Question: Can you burn out an LED on a 1.5 volt battery?

[rockgardenlove]

I vote no, and another fellow votes yes.  By my calculations, an LED with a 1.7 volt drop can't achieve our target current of 15 mA flowing through it, without any resistor.   
This other gentleman is claiming that the inherent resistance in cheap wire (he measures 10-12Ω) is preventing his LED from blowing with a AA battery. 

So, a question for you knowledgeable fellows:
Can you blow an LED on a AA battery?
Can you even achieve a 15 mA current flow?

Peace!

[brett]

Hi
Interesting question....
as far as I know, current through an LED is proportional to the voltage squared. If Vf is 1.7 Volts and I is 15mA, then at 1.5 volts, I will be low (a few mA), but it might still light up a bit.

PS 1.5 V batteries don't generally put out 1.5V, so that might affect things.

[rockgardenlove]

Yeah...the funny bit is that the voltage drop exceeds the supply voltage.  R=(Supply-Voltage drop)/Current, no?  I ended up with -13.3 ohms as the proper resistor voltage.

And yeah, a brand new fresh AA seems to be 1.7 volts (measured several), so that is a factor.  Still worked fine after it started going down in voltage though.

[the_random_hero]

^Hi Christian! Long time no talk :p

Anyway, I was under the impression that current was determined by circuits resistance. The LED's going to drop the maximum voltage it can and the on-resistance is very small, so I'd be saying that the current drawn will be fairly high (giving you a good chance that you might blow some LED's)

[rockgardenlove]

Yes, but I should mention that it's been tested and it does not burn out.  Now we're just trying to decide if it's up to the 10-12 ohms in the cheap wire. 

[the_random_hero]

Yes, but I should mention that it's been tested and it does not burn out.  Now we're just trying to decide if it's up to the 10-12 ohms in the cheap wire. 

10-12 ohms? That's a LOT of resistance for just plain wire to have. I have a two foot length of 20 gauge wire here and it has no measurable resistance. I also have a 2 foot length of 24 gauge wire and it has less than .1 of an ohm resistance. I'm thinking there might be a problem with the wire in question...

[brett]

So the battery voltage and LED drop are the same (1.7 V).  That's handy.  As I said above, the current goes up with the square of voltage, so you can't move too far away from 1.7 V and still have it work.  (I suspect that at 1V it would barely light, and that around 2.5V it would burn out)  That's why we use current-limiting resistors and supply voltages much greater than Vf.

[amz-fx]

Q: Can you blow an LED on a AA battery?
A: Yes.

Q: Can you even achieve a 15 mA current flow?
A: Yes.

In reality, will you blow out an LED by connecting it directly to a 1.5v battery? That depends on a couple of things.

What is the determining factor in this question is the internal resistance of the battery and the fact that the turn-on point of the LED is not a sharp switch-on.

With an ideal battery and an ideal LED, you would draw infinite current with the LED once the voltage on it exceeded its forward threshold of conduction, and this would burn it out.

In the real world, the 1.5v battery has some internal resistance, which will limit the amount of current that can flow. This resistance varies with the age of the cell, material of its construction and the size of the battery. The AAA battery will have a higher internal resistance than a D cell, and carbon-zinc batteries have higher resistance than alkalines. Fresh batteries will be lower resistance than older units.

Also, the LED does not sharply turn-on at a certain threshold...  it will conduct to varying degrees at slightly lower voltages, and with some equivalent resistance. Because the Vth of the LED is about the same as the battery voltage, it will not fully turn-on and the combination of internal battery resistance and LED resistance may limit the amount of current that can flow to a level that the LED can tolerate.

A fresh alkaline D-cell will probably blow an LED connected to it very quickly. A carbon-zinc AAA will probably not damage the LED.

A powerful rechargeable NiMH may be able to blow an LED even though its voltage is measured at 1.25v, because it has very low internal resistance and can supply large currents.

Put a small value resistor in series with the LED for insurance - 10 to 22 ohms should do the job.

regards, Jack