This FanTemperature Controller circuit design technique to adopt a more ancient as its objective is to vary the fan speed in relation to temperature with a fraction of counting and avoid using special-purpose integrated circuits, often difficult to obtain.
R3-R1-R4 and P1 are connected as a Wheatstone bridge in which R3-R4 generates a fixed two-thirds of the supply of "reference" voltage, P1-R1 generates a temperature-sensitive "variable" power and Q1 is used as a balanced bridge detector.
P1 is adjusted so that the "reference" stress "variable" is equal to a temperature just below the critical value necessary, and under the base of Q1 and the condition of the issuer are located in the same tensions and Q1 cut. When the temperature rises above this R1 "balance" the value of P1-R1 voltage falls below the "reference" value, so that Q1 is polarized, pulse charging C1.
This is because the circuit is supplied by a 100 Hz half-wave voltage from the mains supply by means of the D3-D6 diode bridge without filter capacitors and fixed to 18V by Zener diode D1 and R9. Therefore the supply of 18 volt DC circuit is not true, but it has a trapezoidal shape instead. C1 provides a variable lag phase pulse train related to temperature and synchronized with the mains "zero voltage" point of each half cycle, resulting in minimal change RFI SCR. Q2 and Q3 form a firing device, which generates a short pulse suitable for driving the SCR.
