# Request help interpretting LED max Vf spec in spec sheet

Discussion in 'LEDs and Optoelectronics' started by seanspotatobusiness, Aug 6, 2014.

193
4
Sep 11, 2012
This image is a page from a spec sheet for a Philips Luxeon LED. One part of the sheet suggests that the maximum voltage tolerated is 4.95 V. However the LED's maximum tolerated current is 1500 mA and this is reached at 3.85 V. Does this mean that the current remains at 1500 mA between 3.85 V and 4.95 V?

2. ### BobK

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Jan 5, 2010
No, it means the voltage at 1500 mA will be somewhere between 3.03 and 4.95V.

That is an outrageously wide range, though.

Bob

193
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Sep 11, 2012
Okay, I'll have to test it. Thanks a lot.

4. ### Gryd3

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Jun 25, 2014
Many of those sheets are created by testing multiple pieces that come off the line. They are expected and acceptable values.
Take a look at the top of that page you posted and you'll notice they have posted the test conditions.
1000mA and a 25 Degrees Celsius Junction temperature.

If you are using different values than the test conditions, expect different results.
Some specification pages will also plot those values so you have a rough idea of what to expect as conditions change. (ie. Temperature)

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
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Jan 21, 2010
There's nothing to test. You're given a max current of 1500mA. Just use a constant current source set up for a current less than or equal to 1500mA.

The voltage need not concern you other than for you to allow sufficient headroom for the regulator to operate.

KrisBlueNZ likes this.

193
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Sep 11, 2012
No! I want this to make a dynamo-powered light for my bike with a few batteries keeping it lit when I stop at lights.

7. ### Gryd3

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Jun 25, 2014
No what?
We are stating that the LED's don't need to be tested to determine how much current or voltage they consume because the information is in the data sheet.
If that's what you want to make, you need to determine a method of limiting the current to the LED so you don't burn it out. You also need to determine the appropriate battery or capacitor to keep it lit for as long as desired.

There is no need for your response, we gave you the details you requested, and nowhere above have you mentioned anything regarding a dynamo or battery operated device. What's more, is if you 'test' the LED you want to use and use your results to build your device, you are asking for trouble. The LED you are using could vary with time or temperature rendering your test to be inaccurate which could cause the LED to fail. Once the LED is dead, and you order another you will run into the same problem.

Take *Steve*s information to heart and save yourself some trouble.

193
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Sep 11, 2012
No need for my response? I haven't said anything even remotely offensive.

And given that the spec sheet applies to more than one unit, and all the units are not the same, clearly I do have to test my individual unit.

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
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Jan 21, 2010
I now assume he might mean that he will get a constant current source and try that with his power source. The testing will be (presumably ) using his two power sources with diodes to isolate them.

I note he has replied a few moments ago so the post which may appear above this one could confirm that...

10. ### Colin Mitchell

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Aug 31, 2014
Why complicate things with a constant current source????

Just use a resistor from the battery and connect the dynamo to the LED via a diode..
This will allow you to illuminate the LED when riding and the LED will glow dimly when at the lights.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
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Jan 21, 2010
Sure Colin. Thanks for the Chinese design tips.