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fatboy	#1 17/05/2008 - 03h03 Top
Class : Apprenti Posts : 12 Registered on : 14/05/2008 Off line	I have my frame made up for my headphone clip and I'm just testing everything while I figure out a good way to mount it and keep it level on my headphones. Damn Panasonic ergonomic design :S I had them running for about 45 seconds whilst moving around, and felt they were getting warm to the touch, is that normal or should i throw in a 10 Ohm resistor? I have 3 AA batteries at 1.5 V each in series (4.5V total from source) w/ 3 LED's at the moment. I thought theoretically the voltage across the 3 LEDs would equal out and everything would be dandy, but the warmth of the device is somewhat worrying. EDIT: I forgot to mention my LED's have a forward current rating of 150 mA and a 1.3 - 1.7 forward voltage max. The calculator says a 22 Ohm resistor should work. Perhaps this thread isn't to ask whether or not I need a resistor anymore, but are the LED's supposed to be warm? Edited by fatboy on 17/05/2008 at 03h06.
Deimos	#2 17/05/2008 - 11h25 Top
Class : Beta Tester Posts : 120 Registered on : 07/11/2007 Off line	If you look at an examle LED datasheet: http://www.datasheetcatalog.org/datasheets/50/486602_DS.pdf You'll notice that its 100mA forward current is listed in maximum ratings table. It's the same with your LED - 150mA is the maximum current the LED can handle in long term without taking damage. Your LEDs are heating up because they operate closely to their maximum ratings, or possibly above them, because you don't limit the current in the circuit in any way (other than resistance of the wires, which is close to zero anyway). It's true that the voltage drops across the 3 LEDs, but usually the forward voltage is lower than nominal (1.5V) value, and even the little voltage left can cause enough current flow to exceed LED specifications. It's always the safest to limit the current and keep current values at about 50% (80% if you really need the extra brightness) of the maximum ratings. Then the LED will still be bright enough, and won't heat up. Note that maximum operating temperature SFH485P (and probably most other LEDs as well) can handle is 100 degrees Celsius, so anything lower shouldn't damage it. But unless you mount the LEDs in a PCB for better heat dissipation, letting them get hot isn't the best idea.
marcusw	#3 17/05/2008 - 12h16 Top
Class : Habitué Posts : 150 Registered on : 17/04/2008 Off line Www	just off the top of my head, I'm guessing (since you are in a series, and you are using 4.5 volts at 1.3 per led) you needanywhere from 22 to a 33 ohm resistor placed first in the positive side of the series. I suggest you use 33, its just safer, but I'm not sure if 150 ma is the max rating or the suggest rating. I'm used to seeing 100 ma as the suggested rating, which is why i say 33. And for some reason, my leds are 97.7..... weird number lol. Oh yeah, another safety tip. When you are wearing an live circuit on your head, ALWAYS go with the lowest number. If it says 1.3 - 1.7, then its just 1.3. Remember that UL isn't gonna be there to inspect your circuit, and hair takes a long time to grow back, Edited by marcusw on 17/05/2008 at 12h22. The only way to fly!!!
fatboy	#4 17/05/2008 - 20h14 Top
Class : Apprenti Posts : 12 Registered on : 14/05/2008 Off line	Thanks guys. I'll throw in a 33 Ohm resistor.
simonpe	#5 12/07/2008 - 15h33 Top
Class : Apprenti Posts : 9 Registered on : 10/07/2008 Off line	Sorry for waking up an old thread but... ...you don't limit the current in the circuit in any way (other than resistance of the wires, which is close to zero anyway). This is not true. You have to take the internal resistance of the battery into account, I am currently running three LED's in parrallel on a 1.5V AA battery using only the battery's internal resistance to limit the current. The voltage over the battery drops to 1.2V (same as the voltage drop over the diodes), at this point I measure a current flow of 75mA over the diodes, which is nice. But please note that if you use a switched/zener power supply of some sort you will most definately burn the LEDs! Also note that if you run two AA batteries in parallel you will also most likely burn the LEDs since the combined internal resistances will roughly halve.
Deimos	#6 14/07/2008 - 10h00 Top
Class : Beta Tester Posts : 120 Registered on : 07/11/2007 Off line	simonpe @ 12/07/2008 - 17h33 a dit: Sorry for waking up an old thread but... ...you don't limit the current in the circuit in any way (other than resistance of the wires, which is close to zero anyway). This is not true. You have to take the internal resistance of the battery into account, I am currently running three LED's in parrallel on a 1.5V AA battery using only the battery's internal resistance to limit the current. The voltage over the battery drops to 1.2V (same as the voltage drop over the diodes), at this point I measure a current flow of 75mA over the diodes, which is nice. But please note that if you use a switched/zener power supply of some sort you will most definately burn the LEDs! Also note that if you run two AA batteries in parallel you will also most likely burn the LEDs since the combined internal resistances will roughly halve. Yeah i know that. But i didn't want to rise too much confusion there. In my opinion it's best to assume that the current from batteries in infinite and the current limiting resistor is always needed. Someone who really knows what he's doing won't need instructions/LED calculator anyway, and such assumptions might save more than a few LEDs assembled by people not too knowledgeable in electrical stuff Better safe than sorry Edited by Deimos on 14/07/2008 at 10h01.
tristan68	#7 16/07/2008 - 14h35 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	I confirm Deimos's opinion What we need is a setup which works in all cases. That's definitely not the case of a setup relying on the battery's internal resistor, since we cannot guarantee this resistor value. People knowing what to do won't need the wizard, they'll calculate their own setup. The wizard is made for people who aren't familiar with leds. That's also why it includes a few safeties, to be sure to be on the right side. cya
simonpe	#8 17/07/2008 - 01h39 Top
Class : Apprenti Posts : 9 Registered on : 10/07/2008 Off line	Yeah, you are right. Anyways, I really think you should put recommendations on using potentiometers in the manual. Many users (including me) use salvaged LEDs from old remotes etc. with no specifications which causes problems. I rebuilt my setup to include a 1k trim pot with a 11.1V LiPo battery pack and it is really great to be able to adjust the current. Is there any reason this isn't standard?
tristan68	#9 17/07/2008 - 07h39 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	Hello simonpe I usually don't recommend using a potentiometer because most of them aren't able to take the current some people want to use in their setup. Always the same problem, we need a solution working FLAWLESSLY with every people. About using salvaged leds from TV remotes : The problem is you usually don't know the led model so you're uncertain about the voltage drop values and current. My 2 cents opinion would be : Somebody having knowledge with electronics can use salvaged leds, since he'll know how to deal with them (test and measurement...). Somebody unfamiliar with electronics should always use standard leds, provided with their operating values (whatever the model). cya.

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