Temperature-Controlled Fan Circuit Diagram – Build Your Smart System!

The speed gradually increases as the temperature rises

A broadly adjustable temperature range is available

Circuit Diagram

Components:

Device Purpose:

This circuit implements an older design approach, aiming to modulate a fan's speed in response to temperature, with minimal component count and avoiding specialized integrated circuits, which can be challenging to source.

Circuit Operation:

R3-R4 and P1-R1 are connected as a Wheatstone bridge, generating a fixed two-thirds-supply “reference” voltage, and P1-R1 produces a temperature-sensitive “variable” voltage. Q1 functions as a bridge balance detector.

P1 is adjusted so that the “reference” and “variable” voltages are equal at a temperature just beneath the desired activation value. Under this condition, Q1’s Base and Emitter are maintained at equal voltages, and Q1 remains inactive. When R1’s temperature exceeds this “balance” point, the P1-R1 voltage falls below the “reference” value, causing Q1 to be forward biased and to charge C1.

This occurs because the entire circuit is powered by a 100Hz half-wave voltage derived from the mains supply via the D3-D6 diode bridge and regulated to 18V by R9 and the Zener diode D1. Consequently, the 18V supply of the circuit is not strictly DC, exhibiting a somewhat trapezoidal waveform. C1 then generates a variable phase-delay pulse train correlated to temperature and synchronized with the mains supply’s “zero voltage” point during each half cycle, thereby minimizing switching Radio Frequency Interference (RFI) from the SCR. Q2 and Q3 constitute a triggering device, producing a short pulse sufficient to activate the SCR.

Notes: