The lifetime of an LED is inversely proportional to the temperature of its PN junction. In some high-power LED lamps with limited space, reducing the temperature rise of LEDs cannot be achieved by using larger heat sinks. For example, track lights, using DC fans to force heat dissipation has become a last resort choice. In recent years, with the continuous improvement of the manufacturing process, the life of the electric fan has been prolonged, and some manufacturers even claim that the life of the electric fan is longer than the life of the LED. However, the mechanical fan can not be used with high-power LED lamps with high reliability. Add an electric fan operating condition monitoring circuit to automatically cut off the main circuit when the fan fails. To protect the LED from overheating damage, this idea can be evaluated. Based on this idea, a circuit for detecting the cooling fan was developed. The block diagram is as follows: 
The principle of this idea is explained below.
The main failure modes of the fan are open circuit and blocked.
Let's first look at the waveform of the fan when it works:
It can be seen that the waveform of the fan working is a series of alternating waveforms, and the capacitive coupling rectification and filtering outputs a DC voltage value. According to the presence or absence of this voltage, it can be distinguished whether the fan works normally.
The waveform transition of the fan from normal operation to stalled is shown in the figure below. The yellow waveform in the figure is the fan waveform, and the red waveform is the DC voltage waveform after capacitive coupling rectification and filtering. The DC voltage has a voltage difference of more than 3 volts. This voltage difference is applied to the input of the comparator to obtain an amplified signal at the output of the comparator to control the opening and closing of the electronic switch.
When the fan is disconnected, there is no alternating waveform output, and the DC voltage after coupling rectification and filtering is zero. The above analysis is also applicable.
The actual schematic is shown in the figure below. In the figure, 1-3PIN of CON1 is the 36V DC output terminal of LED constant current driver, and 2-4 is connected to the positive and negative poles of LED. CON2 is connected to a 24V1.5W DC fan. The LED uses 10 strings of 10 and 50W light sources. If the parameters of the drive power box light source are different, you can change the R7 and R71-R73 values ​​in the figure and select the appropriate electric fan.
PCB diagram and physical objects are shown below:
The switch tube in this case selects MOS tube IRF1404 with VSS=40VRDS(ON)=0.004 ohm ID=162A, which is used to control the LED of 3.3A100W, and can work continuously without heat sink for a long time. If you use a higher power LED, you can consider using a higher specification MOS tube or a heat sink on the MOS tube.
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