How Do I Calculate the Resistor Value for an Infrared Transmitter Circuit?

[N468989]

Hi all,

i need some help on this circuit that I am building. i want to get the maximum current for my infrared emitting diode (Kingbright L-53F3C)
datasheet->http://www.soselectronic.com/a_info/resource/pdf/king/L-53F3C.pdf"

the peak forward current permitted is 1.2A (condition: 1/10 DutyCycle, and 0,1ms pulse width. but i don't understand how to apply this..

(the idea is more current equals more distance...)

i have attached my IR transmitter block diagram...

i am also using a N-MOSFET BS170 and the maximum current allowed is 500mA continuous and 1200mA pulsed (not really sure what that means either).

datasheet->http://w3.id.tue.nl/fileadmin/id/objects/E-Atelier/doc/Datasheets/Transistoren/BS170.pdf"

so basically i would help on how do obtain the value of the Resistor R for my signal do be sent more efficiently for my specified frequency (90kHz) and dutycycle (5%) (without burning the IR LED).

any help is much appreciated thanks

 

[dlgoff]

Wouldn't you find the resistance that gives 1.2A current continious, then make sure that your pulses are 10 mircosec or less with a 10% maximum duty cycle?

 

[N468989]

i have calculated the mean current for the IR LED for my signal

DATA:
frequency = 90kHz
Period = 11.11us
Ton = 0.55us
Toff = 10.056us
Dutycycle = 5%

Ipk = 1.2A (peak current)

Im= Ipk * 0.05 <=> I am = 60mA (mean current)

now i must calculate the R to limit the current on the diode

R = (Vcc - V_led - Vmosfet ) / I am <=> R = (5-1.2-2.1) / 60mA = 28.3 Ohm

R gives me 28.3 Ohm, but I am not very sure about this result i might be missing something...

then there is the power dissipation which is 100mW

if someone could help me out with these calculations, i have a book on this but its no very helpfull.
thanks




dlgoff : do mean like this -> R = (Vcc - V_led - Vmosfet ) / Ipk , then i just pulse the signal normally?

 

[dlgoff]

"dlgoff : do mean like this -> R = (Vcc - V_led - Vmosfet ) / Ipk , then i just pulse the signal normally?"

Well that's how I would do it, but I'm no expert here.