help with the LED circuit

I am confused by a simple circuit.

Just a resistor and a LED in series. T (temperature) Vf( LED forward voltage) If (LED forward current) Vr (votage on resistor)
The voltage input has no change.
Temperature goes up

T up ---> Vf down (by data sheet) ---> Vr up--->If up

So the current goes up with temperature. This should be correct proved by test.

But T up---> If down (by data sheet too) --->Vr down---> Vf up

Same temperature effect with conflicting result.

As LED is a current drive component the analysis seems to start with current but conclusion is wrong.
Please shed some light.
Thanks.

Hi there Richard,


Perhaps you can provide us with a link to the data sheet that you refer to?
This will help us understand better.

In the mean time, if you power an LED with a voltage source and a series
resistor you will see a decrease in forward voltage as the LED heats up.
Let me give you a real world example here...

Two 1w Stars connected in series, in series with a 10 ohm resistor, driven
by a 9v DC wall wart and using a heat sink.
When the LEDs are at room temperature (maybe 80 deg F)
and the power is first applied, the two LEDs read 3.280 volts
across the two leads of each LED. As the LED heats up, the voltage
across one reads 3.260 volts. This is a decrease of 20mv, and the spec
is 'roughly' -2mv per degree C, so this means the LED got hotter by
20/2=10 degrees C. This is approximate and does not take into account
the change in current as a result of the decrease in Vf of the two LEDs.
To make this a little more accurate, we drive the LED at a constant current
of 247.921ma (or whatever you want to use that is reasonable). Now the
forward voltage is 3.285v at room temperature and 3.260v after it heats up.
This is a change of -25mv, so the LED temperature increase was really
25/2=12.5 degrees C.

Next, i would like to see the data sheet you are referring to.

I agree with MrAl that seeing the data sheet would allow a much clearer answer to be given.

Here is something to think about though.

In engineering, there are two kinds of calculation to consider: design calculations and rating calculations.

Design calculations consider the allowable limits when operating a device.

Rating calculations consider what a device will actually do when subjected to certain conditions.

One calculation is kind of the reverse of the other.

So for instance, if you take an actual LED, it has a negative temperature coefficient. This means that if you increase the temperature the resistance decreases. Therefore: for a given current, Vf will go down, or for a fixed voltage the current will increase. This is a rating calculation.

Now instead consider the allowable operating conditions for the LED. There is a maximum temperature that must not be exceeded to prevent damage. However the current flowing through the LED will heat it up further. So if the LED is already operating in a warm environment the current allowed must be reduced to keep the junction temperature in a safe range. But if the LED is operating in a cool environment more heat can safely be removed -- and therefore the allowable current may be increased. This is a design calculation.

To put the design aspect into plain terms, an LED with a big fat heat sink can run higher currents than the same LED with no heatsinking.

You can see the difference: when rating, a higher temperature leads to higher current and lower forward voltage. When designing, higher temperature requires a restriction to lower current (and still leads to lower forward voltage).

Let's make superflux as example the http://www.lumileds.com/pdfs/DS05.pdf

The figure 4a shows Temp. VS If.
We can see forward current(If) goes down with the temperature goes up

I also understand Forward voltage (Vf) goes down witht he temperature goes up

But I am confused in the circuit of one resistor in series with one LED provided constant voltage input.

Analysis1: By If
Temperature (T) UP ----> forward current (If )decrease ------> voltage on resistor (Vr) decrease -----> forward coltage (Vf) up (Vin-Vr)

Analysis 2: BY Vf
T up------> farward voltage Vf decrease------> voltage on resistor Vr up------>If increase

Two analysis conflict. Analysis 2 is correct by test value.
I am trying to figure where I am wrong. Which one we should think of first Vf or If in a circuit?
A bird learns to fly. Thanks.

It is as I explained in my post above.

Figure 4a is not a rating chart, it is a design chart. It shows the maximum DC current that is permitted by design according to the ambient temperature. You as the circuit designer are required to reduce the current if you think the ambient temperature will be higher. You must reduce the current in your example by increasing the value of the series resistor.

Therefore your analysis 1 is incorrect. It should be like this:

Analysis1: By If
Temperature (T) UP ----> maximum current (I ) must be decreased ------> resistor value must be increased -----> voltage on resistor (Vr) increases -----> forward voltage (Vf) down (Vin-Vr)

Thanks for your explanation.
I unstood I should lower the current to meet that maximume current alowed in Temp VS current chart.
But Let's say that simple circuit is there already after design. We do a circuit study.
The relationship between the temperature and forward voltage and current is there for an LED.
Does forward current decrease when temperature goes up?
Does forward voltage decrease when temperature goes up?
If answers are yes my confusion is still there as posted becasue the results seem conflict.
I still didn't get it.

Mr. happy,
Sorry, I read your first response about the rating calculation. I think I got the answer now. Still need digest a little bit. teach me if you have any more.
I appreciate your great help.

If you have an already installed circuit then the forward voltage will decrease when the temperature goes up, and the current will tend to increase accordingly unless it is regulated.

However, this is where design comes in to play again. When installing an LED such as the one on that data sheet, the design must permit sufficient heat removal from the LED to keep the junction temperature in the allowable range. The manufacturer of that device suggests that the typical junction temperature will be 60ºC when installed with an ambient temperature of 25ºC and sufficient heat sinking is provided.

It means that in general the effect of temperature on Vf is not that important as a design consideration. It is more important to stay within the allowable limits of operating current and environmental temperature. Usually an active current regulating circuit would be used for high power LEDs rather than a simple series resistor.

It is always in my mind that LED is current driven component. I was thinking analysis should start with current. But actually we look at the voltage change. How can I clear this confusion in my mind?
Thanks again.

Richardli said:

It is always in my mind that LED is current driven component. I was thinking analysis should start with current. But actually we look at the voltage change. How can I clear this confusion in my mind?
Thanks again.

Click to expand...

Yes, design and analysis of LED circuits is based on mainly on current. You need sufficient voltage available to overcome Vf, but after that it is not directly involved in the calculations. I don't think I said analysis should be based on voltage. Here is what I said:

It means that in general the effect of temperature on Vf is not that important as a design consideration. It is more important to stay within the allowable limits of operating current and environmental temperature. Usually an active current regulating circuit would be used for high power LEDs rather than a simple series resistor.

Click to expand...

Vf is not that important. It is more important to consider current. An active current regulating circuit is often used to achieve this.

Go back to that simple circuit of resistor and LED in series for the temperature effect.
I understood I shouldn't look at the chart in datasheet which shows max. forward current for design calculation.
But for analysis purpose.
Temperature goes up both Vf and If decrease for an individual LED. Which one I should think first Vf or If for that circuit? You know they will give different result.
I still can't think it through what I am missing. But I am learning.

Richardli said:

Go back to that simple circuit of resistor and LED in series for the temperature effect.
I understood I shouldn't look at the chart in datasheet which shows max. forward current for design calculation.
But for analysis purpose.
Temperature goes up both Vf and If decrease for an individual LED. Which one I should think first Vf or If for that circuit? You know they will give different result.
I still can't think it through what I am missing. But I am learning.

Click to expand...

up front please know,...wading in here midstream w/o carefully reading the entire Thread.

here's my guess (which could be wrong) on the source of your confusion.

you're looking at the charts you referenced analytically when you ought to be looking at them as design limitations.

1. the charts are telling you what you, as the designer, must limit the current to for a given temperature.

2. unless i'm grossly mistaken, the charts are NOT intended to tell you, operationally in a circuit, what happens to current as the t-amb. changes. your circuit analysis will show that the current is going up as the temp goes up. that is correct. the charts are only saying that you MUST NOT allow the current to go beyond the points indicated on the curves for a given temperature. again, if i understand the charts correctly, they are NOT telling you that the I goes down as the Temp incr., but rather that you, the designer, must limit the current to the level specified by the charts for any given temperature. the charts are NOT performing any circuit analysis, just specifying maximum design criteria for a given temp. as such, if your design is operating in warmer t-amb, then you must account for that in your design and NOT permit the current through the LED to go above that indicated on a particular curve for a given temp. the charts are NOT saying that current goes down as temp rises. they are expressing OPERATIONAL LIMITS/MAXIMUMS, *NOT* operational circuit analysis (i.e. if operating temp goes up, then operational current goes down [sic] which would be just the opposite of what your ckt analysis showed; the current actually goes up, but the designer must know this and make sure that it doesn't exceed the I-Maxallowable from the chart).

3. the charts are merely telling you what you must limit the max. current to in your particular design in order to not damage or drastically shorten the life of the LED, in other words the charts specify the "safe"/normal Mfr's recommended upper operational limits. as such, for a given temp, you must limit the I to the indicated max value, or you must make sure that your design removes any additional thermal energy (i.e. heat), through some cooling mechanism.

unless i'm mistaken, this is one of the points that Mr. Happy has been tryin' to make.

[Note: my apologies for the redundancy of description above. i've tried, through somewhat different wording, to make the explanation clear in the hopes that one of the explanations might hit home, so to speak.]

I did understand what I was wrong to use the chart in the datasheet for my operational caluculation. It is clear. I know where my mistake was. Thanks for explanation.

Just look at that simple circuit without look at the chart at all.
I need to know the change in the circuit with the temperature goes up.
Then the LED forward voltage decrease. Why not start with the forward current decrease?

I am not a designer. I just learn how to analyze the circuit so that I can explain some problem here.
My english is not too good. Apologize for some of my wording.

Richardli said:

Just look at that simple circuit without look at the chart at all. I need to know the change in the circuit with the temperature goes up. Then the LED forward voltage decrease. Why not start with the forward current decrease?

Click to expand...

The current does not decrease. In circuit analysis, if you increase the temperature of the LED the forward voltage decreases and the current therefore increases. Why start with the forward voltage? -- because it is the most useful in design calculations to determine the current.

Please note that in a well regulated circuit the change in temperature is not that important, so you may be worrying about something that does not need to be of concern.

The circuit is one of our old products. Most of new products here use voltage regulators and switching PWM circuit.
You are right all.
I do appreciate for all your patience and explanation.
RIchard