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Old 11-30-2019, 03:34 PM
nelliebly is online now
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Which LED mode burns out faster?


I have blinking LED lights that clip on my sneakers. I have to walk in the dark, and they make me more visible to drivers. Blinking mode is supposedly more effective than steady mode. Which mode is more likely to wear/burn out sooner?
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Old 11-30-2019, 03:41 PM
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Regardless of which mode you choose, you'll probably lose them before they burn out. But, at least in theory, they should last longer when they're blinking since they spend a portion of the time not energized.
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Old 11-30-2019, 03:57 PM
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The battery will die long before the LED burns out. The frequency of the flashing is probably directly related to the life of the battery. Flash 2x a second, extend battery life ~2X.

At one time LED throwies were a thing. Tape a LED to a coin battery, add a magnet and throw at a metal surface to stick. Reports are that they lasted for weeks to months depending on the type of battery and LED brightness.
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Old 11-30-2019, 04:06 PM
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For reference, the flashing LED on a smoke detector powered by a 9V battery lasts for years. Replace the battery and you're good to go. The lifespan of a smoke detector is ~8-10 years and the LED is probably still working fine.

Last edited by lingyi; 11-30-2019 at 04:08 PM.
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Old 11-30-2019, 04:07 PM
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Originally Posted by lingyi View Post
For reference, the flashing LED on a smoke detector powered by a 9V battery lasts for years.
And it's doing a whole lot more than just blinking the LED.
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Old 11-30-2019, 05:46 PM
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Turning LEDs on/off repeatedly does not wear them out. In fact, most LEDs in use are blinking faster than the human eye can see. Because that's the most efficient way to control their brightness.
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Old 11-30-2019, 07:59 PM
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Thanks for the replies! I'm going to let 'em flash and not worry about it then. And while I have a genius for losing things (Sounds so much nicer than "I'm a ditz," don't you think?), I'm so conscious of these, I don't think I'll lose them.
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Old 11-30-2019, 09:38 PM
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... most LEDs in use are blinking faster than the human eye can see.
Right. You can notice this by scanning your eyes rapidly across the light (best done in low light). For a short time afterwards you see multiple persistent images.
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Old 12-01-2019, 09:06 AM
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I though this was going to be about the two modes for limiting the power to an LED. And it indirectly is.

An LED, once the power starts flowing, has very little internal resistance. So the current shoots up. This will cause the LED to burn out. (Not long term, but as in "right now".)

So there are two ways to avoid this:

One is to add a resistor in series. Some LEDs come with a resistor built-in. This limits the current and all is well. Except now the resistor is wasting power and reducing battery life.

The other is to flash the LED off and on. Often too fast for a person to notice in most situations. Then the LED won't overheat. But this requires a more complicated circuit than a resistor. And a lot of makers want to save every penny and not everyone does this.

Note that the flashing in sneakers is too slow of a rate for this. But if you're putting in a blinking circuit, may as well to both. A fast blink when when on and a slower blink on top of that.

As to the OP: if done right the LED won't burn out in any reasonable time frame. The issue is battery life. If the LED is "on" (but either quickly blinking or going thru a resistor) half the time, the battery runs nearly twice as long. There is some battery life lost due to running the circuit all the time. Plus there's a background loss whether you're walking or not. (If you put the shoes away for a few years and start using them, expect the battery to be in poor condition if it works at all.)
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Old 12-01-2019, 09:45 AM
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One is to add a resistor in series. Some LEDs come with a resistor built-in. This limits the current and all is well. Except now the resistor is wasting power and reducing battery life.
Keep in mind the technique of using a series resistor may not work if the LED's forward voltage drop is close to the power supply voltage. (The LED will likely fail.) LEDs are "happiest" when powered from a constant current source.
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Old 12-01-2019, 09:50 AM
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Quote:
Originally Posted by ftg View Post
One is to add a resistor in series. Some LEDs come with a resistor built-in. This limits the current and all is well. Except now the resistor is wasting power and reducing battery life.

The other is to flash the LED off and on. Often too fast for a person to notice in most situations. Then the LED won't overheat. But this requires a more complicated circuit than a resistor. And a lot of makers want to save every penny and not everyone does this.
Are you saying you can use PWM with a constant voltage source to drive an LED? I thought even with PWM, you needed another mechanism to limit the current during each pulse of the PWM.
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Old 12-01-2019, 10:05 AM
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Are you saying you can use PWM with a constant voltage source to drive an LED? I thought even with PWM, you needed another mechanism to limit the current during each pulse of the PWM.
The current rating of an LED depends on the duty cycle. A duty cycle of 100% (continuous current) requires a relatively low current. As the duty cycle decreases the allowable peak current through the LED can increase (along with the peak brightness). But there's also a max current rating that can't be exceeded regardless of duty cycle.

According to one datasheet I just looked at, the LED had a max continuous current rating of 20 mA, but it can be driven with a peak current of 100 mA if the duty cycle is less than 10%.
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Old 12-01-2019, 10:16 AM
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Are you saying you can use PWM with a constant voltage source to drive an LED? I thought even with PWM, you needed another mechanism to limit the current during each pulse of the PWM.
Remember, there’s no such thing as a “constant” voltage source. There’s always parasitic resistance. In a battery-powered circuit, there may be enough internal resistance in the battery that an LED can be driven with no dropping resistor, as long as the pulses are short enough.
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Old 12-02-2019, 10:55 AM
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For reference, the flashing LED on a smoke detector powered by a 9V battery lasts for years. Replace the battery and you're good to go. The lifespan of a smoke detector is ~8-10 years and the LED is probably still working fine.
PEDANT ALERT!

It is actually a smoke alarm, as (hopefully) listed under Standard ANSI/UL-217. They must be replaced every 10 years, per NFPA 72.
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Old 12-02-2019, 04:17 PM
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LEDs are semi-conductor devices, and, like all solid-state devices, subject to atomic migration while warm. LEDs run 'hot', and are subject to a brightness decay over their lifetime (as are Florescents).

Bright as the LEDs on your feet are, they probably don't get as warm as residential or industrial lighting. I expect that the brightness decay might be measured in decades, or even centuries.

Like all solid state devices, LEDs are also subject to random sudden failure. That's also related to temperature. And LED lights are also subject to driver failure -- actually, I expect most LED lights to die from driver failure before they get dim enough for you to notice. Driver failure works the same way as LED failure -- temperature, decay and random failure, plus, for mains-power devices, spikes, over-voltages, under-voltages and conducted emissions of all types.

For your feet, I expect the most common causes of failure to be battery failure and mechanical failure.
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