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Author Topic: LED current rating @ duty cycle  (Read 442 times)
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Whatsup people, got some new LED's for a big SSD I'm building and I have a question regarding the current I'm supplying to my LED's.

The setup is a multiplexed SSD at a duty cycle of 30%. The LED's data sheet shows max forward current at 150mA but another place it sais 30mA. So that has already confused me.

I was running them at 38mA and I was wondering to myself "can I increase the current to 50mA seeing as though its only a 30% duty cycle" can I do this or will the LED's get damaged?
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Sometimes the datasheet will specify max current at a particular duty cycle. Do you have a link to that datasheet? Strictly gut feel, but I'd think that 50mA at a 30% duty cycle would be ok.
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I betcha that is Absolute Max of 30mA, and 150mA rated like this:

Peak Forward Current (1/10th duty cycle, 0.1ms pulse width)

so 150mA at 30% duty cycle will overheat them and lead to early failure.
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Sometimes the datasheet will specify max current at a particular duty cycle. Do you have a link to that datasheet? Strictly gut feel, but I'd think that 50mA at a 30% duty cycle would be ok.

za.rs-online.com/mobile/p/visible-leds/4663582/?searchTerm=4663582&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D6265724D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B367D247C5E5C647B377D247C5E5C647B31307D2426706F3D313426736E3D592673743D52535F53544F434B5F4E554D424552267573743D343636333538322677633D4E4F4E4526


That's the mobile link as I'm not around my computer at the moment.
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I betcha that is Absolute Max of 30mA, and 150mA rated like this:

Peak Forward Current (1/10th duty cycle, 0.1ms pulse width)

so 150mA at 30% duty cycle will overheat them and lead to early failure.

Yes it does say absolute max 30mA. So what u are saying is that I can only drive them with 150mA at a 10% duty cycle?
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I am saying read your datasheet to understand its limitations and be sure.  I cannot access the link you posted at the moment.
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I am saying read your datasheet to understand its limitations and be sure.  I cannot access the link you posted at the moment.

Ok I will read it again tomorrow and post a proper link. Thanx for always helping, you are very helpful and always willing to help us noobs. Thanx:)
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No problem. Lets keep all the magic smoke inside if possible smiley-cool
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Here's the link to the datasheet pointed to by that website:

http://docs-europe.electrocomponents.com/webdocs/0332/0900766b80332ad6.pdf

On it it says:
Absolute Maximum Ratings at 25C:
Power Dissipation:150mW
DC Forward Current: 30mA
Peak Forward Current[1] : 195mA

Notes:
1. 1/10 Duty Cycle, 0.1ms Pulse Width

So, the Max DC current is 30mA. The Max Peak current is 195mA (at 25C).

50mA at 30% duty cycle? You're probably fine up to about 60mA (which averages to about 20mA, without ever approaching the max peak value).
« Last Edit: December 10, 2013, 05:14:00 pm by arduinodlb » Logged

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Here's the link to the datasheet pointed to by that website:

http://docs-europe.electrocomponents.com/webdocs/0332/0900766b80332ad6.pdf

On it it says:
Absolute Maximum Ratings at 25C:
Power Dissipation:150mW
DC Forward Current: 30mA
Peak Forward Current[1] : 195mA

Notes:
1. 1/10 Duty Cycle, 0.1ms Pulse Width

So, the Max DC current is 30mA. The Max Peak current is 195mA (at 25C).

50mA at 30% duty cycle? You're probably fine up to about 60mA (which averages to about 20mA, without ever approaching the max peak value).

Thanx for the help, what is the 195mA they specify then? Is this the current you are never allowed to exceed no matter the condition?
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what is the 195mA they specify then? Is this the current you are never allowed to exceed no matter the condition?

That would be it.
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Or are they saying that if u use a 10% duty cycle you can pump max 195mA through them provided your pulse width is 0.1ms?

The frequency I am driving my LED's is 200hz but I'm not sure if I have calculated that correctly. As a simple example of how I'm driving them let's say I have 3 led's, I'm switching between them every 5ms. And they stay on for 5ms. So my duty cycle is 5/15 = 30%. My frequency is 1/0.005 (1 second / switching every 5ms) = 200Hz? Is that right?
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And they stay on for 5ms.
So that is 50 times longer than 0.1mS (100uS).
1/10 duty cycle at 0.1ms width means you go thru 10 cycles/mS. = 10,000Hz.
Otherwise, the LED will get too hot & fail.
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Or are they saying that if u use a 10% duty cycle you can pump max 195mA through them provided your pulse width is 0.1ms?

What Paul said. Unless you like the sound of "pfft".

Or are they saying that if u use a 10% duty cycle you can pump max 195mA through them provided your pulse width is 0.1ms?

They are saying that for their test, their Peak current was 195mA, but it was only on for 10% of the time. This averages to 195/10=19.5mA < 30mA. The frequency is there to say you can't leave it on the 195mA peak for too long. You can hit the peak, but you're rapidly heating up the LED, so you need to turn off the current before it gets too hot, wait for it to cool down, then you can do it again.

So basically, you have to think about the average current, always stay below the peak current, and make sure the frequency is high enough so that the LED can cool down sufficiently before the next pulse. This min frequency is dependent on how close to the peak you are going (which determines how quickly you're heating the LED up) as well as the room temperature. Just stay well away from the maximums, and as long as your frequency is also decent you should be fine.
« Last Edit: December 11, 2013, 12:24:57 am by arduinodlb » Logged

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Ok awesome thanx guys, electronics just gets more interesting by the day!!!!!!!
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