need help understanding datasheet - Page 1
Forum section: electronics / beginners

author: torxt
author_rank: Newbie
timestamp: February 08, 2016, 07:52:08 pm
content: Hi, i have 5v power source and want to turn on 9 leds.i got 9 * SFH 4546:http://www.osram-os.com/Graphics/XPic1/00101982_0.pdfi found this link to calc the resistors:http://led.linear1.org/led.wizim struggeling to pull the diode forward voltage. there are two infos. what number should i use?Forward voltage (IF = 100 mA, tp = 20 ms)(typ (max)) VF 1.5 (? 1.VForward voltage (IF = 1 A, tp = 100 ?s)(typ (max)) VF 2.3 (? 3) Vwill it drive like this:5: Source voltage1.5: diode forward voltage100: diode forward current (mA)9: number of LEDs in your arraywith 3* R= 5.6 ohms?

author: Blackwarrior
author_rank: Unknown
timestamp: February 08, 2016, 09:56:32 pm
content: Hey TorxtHave you tried that link to calculate resistors, it's quite good.Set the supply voltage to 5vForward voltage of LED to 2.3Forward current to 20mAAnd 9 ledsThere are 2 diagrams produced, I'd go for the second one which show a resistor for each led, although this draws more current the brightness of the leds should be quite even..The first diagram is ok tooNot sure where you got the 5.6 ohm resistors from though, don't use that for a single led, it won't last long..Hope this helps a littleEditJust to clarify that calculators results in case your not sure.The data sheet States a forward voltage of 2.3 volts, and requires 20mA.Therefore if we have a 5 volt supply, we need a resistor to drop 5v - 2.3v = 2.7 volts at 20mA.Ohms law here, 2.7v / 0.02amps = 135 ohms, safest common size to use will be 150 OhmsEach led will therefore use 2.3v / 150 ohms 15mA.Make sure your 5volt PSU can handle the current

author: torxt
author_rank: Newbie
timestamp: February 08, 2016, 10:25:20 pm
content: HI Blackwarrior!Thanks for the good explanation.I will try u suggested values. Could you explain (or give me a link, if it takes too much time) how to got those values?for Forward current:in the sheet i found max. Forward current: 100 mA. At the bottom there is a graph with I as a fuction of temparature, with Max. Permissible Forward Current, did u go: "the led will run at max 85 C lets set f. current to 20" ?or is it based on commen sense that u run leds on 20mA?for Forward voltage:there are two values, one for I=100mA(1.5) and one for I=1A(2.3)how do figure out, we need 2.3 not 1.5?

author: Blackwarrior
author_rank: Unknown
timestamp: February 08, 2016, 10:46:29 pm
content: Sorry I didn't realise these were IR LEDs, what I've described would be nominal for standard LEDsif you check out the permissable pulse handling on page 6, it clearly shows the pulse width in time against permissable current. For example at 100mA you can pulse this for 1 second cycle time, that's 500mS on and 500mS off.Hope this helps a little, maybe someone can come in and explain more

author: Ian.M
author_rank: Super Contributor
timestamp: February 08, 2016, 10:59:50 pm
content: IR LEDs typically run at a much higher forward current than visible LEDs in the same package.You will notice that most of the specs in the datasheet are quoted at 100mA If, but that is the maximum continuous forward current. If you are using them as simple IR illuminators, you will need to derate them a bit to have a safety margin. See graph OHR00880 on page 6, and pick a current that will be safe at the maximum ambient temperature they will ever be run in.If you are pulsing them fast enough with a low enough duty cycle, they can be run at significantly higher pulse currents, and as their efficiency rises with forward current there is a significant advantage in doing so.

author: Audioguru
author_rank: Super Contributor
timestamp: February 08, 2016, 11:43:47 pm
content: Each LED will have a different forward voltage so each LED needs its own current-limiting series resistor.