GarageBoy
Flashlight Enthusiast
What exactly is foward voltage? My L bin will light up with only 3 volts, but not that bright. I thought LED brightness was based on current
Foward Voltage?
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The forward voltage (Vf) is the voltage across the terminals of the LED. The rated Vf (which is what binning information is based on), is the voltage across the LED when the rated current is flowing through the LED.
Increasing the current will result in a higher Vf, and decreasing the current will result in a lower Vf. Vf and current are tied together along a curve that the LED operates at. This is the I/V curve.
There are two ways of looking at it, depending on how you think of powering the LED.
First, let's look at things in terms of powering an LED properly, with a constant current power source. In this case, the Vf depends on the current through the LED. So, The I/V curve looks like this: (this is a TV1K Luxeon III)
When controlling the current, the Vf changes only a little across a wide current range. In fact, when powering the LED with a constant current source, Vf isn't of too much concern, which is a good thing (I'll explain this a bit later)
Now, lets look at driving the LED in terms of supplying a constant voltage - which is what we're used to for driving things like motors, light bulbs, etc...
This is the same data, only I've flipped the graph, so now the current varies depending on the voltage across the LED - as if you used a voltage supply to power the LED.
Now, if driving an LED with a constant voltage supply, things get quite a bit more sensitive. Changing the voltage by less than 1V results in a 10X change in current. It would be rather difficult to ensure that an LED is being properly driven by just supplying a constant voltage to it.
That's not the only challenge. Every single LED has a different Vf. Binning ranges of Luxeon LEDs are pretty big. For instance, the K-bin range is 3.51V-3.75V. For Luxeon III LEDs, this measurement is taken at a current of 700mA. So, lets say we want to drive this particular Luxeon at 700mA, using a fixed voltage supply. We would have to set our power supply to about 3.68V. That might be fine for that one particular Luxeon, but if you wanted to set up a bunch of different Luxeons to be driven at 700mA, you would have to tune each voltage source to match that particular Luxeon's characteristics. You'd also have to make sure that your power supply could supply that exact voltage without drifting due to temperature, load, etc. A very difficult method.
But there's more
The Vf varies, not only on an individual basis, but with temperature. As the LED heats up, the Vf decreases, and increases as the LED temperature decreases. The change is only a vew millivolts per degree Centigrade, but, as you can see in the 2nd graph, very small changes in voltage result in large changes in current.
In the most extreme case, the LED heats up a bit, the Vf drops a bit; which, when combined with a fixed voltage source, results in more current going through the LED. Thus, the LED gets hotter. So the Vf drops more, drawing more current, etc, etc. This is called "thermal runaway", and if not stopped, results in the destruction of the LED, either from too much current, or too high of temperatures.
We're still not done yet. /ubbthreads/images/graemlins/tongue.gif
Vf also varies with time. Luxeons experience a reduction in Vf over their operational lifetime. The vast majority of the reduction happens in the first 100 hours or so of operation. I have done experiements verifying and quantifying this change in Vf. Below, is a graph that shows the above Luxeon after its Vf shifted after 100 hours:
And, the two plots together (initial Vf curve in blue, 100 hours later in red)
So, if you had started by using a fixed voltage supply to power this Luxeon, set at 3.68V, you would no longer have 700mA of current. You would now have about 1900mA of current through the Luxeon, or over 2.5 times the initial current. If left at that current level for long, that Luxeon is well on its way to destruction!
So, to conclude:
- Vf is the voltage across the LED
- The rated Vf (what you will see stated for an LED, or what a Luxeon will be binnned as) is the voltage across the LED at the LED's rated current.
- Vf and current are tied to each other across the I/V curve. The LED operates somewhere on this curve
- Vf varies on an individual LED basis. Two Luxeons from the same Vf bin will probably have different Vfs. Vf binning information is really provided to give a hint at characteristics, not specific information for powering. That's why Vf bins are in a range instead of exact values.
- Vf varies with the temperature of the LED. Higher temperatures lower the Vf (temporarily)
- Vf changes with use time (at least in Luxeon LEDs, probably in 5mm LEDs too). As LEDs age (while powered up), the Vf drops. This is a permanent change.
- Driving LEDs with a constant voltage sources is a bad idea. Direct driving LEDs from batteries is just one step from this configuration.
- Driving LEDs with a constant current source is a good idea. Changes in Vf are of no concern when using a current source.
Probably more information than you bargained for /ubbthreads/images/graemlins/tongue.gif
Increasing the current will result in a higher Vf, and decreasing the current will result in a lower Vf. Vf and current are tied together along a curve that the LED operates at. This is the I/V curve.
There are two ways of looking at it, depending on how you think of powering the LED.
First, let's look at things in terms of powering an LED properly, with a constant current power source. In this case, the Vf depends on the current through the LED. So, The I/V curve looks like this: (this is a TV1K Luxeon III)
When controlling the current, the Vf changes only a little across a wide current range. In fact, when powering the LED with a constant current source, Vf isn't of too much concern, which is a good thing (I'll explain this a bit later)
Now, lets look at driving the LED in terms of supplying a constant voltage - which is what we're used to for driving things like motors, light bulbs, etc...
This is the same data, only I've flipped the graph, so now the current varies depending on the voltage across the LED - as if you used a voltage supply to power the LED.
Now, if driving an LED with a constant voltage supply, things get quite a bit more sensitive. Changing the voltage by less than 1V results in a 10X change in current. It would be rather difficult to ensure that an LED is being properly driven by just supplying a constant voltage to it.
That's not the only challenge. Every single LED has a different Vf. Binning ranges of Luxeon LEDs are pretty big. For instance, the K-bin range is 3.51V-3.75V. For Luxeon III LEDs, this measurement is taken at a current of 700mA. So, lets say we want to drive this particular Luxeon at 700mA, using a fixed voltage supply. We would have to set our power supply to about 3.68V. That might be fine for that one particular Luxeon, but if you wanted to set up a bunch of different Luxeons to be driven at 700mA, you would have to tune each voltage source to match that particular Luxeon's characteristics. You'd also have to make sure that your power supply could supply that exact voltage without drifting due to temperature, load, etc. A very difficult method.
But there's more
The Vf varies, not only on an individual basis, but with temperature. As the LED heats up, the Vf decreases, and increases as the LED temperature decreases. The change is only a vew millivolts per degree Centigrade, but, as you can see in the 2nd graph, very small changes in voltage result in large changes in current.
In the most extreme case, the LED heats up a bit, the Vf drops a bit; which, when combined with a fixed voltage source, results in more current going through the LED. Thus, the LED gets hotter. So the Vf drops more, drawing more current, etc, etc. This is called "thermal runaway", and if not stopped, results in the destruction of the LED, either from too much current, or too high of temperatures.
We're still not done yet. /ubbthreads/images/graemlins/tongue.gif
Vf also varies with time. Luxeons experience a reduction in Vf over their operational lifetime. The vast majority of the reduction happens in the first 100 hours or so of operation. I have done experiements verifying and quantifying this change in Vf. Below, is a graph that shows the above Luxeon after its Vf shifted after 100 hours:
And, the two plots together (initial Vf curve in blue, 100 hours later in red)
So, if you had started by using a fixed voltage supply to power this Luxeon, set at 3.68V, you would no longer have 700mA of current. You would now have about 1900mA of current through the Luxeon, or over 2.5 times the initial current. If left at that current level for long, that Luxeon is well on its way to destruction!
So, to conclude:
- Vf is the voltage across the LED
- The rated Vf (what you will see stated for an LED, or what a Luxeon will be binnned as) is the voltage across the LED at the LED's rated current.
- Vf and current are tied to each other across the I/V curve. The LED operates somewhere on this curve
- Vf varies on an individual LED basis. Two Luxeons from the same Vf bin will probably have different Vfs. Vf binning information is really provided to give a hint at characteristics, not specific information for powering. That's why Vf bins are in a range instead of exact values.
- Vf varies with the temperature of the LED. Higher temperatures lower the Vf (temporarily)
- Vf changes with use time (at least in Luxeon LEDs, probably in 5mm LEDs too). As LEDs age (while powered up), the Vf drops. This is a permanent change.
- Driving LEDs with a constant voltage sources is a bad idea. Direct driving LEDs from batteries is just one step from this configuration.
- Driving LEDs with a constant current source is a good idea. Changes in Vf are of no concern when using a current source.
Probably more information than you bargained for /ubbthreads/images/graemlins/tongue.gif
GarageBoy
Flashlight Enthusiast
Thanks, I'm still trying to relate this all to brightness
Brightness is pretty much completely related to current, and almost completely unrelated to Vf.
Doubling the current gets you about 1.6 times the light, up to a point - about 1.3A on a Luxeon III. Brightness is diminished by higher temperatures.
Doubling the current gets you about 1.6 times the light, up to a point - about 1.3A on a Luxeon III. Brightness is diminished by higher temperatures.
L
LITEmania
Guest
evan9162
you are truly an asset here CPF.
Thanks for your lecture.
Warren
you are truly an asset here CPF.
Thanks for your lecture.
Warren
NewBie
*Retired*
Very well explained evan9162!
GarageBoy
Flashlight Enthusiast
Thanks everyone!
Just to expand on brightness vs. current, here's a graph that shows brightness vs. current for the same sample that the above graphs were made from.
The lux measurement was made by using a light meter at 1 meter from the bare emitter (no optics).
The lux measurement was made by using a light meter at 1 meter from the bare emitter (no optics).
chimo
Flashlight Enthusiast
Excellently presented, evan9162!
This chart makes you wonder if a "burn in" period of about 4 days on a test jig may be of benefit (it would beat swapping batteries for 4 days). It's also interesting the curve seems fairly linear between 620 and 1540mA.
Paul
Edit: Deleted question after I read the other thread on Vf shift over time.
This chart makes you wonder if a "burn in" period of about 4 days on a test jig may be of benefit (it would beat swapping batteries for 4 days). It's also interesting the curve seems fairly linear between 620 and 1540mA.
Paul
Edit: Deleted question after I read the other thread on Vf shift over time.
Amorphous
Enlightened
Evan9162.. That's an Awesome presentation.
I've been experimenting with the Vf and current relationship using a HP 6827A. (Adjustable * current limiting powersupply ) My result resonates well with your data. The only variable left is keeping the X/Y coordinate or Spectrum Loci on the CIE 1931 constant over time and over temperature variation. Another word, "Tint preservation" – I want to do everything to keep my priced LED color or tint the way it is for a long long time.
I've been experimenting with the Vf and current relationship using a HP 6827A. (Adjustable * current limiting powersupply ) My result resonates well with your data. The only variable left is keeping the X/Y coordinate or Spectrum Loci on the CIE 1931 constant over time and over temperature variation. Another word, "Tint preservation" – I want to do everything to keep my priced LED color or tint the way it is for a long long time.
NewBie
*Retired*
Here is some more data in a similar situation, snagged from my tests in my Constant Current vs. PWM thread, with numbers added:
Additional information is found here:
Constant Current vs. PWM
Temperature Effects
Additional information is found here:
Constant Current vs. PWM
Temperature Effects
GarageBoy
Flashlight Enthusiast
How does PWM work?
NewBie
*Retired*
Basically PWM is just like switching a light all the way off and all the way on. Basically the LED is just directly connected to the battery, and there is an electronic switch, thats about it.
GarageBoy
Flashlight Enthusiast
And how does that dim? (something with the eyes' perception?)
You basically switch the light on and off hundreds to thousands of times per second. The percentage of on time is the percentage of dimming - so the on time of each pulse can be longer, shorter, or the same as the off time.
Past a certian frequency, your eyes integrate the pulses into constant light. The brightness you see is dimmer than the brightness of a single pulse, but equal to the average brightness.
Past a certian frequency, your eyes integrate the pulses into constant light. The brightness you see is dimmer than the brightness of a single pulse, but equal to the average brightness.
chimo
Flashlight Enthusiast
Very nice chart, Newbie. Interesting to see the broad delta in intensity at the mid power levels.
Paul
Paul
NewBie
*Retired*
chimo,
If you look at the numbers closely, you'll see they get porportionately broader, peaking at the 3% point, running a 3% duty cycle and 3% of 1140mA (so both at 3% power). You'll note that the current dimmed LED is 228.3% more efficient than the 3% PWM dimmed LED.
Here is a chart showing the delta:
If you look at the numbers closely, you'll see they get porportionately broader, peaking at the 3% point, running a 3% duty cycle and 3% of 1140mA (so both at 3% power). You'll note that the current dimmed LED is 228.3% more efficient than the 3% PWM dimmed LED.
Here is a chart showing the delta:
