One problem is that most of us don't study how to burn up parts. Some old rule of thumb says double the lifetime for every 10% less power. If you don't actually melt the LED you are working with a, "shortened" life span. How short? I think you are going to be the pioneer on this research. This seems a stupid way to look at it, but how many times do you have to cut the LEDs life in half to get from 100,000 hours to 10 seconds? Even if you can do the math, the answer is not going to be reliable.

You're just going to have to burn a few of them up.

You'd have to cut it in half 25 times. If each cutting in half required a 10% increase in power (~= current in this case) then that would require right at 10x the current or 220 mA.

I agree that it is pretty unreliable, but that number doesn't seem too far out of kilter.

It might actually make a good local science fair project at this age to plot current versus lifetime for LEDs abused into lifetimes of a month and less.

 

I am on a working on a science experiment with my 5th grade son and my EE knowledge is a bit rusty. It's a basic circuit which involves turning on a 3mm red 20mA LED using Ohm's law to calculate R given different voltage values (3V, 4.5V, 6V, 9V - basically different combinations of AA batteries).

The three goals are to show that when the current is too low (high R value) the LED will not turn on. When current is just right at 20mA (using a calculated R value for the corresponding voltage) the LED lights up nice and bright. When the current is too high (low R value) the LED will (hopefully) burn out.

If I am using basic 1/4 watt resistors, the most I can push through a 100 Ohm resistor is 50mA, right? So if I am using a 4.5V battery configuration and a 100 Ohm resistor, that should get me 45mA.

Will that be enough to intentionally fizzle out my LED?

You can get zero-Ohm resistors - some manufacturers use them because they're easier for automated assembly than wire links.

Then all you need is a battery with more than 3.5V (for white LED) and not much internal resistance.

 

I think some of the replies are making this needlessly difficult. You have the right idea (but do take precautions against an exploding LED).

.

Exploding LEDs are actually pretty rare - The last PC case I got second hand had the front panel LED wired to a Molex connector - it never crossed my mind that anyone would be dumbass enough to do that without including a resistor, but they had.

The LED flashed at randomly varying rates - my best guess is that thermal expansion had fractured the bond wire and it was making only intermittent contact.

 

Why abandon the LED? Just calculate R for different operating currents. Such as 1, 10, 20, 50, 100, 200 mA. You'll see a change in brightness and will end with the LED burned out.

I wouldn't say that I want to abondon the LED but here's my rationale...

I thought the LED would have a very minimal operating range w.r.t. supplied current. In other words, I imagined it would be very faint at 12mA, nice and bright at 20mA, and would fizzle out at anything above 30A. That corresponds to R values of 225 ohms, 135 ohms, and 90 ohms.

If I need to push through 100mA (or more) to kill the LED then my R value would need to be no greater than 27 ohms (which is a huge difference from 90 ohms).

 

http://www.mouser.com/Optoelectroni...Z1z0w16qZ1z0wa5iZ1yzubgzZ1yyg4r3&Ns=Pricing|0

Thanks for the link but I wouldn't even know which one to pick.
What about something at my local radio shack or electronics supply store?
How would these miniature incandescent bulbs be any different than using an LED?

 

Thanks for the link but I wouldn't even know which one to pick.
What about something at my local radio shack or electronics supply store?
How would these miniature incandescent bulbs be any different than using an LED?

A tiny flashlight bulb is so generic! There is no formula for, "the right one to burn out on command". The only advantage might be that incandescent bulbs get spectacularly bright, but an LED won't. Personally, I have a drawer full of 6.3 volt, number 47 bulbs, so they would be cheap for me. Whatcha got laying around for free? That's all it amounts to.

 

A tiny flashlight bulb is so generic! There is no formula for, "the right one to burn out on command". The only advantage might be that incandescent bulbs get spectacularly bright, but an LED won't. Personally, I have a drawer full of 6.3 volt, number 47 bulbs, so they would be cheap for me. Whatcha got laying around for free? That's all it amounts to.

How would you connect that you a breadboard without any soldering involved?

 

How would you connect that you a breadboard without any soldering involved?

Yet another skill to teach your son, soldering.

 

Yet another skill to teach your son, soldering.

Don't even know where my soldering gear is. Do incandescent bulbs come in LED-style housing with leads?
Not looking for an easy way out, just not looking to spend any more money by getting a soldering gun, etc.

 

How would you connect that you a breadboard without any soldering involved?

If you would bother to look at the link I posted, you would see that the bulbs in positions 2, 7, and 9 have wire leads.

 

Exploding LEDs are actually pretty rare - The last PC case I got second hand had the front panel LED wired to a Molex connector - it never crossed my mind that anyone would be dumbass enough to do that without including a resistor, but they had.

The LED flashed at randomly varying rates - my best guess is that thermal expansion had fractured the bond wire and it was making only intermittent contact.

I agree, they are rare -- but imagine how this guy would feel if one of those rarely exploding LEDs blinded his son. And I even admit that if I were doing this I probably wouldn't wear any eye protection. But that doesn't mean that I wouldn't be fully aware that I SHOULD be wearing eye protection! If (hopefully when) I do similar stuff with my daughter, I don't know if I will be fastidious enough to have her (and me, to set the example) wear eye protection or not.

 

I wouldn't say that I want to abondon the LED but here's my rationale...

I thought the LED would have a very minimal operating range w.r.t. supplied current. In other words, I imagined it would be very faint at 12mA, nice and bright at 20mA, and would fizzle out at anything above 30A. That corresponds to R values of 225 ohms, 135 ohms, and 90 ohms.

If I need to push through 100mA (or more) to kill the LED then my R value would need to be no greater than 27 ohms (which is a huge difference from 90 ohms).

Don't overthink this.

Take what you've got and play around with it. If you don't get the LED to burn out with what you have, rub your hands together, make an evil grin, say, "More power!" and go get more batteries. You don't need to appear the all-knowing mage to your son -- make it a fun opportunity for both of you to experiment and discover and learn together.

EDIT: Fix typos.

 

Typo, Bahn. That's an, "evil" grin.
and, yes, I have seen an LED explode.
I love the crackle of exploding thermoplastic in the morning.

 

Had a student use an led as a clamping diode across a relay coil. He said it cycled 3 times before it blew its top..

 

Typo, Bahn. That's an, "evil" grin.
and, yes, I have seen an LED explode.
I love the crackle of exploding thermoplastic in the morning.

Damn autocorrect! I probably typed "evel" and it decided it knew better than me what I meant to type. Oh well, those evil even grins are even evil better!

 

Damn autocorrect!

Another grin for the fact that I have a, "stupid" phone.
It doesn't autocorrect or catch viruses. It just dials, rings, and stores phone numbers.

 

Another grin for the fact that I have a, "stupid" phone.
It doesn't autocorrect or catch viruses. It just dials, rings, and stores phone numbers.

Same here. I guess now the perjorative term is "flip phone". It works fine. I can make calls on the very rare occasions I need to make a call and on an even rarer occasion it receives calls. The few times I have tried to use a "smart" phone to surf the internet I have been extremely frustrated.

 

If you would bother to look at the link I posted, you would see that the bulbs in positions 2, 7, and 9 have wire leads.

Ok, suppose I am interested in this one. It operates at 3V and requires 60mA to operate. How different is this from an LED other than the fact that it requires three times as much current to operate? I'm not being ignorant, I'm just trying to understand how your suggestion of using an incandescent bulb is different from using an LED. Don't forget what I mentioned in my OP...my knowledge of electrical circuits is very rusty. I haven't played with bread boards in over 20 years since my undergrad days.

 

Ok, suppose I am interested in this one. It operates at 3V and requires 60mA to operate. How different is this from an LED other than the fact that it requires three times as much current to operate? I'm not being ignorant, I'm just trying to understand how your suggestion of using an incandescent bulb is different from using an LED. Don't forget what I mentioned in my OP...my knowledge of electrical circuits is very rusty. I haven't played with bread boards in over 20 years since my undergrad days.

Led is interesting.

Green are definitely more durable than red and more interesting.


A standard Green indicator LED with green plastic lens rated at 20/30 mA
100 ohm resistor (positive to LED)
Led voltage is measured anode to cathode

Voltage of power supply/Voltage across LED
2 / 1.8. = Dim green
2.2 / 2.0 = nice green
3 / 2.7 = light green
4.8 / 3.8 = yellowish green
4.2 / 5.2 = yellow
5.3 / 6.6 = orange
8 / 6.8 = red/orange
9.5 = dim red orange and hot smell (resistor was still cool)
Tried turning down power
2.5/2.2 = dim yellow. I could not get the green back
Power supply was adjusted slowly and LED powered the whole time. I am sure that short pulses would have yielded different results.

Red high brightness LED w/clear lens yielded brighter but died at 7 volts. Did not change color that I could see but my near infra-red vision is poor.

I would recommend getting a 100 ohm and a 33 ohm resistor and beating it up with a 3, 4.5, 6 and 9 volt battery pack.

Buy a pack of LEDs from a distributor for a few cents a piece instead of the RadioShack price.
TaydaElectronics.com has them for 3 cents each. Buy a hundred of a few different colors and he can have a live demo.

Shipping takes 2 weeks through their Colorado warehouse (from Thailand) all for a couple of bucks. Buy the battery holders, 1/2 watt resistors while you are at it.

 

yes its the current which counts. old batteries still maintain the voltage, but it breaks down when a load is applied.

So you can test all LEDs with old 9v battery.
If it cant light up a small bulb anymore its depleted.