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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 12-13-2013, 05:06 PM
Morpho Morpho is offline
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Default Science Fair Project Help

To make a long story short, Iím 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 LEDís in parallel (thereís no significance to the number 8, itís just the number of LEDís 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). Iím using a DC power source thatís putting out 14.6V. I figured I could wire the LEDís in parallel and avoid having to provide 8 separate power supplies. I expected the 4 LEDís 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 LEDís light at all. I suspect I'm missing something basic and any help or ideas would be greatly appreciated.
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Old 12-13-2013, 05:09 PM
mcgyvr mcgyvr is offline
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Do you have them wired backwards? They are polarity sensitive..
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Old 12-13-2013, 05:09 PM
gerty gerty is offline
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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..
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Old 12-13-2013, 05:14 PM
Morpho Morpho is offline
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Checked with a 1.5V battery for polarity and they light.
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Old 12-13-2013, 05:36 PM
gerty gerty is offline
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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.
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Old 12-13-2013, 05:56 PM
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JohnInTX JohnInTX is offline
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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?
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Old 12-13-2013, 06:29 PM
BobTPH BobTPH is offline
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How about a circuit diagram? Just becuase you said you hooked them up in parallel does not mean you did it correctly.

Bob
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Old 12-13-2013, 06:53 PM
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t06afre t06afre is offline
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Quote:
Originally Posted by gerty View Post
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.
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
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Old 12-13-2013, 07:22 PM
Morpho Morpho is offline
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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
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Old 12-13-2013, 07:40 PM
wayneh wayneh is offline
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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.

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