LED lamp heat dissipation judgment: half hour illuminance measurement method

December 03, 2020

Recently released "the high value of the value of the heat is not the use of heat dissipation is the core of LED lamps! 》 was heated, the text pointed out: to determine whether a radiator of a luminaire is doing well, the most direct method is to measure the temperature of the LED chip when the luminaire works, the technical term is called "junction temperature", but this is obviously theoretical, when a After the luminaire is completed, the “junction temperature” is hidden, which is why we need to sum up so many measurement methods to help determine the heat dissipation.

In addition to the subjective qualitative judgments pointed out in the previous article, this article introduces a way to indirectly quantitatively determine the junction temperature.

"Half-hour illumination method" measuring junction temperature

Since we can't directly measure the junction temperature, is there any indirect way to know the junction temperature? Fortunately, the general LED junction temperature rises and the luminous flux decreases. Then, as long as we measure the illuminance change of the luminaire in the same position, we can reverse the change of junction temperature.

The specific approach is:

1. Choose a place that is not subject to external light interference. It is best to turn off other lights at night.

2, turn on the light in cold state, immediately measure the illuminance of a position, note that the reading is "cold illuminance".

3. Keep the position of the luminaire and illuminance meter unchanged, and the luminaires continue to work.

4. After half an hour, read the illuminance value here and note that the reading is “hot illuminance”.

5. If the two values ​​are similar (10~15%), the heat dissipation system of the lamp is basically good.

6. If the two values ​​are far apart (greater than 20%), then the heat dissipation system of this fixture is questionable.


Figure: The “half-hour illumination method” measures the junction temperature change indirectly.

Do general designers or engineers have illuminance meters? what? not yet? Hurry and buy one, it is not expensive, more than 100 dollars can be used!

Just what proportion of the specific illuminance will fall is reasonable? This depends on the specification performance of the specific LED chip, and it is not possible to simply specify the range ratio.

The scope of application of the "half-hour illumination method"

We cite the "luminosity vs. junction temperature" curve of several commonly used chips. From this curve, we can see how much lumen loss of the luminous flux can indirectly know how many degrees Celsius the junction temperature has risen.


Figure: OSRAM S5 (3030) chip, when the luminous flux is 20% lower than at 25 ° C, the junction temperature has exceeded 120 ° C

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