Science Fair Project Help

Discussion in 'The Projects Forum' started by Morpho, Dec 13, 2013.

  1. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    To make a long story short, I’m having trouble helping my son with his science fair project. We decided to look at the relationship between resistance and the life of an LED. I wired 8 LED’s in parallel (there’s no significance to the number 8, it’s just the number of LED’s I had in the box that were all 1.9V and 20mA) with 8 different resistors (0, 1, 10, 33, 82, 220, 510 and 1K ohms). I’m using a DC power source that’s putting out 14.6V. I figured I could wire the LED’s in parallel and avoid having to provide 8 separate power supplies. I expected the 4 LED’s below 82 ohms to burn out and the other 4 to continue to be lit for the duration of the experiment. However, when I built the circuit, none of the LED’s light at all. I suspect I'm missing something basic and any help or ideas would be greatly appreciated.
  2. mcgyvr

    mcgyvr Senior Member

    Joined:
    Oct 15, 2009
    Messages:
    3,297
    Location:
    North Carolina, USA
    Do you have them wired backwards? They are polarity sensitive..
  3. gerty

    gerty Well-Known Member

    Joined:
    Aug 30, 2007
    Messages:
    939
    Location:
    Tennessee GMT -5
    The first thing I would check is the polarity of the leds.
    That is of course that you're certain they're good.

    Beat me by a few seconds..
  4. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    Checked with a 1.5V battery for polarity and they light.
  5. gerty

    gerty Well-Known Member

    Joined:
    Aug 30, 2007
    Messages:
    939
    Location:
    Tennessee GMT -5
    Usually connecting a led to a battery without a current limiting resistor is fatal to the led. Yes it'll light up the first and maybe second time, but it usually won't live long after that.. Kinda like striking a match to see if it's good, it'll never strike the second time.
  6. JohnInTX

    JohnInTX Well-Known Member

    Joined:
    Jun 26, 2012
    Messages:
    1,013
    Location:
    Texas GMT-5
    What is your DC source? With a power supply and your hookup, it may be that it goes into current limit. If its a foldback current limit, it will drop the applied voltage until the overload is reduced. With 0 ohms, you've essentially shorted the supply. If the LED fails shorted, the others won't light.

    If the source is a battery, it also may not have the current capacity to overcome the overload.

    Have you measured the voltage on the 14.6V bus when its not lighting?
  7. BobTPH

    BobTPH Active Member

    Joined:
    Jun 5, 2013
    Messages:
    560
    How about a circuit diagram? Just becuase you said you hooked them up in parallel does not mean you did it correctly.

    Bob
  8. t06afre

    t06afre AAC Fanatic!

    Joined:
    May 11, 2009
    Messages:
    5,939
    Then using a 1.5 volt battery. The voltage will be safe for the LED. The picture shows a typical voltage current curve for a typical red LED. At 1.5 and up to 1.7 volt the current will be in a safe area. However at about 1.8 the voltage current curve gets very steep. And with a very small increase in voltage you will have currents that might destroy the LED. So the bottom line. With a 1.5 battery the voltage will be safe for say testing since it is the battery chemical properties that decide the voltage. But by using other voltage sources that may drift some. A resistor must be used to protect the LED

    Attached Files:

  9. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    The power source is an power supply from an old printer.

    This is new to me so I'm not sure I got the schematic uploaded properly. If not, I'll keep trying.

    You're making me wonder now whether the circuit is even viable. Would there be a better way to do this without using a separate power source for each LED?

    Thanks

    Attached Files:

  10. wayneh

    wayneh AAC Fanatic!

    Joined:
    Sep 9, 2010
    Messages:
    8,380
    Location:
    Roscoe, IL
    I agree you should eliminate - for now - any LED with less than the 510Ω resistor. One or more of them may have failed to a short.

    [​IMG]

    Attached Files:

  11. t06afre

    t06afre AAC Fanatic!

    Joined:
    May 11, 2009
    Messages:
    5,939
    It would help if you could take a readable picture of the information label for your power supply. This may be hard to do if the information is stamped into the plastic with heat. A problem I may see is that the LED wit zero ohm shorts out your power supply. And some protection mechanism lower/cut off the output voltage. Do some experiments by removing the lowest resistors values one by one. Until your setup works (probably then removing the 0 and ten ohm resistor. For this experiment you may require a power with more juice. LIke some old laptop power supply. But I will recommend not using zero ohm as series resistor in any case.
  12. BobTPH

    BobTPH Active Member

    Joined:
    Jun 5, 2013
    Messages:
    560
    Yep, that is the correct circuit. Agree with Wayne, one of them is probably failed shorted and taking away the voltage from the others. Did you measure the power supply voltage with them connected?

    Bob
    inwo likes this.
  13. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    I think the circuit design is bad overall. I thought we would be able to measure how long the LED's would last with different resistances. However, it just looks like we created a short circuit. Any ideas how accomplish the same goal without setting up 8 different circuits?
  14. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    I think the circuit design is bad overall. I thought we could see the effects of the different resistances in a single circuit. It appears we only succeeded in creating a short circuit. Any further ideas how to achieve the same goal without setting up 8 different circuits? Thanks again for all the input.
  15. tshuck

    tshuck E-book Developer

    Joined:
    Oct 18, 2012
    Messages:
    3,199
    Location:
    Long Beach, CA
    The LED lifespan isn't affected by the series resistance, it is the current through it that affects it. Now, your setup uses resistors to set a current, but this is dependent on voltage supply ripple and thermal effects from use. You could try constant current sources.

    I would say that an experiment to pulse the LEDs with a much higher current for a short amount of time, but varying in how much current and how much time with the average of 20mA might make for a more interesting experiment, but is considerably more complicated...
  16. tracecom

    tracecom Well-Known Member

    Joined:
    Apr 16, 2010
    Messages:
    3,300
    Location:
    West Tennessee
    I don't know how complex a project you want, but here's a simple suggestion. Use a 6V lantern battery in the attached circuit. Demonstrate how varying the resistance with VR1 varies the brightness of the LED by allowing more or less current through the circuit. Calculate and post the current at maximum resistance (5220 ohms) and minimum resistance (220 ohms) and some points in between. Explain how you use ohm's law to determine the current. Add in a current meter if you have one. As a finale, set VR1 to minimum brightness, short out R1, rotate VR1 to maximum brightness, and explain that the LED briefly was subjected to unlimited current and therefore burned out, at which time all current ceased to flow. Make the LED easily replaceable and repeat the demonstration as many times as you want, or until you run out of LEDs.

    Attached Files:

  17. Morpho

    Morpho Thread Starter New Member

    Joined:
    Dec 13, 2013
    Messages:
    13
    Thanks to all who offered criticism and suggestions. It's very much appreciated!
Similar Threads
Forum Title Date
The Projects Forum Faraday Flashlight for 7th grade Science Fair Oct 12, 2012
The Projects Forum Science Fair project Sep 28, 2011
The Projects Forum Science project Feb 5, 2014
The Projects Forum Help for a project : Sciences PI Plate Oct 22, 2013
The Projects Forum Help with son's Faraday Cage Science Project Apr 3, 2012

Share This Page