I used an online calculator to get the resistor values.
My problem is, the IR LEDs are getting really hot. They're on top of a piece of plastic (monitor), and if they run for more than 2 minutes, I'm afraid that they will melt the plastic. How can I cool these down? Are heatsinks necessary? Or should I just get a bigger resistor?
I'd imagine they're supposed to get hot, the ones in the official sensor bar get very hot even through the case. IR LEDs are usually only used for short bursts in the case of remote controls.
For the IR Led : ( 5 V - ( 1.28 + 1.28 ) ) / 100mA = 24.4 ohms 33 is OK ( 5 - ( 1.28+1.28 ) / 33 = 73.939393 mA or 74 mA
Power dissipation : 1.28 * 74 mA = 94.72 mW per IR LED The resistor : 33 * 74 mA ^2 = 180.708 mW
For the Power LED : 5 V * 30 mA = 150 mW The resistor : 1 * 30 mA ^2 = 0.9 mW
As for getting hot, well that one is ??? According to calculation is not suppose to get THAT HOT !!! The IR led is lower than 1/4 Watt power dissipation. If the current is correct.
Check for short, maybe LED is faulty ( shorted ? ) Check for current --> put a DMV in series with the circuit. Plce the DMV in mA Mode. And check voltage too. Use an another supply voltage, NOT your computer !!!
@nexekho
According to calculations... NO nut suppose to get hot.
I had slightly bad data - the PC is putting out ~4.5v, not 5v.
Nope. It's practically cold.
TFel:
100mA though a 5mm LED will always get hot.
If you are worried about the heat affecting your monitor, put them in an enclosure or lower the current.
How low should I make the current? 50mA? I'm worried that the LEDs won't be bright enough, but hey, I can't see them, so all I can do is trial and error, I guess.
Also, would this enclosure provide any kind of heatsink capability?
I was thinking more along the lines of a plastic project box you can put them to insulate your monitor from them. If you mount the LEDs on a PCB and mount the PCB inside a project box with a cutout in the side for the LEDs to poke though, you should be fine.
Can you measure the current of the IR LED and voltage across the IR Led ? I am curious to know why it is hot. It is hot like, "I burning my finger here ..." or just warm... because a 1/4 Watt at the IR LED ( according to calculation ) can not be that HOT .. I am having a hard time to believe this. A 100 W light buld is " I am burning my finger here ..." is HOT to touch, but NOT 1/4 W !!
I know you can not see the IR LED, but you can still measure the current & voltage.... TIP : place a DMV ( in current measument mode ) in parallel with the switch to measure the current. Just don't turn ON the switch, the meter will connect your circuit. But, you will get a total current value ( according to Kirkov Current Law -- I total = I 1 + I 2 )
Firstly IR LEDs are rated for pulsed operation I suspect - that's how they are almost always used.
Secondly LEDs voltage ratings are very approximate (wide variation with batch and temperature). You should not be using a 1 ohm resistor for the blue LED, that will risk overloading the USB port. To drive LEDs you want to set the current, not the voltage, hence the common practice of a series resistor and a power supply voltage greater than the LED requires (typical blue LEDs are about 3.2 to 4.0V range at room temperature BTW, suggesting a 47ohm series resistor - that Radio shack one seems unusual - perhaps its violet instead of blue? Perhaps the specs are nonsense?).
Can you measure the current of the IR LED and voltage across the IR Led ? I am curious to know why it is hot. It is hot like, "I burning my finger here ..." or just warm... because a 1/4 Watt at the IR LED ( according to calculation ) can not be that HOT .. I am having a hard time to believe this. A 100 W light buld is " I am burning my finger here ..." is HOT to touch, but NOT 1/4 W !!
I know you can not see the IR LED, but you can still measure the current & voltage.... TIP : place a DMV ( in current measument mode ) in parallel with the switch to measure the current. Just don't turn ON the switch, the meter will connect your circuit. But, you will get a total current value ( according to Kirkov Current Law -- I total = I 1 + I 2 )
I can't measure the current right now, but it was definitely "I'm burning my finger here". Heck, it's possible it could have gotten to "it's melting the tape on it right now".
MarkT:
Firstly IR LEDs are rated for pulsed operation I suspect - that's how they are almost always used.
Secondly LEDs voltage ratings are very approximate (wide variation with batch and temperature). You should not be using a 1 ohm resistor for the blue LED, that will risk overloading the USB port. To drive LEDs you want to set the current, not the voltage, hence the common practice of a series resistor and a power supply voltage greater than the LED requires (typical blue LEDs are about 3.2 to 4.0V range at room temperature BTW, suggesting a 47ohm series resistor - that Radio shack one seems unusual - perhaps its violet instead of blue? Perhaps the specs are nonsense?).
It's blue, and the specs don't seem nonsense. It's a weird LED I guess.
Anyway, I was able to fix the problem. I replaced the 33 ohm resistor with a 47 ohm one. It basically halved the current. It also cooled the LED significantly. I looked at it with a camera (thanks pwillard!) and they seemed plenty bright enough. They work fine.
Don't forget that IR is just another word for HEAT! You're driving these IR LEDs pretty hard (20 mA is probably more than enough) and they are emitting a lot of IR radiation! Your finger perceives that radiation as heat! As mentioned earlier, they are usually pulsed and not left on continuously - that will only make them appear hotter.
No they don't not at those sort of wavelengths, you fingers peeve them just the same as if they were visible light.
I beg to differ, Mr. Grumpy! Typical IR LEDs emit around 940nm and that is used extensively in the medical industry because it does penetrate the skin. Here's some light reading for you:
You will notice that the title of that paper says:-
a theoretical investigation
Have you ever stuck your finger in front of an IR LED? Have you felt it get hotter? Was this hotter than when the same power was given off by a red LED?
Perception is everything in this question.