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HI guys I'm new, i bought some IR leds to do some experiments, the shop clerk claimed that they were 5 volts (AFAIK this kind of leds do not exist) but other that that they give me no tech specs.
These leds were really cheap so i bought some to do some testing, a single led cam be powere up with a single AA alkaline batttery without aany kind of resistor , trying them with a 4.5 battery fried one in a couple of seconds (i expected that but i wanted to verify claims of the shop keeper), 3 leds in series worked good with a 4.5 v battery and were very bright with a 4x aa barrtey pack thought they were very warm to touch they seemed to be fine (i kept them on for 5 minutes).
Is this warmness normal? if not what should be a correct resistor generic value to use them with a 6v (4x AA battery) and/or with USB power, id l'ike to avoid to fry my computer USB and more than that my face.

Thanks in advance to all.

The typical Vf (Voltage forward) of IR LEDs is 1.2 to 1.5 Volts.
The typical maximum If (Current forward) is 50 to 100 mA.

Three in series will be 3.6 to 4.5 volts.
With a 6 volt (4 AA) battery and a nominal 50 mA current the current limit resistor will have a value of 30 Ohm to 48 Ohm.

The most logical value to use would be a 47 Ohm.
For 4.5 (3x1.5) on 6 volt battery the current would be 32 mA. Resistor drops 1.5 volts.
For 3.6 (3x1.2)on 6 volt battery the current would be 51 mA. Resistor drops 2.4 volts.
You could measure the voltage across each LED to verify Vf under these 47 Ohm conditions. This would be a good first step.

The LEDs should not be getting "warm", if they are, then to much current is flowing.
The lowest current limit resistor value to start with is 27 Ohm.

Word of advice: Do not operate LEDs at the maximum rating unless you know Electronics and how to do so properly!

Use a USB power pack (5 Volts) rather than hooking up directly to the PC.
10 Ohms is a good value for series IR LEDs with Vf of 1.5 Volts.
27 Ohms is a good value for series IR LEDs with Vf of 1.2 Volts.

Thanks for your help I should have some 10 ohm resistor laying around somewhere, i can add them in series until the led are "cold" (actiually I'll try to reach the value most near to the ones you are suggested and verify them with a multimeter) . im planning to use the 4x AA power pack (since it has it's own power switch) so i will use your calculation for the 6v assembly. if either the led or the batteres are warm I will know that something is wrong.

dewey1 @ 06/03/2012 - 18h12 a dit:

The typical Vf (Voltage forward) of IR LEDs is 1.2 to 1.5 Volts.
The typical maximum If (Current forward) is 50 to 100 mA.

Three in series will be 3.6 to 4.5 volts.
With a 6 volt (4 AA) battery and a nominal 50 mA current the current limit resistor will have a value of 30 Ohm to 48 Ohm.

The most logical value to use would be a 47 Ohm.
For 4.5 (3x1.5) on 6 volt battery the current would be 32 mA. Resistor drops 1.5 volts.
For 3.6 (3x1.2)on 6 volt battery the current would be 51 mA. Resistor drops 2.4 volts.
You could measure the voltage across each smd-5050 to verify Vf under these 47 Ohm conditions. This would be a good first step.

The LEDs should not be getting "warm", 12v led lighting if they are, then to much current is flowing.
The lowest current limit resistor value to start with is 27 Ohm.

Word of advice: Do not operate LEDs at led "flexible strip" the maximum rating unless you know Electronics and how to do so properly!

Use a USB power pack (5 Volts) rather than hooking up directly to the PC.
10 Ohms is a good value for series IR LEDs with Vf of 1.5 Volts.
27 Ohms is a good value for series IR LEDs with Vf of 1.2 Volts.

Can I use any type of solder with flexible LED strips?
I am trying to wire together several smaller pieces. The leads on the strips are copper.