locked

    check your discusting attitude

    and it was you who started your OP with
    you were treated as such and were given information based on your admitted lack of knowledge

    PM me when you have calmed down and are ready to make an apology and I will reopen the thread

    Dave
     
    Last edited: Dec 26, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Read section 2 on the page I linked to you earlier.

    If you already know all of that then you wouldn't be asking the questions.

    I won't go through your post point by point, but it is you that is misinformed, not us.

    The value of the resistor required for a constant voltage source changes as you add more LEDs in series. That link I gave you has the formula you need to use as well as lots of other information including links to sites that will calculate the value for you.
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    The battery might be able to supply 85mA but what voltage does it drop to when doing this. If drawing 20mA drops the voltage of the battery to below what is required to run both LEDs then this could be the issue. Would explain why the LEDs didn't blow up when you tried it without any resistance.
    just a thought
    Adam
     
  11. BobK

    BobK

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    Jan 5, 2010
    I am sorry, but you do not understand series and parallel circuits if you could make the statement in the first quote. My conclusion is that you had confused them. I was wrong. The correct conclusion was that you gave us incorrect information in the OP.

    I also prefaced my post with "If it is working like that", which means my post was not incorrect, since it was not working like that, the current was not reducing after each LED.

    Bob
     
    Last edited: Dec 26, 2013
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