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Old 17th December 2003, 05:21 PM   #11
Magura is offline Magura  Denmark
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All this calculating resistor stuff is ok....but the easy way in most cases is to use a driver.
A LED driver will cost you 0.2usd, and you can even run it with current feedback in order to maintain the correct intesity of the LED.

Add an LDR to the circuit, and you got a LED array that compensates for the ambient light.

Magura
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Old 17th December 2003, 06:21 PM   #12
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Quote:
Originally posted by Magura
All this calculating resistor stuff is ok....but the easy way in most cases is to use a driver.
A LED driver will cost you 0.2usd, and you can even run it with current feedback in order to maintain the correct intesity of the LED.

Add an LDR to the circuit, and you got a LED array that compensates for the ambient light.

Magura

What's the use of a LED driver ( ? ) if a resistor is enough ?

BTW I made the mistake myself to think a resistor is *always* needed. I searched for the thread about blue LEDs directly wired to a newly wound winding on a toroid ( P.Daniel ) but I didn't find it. When the LED is operated in the right spot in its curve ( supply voltage just above turn-on voltage of the LED ) a resistor can be omitted but it is really wiser to use one in all cases for safety reasons. I would measure first if you have 2.4 V exactly. When it is higher you certainly need the resistor.

When you want to feed the LED from a low voltage AC winding as the suggested extra winding I would use a series resistor and a series diode like a BATxx type ( Uf of 0.2 V ). LED's don't like reverse voltages too much.

edit: found the thread:

Y B Blue - how blue LED improves the CD playback
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Old 17th December 2003, 08:37 PM   #13
lgreen is offline lgreen  United States
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Default What about Question#2?

Thanks for the posts, but what about question #2? What is the best way to turn off the LED, open circuit or short both ends to ground?
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Old 17th December 2003, 08:55 PM   #14
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Quote:
Originally posted by jean-paul
I would use a series resistor and a series diode like a BATxx type ( Uf of 0.2 V ). LED's don't like reverse voltages too much.
I would use a diode (any diode) in parrallel with the LED but in opposite directions. LEDs usually have a reverse voltage of 4-5v. so your method would also work as the overwhelming majority of the reverse voltage will be on the diode, not the LED. but I just feel safer knowing my LEDs aren't hugely reversely biased.
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Old 17th December 2003, 10:10 PM   #15
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Default Re: What about Question#2?

Quote:
Originally posted by lgreen
What is the best way to turn off the LED, open circuit or short both ends to ground?
Open-circuit.
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Old 18th December 2003, 06:57 PM   #16
Magura is offline Magura  Denmark
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Quote:
Originally posted by jean-paul



What's the use of a LED driver ( ? ) if a resistor is enough ?


Since a LED dosnt maintain the same intensity over the years, a driver with current feedback will compensate for this. Just take a look at older LED signs, the intensity matching simply sucks.

Thats part of the point of using a driver, but especially if you run more than 1 LED, id say a driver is a must. You could buy intensity matched LED's, but at a fairly high price, and with a limited time horisont of the matching anyway.

Magura
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Old 18th December 2003, 07:05 PM   #17
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A constant current driver won't compenste for an LED dimming due to ageing unless the reduction in luminence is due only to increased forward-voltage.
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Old 18th December 2003, 07:09 PM   #18
Magura is offline Magura  Denmark
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Wich it is as far as i know....am i right on this one??

All i know for sure is that it works(keeping the intesity at level using a driver with current feedback).

Magura
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Old 18th December 2003, 07:23 PM   #19
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Take an LED with Vf=2V powered from a 12V supply via a 500 Ohm resistor for a current of 20mA.

If Vf were to increase by 25% to 2.5V (unlikely) the current would only decrease to 19mA, there would be no perceivable decrease in brightness.
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Old 18th December 2003, 07:27 PM   #20
Magura is offline Magura  Denmark
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Hmm...will have to investigate what actually happens when a LED grows old and looses intensity. I guess this could be done by over-powering a LED for a couple of hours.

Magura
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