# Led's in series light power degradation with strange results

I am doing life cycle test for 7 led's connected in series with current limiting resistor.(picture for reference)

I am measuring light power(mW) with Thorlabs photo resistor for each led. Led's drives from constant voltage supply. After ~ 6 month i see strange results: while 2 led's light power decreased (~5%) other led's light power increased ~4-7%. I don't have the datasheet since this is some custom made module(but i know for sure they have current limiting resistor and the led's connected in series).

Please help to explain and understand these strange results. Will appreciate technical,intuitive and physical explanations.

Thanks you!

• If the dimmer leds were deteriorating and their terminal voltage lowered, more current would be taken by the stack thus boosting the brightness of the non-deteriorating leds. Apr 22, 2015 at 10:47
• @Andyaka You say, the forward voltage(of some led's) is lowered hence overall current was boosted. But the current should be the same for all 7 led's(from ohm's law) following higher brightness(more current more flux). I think i miss something.. Apr 22, 2015 at 11:06
• Not true - the current thru the LEDs is dictated by all the forward volt drops of the LEDs, the current limiting resistor and the applied DC power supply voltage. What is the difficulty you are having here? Apr 22, 2015 at 11:08
• @Andyaka Let's take example. I have 2 led's in series with 9V battery and 2V forward voltage. let's say according to datasheet with 2V forward voltage should be 20mA led current.So the resistor would be R=(9-2*2)/0.02 =250 ohm. Now from some reason first led forward voltage deteriorated to 1.5V hence with 250 ohm resistor the current should be I=(9-2-1.5)/250=22mA. If overall current boosted how the brightness for led with 1.5v forward voltage deteriorated (bigger current = more flux). Apr 22, 2015 at 11:21
• For whatever reason (known only to the deteriorating LED), it develops a lower forward voltage and lowers it's brightness, despite more current. If the LED is slowly dying then who can say why it does this? Maybe some form of increased leakage resistance around the LED is simultaneously lowering its forward voltage and diverting current away from the silicon that generates light? It's just speculation - you cannot expect a definitive reason from the info in your question. Apr 22, 2015 at 11:36

## 2 Answers

Relevant: I have personally carried out a large number of LED-years of testing, operating numerous strings of LEDs in series for many months each and monitoring light output change with time.

It is common for LED light output to increase during the early stages of their life, in some cases. What you are seeing is completely usual behaviour.

Light output and Vf (LED forward voltage may not be well correlated here)

While Andy's explanation about changing Vf with aging MAY be true and may even be the predominant affect that you are seeing, I would expect that variation in brightness with aging would explain what is being seen.

Factors of relevance to aging include:

• Whether the LEDs are all nominally the same or differ in batch or model or manufacturer.

• Whether the LEDs are run near or above or well above or well below their current and power ratings

• What temperature they are operated at, whether they are heatsunk and whether they all experience equal cooling and ambient conditions.

• More ...

If they ar all the same and treated the same way then it would be less usual to see significant differences in aging BUT what you are seeing is within the range of normal.

If they are different products or batches or if some are more exposed to cooling or they are on a heatsink with a temperature gradient, then differences would be expected.

In my testing I used a simple constant current supply per string (using one LM317 and one resistor). This is easy, not very expensive and removes the sort of uncertainty that you now have. All LEDs were nominally identical I also had a reference LED in the string which was shorted during normal operation and allowed to operate during testing. As the string was constant current driven, adding or removing the reference LED made no difference to the operation of the other LEDs but gave a sanity check on current constantness, instrumentation and measurement variations and measurement techniques.

Getting repeatable measurements between LEDs and between sampling periods requires careful design and operation. I made a sampling cone that seated very repeatedly over each LED fixed permanently to a light meter head. The reference LED provided a check on method etc.
Introducing the LED into an averaging sphere would work if you have such.

Lifetime depends on current and temperature independently. While the two are related thermally via power and heat sinking and ..., LEDS at the same temperature and varying current age differently. LEDs at the same current and differing temperatures age differently. Philips has some good material on this sunder Lumen maintenance.

Some real LEDs out to 300 hours: This is for 60 mA rated LEDs run at 40 mA. Look half reasonable.

Oops!
Same LEDs run at 60 mA. Result was similar at 40 mA but slower. Red line is ~= manufacturer's claim.
Utter rubbish.

• In my specific case i can drive them only with 24v DC (according to manufacturer). I have to say the increase and decrease in light power was acquired every 3 hours for ~ 6 month with 100% repeatability in lab conditions. If i plot 7 led's i see half of them increasing and half decreasing with linear behavior. Apr 22, 2015 at 13:15
• @PavelGolubev I cant't afford to spend time on this if you are going to eke out information a small amount at the time. I have asked or implied a range of questions above. I don't know LED models, power levels, closeness to ratings, cooling, differences in temperature across the sample, what is does etc. I don't know how the LEDs are arranged and if the fading ones may be preferentially cooled. You said 2/7. Now it's 3.5/7 (half) and before that 3/7. We either need a much better picture or you have what help I at least can give. (I lost a lot of notional hair over LEDs in China over some.... Apr 22, 2015 at 14:14
• ... years. If you want some benefit from the hair tearing you need to help the process with information. | 7 LEDs - are they white, red or Octarime? | 24V - from where and why do they say? If its not designed for a lab supply what will the REAL result be? If say 3V/LED = 21V headroom = 3V. If 3.1V/LED headroom = 2.3V. 3.2V = 1.6V. 3.3V = 0.9V headroom. | If this uses a resistor in series it will be immensely sensitive to V change. If LEDs are run near Vmax the Vf per LED will vary and power will vary (not vastly). | This looks soluble BUT only if we know what THIS is. Apr 22, 2015 at 14:20
• I appreciate your time. I don't know what is the Model/Manufacturer for each led since they are packaged by other manufacturer who don't want to reveal the led manufacturer.The only information i know is that the led's connected in series with resistor. I drive them with PWM 90% 24V the result is 2 of them decreasing to 95~ and 3 of them increasing to ~105. My question was very specific, let's assume 100% repeatable measure,same ambient temperature,same humidity same heat sink etc...How can i explain the increasing in light power above 100% with linear behavior. Thanks for your time. Apr 22, 2015 at 14:32
• Led's are Cool white. Apr 22, 2015 at 14:36

For whatever reason (known only to the deteriorating LED), it develops a lower forward voltage and lowers it's brightness, despite more current. If the LED is slowly dying then who can say why it does this? Maybe some form of increased leakage resistance around the LED is simultaneously lowering its forward voltage and diverting current away from the silicon that generates light? It's just speculation - you cannot expect a definitive reason from the info in your question.