3 ms·

Not sure what you mean. LED’s don’t have internal resistance. Can you ckarify?

zaarn 7y ago
LEDs have an internal resistance, they're not superconductors. When you switch them on, they have a specific value on that resistance. Due to their internals, as they heat up, this resistance goes down, causing current and thermal output to go up.
- robomartin 7y ago
  They have something that is sometimes referred to as "dynamic resistance", which is the inverse of the slope of the forward portion of the V-I curve, and yes, it's mostly constant (at constant temperature). For example, a typical lighting class LED will have a dynamic resistance in the order of 0.2 to 0.3 ohms. However, in as much as this varies a bit with temperature the change is mostly negligible. I am also not sure about your assertion that this dynamic resistance decreases with temperature. I took a few minutes to review the datasheets of a dozen OSRAM and Seoul Semiconductor 1W class LED's we use. In all cases the slope of the V-I curve increases slightly with temperature, which means the dynamic resistance decreases slightly with an increase in temperature. Can you explain the effect you are referring to in greater detail? Always interested in learning something new. Thanks.
  - zaarn 7y ago
    Dynamic and internal resistance probably mean different things, afaict, but essentially the internal resistance of an LED is a function of it's temperature. An exponential function in fact; the internal resistance of an LED is an exponential function with the temperature as the inverse in the exponent. Your dynamic slope would also be sufficient to get runaway meltdown. Essentially, imagine it like this; The LED has a starting temperature of 20°C and no power applied. When the appropriate voltage is applied, the LED heats up to 40°C. The dynamic resistance has a small slope, so as a result the dynamic and internal resistance both decrease. The current increases, the voltage is the same, the power output of the package goes up. The LED now heats to 45°C. The process repeats but because the internal resistance is exponentially inverse related to temperature, it speeds up. The LED is now 55°C. It repeats again and the LED reaches 75°C. Resistance drops again at the higher temperature, the current increases more. 115°C. At the next step the LED will reach 195°C, due to the exponential increase and likely the solder points will liquify. If the LED still works it'll hopefully melt itself from the PCB before it catches fire. This can happen incredibly fast, you can counteract it by running the LED well below maximum spec but then you're one power surge or heat surge away from meltdown. Similar to burning wood, the LED's meltdown requires a sufficient amount of extra heat to trigger, otherwise the exponential heat output is too small and is dissipated. This is why you need a current controller or atleast limiter so that the LED cannot pull more current than it's designed for. 0: https://www.researchgate.net/publication/49586062_Internal_Resistance_of_an_LED_as_a_function_of_Temperature https://www.researchgate.net/publication/49586062_Internal_R...
    - robomartin 7y ago
      Thanks, I’ll dig further and talk to the LED applications engineers to see what they have to say. I can’t find anything other than dynamic resistance in the datasheets or manufacturer application notes.
gingabriska 7y ago
The amount of current LED consume varies (due to varying internal resistance) with its temepture assuming voltage is kept same. Now, if your LED heats up, it's internal resistance goes down, it sucks more current and it heats up even more and this shortens its lifespan or can kill the LED instantly. A quality driver should handle grid fluctuations, limit the current (by varying the voltage) and LED should have enough cooling (to not disturb its internal resistance much or go above its optimal operating temperature) but often due to space and cost constraints, it doesn't happen. Some even use bad capacitors (low capacity, dries faster, low voltage rating so blows up when power surge happens) this induces flicker. Some use crap LEDs array which does not lit evenly, test for it by lowering the voltage if you see LEDs with uneven brightness across the array or grid, you've bad LEDs. Low quality components, insufficient design, poor QA, inability handle power surge (both grid and local ones) are primary reason for LED flickers and short lifespan Why they don't fix it? To minimize cost, which helps bottom line and boosts profit. Customer's experience with LED will vary (depending on if they've surge protectors or not or grid fluctuations, these are local to you and others might have different situation), so you won't instantly know from reviews or feedback online - which LED is good for you unless ofc you reverse engineer and figure out how it works even then you might not be able to figure out quality of components and degradation with time. Marketing/Sales can fix (hide?) lots of bad reviews and profits can be boosted regardless of underlying quality. Many customers won't pay more because they can't differentiate between quality. And everyone seem to agree that LED as a technology might not last forever, so why bother boosting lifetime/quality of current bulbs when they'll be replaced and an opportunity to make more money will come along? And when a new better bulb technology will come along, they all will blame their cost cutting induced quality / life issue on LED technology and then they'll move on doing same with the new technology.

LEDs have an internal resistance, they're not superconductors. When you switch them on, they have a specific value on that resistance. Due to their internals, as they heat up, this resistance goes down, causing current and thermal output to go up.

robomartin 7y ago
They have something that is sometimes referred to as "dynamic resistance", which is the inverse of the slope of the forward portion of the V-I curve, and yes, it's mostly constant (at constant temperature). For example, a typical lighting class LED will have a dynamic resistance in the order of 0.2 to 0.3 ohms. However, in as much as this varies a bit with temperature the change is mostly negligible. I am also not sure about your assertion that this dynamic resistance decreases with temperature. I took a few minutes to review the datasheets of a dozen OSRAM and Seoul Semiconductor 1W class LED's we use. In all cases the slope of the V-I curve increases slightly with temperature, which means the dynamic resistance decreases slightly with an increase in temperature. Can you explain the effect you are referring to in greater detail? Always interested in learning something new. Thanks.
- zaarn 7y ago
  Dynamic and internal resistance probably mean different things, afaict, but essentially the internal resistance of an LED is a function of it's temperature. An exponential function in fact; the internal resistance of an LED is an exponential function with the temperature as the inverse in the exponent. Your dynamic slope would also be sufficient to get runaway meltdown. Essentially, imagine it like this; The LED has a starting temperature of 20°C and no power applied. When the appropriate voltage is applied, the LED heats up to 40°C. The dynamic resistance has a small slope, so as a result the dynamic and internal resistance both decrease. The current increases, the voltage is the same, the power output of the package goes up. The LED now heats to 45°C. The process repeats but because the internal resistance is exponentially inverse related to temperature, it speeds up. The LED is now 55°C. It repeats again and the LED reaches 75°C. Resistance drops again at the higher temperature, the current increases more. 115°C. At the next step the LED will reach 195°C, due to the exponential increase and likely the solder points will liquify. If the LED still works it'll hopefully melt itself from the PCB before it catches fire. This can happen incredibly fast, you can counteract it by running the LED well below maximum spec but then you're one power surge or heat surge away from meltdown. Similar to burning wood, the LED's meltdown requires a sufficient amount of extra heat to trigger, otherwise the exponential heat output is too small and is dissipated. This is why you need a current controller or atleast limiter so that the LED cannot pull more current than it's designed for. 0: https://www.researchgate.net/publication/49586062_Internal_Resistance_of_an_LED_as_a_function_of_Temperature https://www.researchgate.net/publication/49586062_Internal_R...
  - robomartin 7y ago
    Thanks, I’ll dig further and talk to the LED applications engineers to see what they have to say. I can’t find anything other than dynamic resistance in the datasheets or manufacturer application notes.

gingabriska 7y ago

The amount of current LED consume varies (due to varying internal resistance) with its temepture assuming voltage is kept same. Now, if your LED heats up, it's internal resistance goes down, it sucks more current and it heats up even more and this shortens its lifespan or can kill the LED instantly. A quality driver should handle grid fluctuations, limit the current (by varying the voltage) and LED should have enough cooling (to not disturb its internal resistance much or go above its optimal operating temperature) but often due to space and cost constraints, it doesn't happen. Some even use bad capacitors (low capacity, dries faster, low voltage rating so blows up when power surge happens) this induces flicker. Some use crap LEDs array which does not lit evenly, test for it by lowering the voltage if you see LEDs with uneven brightness across the array or grid, you've bad LEDs. Low quality components, insufficient design, poor QA, inability handle power surge (both grid and local ones) are primary reason for LED flickers and short lifespan Why they don't fix it? To minimize cost, which helps bottom line and boosts profit. Customer's experience with LED will vary (depending on if they've surge protectors or not or grid fluctuations, these are local to you and others might have different situation), so you won't instantly know from reviews or feedback online - which LED is good for you unless ofc you reverse engineer and figure out how it works even then you might not be able to figure out quality of components and degradation with time. Marketing/Sales can fix (hide?) lots of bad reviews and profits can be boosted regardless of underlying quality. Many customers won't pay more because they can't differentiate between quality. And everyone seem to agree that LED as a technology might not last forever, so why bother boosting lifetime/quality of current bulbs when they'll be replaced and an opportunity to make more money will come along? And when a new better bulb technology will come along, they all will blame their cost cutting induced quality / life issue on LED technology and then they'll move on doing same with the new technology.