One of the cooling fan automatic control circuits

When the temperature reaches the upper limit (for example, 850C for a power transformer), contacts 1 and 2 of the electric contact pressure thermometer
KP are connected, relay KA is closed, and the fan runs. When the temperature drops to the lower limit (65°C),
2-3 of KP are turned on, KA is released, and the fan stops running.
    Since the contact of KP does not directly start the relay, but controls the base circuit of the triode, it can avoid
sparks and singeing of the contacts.