# Fire risk?

Discussion in 'General Electronics Discussion' started by cdoghw, Feb 12, 2015.

1. ### cdoghw

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Feb 12, 2015
Hey guys! I am doing a simple project with kids to teach them about electric circuits and want to know if anybody has insight on any fire dangers with my project. Basically we are connecting 1 simple cheap LED to 2 AAA batteries with basic electrical tape....if they left the light on would it cause a fire (assuming it was laying on paper or fabric or something potentially flammable)? Any info would be helpful!

2. ### Bluejets

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Oct 5, 2014
Might need a resistor in there also.
Otherwise, it depends on what LED.

3. ### Gryd3

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Jun 25, 2014
LED's have a 'forward voltage drop' when they are used. So if you use a 5V battery, and an LED with a 1.2V forward voltage drop, that means 3.8V is left over to do... nothing, something, burn?
So the standard is to use a resistor, not only will the remaining voltage fall across this component (The 3.8V left over from above), but the value of this resistor will dictate how much current goes through the LED. Say you only want 20mA (which is pretty common) you would use a value of 3.8V / 0.02A = 190Ω . This resistor is important to protect the LED.
All batteries have a property called an internal resistance. On some special cases, the internal resistance of the battery will be good enough to use instead of a resistor... I say special cases, because this internal resistance is usually very low... so if there is any more than perhaps a volt left over, the internal resistance may not be enough which will result in a broken and possibly smoking LED.
If you can keep the left over voltage as small as possible, you will usually be fine. (Like a 1.5V battery and an LED with a 1.2V forward drop)

Just a tidbit of info. That's exactly how I started with electronics... was taking old LED's to battery packs of taped up AA's, C's D's... anything I could find. I have also watched many LED's light up green, and begin to shift to a yellow color... then red... then off (followed by smoke).
If I had access and knowledge of resistors when I was playing, it would have made my projects easier than guessing to see if a battery pack would work. I highly recommend it. (They don't need to know the specific value of the resistor, just so long as they understand there is a value and what it does.)

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4. ### Harald KappModeratorModerator

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Nov 17, 2011
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5. ### cdoghw

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Feb 12, 2015
Thank you !!!