Resistor rating for IR LED for WiFi IoT remote

Hi Nico,

It is easiest to look at the 1A case.  There will be a 1A x 3.4R = 3.4v drop across the resistor and 1.6V drop across the LED.  Since power = I*I*R the resistor sees spikes of 3.4W and the LED sees 1.6W.  That doesn’t sound good. Are you using a transistor to control on/off?  Have you considered a constant current source without a resistor.  I’m curious what people more knowledgeable than me think. 

Frank

Here is another approach: https://forum.43oh.com/topic/10098-msp430-infrared-controlled-wearable/page/2/?tab=comments#comment-77320

i used multiple IR LEDS with their own resistors to control current and still get lots of photons. The project involved controlling LEDs on tiaras my grand daughters were wearing so I could blink them in unison 

Hello Nico,

some comments:

1) Remotes usually send a modulated signal (38 kHz is a very common frequency, but anything between 34 and 56 kHz could be possible). This is good for your use case, as it cuts the duty cycle of your LED on time basically in half already. Unfortunatelly the datasheet is missing a standard diagram that shows if 50% or 25% duty cycle is OK at 1 A. The data might be UART or with a custom protocol encoded which again reduces the duty cycle, but to be on the safe side let's assume 50%.

2) Thanks for sending the LED part number, that always helps to trigger better comments. The 1.6 V max forward voltage is only valid for a current of 100 mA. Figure 5 on page 3 shows a forward voltage of ~2.95V at 1000 mA. So your resistor to limit the current would be (5V - 2.95V)/1A = 2.05 Ohm. This would be 2.05 W through the resistor, with the duty cycle 1W. As Frank already said, there must be a switching transistor, which also has a resistance, even if it is a MOSFET. Tolerances of all components are also important, because small changes with low resistor values like this change the current a lot.

3) IR wavelength also might be important, they also come in 850 nm and 1130 nm.

A little drawing how you plan to solder things together also might help for some more advise.

Cheers,

 - Wolfgang.

Some good info already in the answers. It is high temperature that would kill the resistor or LED, and even 1 Watt in a component that size will heat up quickly. You would need to determine how much energy and time it takes to reach a dangerous temperature. Then you could figure out how to keep the components cool enough for the time you need them to operate.

I would just keep the power down in each component simply to avoid temperatures that could burn you or melt plastic. The components will last longer and the calculations are much simpler. You could use several resistors in parallel as you mentioned, or LEDs and resistors as Frank mentioned. Wolfgang's calculations indicate you might get away with 2 LEDs with 2 resistors on each LED.

If you can find a cheap lens that will work at the wavelength of your LEDs, it could substantially improve range without having to boost power. I once used an 8 inch lens to make a 0.16 Watt LED visible from one mile away.

One other note is the driving transistor may need a strong drive (low impedance source) to keep up with the switching rate. This could mean adding a second transistor.

ntewinkel said:

Should I be using a set of parallel resistors instead (knowing I'll have to up the resistor values to compensate) ?

I don't believe so. the problem you may have is if you lose one of the parrel Resistors things can get very ugly. As R_t will change and your LED could burn out. 

~~Cris

Thanks Frank! I knew I was missing something big but didn’t know what! - I missed the math on the voltage drop on the resistor being vcc-(led voltage drop).

And yes, early on I found that I had to use a transistor to get any results at all.
I haven’t checked the ratings on it, I think it’s a pn2222a - in the most basic circuit where the I/O pin drives the transistor, connected through a resistor of course.

I’m not familiar with constant current sources, I’m just using a phone charger cube rated at 1amp/5v

I’m aiming for just a basic “works ok”, so if I can reduce the amperage to something within easier limits, I’ll do that. It only really needs to be in one spot aimed at the heat pump and send a packet twice a day, so I might be able to get away with some bursts of over current.

I have a few more days away but will try some hands on testing later.

Multiple LEDs is a good idea too, thanks!

Thanks Wolfgang! I really appreciate all of your help!

That’s a lot of learning for me  

I wasn’t aware of the change of forward voltage with changing current, that’s interesting.

I replied to Frank’s comment with more details, but in a nutshell yes a transistor in the most basic Arduino-101 circuit. I might be able to reduce the output to a more manageable level while still getting enough light to reach the heat pump.

I’ll do some more math on the Daikin IR packet to see about the duty cycle.

Thanks Doug!

You make a really good point about fire safety that I had not really been paying attention to.

I’ll have to do more testing to see if lower power levels will work.

I think that part might be ok, assuming a failed resistor leaves it open, not shorted, because of that inverse math for parallel resistors. For example, 4x 100 ohm in parallel gives 25 ohms. if one fails open, the 3 result in 33 ohms, which (I think?) will reduce current instead of increasing it.