Author Topic: Whats the difference?  (Read 5740 times)

0 Members and 1 Guest are viewing this topic.

Offline shane_95Topic starter

  • Regular Contributor
  • *
  • Posts: 82
  • Country: au
Whats the difference?
« on: July 02, 2012, 03:57:11 pm »
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?
 

Offline 8086

  • Super Contributor
  • ***
  • Posts: 1084
  • Country: gb
    • Circuitology - Electronics Assembly
Re: Whats the difference?
« Reply #1 on: July 02, 2012, 04:01:37 pm »
IF typical is a safe typical current

IF max is the maximum permissible current and if you go higher damage may occur.
 

Offline Stephen Hill

  • Regular Contributor
  • *
  • Posts: 178
  • Country: gb
  • M3VXY
Re: Whats the difference?
« Reply #2 on: July 02, 2012, 04:02:11 pm »
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.
 

Offline shane_95Topic starter

  • Regular Contributor
  • *
  • Posts: 82
  • Country: au
Re: Whats the difference?
« Reply #3 on: July 02, 2012, 04:07:03 pm »
Ok cool thanks :)
 

Online PA0PBZ

  • Super Contributor
  • ***
  • Posts: 5121
  • Country: nl
Re: Whats the difference?
« Reply #4 on: July 02, 2012, 06:11:50 pm »
Running the LEDs at their maximum will reduce their life.

Is 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...


Keyboard error: Press F1 to continue.
 

Offline T4P

  • Super Contributor
  • ***
  • Posts: 3697
  • Country: sg
    • T4P
Re: Whats the difference?
« Reply #5 on: July 02, 2012, 06:51:15 pm »
Running the LEDs at their maximum will reduce their life.

Is 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...

There's no general rule but to err on the side of caution derate them by at least 200% at max
and anything beyond that ... 10000%
 

Offline digsys

  • Supporter
  • ****
  • Posts: 2209
  • Country: au
    • DIGSYS
Re: Whats the difference?
« Reply #6 on: July 03, 2012, 01:44:50 am »
Quote
Is 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...
Here's where it gets complicated :-)   It totally depends on how / where it was made, and you will probably
never 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 only
lose 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 set
up 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.
Hello <tap> <tap> .. is this thing on?
 

Online Psi

  • Super Contributor
  • ***
  • Posts: 9889
  • Country: nz
Re: Whats the difference?
« Reply #7 on: July 03, 2012, 02:56:13 am »
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.
Greek letter 'Psi' (not Pounds per Square Inch)
 

Online PA0PBZ

  • Super Contributor
  • ***
  • Posts: 5121
  • Country: nl
Re: Whats the difference?
« Reply #8 on: July 03, 2012, 02:31:21 pm »
Here's where it gets complicated :-)   It totally depends on how / where it was made, and you will probably
never know for sure.

Exactly 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  ;)
Keyboard error: Press F1 to continue.
 

Offline digsys

  • Supporter
  • ****
  • Posts: 2209
  • Country: au
    • DIGSYS
Re: Whats the difference?
« Reply #9 on: July 03, 2012, 03:22:27 pm »
Quote
  Exactly 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?
OK, 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 cheapo
manufacturers 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.
Hello <tap> <tap> .. is this thing on?
 

Online PA0PBZ

  • Super Contributor
  • ***
  • Posts: 5121
  • Country: nl
Re: Whats the difference?
« Reply #10 on: July 03, 2012, 05:59:42 pm »
OK, my mistake explaining ..

No, 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.
Here http://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 time  :D Not being native English often makes stuff more complex..
Keyboard error: Press F1 to continue.
 

Offline digsys

  • Supporter
  • ****
  • Posts: 2209
  • Country: au
    • DIGSYS
Re: Whats the difference?
« Reply #11 on: July 04, 2012, 12:04:54 am »
Quote
... there must be some linear or exponential function for these values where you probably shorten the lifetime ..
Then 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.
Quote
...  Not being native English often makes stuff more complex ...
LOL, naaa don't worry, many of us "native" english speakers don't understand each other :-)
Hello <tap> <tap> .. is this thing on?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf