IR LED, do I need a resistor?
I am building a remote for my AC that uses IR. I am planning on using a 2N3904 transistor and I need to calculate the resistor for the IR LED. I have chosen one that has a 1.3V forward voltage (1.6 max) and a forward current of 800mA. I calculate it to need a 4.265 ohm resistor and it will consume 2.96W. I don't want to have to get a resistor that can handle more power. I could run 2 2ohm 1/2W resistors but that is still 1/3 of the power.
My question is, since it is such a low value resistor needed and an IR LED would not be continuously lit, could I get away without the resistor? Or could 2 in parallel at a 1/2 watt rating work considering the short duration of use? The AC unit does have a long code to send compared to a normal remote.
Any advice would be greatly appreciated.
You would want one. Damage is cumulative and at some point, the LED will probably stop working before it's time.
Thank you @triform,
I was hoping that I could sacrifice the longevity on a component that only costs a few cents and could be replaced easily. I don't think anyone can answer how long it will really last so I guess the best decision would be to use a resistor.
But that leaves me the question of the resistor. Can 2 half watt resistors handle the short bursts? Otherwise I would have to use 3 1ohm 1 watt resistors. Unless there are 3 watt resistors. I was hoping to use common components.
With LEDs, I typically add a resistor that I like for the brightness. For IR, I would use the datasheet recommendation though. For small bursts that happen occasionally, you could probably get away with a .25watt resistor in the 10R or so. You never said what your circuit voltage is?
@triform, Yeah sorry for leaving out that bit of relevant information. I am using a 5 volt supply.
I have some .5 Watt carbon film resistors I will probably use. I might use 2 5R in parrallel. I will begin testing of that soon. I do have few lower ohm resistors to bring it closer to the calculated voltage but I will play it safe at first.