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Topic: So what (grin) resistor should i use for this LED then? :D (Read 2802 times) previous topic - next topic

Osgeld

yes temperature is what I meant, but more importantly OMG someone understands!
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cjdelphi

True Or False?

1. LED's are current controlled, not voltage?  True or False
2. Regulators have a Maxium Limit they can supply before thermal regulation steps in...?  True or False?

If (Answer1==TRUE & Answer2==TRUE)
  {
       //LED is safe.....
  }


is that Logic incorrect? I know my cheap switching reg's can supply 15 - 30 watt (if heatsink is adequate) .. how
will a 100watt ever "dim" from a max of a 30watt power supply?....

I'm trying to explain why in some, yes SOME instances, it's safe not to use a resistor... providing you have perfect
regulation of voltage with no deviation whats ever from heat or otherwise.

http://www.candlepowerforums.com/vb/showthread.php?284177-Direct-Drive

You'll find plenty of Lithium DD flashlights/torches on the market, they don't dim after 2 days lol, point is, anyone who claims "EVERY LED" need's some kind of current limiting device eg a Resistor or
Current Limiter (via a Semiconductor) is wrong, people in the flashlight world do it all the time, providing you don't supply > 4.2v (if it's possible, then sure add a limiter) on some LED.


Be 1 lumen from a 5mm LED or 50,000 from a monster sized emitter of LED arrays, as long as they don't go over that magical voltage number, I don't see the point of an LED on HIGH POWER LED's
if your LED requires a max of 50ma and anything over will kill it, yes give it a 300ohm resistor, protect it, but if your LED requies 100watts, and your power supply can only dish out 50watts at MAX.

Let's say we had a CV of 12v out (15v in via the Regulator) (30watt supply MAX before thermal shutdown kicks in)

12v in +(3.8v LED)- GND

POOF.

Dead, if it were a 5mm LED.


BUT, after all, LED's are "Current Controlled"

In this instance, let's use a 3.8v LED that supplies an array of LED's but each LED can handle 5amps easy, the entire array let's assumes 100watts of energy.
it sees 12v, it does the same as the little 5mm, it tries to take as much current as possible... the Regulator does so and provies it's full 50watts
of energy to the LED.

Now, we have the LED running at half it's power, it's not dead, it's emitting very brightly and getting very very hot, but we have a nice big 500watt heatsink in the form of a huge finned
heatsink i ripped out from a very old square more heavily armored than a tank and did my back in getting it into the car... so let's now take a look at the regulator.

for argument sake it does manage to supply 50watts and it's not oscillating between low and high power because it's maxed out..

The voltmeter, will show 2-3.8v (since it's 50% capacity, i'd guess the voltage across - and + terminals to be around 50% around 2.8-3.v and a huge voltage drop because it can't supply enough current
as a result the voltage would drop to whatever the LED's is comfortable at emitting at a specific voltage, but sure enough that LED will be alive, unless that Semiconductor peaks > 100watts I really
can't imagine it dying.

but i'd love to see myself proved wrong, on these cheapy regulators, if you show me it going POOF from a really expensive regulator >$100 which supplies > 100watts then fair enough, goahead limit the current
because i would..... I'd also limit the current if i connected it directly to a 12v battery...


I just don't see the need to protect it from something that could never supply enough current to kill it.... take a lithium 4.2v battery, connect a >3watt LED, it should run perfectly fine with no next to no dimming for thousands of hours, LED's fade i'm not against using Resistors, i always do use them always, what i can't accept is people telling me ALL LED's regardless need some kind of current limiting, no not always, if you're careful enough all you need is a voltage controlled regulator and nothing more and be totally safe even with a 5mm LED.


Set 2v via an Adjustable Regulator, connect it in series with an ammeter and adjust until it hits 15-20ma, and stop, read the voltage....

even a 5mm is not going to dim if all you supply is a Precise Voltage, you know the regulator would never produce enough heat to drift off
desired voltage, just how is the LED going to dim without a resistor then?



Osgeld

#17
Jan 21, 2013, 05:42 am Last Edit: Jan 21, 2013, 05:45 am by Osgeld Reason: 1
Quote
Set 2v via an Adjustable Regulator, connect it in series with an ammeter and adjust until it hits 15-20ma, and stop, read the voltage....


and that number will be wrong in a matter of time, the Fv of a led changes with heat, what part of that are you not grasping?

Quote
even a 5mm is not going to dim if all you supply is a Precise Voltage


if your going through the trouble of providing a precise voltage for a moving target why not just do it right in the first place?

And bull, any led will dim just by using it, now if its abused it will crap out much quicker than one kept in absolute perfect conditions, but it will still dim
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be80be

#18
Jan 21, 2013, 05:49 am Last Edit: Jan 21, 2013, 05:52 am by be80be Reason: 1
Na the op is using the regulator as a current source and if you read the data sheet on most they lower the voltage to hold there output of current. I posted this before but some people don't read data sheet's  and think that A plus B is C Na A can be A LOL.

     

cjdelphi


Quote
And bull, any led will dim just by using it, now if its abused it will crap out much quicker than one kept in absolute perfect conditions, but it will still dim


ok, so if i set a spare regulator to a a mid range level until it reads say 15ma, i'll leave it at 2.1v or 2.3v or whatever it is, i'll have an identical one to that i wont touch (only to power it up once to compare brightness), and i'll leave it on for a week and i'll take a before and after picture... don't expect me to do it right now, but I will tomorrow... and i'll see how much visible differences they are


GoForSmoke

There's voltage regulators and there's current regulators.

Quote
Characterizing the Thermal Resistance Coefficient of LEDs

http://www.lrc.rpi.edu/programs/solidstate/cr_thermalresistance.asp

Quote
Do LED bubls weaken/lose brightness over time?

http://www.candlepowerforums.com/vb/showthread.php?132057-Do-LED-bubls-weaken-lose-brightness-over-time

But why ask people who have spent more than a little time actually checking these things when you can simply misinterpret a datasheet?

Hmmmm, led brightness drops off over the lifetime (50,000 - 80,000 hours?) by perhaps 30% before it dies. Then the loss is 100%.

So getting old sucks for leds too.
Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

dhenry

Quote
someone understands!


Surprisingly, few in the "you have to have a resistor" crowd understand the reason you had articulated as to how a resistor is needed.

GoForSmoke



Quote
And bull, any led will dim just by using it, now if its abused it will crap out much quicker than one kept in absolute perfect conditions, but it will still dim


ok, so if i set a spare regulator to a a mid range level until it reads say 15ma, i'll leave it at 2.1v or 2.3v or whatever it is, i'll have an identical one to that i wont touch (only to power it up once to compare brightness), and i'll leave it on for a week and i'll take a before and after picture... don't expect me to do it right now, but I will tomorrow... and i'll see how much visible differences they are




A week? Try 3 months maybe more to get a real drop.

I wonder if leds kept colder than room temp, say in a walk-in freezer, would last appreciably longer, more than 10-12%?

Or what using a led as a light sensor does to its lifetime?
Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

Osgeld

#23
Jan 21, 2013, 05:35 pm Last Edit: Jan 21, 2013, 05:45 pm by Osgeld Reason: 1
be sure to record your current as well over the time. I doubt you will be able to see much in a week without using a lux meter ... but then I tend to design stuff that will last more than a week anyway

let me ask, do you also drive your car with it always in first gear cause it works fine and  you see no immediate damage?
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GoForSmoke

LOL, you ask about driving a stick in this day and age?
Not that I don't have an abiding dislike for automatics but I'm approaching 60.
Nick Gammon on multitasking Arduinos:
1) http://gammon.com.au/blink
2) http://gammon.com.au/serial
3) http://gammon.com.au/interrupts

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