What is the current flowing through my green LEDs, and why did one burn out before?

[aegimius]

This is for a green LED night goggle project. Originally, I foolishly connected two green 3 mm LEDs to a 3.6 power source(3 rechargeable Eneloop AA batteries) in parallel, without resistors. As soon as they were connected to the batteries, one of the LEDs burned out(smoke was rising from it, although it did produce green light for about a second before burning out), and the other was damaged but continued to produce green light. According to the manufacturer, the forward voltage of these green LEDs is 3.2 V. Here it is - https://www.alliedelec.com/search/productdetail.aspx?SKU=4050041#tab=Specs

Unfortunately, I can't get what the ideal operating current(mA) of these green LEDs is since it didn't come with the packaging or on the website. Nor can I get the peak operating current which I obviously exceeded since one of them burned out. For 5 mm and 3 mm LEDs, 20 mA is the most common current used, but I'm not sure at what current they burn out - 30 mA, 50 mA? Which current caused them to burn out, and why did the resistors make the difference?

Luckily I have plenty of spare 3 mm green LEDs, so I rebuilt this little device(LEDs in parallel again) but this time used two 33 ohm resistors(1/4 watt), and used the same 3.6 v power source(batteries 1.2 V each). This time they don't burn out and shine brightly(at peak or near peak brightness I'm guessing). I'm not even sure if I connected the resistors(one is on one of the LED's cathode, the other resistor on the other LEDs anode) correctly, since I am new to this. Now I'm wondering what the current going through my LEDs is now, since I am now using resistance.

Using a multimeter has revealed that my LEDs are operating at a 3.0 voltage, whereas the manufacturer says they run on a 3.2 V. Unfortunately, I already soldered and taped and everything, so I can't disconnect what I need to disconnect to measure the amps going to my LEDs.

The LEDs are connected in parallel, and green LED 1 has a resistor(33 ohm, 1/4 watt) attached to its anode, while LED 2 has a resistor attacked to its cathode. So I'm wondering what the current is going through these LEDs and hope it won't damage them soon. Is there a way to tell without taking this apart? Thanks.

 

[MikeMl]

Assuming the LEDs are similar (from the same batch), and both have a 33Ω resistor in series with them, then the currents through the two LEDs will be close to the same. Connect your DMM in mA measurement mode between the positive pole of your power supply and the wire going into the LED assembly. Whatever the current reads, divide that by two, and that will be pretty close to the actual individual LED current.

Most garden-variety LEDs used as indicators have a max current rating of ~30mA.

Without a current-limiting resistor in series with a LED, the current is limited only by what the supply can source. Just before you smoked it, the current through the dead LED was likely in excess of 1/2A.

 

[Pommie]

If the voltage across your LEDs is 3V then the voltage across the resistors is 0.6V and so the current is I=V/R = 0.6/33 = 18mA.

Mike.

 

[Boncuk]

According to LED data sheets green standard LEDs normally have a forward voltage Vf=2.4 to 2.6V and a forward current of 20mA.

Superbright LEDs are rated 30mA.

33Ω for a current limiting resistor will cause an LED forward current of 36.4mA (for Vf=2.4V) and 30mA for Vf=2.6V

In any case using 33Ω current limiting resistors at VS=3.6V will shorten LED life time considerably.

Better measure the current flow as suggested by MikeMl and adjust the resistors accordingly.

Boncuk