12V Fan Temperature-Controller

this simple design allows|permits an correct speed control of 12V dc fan motors, proportional to temperature. A n.t.c. Thermistor (R1) is employed as temperature sensor, driving two directly coupled complementary transistors wired during a dc feedback circuit. An optional circuitry was added to remotely monitor fan operation and to permit some type of rough speed indication by means that of the increasing brightness of a LED.

Notes:

• R5 must be set to allow motor just starting at the desired temperature.
• Any n.t.c. Thermistor in the 6K8 - 22K range value might work, provided R2 value is one/tenth of Thermistor's value.
• R6, R7 and D1 are optional: R7 must be adjusted until the LED glows faintly when the motor is just running.

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Temperature Control using LM35DZ

The temperature control circuit is a circuit thermal simple but very precise control that can be used in applications where automatic temperature control. The circuit switches a miniature relay on or off depending on the temperature detected by the sensor chip LM35DZ single temperature.

When LM35DZ detects a temperature higher than the preset level (set by VR1), the relay is activated. When the temperature drops below the set temperature, the relay is deactivated. The circuit can work with any 12V DC source or battery (100 mA min.)

Part list:

IC1 : LM35DZ
IC2 : TL431
IC3 : LM358

LED1 – 3mm or 5mm LED

Q1 – General purpose PNP transistor ( A1015,…) with E-C-B pin-out)
D1, D2 — 1N4148
D3, D4 — 1N400x (x=2,,,,.7)

ZD1 — Zener diode, 13V, 400mW

Preset (trim pot) : 2.2K (Temperature set point)
R1 – 10K
R2 – 4.7M
R3 – 1.2K
R4 – 1K
R5 – 1K
R6 – 33Ω

C1 – 0.1 µF ceramic or mylar cap
C2 – 470 µF or 680 µF electrolytic cap. (16V min)
Miniature relay – DC12V DPDT, Coil = 400 Ω or higher

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