Newbie needs help with LEDs

Discussion in 'The Projects Forum' started by pcaston2, Dec 17, 2015.

  1. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    I am trying to hook up a simple circuit for a prop. I need to illuminate 3 LEDs and have them each wired so they can be turned off individually (in this case cut).

    I have at my disposal:
    Multicolored LEDs (red, yellow, green, etc.) With no voltage or amperage information.
    Resistor multi-pack
    Wires
    9V battery snap on adapter
    2x AA adapter
    4x AA adapter
    Multimeter

    I have read a bit on the topic. If possible I want to have each light 'cuttable' without affecting the rest of the lights. I understand this might not be possible with one power source. I know a series would likely need to be used so they don't all go out when one is cut, but the amperage would change as some are removed, and I'm not sure if LEDs can work with those fluctuatuons.

    My biggest issue at the moment is I have the LEDs without any information. Can I measure them somehow using the multimeter to get the V and A required? I have some idea how to identify the resistors and there amounts (or read them off the meter) but I can't find the right resistor without knowing the LED specs.

    I imagine I would have the positive lead off the battery going to the 3 resistors which are connected to the LEDs long leg (cathode?) And then the short legs would link back to the negative terminal on the battery.

    Any help would be fantastic, I can provide more information if needed. Thanks!
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    They should be wired in parallel with every branch having a switch on its own.

    [​IMG]
     
  3. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    Okay, thanks for the chart. If one of the lights is disconnected, it won't affect the other lights (in terms of V or A?) And they will go on working fine? If I am powering more lights do I need more battery power?
     
    Last edited: Dec 17, 2015
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    and each having their own resistor..
     
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  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    No.. removing one won't effect the others as each is in parallel with the other and essentially its own circuit..

    battery power will dictate how long it runs before the LEDs dim/go out..
    Battery voltage dictates how many you could put in series (not parallel)

    The power source needs to be greater than the sum of the forward voltage rating of the LEDs in each series string..
    So (examples)
    If the battery voltage is 3Volts and the LED has a Vf of 2.2V then you can only have 1 LED in series.. as 2.2 < 3 but 4.4 (2 LEDs in series) is > 3
    If battery voltage is 6V then you could have 2 LEDs in series as 4.4 < 6 but 6.6 (3 LEDs in series) is > 6

    If you had 10 LEDs in series (10 x 2.2 = 22) you would need a supply voltage of greater than 22 V
    make sense?
     
  6. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    Yes, that's helpful! I am going with green yellow and red. I have 3mm and 5mm, and am assuming the 3mm take less voltage. So if I have 3 3mm I could get away with a 9V? I am assuming once I add 4 the voltage isnt sufficient and they turn off? If there's too much voltage does it just draw what's needed, or does it fry if there's too much? Also, in parallel, how do I find out which resistor to use with each LED?
     
  7. ISB123

    Well-Known Member

    May 21, 2014
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    Size of led doesnt dictate its Vf.
     
  8. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    Okay. Well I'm assuming the reds are 1.7 and yellow/green are 2.2
    Wanting 20 mA I took 9 away from 1.7 and 2.2 respectively and divided that by 0.02

    The result was 365 and 340 ohms respectively. Would that do the trick (assuming the LEDs are close to right)?
     
  9. ISB123

    Well-Known Member

    May 21, 2014
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    Yes it should work fine.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    No.. You are thinking about typical resistive loads (sort of) which LEDs are not ..
    A regular resistive load that (for example) requires 12V at 1 Amp can be powered with a constant voltage power supply of greater than 1 Amp.. It will only draw the current it needs.

    With an LED you power it with a voltage greater than the sum of the Vf and you use a resistor to "set" the current of that circuit..
    In general, the forward voltage is the minimum voltage required to cause the LED to start "passing" electricity through it.. Not enough voltage and that led door is just shut and no current is flowing in that circuit..
     
  11. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    Awesome, thanks. Quick question. The difference of 240 and 265 is 25 ohms. Can I put 240 before the parallels and a 25 on the red? Does 'factoring' work this way or no?
     
  12. mcgyvr

    AAC Fanatic!

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    No idea what you are talking about now..
    240/265?
    parallels?
    Draw a diagram..
     
  13. ISB123

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    He wants to put 240 in series and 25 in parallel.
     
  14. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    Yes, sorry I actually meant 340 and 365 as mentioned earlier. If I did as mentioned, 340 in series and 25 in parallel for the lower voltage (red) LED, would the math work out the same? Is 25 ohms enough to even worry about, or should I just run it all on 350 or so.
     
  15. ISB123

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    Run all on 350.
     
  16. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    I've found a orange orange brown gold (330 ohm +- 5%) and a red red black gold (22 ohm +- 5%) and plan to wire these in series for 352 ohm then connect that to the parallel circuit. I should be able to wire 1 of these to test then add more in parallel as needed (up until the maximum voltage of the power source)
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The two battery boxes 2XAA & 4XAA, with alk. could be connected in series for 9V at much longer life than std. 9V battery.
    125 ohm in series with 3 LED's should be about right for 9V. Measure V across 125 ( 2.5V ) then modify as needed.
     
  18. pcaston2

    Thread Starter New Member

    Dec 17, 2015
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    The 3 lights may be switched off independently so I am planning to do it in parallel, but if possible I am putting the resistors in series to avoid one per LED.
     
  19. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Still not getting it are you..
    With (for example) a 3V power source and a 2Vf LED you could have thousands of single LED+resistors circuits in parallel and all will be fine (battery won't last long)
    Its when you start putting multiple leds in series that summing and voltage rating comes into play..

    You should really post a diagram of your intended circuit before doing anything..
    I'm thinking the concepts aren't sticking for you.
     
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