# Can running an LED on too low power cause damage to it?



## Ilikeshinythings (Nov 12, 2006)

I haven't posted in some time, but I have a question. 

Can running an LED on too low of power for a given amount of time cause damage to the LED, similar to running an LED at too high of power?


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## James S (Nov 12, 2006)

I'd be tempted to just say "no" 

But I'll say that no, not that I've ever heard of or seen in my own messing around. If that were the case then current or voltage limiting as a way of dimming them wouldn't work as well as it does.


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## D-Dog (Nov 12, 2006)

I too say no, as an LED is nothing more than a diode (...) and if you don't have enough voltage, light just wont be emmited. Unlike an incandescant bulb, running on a lower voltage will not increase lifetime. Overdriving an LED is another story, however, they still usually last a long time.


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## Meduza (Nov 12, 2006)

No

D-Dog, if it is a white led you will propably extend its lifetime due to that it run cooler and thus reduce the degradation of the phosphor...


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## Ilikeshinythings (Nov 12, 2006)

cool! Thanks for the quick response's as usual!


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## 45/70 (Nov 12, 2006)

D-Dog said:


> I too say no, as an LED is nothing more than a diode (...) and if you don't have enough voltage, light just wont be emmited. Unlike an incandescant bulb, running on a lower voltage will not increase lifetime. Overdriving an LED is another story, however, they still usually last a long time.


D-Dog is correct! :thumbsup: A very good explanation. I would point out though, that with an incandescent, there is a point where too low a voltage actually decreases bulb life. If the filament is not run at a high enough temperature, the vaporized tungsten will not redeposit itself back onto the filament. When this has happened, you will notice a grey/black film on the inside of the envelope. This tungsten is displaced from the filament, making it thinner and eventualy it will break, usually with a bright flash. 

Dave


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## PhotonBoy (Nov 12, 2006)

As I understand it, there are two basic ways to lower the output from an LED: lower the voltage, or supply square wave voltage at a relatively high voltage to the LED. The latter is known as PWM or Pulse Width Modulation. (I hope that explanation is clear).

Anyway, my question is this: Does PWM stress the LED more than simply running the light at a constant, lower voltage? A real world example would be to drive your car more slowly by flooring it for half a second and then letting up and allowing the car to coast for ten seconds, then repeating.


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## 45/70 (Nov 12, 2006)

PhotonBoy said:


> Anyway, my question is this: Does PWM stress the LED more than simply running the light at a constant, lower voltage? A real world example would be to drive your car more slowly by flooring it for half a second and then letting up and allowing the car to coast for ten seconds, then repeating.


PWM does not stress the LED any more than it would if the LED were driven at the peak current. Your car analogy is more or less correct however, it'd be more like flooring and coasting at, for example, 200 times per second thus, smoothing it out so, you wouldn't really notice it! 

Dave


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## wasBlinded (Nov 12, 2006)

45/70 said:


> PWM does not stress the LED any more than it would if the LED were driven at the peak current. Your car analogy is more or less correct however, it'd be more like flooring and coasting at, for example, 200 times per second thus, smoothing it out so, you wouldn't really notice it!
> 
> Dave


 
Actually, you *can* damage an LED with high peak PWM currents, even though the average current is well under specifications. As long as the peak current is within specs, there should be no problems.


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## MrAl (Nov 12, 2006)

Hi there,

There is nothing wrong with running an LED under the current spec except
for color output. Most of the LED colors are spec'd at their nominal current.
This is necessary in order to produce the correct color output.
Usually it doesnt matter that much because the color doesnt change
that much with reduced output. I've had no problems with this ever.
There's never been a problem with flashlights that i've heard about.

There are other types of applications that are more sensitive to the
wavelength of light however...

In dental applications it is possible that the dental epoxy used
for tooth repair may not cure properly if the wavelength of the emitted LED
light is not correct (special blue LEDs). For this kind of application it is mandatory to 
supply the LED with the correct current level.


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## TORCH_BOY (Nov 12, 2006)

Nope,


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## chesterqw (Nov 12, 2006)

will running on low voltage and super high current damage LED?

lets say luxeon I white, 0.05V,4A


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## wasBlinded (Nov 12, 2006)

chesterqw said:


> will running on low voltage and super high current damage LED?
> 
> lets say luxeon I white, 0.05V,4A


 
It simply cannot be done!


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## Handlobraesing (Nov 12, 2006)

PhotonBoy said:


> As I understand it, there are two basic ways to lower the output from an LED: lower the voltage, or supply square wave voltage at a relatively high voltage to the LED. The latter is known as PWM or Pulse Width Modulation. (I hope that explanation is clear).
> 
> Anyway, my question is this: Does PWM stress the LED more than simply running the light at a constant, lower voltage? A real world example would be to drive your car more slowly by flooring it for half a second and then letting up and allowing the car to coast for ten seconds, then repeating.



Depends on the duty cycle and duration of on-cycle. 
LED should be able to take 400Hz 100mA @ 25% duty cycle as if it was continuously 25mA, but if you were to drop it to say 1/4 Hz 100mA @25%, 1 second on, 3seconds average current over a long time is the same, but the on-time at 100mA exceeds the acceptable on-time duration.


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## TMorita (Nov 12, 2006)

chesterqw said:


> will running on low voltage and super high current damage LED?
> 
> lets say luxeon I white, 0.05V,4A


 
You can't push 4 amps through a LED with only 0.05 volts of pressure.

This is like asking if you can damage a 1 inch water pipe with 0.05 pounds per square inch of pressure with 4,000 gallons per minute flowing through it.

Toshi


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## matrixshaman (Nov 12, 2006)

Nope - no damage potential. BTW I've got an LED in a garage door switch that's been flashing on and off about 2-4 times per second for many years - it runs this way 24/7 to indicate the door is electronically locked. It's not showing any sign of dimming or quitting (it is probably just a 5mm LED).


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## chesterqw (Nov 13, 2006)

meep. my physics suck 

how about 2.5V and 4a?


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## evan9162 (Nov 13, 2006)

It still can't be done. The LED must operate on its I/V curve. 

Read these and be enlightened:

http://candlepowerforums.com/vb/showthread.php?t=77221
http://candlepowerforums.com/vb/showthread.php?t=72528


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## MrAl (Nov 13, 2006)

Hi there,

You have to have enough voltage AND current to run an LED in the first place.
The voltage is not proportional to the current, but as the current rises so
does the voltage. In other words, if you have a 3.3v LED at 350ma to get four
amps through it (not recommended) you would have to raise the voltage above
3.3v.


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## Christexan (Nov 13, 2006)

Too low won't hurt it, and most LEDs are rated for "pulse" modulation for higher output situations... for instance a Cree XR-E is rated (from memory, correct me if I'm wrong) for 1.8amp operation at 10% of a 1khz cycle. 
If you want to dim them, 2 chioces are "lower power" or "pulse" modulation... 
Pros/cons...
Lower power - 
Pros: Simple (stick in a resistor/take out some current, etc...), cheap, LEDs are more efficient at lower powers (to their Vf cutoff point).

Cons : Wasteful... a 20mA rated LED (or a larger LED/array for better comparison), at 10mA uses 1/2 the power of it's rated output. (Sounds good, but see below)...

"Pulse" modulated LED
Pros : Extremely efficient, 
--if a 1000khz frequency at 10% duty cycle is used, at rated 20mA current, the LED (stationary) will "look" the same to the eye as a full 20mA LED, but using only 1/10th the power. Eyes cannot easily detect frequencies above around 100hz from a stationary source. If source is moving, "flicker" can be detected, frequency and duty cycle can be tweaked further to make this undetectable. 
-- Using lower power input, (10mA), combined with a "visually stable" pulse (say 500hz/20% duty cycle), the LED would appear roughly identical to a 10mA constant, but using only 1/5 the power (or 2.0mA effective).
-- Using 20mA input, 1000khz, at a smaller duty cycle (5%), the eye might see a drop in output... tweak as desired for "50% dim", etc). Your lower limit is the speed of the LED itself (Off-On-Off cycle time, typically in the 10 nanosecond to 1 microsecond range)
Cons:
Cost of circuits - ($$)
Cost of circuits - power to run the circuit
Space required for circuits
Tweaking to eliminate "flicker"


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## TMorita (Nov 13, 2006)

chesterqw said:


> meep. my physics suck
> 
> how about 2.5V and 4a?


 
It doesn't work that way.

Let's say a 1 inch pipe has a flow of 12 gallons per minute at 5 pounds per square inch of water pressure.

You can't just make up random pressure/flow combinations like, "Oh, what if the 1 inch pipe has 20 gallons per minute at 5 pounds per square inch?"

If you have a certain water pressure in the pipe, you're going to have a certain number of gallons flowing through it, which is proportional to the current. You can decrease the water flow by adding a valve which you can open/close/open partway, but you can't increase the water flow through the pipe without increasing the water pressure.

The water pressure (voltage) determines the amount of water flow (amps) going through the pipe (LED).

Toshi


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## wasBlinded (Nov 13, 2006)

Christexan said:


> "Pulse" modulated LED
> Pros : Extremely efficient,
> --if a 1000khz frequency at 10% duty cycle is used, at rated 20mA current, the LED (stationary) will "look" the same to the eye as a full 20mA LED, but using only 1/10th the power. Eyes cannot easily detect frequencies above around 100hz from a stationary source. If source is moving, "flicker" can be detected, frequency and duty cycle can be tweaked further to make this undetectable.
> -- Using lower power input, (10mA), combined with a "visually stable" pulse (say 500hz/20% duty cycle), the LED would appear roughly identical to a 10mA constant, but using only 1/5 the power (or 2.0mA effective).
> -- Using 20mA input, 1000khz, at a smaller duty cycle (5%), the eye might see a drop in output... tweak as desired for "50% dim", etc).


 
This is not true. The retina/brain combination is an integrating machine, and running an LED at a short duty cycle will not fool the brain and retina into thinking the light output is the same as if it were running the same current at a 100% duty cycle.

PWM is also less efficient than straight current regulation at a given average power because the LED operates less efficiently at high currents than low currents. There are other advantages to PWM, though: cheaper circuit, less LED tint shift with dimming.


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## evan9162 (Nov 13, 2006)

Christexan said:


> Too low won't hurt it, and most LEDs are rated for "pulse" modulation for higher output situations... for instance a Cree XR-E is rated (from memory, correct me if I'm wrong) for 1.8amp operation at 10% of a 1khz cycle.
> If you want to dim them, 2 chioces are "lower power" or "pulse" modulation...
> Pros/cons...
> Lower power -
> ...



No.
Absolutey no.
PWM is less efficient than constant current:
http://candlepowerforums.com/vb/showpost.php?p=1490442&postcount=2

Seems we have to refute the myth of PWM efficiency about every 6 months around here.


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## chesterqw (Nov 13, 2006)

pwm is less effcient.

it uses more energy after some time!

pwm is useful only for dimming where the pulsing doesn't matter.
pwm is often use as it is easy to do and cheaper then constant current!


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## elgarak (Nov 13, 2006)

Just to correct some minor, but unfortunately common, errors made in this thread:

The lifetime and output of an LED does not depend on the voltage. The LED gives off no light with voltage below the forward voltage, and give off light above. The voltage can be quite high above the forward voltage without affecting anything.

What is important for an LED is the current going through it. Overdriving means driving the LED at a higher CURRENT than the manufacturer recommendations. Underdriving, of course, too low a current. Underdriving will not damage the LED, in fact will increase its lifetime.

One cannot underdrive an LED with too low of a voltage, since the LED will simply give off no light if the voltage is too low. 

(The difference in voltage needed for lowering the light output is very small. A constant current circuit in which you change the voltage to change the light output [as some have implied here, though I hope this was just sloppy language] is, frankly, crap. One changes the light output by changing the driving current, or PWM.)


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## CanadianGuy (Nov 14, 2006)

Hey, maybe it's worth mentioning Ohm's Law? I remember learning it back in College.

E=I x R

(E is electromotive force, or Voltage)
(I is inductance, or Amps)
(R is Resistance, or Ohms)

This should apply to what you're talking about here, eh?


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## NewBie (Nov 14, 2006)

Christexan said:


> Too low won't hurt it, and most LEDs are rated for "pulse" modulation for higher output situations... for instance a Cree XR-E is rated (from memory, correct me if I'm wrong) for 1.8amp operation at 10% of a 1khz cycle.
> If you want to dim them, 2 chioces are "lower power" or "pulse" modulation...
> Pros/cons...
> Lower power -
> ...




Most definitely a major fallacy that people have promoted for many years.

The urban legend is just that. The human eye integrates the light over time, and pulsing does not increase the brightness, even though the instantaneous brightness is higher. In fact, it actually takes time for the human eye to integrate the light, and not being a theoretically perfect integrator, and non-perfect response, part of the pulse can be attenuated for very short fast rise time pulses.

Most the stuff you mentioned under PWM is flat out wrong, no if ands or buts about it.

Here are actual measurements for the LED's efficiency under constant current dimming vs. PWM:


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## Calina (Nov 14, 2006)

CanadianGuy said:


> Hey, maybe it's worth mentioning Ohm's Law? I remember learning it back in College.
> 
> E=I x R
> 
> ...


 
Silly me, I always thought I was current.


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## Gnufsh (Nov 14, 2006)

I is current and inductance is not measured in amps.
http://en.wikipedia.org/wiki/Ohm's_law

Anyway, aren't diodes non-ohmic devices? Maybe I'm mis-remembering...


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## evan9162 (Nov 14, 2006)

elgarak said:


> Just to correct some minor, but unfortunately common, errors made in this thread:
> 
> The lifetime and output of an LED does not depend on the voltage. The LED gives off no light with voltage below the forward voltage, and give off light above. The voltage can be quite high above the forward voltage without affecting anything.
> 
> ...



Most of what was said here is wrong as well (lots of bad info in this thread).

The Vf quoted for LEDs is the typical voltage drop across the device at the device's rated current. Down to a certian voltage, supply it with less than that voltage, and the current will be smaller, and less light is produced. Supply it with more, and more current flows, and more light is produced.

Lets take an example of a luxeon with a Vf of 3.3V at 350mA. If you supply it with 3.3V, then 350mA will flow through the device. Supply it with 3.6V, and about 700mA will likely flow through the device. Supply it with 3.0V, and about 100mA will flow through the device. In fact, the turn-on voltage for a luxeon is around 2.4-2.5V. In that range, the luxeon starts to produce a dull glow, increasing in brightness with increasing voltage/current. So an LED will produce light with less voltage than its rated Vf. Of course, supplying an LED with a fixed voltage is a bad idea - you won't really have any control over the current so you're just playing with fire at that point. If you use a proper current regulator, you really don't care about Vf except when choosing what kind of regulator to use vs. the power source. After you've chosen the appropriate circuit topology, you can just forget about Vf all together.

This statement:


> The voltage can be quite high above the forward voltage without affecting anything.


Is just flat out wrong and dangerous. If my Luxeon has a Vf of 3.3V at 350mA, and I put 4V across its terminals, I'll have way more than 1A flowing through the device. How is that not affecting anything?




> A constant current circuit in which you change the voltage to change the light output [as some have implied here, though I hope this was just sloppy language] is, frankly, crap


Constant current circuits do change the output voltage in order to change the current - how else to you expect them to work? The difference is in what they use for feedback. A voltage regulator measures the voltage across its output for feedback. A current regulator measures the voltage across a sense resistor that has the output current flowing through it. In both cases, the regulator modifies the output voltage so that the regulation criteria is met.


You really need to read the two threads that I linked above - it is imperative to fully understand the relationship between Vf and current in an LED before trying to give advice to others about how they work.


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## Ilikeshinythings (Nov 14, 2006)

Sometimes I wish I would have been an EE like my buddy's instead of a Business Major so that I could understand all these details!


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## amanichen (Nov 14, 2006)

Ilikeshinythings said:


> Sometimes I wish I would have been an EE like my buddy's instead of a Business Major so that I could understand all these details!


If you're still in school you might be able to finagle your way into a basic circuits class or two (but some schools restrict them to engineering and physics majors only.) You don't have to be a double E to understand the introductory material, but you will probably need some calculus and physics as prereqs.


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## MrAl (Nov 14, 2006)

Hello again,

It's not too hard to understand the voltage and current character of an LED, but
it is a little different than a resistor. A resistor is a device whos current and voltage
have a very particular relationship that is different than what an LED has.
For the resistor, if you increase the voltage by a factor of 10 percent then the
current rises by 10 percent also. If you increase the current by a factor of 10 percent
then the voltage rises by 10 percent. Note here we could either 'drive' with
voltage OR current, and let the other find it's own spot. We can apply a current
and let the voltage go to whatever it has to be for that value resistor, or we can
apply a voltage and let the current go to whatever it has to be for that value.

For an LED however, when we increase the current by 10 percent we might only
see a 1 percent change in the voltage. Then at a higher voltage, if we increase
again by 10 percent we might see a 10 percent change in current. Here, when
the current was lower and we increased it by 10 percent we saw a 1 percent
increase in voltage but when the current was already high we say a 10 percent change.
This is very unlike the resistor, but there is still a unique relationship here, it's just 
a little more complex than a resistor. This kind of relationship is usually called
"Nonlinear". The relationship between current and voltage in the resistor is called
"Linear".
Devices fall into one of these two catagories, either linear or nonlinear.
The resistor is linear while the LED is nonlinear.

What nonlinear really means to us is that we have to look at that kind of device
a bit differently than with a resistor. We have to know what the current (or voltage)
is already before we can make an estimate as to what the step change will bring.

Also, because after looking at various LED curves we find that the voltage
changes only a little as the current changes a lot this means the LED is more
sensitive to changes in voltage than it is to changes in current. Besides that,
as the temperature changes this relationship changes too. Add to that the fact
that an LED has a rating that is more dependent on current than on voltage, and
we end up finding out that it's better to drive an LED with a current source than
with a voltage source.

Here is a chart of a 10 ohm resistor for various currents:
0.001A 0.010v
0.010A 0.100v
0.100A 1.000v

Here is a chart of an older Nichia type LED:
0.001A 2.800v
0.010A 3.300v
0.100A 4.750v

Note that with the resistor you can just multiply the current by 10 to get
the voltage, but with the LED this isnt so. The LED still has a relationship,
but it's more complex than the resistors.
To understand the relationship between the current and voltage in an LED
it is necessary to look at the spec sheet for that very LED. One thing
to note is that the above LED at 10ma will have a forward voltage of 3.3v,
and that will always be the case (unless the temperature changes). Thus
if you are driving at 10ma you will see 3.3v across the LED (and not say
4.3v). Although the relationship is not linear, there is still a relationship
that correlates one current to one voltage.
If the temperature does change, this relationship changes, but it still
exists (for some LEDs the voltage change is about -2mv per degree C).

Edit: Sign on voltage change.


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## elgarak (Nov 14, 2006)

evan9162 said:


> Most of what was said here is wrong as well (lots of bad info in this thread).
> 
> The Vf quoted for LEDs is the typical voltage drop across the device at the device's rated current. Down to a certian voltage, supply it with less than that voltage, and the current will be smaller, and less light is produced. Supply it with more, and more current flows, and more light is produced.
> 
> ...


Maybe it helps to tell you that I am a physicist and have a slightly different view on things.

You need a certain driving voltage to make an LED light up; this voltage is independent of the driving current and depends mostly on the band gap of the used LED and therefore the color of the LED. Below this voltage, the LED will not light. You can apply anything (within limits) above this voltage, as long as you make sure that the current going through the LED is limited (maybe I was guilty of sloppy language myself in my previous post). The way you do the latter depends on what type of power supply you use (constant current or constant voltage). However, no matter what, you have to supply a certain minimal voltage to make the LED light. 

A battery is a constant voltage power supply, and can supply quite high currents if allowed. So you need a current limiting circuit, at least a resistor, to drive the LED safely. If you have way more batteries (=higher voltage) than needed to light up the LED, a lot of power gets dumped and wasted in this resistor. Cheaper flashlight designs do this all the time.


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## NewBie (Nov 14, 2006)

As you move into higher power LEDs, the current vs. Vf relationship gets much flatter. A good example is one of the current LEDs talked about a bit lately:






As these curves are much flatter than with 5mm LEDs, you will note for less voltage change, you get a larger current change. Where the 5mm LED Mr. Al showed did 100mA @ 4.75V, this LED would be drawing well over 5 Amps @ 4.75V. So, for small changes in voltage on the power LEDs, they have large changes in the current that flows thru them.

Another item of note is that as and LED heats up, it's Vf drops. If you were to use a voltage regulated power supply (instead of a current regulated power supply), as the LED heats up, this would cause the LED to pull more current. Since power is the product of Volts * Amps, the LED would run harder, and get warmer. As the LED gets warmer, it's Vf drops further, causing more current flow. This leads to more power and further heating. And the cycle repeats.

This is why, at a minimum, a resistor is used with an LED, and in the present day and age, it is typical practice to use a constant current source or a constant current power supply, with power LEDs.

As Mr. Al mentioned, a resistor just burns up the excess power as heat. More advanced supplies will use what is called a switching power supply that actually converts the power into something suitable for the LED. These can be low efficiency supplies (80% and below), or they can be good efficiency (90% range), or high efficiency, such as 95% and above.

Atypically, efficiency, cost, and space do not all work together. Usually, supplies can be made more efficient with more space, and more efficient at a higher cost. However, as you shrink the power supply in size, usually they become less efficient.

Switching power supplies come in to major types. One is voltage regulated, and the other is current regulated (current regulated=constant current). Normally with LEDs, you use a current regulated switching power supply, but some people will still use a constant voltage switcher, with a resistor to help "regulate" current.


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## elgarak (Nov 15, 2006)

There's a problem with language. A lot of people use "Forward Voltage" as any positive voltage *applied* in forward direction of the LED. There's also the "forward voltage drop", which some people abbreviate to "forward voltage". You'll notice that I avoided the term altogether in my last post.

The main points I wanted to make, especially with regards to the OP's question are the following:

1) With too low a _voltage_ *applied*, the LED will not light.
2) The brightness and lifetime of an LED are mostly controlled by the _current_ you let pass through the LED. A _current_ higher than the specified one will kill the LED faster. (A voltage *applied* that is higher than necessary to light up the LED, with very low limited current, will not.)


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## MrAl (Nov 15, 2006)

elgarak:
what are you talking about here? How can you apply a high voltage to the LED
and not also at the same time apply a high current? If you apply high voltage
you get high current; if you apply high current you get high voltage. This is
why i dont see what you mean?
The LED acts sort of like a zener diode, where the voltage doesnt rise as
much as the current does once it enters the zener region, but it does still
rise.
Another way of stating this is that there is no place on the curve where the
voltage is high and the current is low. There is no way to control the voltage
independently from the current.
You can't say: "ok, ill put 5v across the LED but i'll only let 1ma pass through it",
cause that cant be done. If you put a certain voltage, you get a certain current
and only that one current level for that one voltage level. Do you understand this?

Here's an equation which appoximates an LED very well at 25 deg C:

v=N*026*ln(i/IS+1)+Rs*i
where
v is the voltage across the LED
i is the current through the LED
N, IS, and Rs are constants for that particular LED and never change
(unless you use a different type LED of course).

N, IS, and Rs change for other LEDs, but for the same LED they remain 
constant for any v, i.

From the above equation you can see that if you plug in one value for i,
you get one value for v. If you plug in another value for i, you get a different
value for v. That's the way it is.

The only way to get a different value for v for the same i is to change the 
temperature, which we usually dont have control over.


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## elgarak (Nov 15, 2006)

Extreme case: Electrostatic generator. Generates extremely high voltages, but cannot sustain much current (and will probably kill the LED due to extreme high voltage). 

The voltage you're talking about is the voltage drop of the LED associated with a certain current. There are power supplies out there (which I happen to work with, for ion beam applications) that cannot provide significant current at moderate voltages. They do not kill LEDs (I tried). The LED tries to draw the current associated with the voltage, but the power supply simply cannot supply it. The measured voltage was the open circuit voltage of the PS minus the voltage drop of the LED. Current was very low, so the LED just dimly glowed.

It's not a suitable PS for running an LED long-term, mostly because the PS has to dump excess power somewhere.

The main point I try to make: The parameter to observe the life of an LED is the current, not the voltage. (Both need to be selected properly to design a proper driving circuit, here you're right.)

As I said, I am a physicist, and may have a different view of things.


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## chimo (Nov 15, 2006)

elgarak, hope you don't feel ganged up on.  

I have a question about this sentence:



> The measured voltage was the open circuit voltage of the PS minus the voltage drop of the LED.



Where was the voltage measured? Since you mention the voltage drop across the LED, I would guess it was not there. 

Paul


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## Christexan (Nov 15, 2006)

Newbie - first off, thank you for the excellent work you've done, I enjoy seeing your graphs and IR photos and such, they are excellent material.

Regarding my being "incorrect" however, we are talking apples and oranges here I believe, but I understand that confusion... I'm not saying that in a free energy environment, PWM dimming is more efficient than lowering the current to an LED, (although at certain levels that may actually be true, but at high levels, the decreasing LED efficiency does make that statement pretty accurate)... what I'm saying is that given a battery-based voltage source of a known constant (let's say 4.8V (4*1.2V NIMH), running a known LED (say Cree XR-E, at 350mA/3.1Vf from your testing graphs), you have two choices to limit the effective current (and adjust brightness), "resistor" based (linear regulator, inline resistance, etc), or switchmode (PWM/PFM) average over time based. Between these two options, the switchmode is going to be much more efficient (assuming no huge losses in the circuit itself), because of the losses in the resistor/linear regulator. In this example, a 4.8V (nominal, it's higher to start off of course) circuit running this LED at 350mA needs to drop 1.7V from the source to reach the 3.1Vf of the LED at 350mA (whether in resistors, regulator, combination of stuff), or 0.6 watts approximately. If a switchmode circuit requiring 0.5V to operate (reasonable) that can handle this is used (at 350mA), it would consume only 0.175 watts in the circuit, for the same average output over time. Now it would probably run a higher mA, over a lower % of duty cycle, to achieve the same average of 350mA per second (and roughly same effective light output)... for instance, running 400mA at 87.5% duty cycle would give roughly the same mA average per second. Now LED efficiency does decrease with increasing amperage (but probably not as sharply in a "pulsed" duty cycle as in a a constant-on cycle), so let's add another 5% to that, for 420mA pulses, at 87.5% (or a longer cycle at 400mA)... ANYHOW.... the point being, the watts consumed in the LED itself may be roughly the same in either situation, but you are gaining the efficiency in the regulation circuitry itself (the battery will last longer because less waste is happening before the power ever reaches the LED)... if you want 50% dimming, you can either add a resistor to drop the current 50% (or the amount needed to reach 50% luminous flux) (wasting the other 50%/whatever of the power as heat), or you can design a different frequency/duty cycle in a switchmode, only slightly changing the efficiency from one setting to the next, but adding little to no more watts of waste as heat.
In fact, the switchmode becomes much more efficienct as desired output drops, because to drop the current in a resistive application, every % drop has a corresponding increase in waste heat, compared to switchmode dimming where it's just "on" for less of a cycle, or less cycles at the same duration. 

Now, looking at it from a "free" energy perspective, yes, if you could have a source that only outputs a given current, with no waste involved to change the current level desired, your statements are correct, but for regulating from a battery source, as I was discussing, PWM is much more efficient than linear/resistive methods of dimming.


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## chimo (Nov 15, 2006)

Christexan, I think you may have missed a method of lowering power.

The term Switch-mode power supply (SMPS) does not imply PWM output. 

PWM/PFM may be used inside the SMPS to adjust the switch timing (and therefore output level), but the output is not PWM'd - it is constant voltage/current (plus ripple). Most constant current drivers are regulated by measuring the voltage across a sense resistor in the output loop. The resistor is sized such that the desired current will yield a voltage drop across it that is equal to the feedback voltage of the particular switcher chip.

These constant current implementations are a much more efficient methof of dimming than PWMing the LED. Recall that the LED becomes much more efficient (Lumens/Watt) with lower drive currents. By using PWM, you are driving the LED at a less efficient point in its Lumens/drive current curve.

Paul


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## Christexan (Nov 15, 2006)

Thank you, now I see where my posts are confusing, and I apologize, I shouldn't have said PWM, I'm referring to switchmode power supplies (I don't even remember where this thread started, but that was what I meant to be referring to in my initial post, I mistakenly used PWM when I meant SMPS), and adjusting a switch mode power supply to different current outputs is more efficient than using resistor based methods of current limiting. Ultimately more of the source power is available in the form of output power to the LED, with less going to waste heat. Sorry for my confusing my terms, I've either been writing in the middle of the night, or at work where I spend 2 hours writing a small post because of interruptions and "lose my place" so to speak, but I think I was originally talking about switchmodes and somehow got sidetracked to PWM (which IS typically more efficient in filament applications where the filament maintains significant light output between pulses (shorter pulses than the filament can cool down during), but that's another story).


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## Christexan (Nov 15, 2006)

Oh, having said all that, I am curious though, maybe Newbie could test this (I'm broke and have no time or materials to do so, but he seems well setup for such testing) to actually test the "effective" visual output of an LED using PWM (above a detectable flicker frequency, say 1000hz to match Cree's max rating at 1.8A or whatever it is (PDF not working for me right now)) vs constant current, at different levels, and see what the real-world effect is... 
For instance: 
A 350mA constant vs 350mA 80% cycle output (1.085 watts vs .868 watts effective)
A 350mA constant vs 600mA 50% cycle (1.085 watts vs .96 watts effective)
A 350mA constant vs 1000mA 30% cycle (1.085 vs 1.02 watts effective)
A 350mA constant vs 1.5A 20% cycle (1.085 vs 1.05 watts effective)
(rough estimated Vf shifts used for calculations above, not exact)... 
Anyhow, I wish I could do it myself, I'd like to see how the eye really reacts (rather than a numerical analysis) to these scenarios, maybe at some point I'll have the time, tools, and money to do it. I have a feeling though that a 1.5A/20% is going to "appear" brighter to the eye than a .35A/100% light, due to the way the eye reacts to light, even though it may not seem to add up numerically.


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## chimo (Nov 15, 2006)

Christexan, I think what you are looking for is fairly well represented by Newbie's second graph above. (Current Dimming vs PWM Dimming).

Paul


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## Christexan (Nov 15, 2006)

Not really, because that's the "statistical" light output (as gathered by a mechanical instrument), however the eye doesn't necessarily follow the statistical evidence due to the nature of photoreceptors, so I'm curious about what it actually "looks" like, rather than the statistical "photons gathered per unit area" numbers. His graphs are great, and I'm not questioning the statistical accuracy, I'm just curious how it relates to real-world visual appearance. Shine two beams at a white wall, one at 100% at XXXmA with a total output of XYZ watts, and one much higher in mA, same relative XYZ watts though, and "see" if one looks different. 150dB of noise at 5khz doesn't "sound" like anything to the ear, but an instrument can measure it, and it can make you deaf all the same. 
Same thing here, a machine can say so many photons fell in a given area per unit time, but it's not necessarily a one-to-one relationship with actual visual perception. In a "constant" mode, yes, since that's how most machines are designed to measure, but how does it relate when source is pulsed, although the eye does work by gathering photons over time, at some pulse rate it may not APPEAR visually different than a constant source. 
Anyhow, wishful thinking, no further desire to belabor the point honestly 9and way off topic of low-powering LEDs), I'm just curious what human eyes might actually perceive compared to raw numerical analysis. 
Afterimage, efficiency of photoreceptors per unit time, etc all could affect perceptual results. Wave a bright light in a dark room quickly, and the machine won't "see" a bright stripe one second later after turning it off, but the eyes will.


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## evan9162 (Nov 15, 2006)

You're really confusing terms here.

You're interchanging "switchmode" with PWM dimming, and the two are NOT the same thing. Switchmode regulators/supplies use inductors or high frequency transformers to store convert/store energy. The result is a reduced current draw from the power supply when the output voltage is less than the input voltage. In this case, the current to the LED is smoothed out and is constant, not pulsed. So switchmode DC-DC converters still send constant current to the load.

PWM dimming is less efficient than constant current at any level. Period. 

Here's the data for a Luxeon III LED:
I = 350mA, Vf=3.3V, out=35 lumens, eff = 30lm/W
I = 700mA, Vf=3.6V, out=60 lumens, eff = 24lm/W
I = 1000mA, Vf=3.8V, out=75 lumens, eff = 20lm/W

If we have 3.6V in, and we want to drive at 350mA, then using a resistor to get constant current, our power efficiency is 3.3/3.6 = 91.6%, with 8.4% (100mW) going up in the resistor. Operating at 350mA, our LED is producing 35 lumens. LED efficiency is 35 lumens/W. Overall efficiency is 32 lumens/W. 

Now, lets take the same setup and drive it with PWM dimming. Since the input voltage is 3.6V, then that means that direct drive (100% duty cycle), the current will be 700mA. Since we want 350mA average, we'll have to use a 50% duty cycle. Each pulse is 700mA, 3.6V, so we use the second set of data, we produce 60 lumens per pulse. But, since it's a 50% duty cycle, average output is only 30 lumens. Each pulse uses 2.52W of power, so the average power draw is 1.26W

Lets summarize real quick:
Constant current with a resistor:
350mA to the LED, 1.15W to the LED, 1.26W drawn from the power source, 35 lumens out, 32 lumens/W overall
PWM dimmed by 50%: 
350mA to the LED, 1.26W to the LED, 1.26W drawn from the power source, 30 lumens out, 30 lumens/W overall

With constant current, you get more light from the LED, for the same power drawn from the supply (so the same runtime), higher overall efficiency, and less power to the LED, so it will run cooler. And you get to use a 10c resistor rather than mucking around with a PWM circuit.


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## Moat (Nov 15, 2006)

Christexan said:


> For instance:
> A 350mA constant vs 350mA 80% cycle output (1.085 watts vs .868 watts effective)
> A 350mA constant vs 600mA 50% cycle (1.085 watts vs .96 watts effective)
> A 350mA constant vs 1000mA 30% cycle (1.085 vs 1.02 watts effective)
> A 350mA constant vs 1.5A 20% cycle (1.085 vs 1.05 watts effective)



No one has mentioned (what I thought was...?) the possibility of LED damage when short, high current (beyond max spec) pulses are used. Isn't it so, that even tho the *average* power consumed (and thus heat produced - 1 watt in the above example) may be within spec, the "instantaneous" temperatures @ the diode junction could prove damaging? IOW - cycling through too hot, cool off, too hot, cool off, etc... as it takes *time* for the heat to carry away from the immediate area of the junction/chip, the "ON" portion of the cycle could be too hot locally, thus damaging the diode...? 

I'd think, aside from lumen efficiency, something to at least consider as well. It would be interesting to see a lumens maintenance/time graph of the last example (20% cycle), for instance.

Also - at high currents - it seems at some point resistive losses in the chip's fine lead wires would come into play, lowering efficiency (as they're minimally sized to diminish light transmission blockage).


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## NewBie (Nov 15, 2006)

Luxeons can be pulsed at higher currents, and they themselves have done just this, up to 5 Amps, but kept the duty cycle rather low. The study was for DLP projectors.

I do know that some folks who owned the LionHeart and LionCub have reported rapid emitter degradation.


Christexan,

Back when I ran those tests, I did another item, where I compared the PWM, and adjusted the linear to match, to my eyes for brightness. I cannot find the excel spreadsheet, but I did learn that during very narrow pulses (low on duty cycle), I needed a tad more than what the instrument said, to make them appear equal brightness. As the pulse duty approached 10% and higher, my eyes agreed with the instrument. My rise and fall times at that time were about 100nS, and the current was regulated during the pulse, within 400nS. I think the repetition frequency was one or two KHz. I've got some circuits I've done that are 100 times faster now. Maybe when winter gets long, dull, and other items are not pressing, I will have a chance to repeat it.

While I've been fiddling with new circuits, here are some of the results (not all are from the same circuit):


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## TENMMIKE (Nov 16, 2006)

At this point i would like to thank the main participants of this thread. i enjoy reading this type of thread. 

You men have passed and are passing along information that is of great interest to many of us who have only a little better then a superficial understanding of the dynamics being discussed in some detail here and thus it becomes a educational thread.(to me anyway)
It is rare these days that we see this type much any more other then newbies and a few others excellent tech threads ,which are of course are still very informative ,as is this one . thanks again.


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## NewBie (Nov 16, 2006)

You are very welcome.

Just for reference, 1nS = 0.000000001 seconds, or one-billionth of a second.


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## Calina (Nov 16, 2006)

What was the name of this thread again? lol

It got side tracked quite a bit but let me add my praise for all the information that was shared here.


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## amanichen (Nov 16, 2006)

Elgarak, I've been following the thread and here's why I think people were getting confused...

It seems you're arguing that given the V-I relationship of an LED, many power supplies can't actually provide the current that an LED would draw given a certain voltage. This is quite true. But I think people were confused because you didn't distinguish between steady state and transient operation.

Remember, current isn't some magical thing that you can vary independently of the voltage (at least not in steady state operation.) A difference in voltage causes current to flow, and if there's less current, there's going to be less voltage regardless of whether the relationship is linear or exponential.

So, if you end up with a Luxeon with 3V across it that only has 20mA flowing through it, then this situation will only last for a short time as whatever charge has built up will dissipate. The voltage across the LED will settle out to correspond with the V-I curve of the LED. This is a capacitive effect, and only works in transient operation.

I think some people were becoming confused by this because it seemed you were implying that you can just pick and choose any combination of voltage and current for a LED in steady state operation. This only works for brief moments during transient operation. In steady state operation you're bound by the V-I relationship of the LED.



elgarak said:


> As I said, I am a physicist, and may have a different view of things.


I'm a mechanical engineer so that means I don't care much for electricity unless I can DO something with it. The nitty gritty details of fields and electrochemistry don't interest me (most of the time anyway :naughty, but that doesn't mean I have a different "view" of how electricity *works* than an electrical engineer would have. It works the same either way, I just care about different aspects of it. I care about how I can use motors to move fluids in creative ways (thermo fluid science gets me all hot and wet :naughty, as opposed to how to deliver power, perform calculations, or create those wonderfully addicting devices known as LEDs. A physicist might get excited about the quantum side of things with electrons jumping energy levels and emitting photons. It all works the same way though.
I think it's time I stopped  now.


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## MrAl (Nov 16, 2006)

Hi again Elgarak,

Ok, i think i see where you are coming from here, but i think i can summarize...

Normally when we characterize an electrical element (especially a two terminal 
element) we try to characterize it apart from any other element (such as a certain
type of power supply). If we do it this way then we can understand how it
works when connected to ANY other element. This is certainly possible in the
static case where voltage and current both reach a constant at some point. This
is why i pointed you to the curve
v=N*026*ln(i/IS+1)+Rs*i
because this is true independent of the type of static power supply used.
This of course means that if you understand that curve you understand the device
very well for 99.99 percent of all applications out there.

Should we need more dynamic info, we add a small parallel cap and series inductor,
but this is rarely needed for our simple flashlights, even when they are PWM at
moderate pulse widths.


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## Halibut (Nov 16, 2006)

Yikes. It looks like direct-driving an XR-E with a 4.2V li-ion would be a very bad idea indeed...

Thanks for posting this chart, Newbie!

-Dan



NewBie said:


> As you move into higher power LEDs, the current vs. Vf relationship gets much flatter. A good example is one of the current LEDs talked about a bit lately:
> 
> 
> 
> ...


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## wasBlinded (Nov 16, 2006)

That may be an atypically low Vf for the Cree XR-E. Time will tell.


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## NewBie (Nov 16, 2006)

wasBlinded said:


> That may be an atypically low Vf for the Cree XR-E. Time will tell.




Actually, it was the very first production version of the XR-E that I ever soldered down.


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