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Resistors

Discussion in 'Cases, Modding & Electronics' started by specks, Apr 20, 2011.

  1. specks

    specks New Member

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    In a simple DC curcuit consisting a battery and a LED, where do you place the resistor? on the positive or negative wire?
     
  2. AthlonX2

    AthlonX2 HyperVtX™

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    i believe it goes on the positive side the longer leg
     
  3. Loosenut

    Loosenut

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    Last edited: Apr 20, 2011
  4. Iceni

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    I didn't think it matterd in a series circuit could be wrong tho, Been a long time since i did electronics. Always thought DC series circuits normalised. If it was parallel then it would only matter as to which "loop" you were on. Since the resistor is only there to soak current and/or draw voltage anyway.
     
  5. qubit

    qubit Overclocked quantum bit

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    It doesn't matter which side you put it on, it works just the same.

    However, circuit design convention might favour one side or the other.
     
  6. Iceni

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    also V=IR.

    you know the voltage on the battery. 1.5V (1.2 if its a rechargeable AA).

    You know the rated ampage of the LED. 20mA 30mA ect.

    therefore dividing the voltage by the rated ampage you can get the absolute min resistor you need in Ohms. 1.5/20x10-3 = 75 ohms. Add a little more to the resistance to lower the ampage of the circuit tho to improve longevety in the circuit.

    You can also add more LED's into the ciurcuit then rebalance, using the same equation just multiplying the rated ampages of the LED's together.


    so 25 LED's would need 1.5/(20x10-3 X 25) = 3 Ohm resistor.
     
    Last edited: Apr 20, 2011
  7. W1zzard

    W1zzard Administrator Staff Member

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    makes no difference, resistors have no sense of polarity
     
  8. Eric_On_Web New Member

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    W1zzard is right...bipolarity makes no sense :D
     
  9. Zyon

    Zyon New Member

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    Think of the LED and the resistor as a combined unit, doesn't matter where you place the resistor as long as it is on the same series circuit.
     
  10. specks

    specks New Member

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    From my POV, since electrons flow from the positive pole to the electrical device then to the positive pole, i think the resistor should be placed before(on the negative side)the electrical device to limit the electricity before going to the device.

    In my own understanding, if i put the resistor on the positive side then the full current would pass through the device before it could be limited.

    Correct me if I'm wrong.
     
  11. Fourstaff

    Fourstaff Moderator Staff Member

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    Elementary physics I did suggests that it doesn't matter where you put the resistor, before or after the LED since that the current flow is proportional to the total circuit resistance if voltage is kept constant. However, my EEE friend told me that in advanced circuit designs, sometimes the position of the components might affect the setup. How so I have no idea.
     
  12. slyfox2151

    slyfox2151

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    this is incorrect :D

    electricity doesn't exactly work like that. it really does not matter where the resistor is placed in this design, it wont make a difference at all witch side its placed on.




    electricity works in mysterious ways:D




    Note:
    i know fuck all about electricity, only what i learnt in high school and found on the web/ Common sense and logic :D
     
  13. qubit

    qubit Overclocked quantum bit

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    No. Complete rubbish.

    Look up how series and parallel circuits work and you will understand why. Searching Wikipedia for "series circuits" or "parallel circuits" is a good starting point. :)
     
  14. MLG The Canadian

    MLG The Canadian New Member

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    I'm studying Electronics Engineering, and I've got three classes on just circuit analysis and design.

    W1zzard nailed it, resistors have no polarity because a traditional resistor has a fixed value, and the current passes through it the same regardless of how it is ... biased? faced?

    If you're building a circuit, and this is my personal recommendation: face them all the same way, with the tolerance band on the right, so you can read the values easier.

    Edit: Also, facing them the same way is a great way to get into the habit of organizing your designs for more complex electronic components. Once you get to transistors and capacitors and such it gets pretty hairy pretty quick.
     
  15. Brandenburg

    Brandenburg New Member

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    well.. to all that says it doesnt matter which side obviously has no clue how a diode works.. or what a resistor does with in the circuit.. a resistor in this circuit is used as a current limiter so that you dont burn out the LED... The resistor MUST the placed on the anode side...(positive) because a diode only allows current ONE way... if you put the resistor on the cathode(negative)side, the resistor wouldnt be able to do its job

    the correct way to figure out the correct resistor for the LED is to:
    1) determine the voltage drop for color/type of the led.. use the LED package or WIKI http://en.wikipedia.org/wiki/Light-emitting_diode
    2) determine the current for the led... usually 10,20 or 50ma

    the higher the resistor the dimmer the led
    source-LED Voltage/current= resistor

    if you use a RED LED the voltage drop is 1.63 to 2.03 for that type of resistor.. i usually use a rough approximation of the average of the min and max

    12V-1.8V/50mA=204 ohms
    10.2V /50mA

    the last LED's i Used actually had what resistors values would work on the back of the package

    usually a 470 ohm resistor is plenty
    i might be wrong too but i have a degree in electronics:)
     
    Last edited: Apr 20, 2011
  16. MLG The Canadian

    MLG The Canadian New Member

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    I was under the impression he was asking if resistors had polarity in a typical series circuit.
     
  17. Brandenburg

    Brandenburg New Member

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    i was under the impression he was askin what side of the diode to place the resistor

    EVERYONE knows that resistors have no polarity.. DIODES do on the other hand

    in anycase.. i gave him the correct way to figure the resistor for said circuit
     
  18. MLG The Canadian

    MLG The Canadian New Member

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    Yes, you're right.
     

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  19. Eric_On_Web New Member

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    I correct you it dont go from positive pole TO the positive pole again hehe

    BTW we had a full electronis course when i made my car mechanic class and its pretty pointless since car mechanics changes the full modules anyway. If the computer tells its broken replace!

    More than pointless i dont even worj in the domain so icouldnt care less lol. The last stuff i made was a repair of headlights on my car. Short in the frame with partially cut and rusted positive wire.

    Solution: After 2 days I found out the positive was the problem. Seeked the wire cut and it was too messy with near 75 wires around there. Soldered a wire there pluged it with a fuse right onto the car battery. Problem solved. I'm a practical man but i like to see what other people makes in electronics.
     
    Last edited: Apr 20, 2011
  20. slyfox2151

    slyfox2151

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    why not? this will give you unlimited free electricity :D
     
  21. Eric_On_Web New Member

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    Yeah right but its not alternative current :p
     
  22. specks

    specks New Member

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    Big Thanks to all of you for clearing up things for me!:respect:
     
  23. Brandenburg

    Brandenburg New Member

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    np.. glad we could help..
     

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