3 ms·

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,

by 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.

by zaarn 7y ago

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.