This cheap component is used for the input modulation for the W1209 heat controller. W1209 is a temperature control module for use in egg hatching incubators. Therefore, the W1209 is designed to operate up to a maximum temperature of 110 ° C. By using a diode, we ensure that the value at the input of the temperature controller drops sufficiently for use up to 250 ° C.
How is diode 1N5408 wired in project?
This you can find in W1209 tutorial video: