HUGE NOOB, need help please!

[xScorchMuffin]

Hello! This is my first post, and I could really use some help on a very basic project i'm working on. Basically, it's an LED attached to a push button, which is attached to a 2 AA battery housing, like the kind you'd see in the back of a remote or something. My problem though, is that no matter what I do, the batteries seem to heat up to temperatures almost hot enough to burn. I was wondering if there was anyway someone could help me figure out why the batteries are doing this? The LED is a 3V to 3.6V white LED. I have a very limited knowledge of anything electrical, and this seemed like a very easy stepping stone, but it's turned out to kind of a headache! Any help would be greatly appreciated!

 

[daGenie]

This sounds very strange, because a AA battery is 1.5 V (or a little greater for a fully charged battery) and connecting 2 in series should produce 3 V (or more). Anyway, the only reason I can imagine would be that the current being draw is too much for the batteries to handle. If this is the case, one way to correct it would be to connect a resistor in series.

To determine the value of the resistor, you need to know the voltage of the batteries without load (i.e. when not connected to the LED). Then, you can use the formula:

R = (Vs - Vled) / I

where Vs would be the battery voltage unloaded, Vled can be safely assumed to be 3 V and I is the current flowing through the LED. For maximum brightness, choose a value between 20 and 25 mA. Also, you can calculate the power rating of the resistor by using the formula

P = I*I*R

Hope this helps.

 

[BobK]

And the formula will give a 0 Ohm resistor since 3.0 - 3.0 = 0.

Is the LED lighting? I suspect you are simply shorting the battery. An LED with a forward voltage of 3.0 to 3.6 should not draw too much current from a 3.0V battery.

Bob

 

[xScorchMuffin]

And the formula will give a 0 Ohm resistor since 3.0 - 3.0 = 0.

Is the LED lighting? I suspect you are simply shorting the battery. An LED with a forward voltage of 3.0 to 3.6 should not draw too much current from a 3.0V battery.

Bob

Yes, it does light, both with the button and without the button in the circuit.

 

[BobK]

What is the max current rating of the LED?

Bob

 

[xScorchMuffin]

What is the max current rating of the LED?

Bob

I don't have it with me as i type this but, i'm pretty sure it's 3.6v.

 

[BobK]

Well, there is a big difference between 3.6V at 20mA and 3.6V at 10A.

Is this a small indicator LED or is it a power LED designed for lighting?

If the circuit is correct and the LED is lighting as expected, then it is probably too powerful to be run off AA batteries, though is it hard to imagine that it would be when run at the low end of it's voltage range. I would expect it would have to be drawing several amps for the batteries to heat up.

Bob

 

[xScorchMuffin]

Well, there is a big difference between 3.6V at 20mA and 3.6V at 10A.

Is this a small indicator LED or is it a power LED designed for lighting?

If the circuit is correct and the LED is lighting as expected, then it is probably too powerful to be run off AA batteries, though is it hard to imagine that it would be when run at the low end of it's voltage range. I would expect it would have to be drawing several amps for the batteries to heat up.

Bob

Hm. It's for sure not an indicator light, it's white. I can't for the life of me find the packaging for it. I'll probably just end up buying a new one. The project i'm working on is an altoids tin flashlight, so could you recommend what i should look for in an LED?

 

[BobK]

Then I am quite puzzled. It cannot be pulling enough current to heat the batteries and if the batteries are shorted it should not light.

Can you post a picture where we can clearly see how everything is connected?

Bob

 

[sndscientist]

altoids tin? it sounds like the batteries may be shorting to the tin. also what kind of AA's are we talking? alkaline, nicad, nimh? reason i ask that is the rechargeable types can supply enough current that it still may light the lamp even with a slight short. maybe it's not a great connection to the tin but passing 400 mah will sure heat up a battery and still light an led for a while

 

[xScorchMuffin]

altoids tin? it sounds like the batteries may be shorting to the tin. also what kind of AA's are we talking? alkaline, nicad, nimh? reason i ask that is the rechargeable types can supply enough current that it still may light the lamp even with a slight short. maybe it's not a great connection to the tin but passing 400 mah will sure heat up a battery and still light an led for a while

It's not connected to the tin in any way. the tin just serves as a project housing. The batteries were heating up even before i put them in the housing. And i used several brands of batteries, Duracell, Energizer, etc. I'm pretty sure those are all alkaline though.

I can't currently post a picture as i don't have it with me, but i will here in a day or so.

this is the switch i'm using, could that maybe be the problem?

http://www.radioshack.com/product/i...ce=CAT&znt_medium=RSCOM&znt_content=CT2032230

 

[CDRIVE]

Yes, it does light, both with the button and without the button in the circuit.

This makes no sense at all. Especially since this is such a rudimentary circuit. Schematic or good photos... Your choice.

Chris

 

[xScorchMuffin]

This makes no sense at all. Especially since this is such a rudimentary circuit. Schematic or good photos... Your choice.

Chris

Best i can do, i'm sorry it's terrible haha

 

[BobK]

Ouch! The switch is shorting the batteries.

Try this:



Bob

 

[xScorchMuffin]

Ouch! The switch is shorting the batteries.

Try this:

View attachment 8975

Bob

Ahhh! Alright thank you so much, told you I was a noob

 

[CDRIVE]

I'm still confused. If the switch is wired across the cells then the switch would short the Cells and the LED when the switch is closed and the LED would turn off. The cells would get hot only when the switch is closed. You said that the LED lights whether the switch is on or off.

Chris

 

[CDRIVE]

Since you're not using a limiting resistor your circuit should look like FIG A. Normally we use a current limiting resistor in series with the LED, as shown in FIG B. Personally, I would still like to see one in there even if its only 1 to 2.7 Ohms.

Chris