"flashing" LED

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phurm
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"flashing" LED

Post by phurm »

I got tired of the light bulb burning out in my small flashlight so I broke open a dead bulb and soldered a LED in place. I did not put in any kind of current limiting resistor, just an LED.<p>This worked fine for about 5 or 10 minutes total of intermittant use. At that point the LED started strobing/flashing at a rate of perhaps 5 to 10 per second (?) - fairly fast. Putting in fresh batteries made no difference so I assume this is being caused by "damage" to the LED.<p>Can anyone explain this to me? Is this a known behavior of LEDs that get "damaged" from excess current caused by no resistor? (or what?)<p>As an aside, how do they make LEDs that are supposed to flash? Is this somehow related to the behavior I am seeing above?<p>TIA for your thoughts.<p>Paul Hurm
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viveguy
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Re: "flashing" LED

Post by viveguy »

Your led probable burned up due to passing too much current.You need to have a limiting resistor in series.<p>Yes, there are led's made with built in flasher.They are available from Digikey.Youcan also find direct replacements for incandescents.Search through the company listings at ledmarketplace.com or epanorama.net.
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jollyrgr
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Re: "flashing" LED

Post by jollyrgr »

I will try to explain how and why you must bias an LED. First an LED, like most semiconductors, will have a certain forward voltage drop, depending on its chemical makeup. For a red LED typical forward voltage drop is about 1.5 volts. A standard silicon diode for rectification in power supplies is about 0.67 volts. A white LED is (if I remember correctly) about 3.6 volts forward voltage drop. The rating is usually specified as a "TYPICAL" with a range of what would be considered "ACCEPTABLE" such as 3.0 volts minimum, 4.0 volts maximum. This is the voltage that causes the device to "turn on". This does not mean "light up" but it means to start fully conducting current. If you keep your forward voltage below the turn on voltage, current levels will be low and the device will not be damaged. <p>As with all electronic devices there is only so much current a device will conduct before it fails. So what does this mean? Unlike a linear device, such as a resistor, the voltage drop across the semiconductor will remain constant over a wide range of current through the device. Therefore you must use a CURRENT LIMITING resistor. If you do not (as you did with the flashlight), the LED will conduct too much current, over heat the PN junction, and fail.<p>You did not mention what kind of LED you were using so I will assume you used a white LED. A white LED has a typical forward voltage drop of 3.6 volts with a typcial current of 20 mA or 0.020 amps. Using ohms law, where E=I*R, you can calculate the resistor needed in series. <p>Take your supply voltage (the batteries) and subtract the forward voltage of the LED. This will give your "loop" voltage across the resistor. (The sums of the voltages around the loop must equal zero.) So if you have a six volt battery, and a white LED you get 6v-3.6v=2.4v. This means your series resistor will have 2.4 volts across it. Since you want to limit the current through the LED, you will calculate a value for the resistor. The current in series elements will be the same. Since you want to limit the current to about 0.02 Amps, you will need solve Ohms law for a value of resistance.<p>Vr=Ir*R Where Vr is resistor voltage, Ir is resistor current and R is resistance.<p>Vr=2.4v
Ir=0.02A<p>2.4v=0.02A*R
Solving for R gives:
120 Ohms<p>Find a resistor of a standard value that is at this value or just higher.<p>Just a side note, I have a keychain white LED flashlight that does not use any series resistor. The battery value is too low to fully turn on the LED. Do not think that this means the LED is dim; it hurts to look into this thing! And this is from a watch battery and LED. What you MIGHT consider doing is hooking two or more LEDs in series. This will cause the voltage drop of the LEDs to be lower; thus they never fully conduct and limit the current themselves.<p>This is somewhat over simplified but I didn't want to get into the VI curves and what not of LEDs. Another thing you can do to make REALLY BRIGHT LED flashlights is to vary the DUTY CYCLE. What you do is you dump a lot of current at a somewhat high voltage for a VERY SHORT time period. You do this repeatedly. To the human eye the light is constant. In reality the LED is on for only one tenth (or so) the time. This involves timer circuits and a few more descrete parts. Maxim makes a BUCK/BOOST chip that can be used to drive LEDs in this manner. If you REALLY want to get clever, see this article:<p>
http://www.maxim-ic.com/appnotes.cfm/ap ... umber/1021<p>They do make LED assemblies with internal limiting resistors. Some are made for direct replacement of incandescent lighting. I have seen a number on eBay for automotive replacement. But I do not know if these are DOT accepted or not. There are others that are specifically designed with built-in inverters to allow them to run on flashlight batteries. The typical two cell flashlight uses a PR-2 lamp. See this web page for an LED replacement lamp. (I have seen cheaper prices at electronic shows (read Ham Fests) and only give this web site as an example of what is available.)<p>http://www.techass.com/el/versalux/pr2/pr2.php<p>[ October 18, 2002: Message edited by: Jolly Roger ]</p>
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dacflyer
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Re: "flashing" LED

Post by dacflyer »

RADIO SHACK also sells flashing leds..you can wire them in series to get even more leds to flash.i made a excellent christmas tree out of several scrap leds and some flashers...have fun
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