Whats the difference?  - Page 1
Forum section: electronics / beginners

author: shane_95
author_rank: Regular Contributor
timestamp: July 02, 2012, 03:57:11 pm
content: im planing out some parts I need for a concept design. One of the parts I nerd is 10xLEDs, and is has "IF typical (mA)" and "IF max (mA).So whats the difference?

author: 8086
author_rank: Super Contributor
timestamp: July 02, 2012, 04:01:37 pm
content: IF typical is a safe typical currentIF max is the maximum permissible current and if you go higher damage may occur.

author: Stephen Hill
author_rank: Regular Contributor
timestamp: July 02, 2012, 04:02:11 pm
content: The typical is the current in mA you should aim to drive these LED at. The maximum is the current which you should not exceed.Running the LEDs at their maximum will reduce their life.

author: shane_95
author_rank: Regular Contributor
timestamp: July 02, 2012, 04:07:03 pm
content: Ok cool thanks

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 02, 2012, 06:11:50 pm
content: Quote from: Stephen Hill on July 02, 2012, 04:02:11 pmIs there a general rule for that? I mean, of course there is a maximum current, but what happens at or after that?Will the life be halved when you run it at maximum current, or will it decrease by let's say 10%?What happens when you exceed maximum current?Yes, you will kill it eventually of course, just wondering, "maximum current" by itself doesn't tell you a lot...

author: T4P
author_rank: Super Contributor
timestamp: July 02, 2012, 06:51:15 pm
content: Quote from: PA0PBZ on July 02, 2012, 06:11:50 pmThere's no general rule but to err on the side of caution derate them by at least 200% at maxand anything beyond that ... 10000%

author: digsys
author_rank: Unknown
timestamp: July 03, 2012, 01:44:50 am
content: QuoteHere's where it gets complicated :-) It totally depends on how / where it was made, and you will probablynever know for sure. If it's a poor die, at MAX current, it may reduce brightness 20-30% a year or more.Even at TYP current, it may reduce 10% a year, which is considered "the higher end". A GOOD doping, may onlylose 1% a year max. So just ask yourself, how long do you want it to last (HRS running).If you run them at 200-300% Max+, then all the curves rise exponentially. IF you really want to know, why not setup a precise LED -> LDR test jig. Feed say 300%max Constant current into the LED, Note the initial LDR voltage,wait until the LED is up to Temp, then make a 2nd Note: , then check it every day.

author: Psi
author_rank: Super Contributor
timestamp: July 03, 2012, 02:56:13 am
content: You often see an exception for max current on leds.Often it will say "Pulsed 10% 1kHz" and give you a current value well above the max current.That's due to the 90% (duty cycle) off time, where it can cool down.So when you run them in pulse mode you can exceed the max current safely in some configurations.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 03, 2012, 02:31:21 pm
content: Quote from: digsys on July 03, 2012, 01:44:50 amExactly what I was thinking. For some reason when I read "max current" I thought "what?".How do manufacturers make up this value anyway, is it based on testing, on die design or something secret?Hmm, you could probably ask the same question for a lot of parts where specs say max this or max that...Maybe sometimes it's just there to let you buy the more expensive version

author: digsys
author_rank: Unknown
timestamp: July 03, 2012, 03:22:27 pm
content: QuoteOK, my mistake explaining .. IF you are lucky to buy from a "respectable" manufacturer, they will guarantee their specs.The figures quoted WILL be worst case, you can definitely trust them.But sadly, there is a serious amount of fraud / counterfeiting going on, and getting worse. PLUS many cheapomanufacturers have lousy QC. YES, a good company WILL life test their products, it's their reputation.I've occasionally bought stuff from Motorola, to find I've had cr@p silicon swapped somewhere in the supply chain.I've heard counterfeiting runs as high as 25% in some devices, LEDs is an easy one for them.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 03, 2012, 05:59:42 pm
content: Quote from: digsys on July 03, 2012, 03:22:27 pmNo, I think your explanation was ok, I think I was not clear in what I wrote.I see a lot of datasheets with max this and max that, but I mostly miss the "or else..."I can see what you mean by respectable and not so respectable, and in the last case I would not be surprised if the component didn't even meet the typical values. Let me see if I can find an easy example.Herehttp://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXPE.pdf,on page 5 they show the maximum DC forward current, the maximum forward voltage and the maximum reverse voltage, but they don't say why, what the penalty is for exceeding these values. Of course it is good practice not to exceed these or even approach them, but I never see what is going to happen when you do. It's probably not even a hard value, there must be some linear or exponential function for these values where you probably shorten the lifetime, but it is never specified.I guess that is what I was trying to say all the timeNot being native English often makes stuff more complex..

author: digsys
author_rank: Unknown
timestamp: July 04, 2012, 12:04:54 am
content: QuoteThen you're a perfect candidate for a CURVE TRACER !! Send a silicon device to heaven, then bring it back to life :-)IOW - You can actually SEE how a device fails, voltage / current / temperature and bring it back to life. I used to love my Heathkit Curve Tracer.QuoteLOL, naaa don't worry, many of us "native" english speakers don't understand each other :-)