LED's

[Baske7Cas3]

Can someone please explain to me why the LED's I have will light up when I use a 9v battery but not when I use a AA battery? I am really confused by this... Don't both the 9v and AA batteries have an output of 1.5v? And if not, shouldn't I still see some kind of reaction to the AA battery?

 

[philba]

wow, where to start? First off, you really need to spend a little time reading up on LEDs.

An LED has a specific forward voltage drop - the Vf value from the datasheet. Think of this as the amount of voltage the LED consumes. This can range from 1.7V up to >3V, depending on the LED. Look on the LED datasheet. Most are typically in the 1.9 to 2.1V range. You would think that 1.5V is about 3/4 of that so it should light up. well, no. when you apply the LED with no resistor to the terminals of a 1.5V battery, it effectively creates a short circuit and the voltage will drop significantly. Probably to less than 1V. That's why the LED doesn't light.

However, the LED will be damaged if the current exceeds a device specific limit - If from the datasheet. Typically it's something like 100 mA. There is also a recommended If for continuous operation, probably 20 mA. Since you did nothing to limit the current through the LED, it was probably pretty high - depending on the internal resistance of the battery. If you left it hooked up to the 9V, it would eventually burn out or the battery would overheat and burst open.

 

[things]

9 volt batteries output 9 volts, and AAA, AA, C, D batteries output 1.5 volts. what colour is the led? i'm suprised they lasted on a 9 volt battery

 

[things]

oh you posted the same time as me

 

[Baske7Cas3]

As for Philba thank you a lot, i know i need to read up on LED's more i am a beginner after all, i think i was confusing a D battery though for a 9v lmao... well the voltage output that is (i know im dumb). And actually thing, i used quite a few LED's lol and none of them burned out...

But why did you say the AA that the circuit uses would light my LED?

 

[things]

well an led will light on a AA battery, but only a little red one. red led's need about 1.2 volts, yellow led's are 2 volts, blue and white are about 3 volts.

 

[Baske7Cas3]

hmmm... I have a tiny red one that wont light... I will try again.

 

[RODALCO]

Or build a joule thief, it will run any colour led on 1.5 volts.
see attached link

**broken link removed**

 

[Baske7Cas3]

Thank you Rodalco but for the project I am working on an LED is definitely not a big issue, just a cool addition (but just so you know i saved the think for future use, I wasn't just saying thx to be nice lol). Also, thing, I guess the LED i was trying to light was orange, i couldn't tell lol, but i can get a red LED (which works for sure) off of the first flash circuit that fired, WOOT!!!! now i just need to de-solder it.

 

[Hero999]

You would think that 1.5V is about 3/4 of that so it should light up. well, no. when you apply the LED with no resistor to the terminals of a 1.5V battery, it effectively creates a short circuit and the voltage will drop significantly. Probably to less than 1V. That's why the LED doesn't light.

That's not ture, connect an LED with a voltage drop of 1.9V to a 1.5V battery and it won't draw any current al all! This is bacause it's below the turn-on voltage of the LED. If you connected an infrared LED with a foward voltage drop of 1.2V to a 1.5V battery a large current would flow and destroy the LED.

Whilst it might be possible to hook up an LED (no series resistor) to a variable voltage supply and tweak it so it draws exactly 10mA this isn't desirable as different LEDs of the same make and model will require different voltages to turn on and draw 10mA. This is why you need to use a current limiting resistor and a power supply voltage considerably higher than the LED's forward voltage drop.

To calculate the value of the series resistor use the following formula:
[latex]R = \frac{V-V_F}{I_F}[/latex]
Where:
V = Supply voltage.
Vf = LED forward voltage drop, depends on the colour, normally about: 1.9V for red and orange, 2.2V for yellow and green and 3.5V for cyan, blue and violet.
If = Normal foraward curent (usually about 10mA).
The above is a rough guide only, look at the datasheet for the LED you're using.

 

[HarveyH42]

You should also be a little careful with the 9 volt battery... I had the lens blow off an amber LED. Before I had an LED tester, I would just grab a spent 9 volt battery to test, but once the battery was fresh not spent, and pop, bounced off the ceiling. Might not happen with all packages, this one was a super-flux square 4 lead package.

 

[RODALCO]

Good reason to wear safety goggles during testing !

 

[HarveyH42]

Didn't know they could go projectile, that's why I keep mentioning it when people are heading down that road.

 

[things]

an led and a 9 volt battery for testing you will soone find out you had a faulty led

 

[TrevorP]

Well this all explains why I burnt out two leds with a 9V.

Mind you I expected it to happen.

 

[audioguru]

An ordinary 5mm LED has a max allowed current of only 30mA.
A brand new 9V alkaline battery can provide 10A into a short circuit and 7A into an LED for a moment which will blow it for sure.

LEDs need a regulated current circuit or a current-limiting resistor.

 

[Hero999]

10A from a little 9V battery?

I don't think so, you'd be lucky to get 1A.

 

[audioguru]

10A from a little 9V battery?

I don't think so, you'd be lucky to get 1A.

Sorry, I was wrong. A little 9V alkaline battery can supply only 9A into a short circuit for a moment.

Go to www.energizer.com and click on Technical Info then on Energizer Alkaline Datasheets. In the Alkaline Battery Applications Manual they show that their 9V alkaline battery can supply 9A flash current and to beware af a fault in your wiring.

 

[RODALCO]

LED's blown up

Attached is a photo of two LED 's minus lenses which popped when lightning hit the parrallel to serial ports overhead cable on our cyclo plant in Warkworth during the storm.
The read switches next to it welded closed and the 1 N914's survived.
Ok surge diverters are in place which clamp the voltage to below 48 Volts but the LED's didn't like it.
OK a million or so volts does a bit of damage or so.