FreeTrack Forum

Hi, I'd like to make IR cap, but I'm somewhat new to this whole 'electronics' thing Ok, I'm serious now: I found interesting IR LED with 90 degrees view angle, HE3-290AC, but I don't now what parameters to use when calculating resistors. Here is a link to datasheet:

http://www.produktinfo.conrad.com/datenblaetter/175000-199999/181713-da-01-en-IR_LED_5MM_HE3_290AC.pdf

1,7 V / 100 mA values are listed as absolute maximum rating, so I think, that those are 'do not exceed' values, am I right? I think 50 mA should be good, as almost all values are referenced to 50 mA current, but what about voltage? Should I go for 1,35 V (listed in Electrical and Optical Characteristics table)? Or should I choose any point above the point, where current/voltage curve begins to rise and use parameters from this diagram (but not exceed 1,7V / 100 mA of course), remembering, that more current = more intensity but shorter lifetime?

Thanks in advance for help. Cheers!

P.S. If there are better diodes (wider angle, lower voltage, anything that is better than He3-290AC) please tell me. In worst case I'll scrap this idea and use proven SFH485Ps.

1,7 V / 100 mA values are listed as absolute maximum rating, so I think, that those are 'do not exceed' values, am I right?

Yes I think (am almost sure) it is.
If I take as an example a SFH485p led, it has a forwarding voltage of 1,5V.This means that if I connect it to a 5V powersource it wants to get get rid of that  5V-1,5V=3,5, what happens is more current will go through your led and destroys it.Therefor we need a resistor to limit the current.
And how do you find the right value for the resistor?
(psuVoltage-total forwarding voltage leds)/ desired current through led
(5V-1,5V)/0.05A=70 ohm
if you use more leds and put them in serie
(5V-3x1,5V)/.05A=10 ohm

First you need to think of whatpowersource and voltage you want to use, then you can think of what resistor you want.I ordered a few different resistor values just to find the right value what gives the leds enough current to make them bright enough for tracking.

Those He3-290AC of yours I never used them, so I can't tell what to expect of them.I use SFH485p leds and I know for sure that they do the job.If you like some experimenting with this why don't order them both and be the first to use them and let us know if they are maybe better then the SFH485p leds?

regards Stormrider

Thanks for your reply. I know about Ohm's law and how to calculate needed resistors, and I plan to doublecheck everything with online calculator. I simply don't know which values of voltage/current to use.

I plan to use USB charger as a powersource, so I can use 5V and up to 1A (so I think up to about 300 mA with diodes in parallel if I need to should be safe). I tried looking up the USB source/SFH485P circuit available on forum, reverse calculating voltage with current listed and comparing it with SFH485P datasheet. I ended up with 1,5 V but it's not listed as 'absolute maximum' but only as one of characteristics, so in this case I wouldn't have any problem.
In case of lack of info I'll try 1,4-1,5 V and 50-80 mA (different values of resistors for testing should be a good idea) on HE3-290AC.

My biggest problem is actually insufficient knowledge of technical (electrical) english; I'm chemist by education and job actually. Probably I can try and compare different diodes; I planned to maybe use diode sockets (of course if I can find any) in case of diode failure or bad performance of diodes used.

I don't need to know what I need fast, I think I will be able to do this circuit in about 2-3 weeks, so I have enough time to gather info.

If you want to use usb-power, it is better to use a serial circuit.Serial uses 50mA, the same setup paralel uses 3x50mA.

Hi,

no problem for these LEDs.
Use it like a SFH485. Forward voltage is approximately the same (<1.8V).
Using a 10 Ohm resistor in serial circuit with 5V USB-Charger does the job.

(Hi Storm   )