How to judge the heat dissipation of LED track lights: half hour illuminance measurement method

We all know that the heat dissipation of LED track lights is very important, but how to evaluate the heat dissipation of a lamp?

Why do traditional light sources do not pay attention to heat dissipation, but in the LED era, so pay attention to heat dissipation? Because the traditional light source is not afraid of heat, although the LED does not have much heat, the built-in chip is very hot .Only when the heat is dissipated, the chip can work steadily, otherwise the heat is concentrated in the light source to affect the work of the lamp.

Here Author still wants to tell everyone a cold knowledge. In the era when LED is rising, we have to distinguish the difference between traditional lamps and LED track lights. The key is to look at the radiator. When we see a radiator, we know that it is an LED lamp.

Then let’s take a look at the look of the radiator. The front of the embedded spotlight is not much different from the ordinary light source, but there are obvious ribs behind it.

Why do you want to do this? The traditional luminaires are flat behind, and these are actually fins specially made for heat dissipation.

There are some, for example, rail spotlights, which are as smooth as possible for good looks. At first glance, it is a LED light with a radiator.

For example, if you look at this track light, you can see the large lens from the front, and you will soon be able to recognize the LED light.But if you look at it from the side, you can’t see whether it is an ordinary light source or an LED light source.

In fact, the appearance looks very smooth and beautiful, but from another angle, you can still see a lot of ribs, these are for heat dissipation.

So how is the heat of the LED light conducted out?

First, the chip emits heat, and the heat sink is transferred through the substrate. The heat sink then dissipates heat through the heat sink surface where the fins are located. Therefore, the size of the heat sink and the area of ​​the heat sink determine the ability of the heat sink.

In order to increase the area, it is generally formed into various shapes such as a column shape and a mesh shape. Because the heat sink has such a complicated process, its cost is relatively high. This has led many factories to cut corners and shoddy in order to save costs.

Then the question is coming. How do we judge whether the factory is shoddy? The most direct way is to test whether the temperature of the chip is too high. In the terminology, this is called “junction temperature”, which is the temperature of the PN junction of the LED.

As long as the junction temperature is controlled within an acceptable range, the quality is passed. On the contrary, it is to cut corners.

However, this junction temperature is not good, we all know that the LED chip is hidden inside the lamp, so the temperature measurement can only be carried out by a professional factory with professional equipment, and the thermometer cannot be plugged into it for testing.

Even if you take a professional thermometer or imager to shoot, you can only see the temperature distribution outside, I don’t know how the junction temperature of the most core LED chip inside is.

Therefore, many people use the most “earth” method when selecting lamps. Turn this light on for a while, then touch it with your hand or not.

Some people say that my lamp is very good, the surface temperature is only 27 degrees, is this really good? In fact, when the LED luminaire is working normally, the temperature of the radiator will be low, but it will not reach a temperature of 27 degrees.

In addition, you touch this temperature is only the temperature of the outer heat sink, so even if the surface temperature is low, it is definitely good.

And the temperature of the heat sink is not high, there are two possibilities.

One possibility is that the heat from the LED chip is well dissipated. This way of big horse-drawn car is definitely a good thing for the buyer, but it is a waste of material for the manufacturer.

Another possibility is that the heat of the chip inside is not scattered at all, and it is lying inside. If there is no good contact between the chip and the heat sink, the gap in the middle is a little impurity, and the heat of the chip does not emanate, and only a small part of the heat is transmitted to the surface. Therefore, if you don’t feel hot outside, it may not be good. In fact, it may not be hot in the inside.

However, the answer to one case is yes. If a light is only opened for a while, it will be very hot when touched. It must be that the heat is not enough. It means that the radiator is like a small horse-drawn cart. It is not enough to dissipate the heat generated by the LED chip, so the heat is concentrated. The surface of the heat sink, thereby raising the temperature.

What method can I use to better distinguish the heat sink?

We recommend that you use a method called “half-hour illumination method” , open the illuminometer, fix the luminaire in one position and illuminate the illuminance meter to read the initial illuminance, such as 1000lx.

After half an hour, I will read the illuminance meter. Generally speaking, its illuminance will drop. Because the LED chip has a characteristic, as the junction temperature increases, its light output will decrease, then it can be pushed back by illumination. How high is the temperature in the end.

It is no longer important to touch the surface to burn it at this time, but the number it displays.

Is that a good number or a good decline?

Generally, if the illuminance is less than 10% within half an hour, it is qualified or even better. If the illuminance drops by 20% in half an hour, then this is a bit of a problem. If the half-hour thermal illuminance is 20% lower than the cold illuminance, then the junction temperature has basically exceeded the tolerance range of the chip, so it is basically judged that the radiator is unqualified.

Heat dissipation is especially important for LEDs, and the most critical is the junction temperature of the chip inside. Evaluate the heat dissipation performance of the luminaire, and touch it by hand. It is recommended to use the half-hour illuminance method. This is at least a little more reliable than the hand touch.

If you want to get accurate information, you need to get the temperature and light output curve of the chip, which is not applicable to all LEDs.

Some manufacturers’ LED chips have a light output that is independent of temperature and can withstand relatively high temperatures. In this special case, it is best to take a look at the measured curves of the manufacturer’s products , but for most LEDs. In terms of this, this method is suitable.

*The above part is excerpted from Xu Qinghui’s “How to judge the heat dissipation effect of LED lamps? “, edited by the Yunzhiguang Lighting Institute.