Automatic Temperature-Controlling Fan Speed System
Overview
Thermal management is one of the fundamental problems in electronics and industrial equipment: operating temperature must be kept within bounds, yet running cooling fans at maximum speed continuously wastes power and accelerates mechanical wear. This project implements a proportional analog controller that modulates DC fan speed continuously in response to ambient temperature, with no on/off hysteresis and no microcontroller. As temperature rises, fan speed increases proportionally; as temperature falls, speed decreases. The control law is entirely realized in passive and linear components.
The sensing element is a negative-temperature-coefficient (NTC) thermistor, whose resistance decreases nonlinearly as temperature increases. Placed in a Wheatstone bridge configuration, the thermistor's resistance change is converted to a differential voltage that is amplified and used to modulate a pulse-width modulation (PWM) circuit. The bridge-to-PWM signal chain ensures that fan speed tracks temperature smoothly across the 20°C–60°C operating range without the speed jumps that on/off bang-bang control would produce.
Technical Approach
The NTC thermistor (10 kΩ at 25°C, B-constant approximately 3950 K) occupies one arm of a balanced Wheatstone bridge; the opposing arm uses a matched 10 kΩ fixed resistor, and the two remaining arms are equal-value 10 kΩ resistors. At the balance temperature, the bridge differential output is zero. As temperature rises above balance, the thermistor resistance drops, unbalancing the bridge and producing a positive differential voltage. This differential is amplified by a non-inverting instrumentation amplifier stage built around an LM358 op-amp pair, configured for a gain of approximately 20 to scale the millivolt-level bridge imbalance into the 0–5 V control range needed by the downstream PWM circuit.
The PWM generator uses an LM555 timer configured in astable mode at a fixed carrier frequency of 1 kHz, with the control voltage input (pin 5) driven by the amplified bridge output. As control voltage rises, the comparator thresholds inside the 555 shift, increasing the duty cycle from approximately 20% at 20°C to 95% at 60°C. The PWM output drives the gate of an IRF540N N-channel MOSFET, which switches the DC fan motor supply at the 1 kHz rate. A 1N5822 Schottky freewheeling diode across the motor terminals recirculates the inductive energy during the MOSFET off-interval, maintaining smooth motor current and preventing voltage spikes that would otherwise exceed the MOSFET's drain-source rating.
The bridge balance point — which sets the temperature at which the fan begins to accelerate — is trimmed by replacing one fixed bridge resistor with a 20 kΩ potentiometer, allowing calibration without component substitution. A second potentiometer on the amplifier gain sets the slope of the speed-versus-temperature characteristic, independently of the balance point.
Outcomes & Learnings
The assembled circuit was characterized by stepping temperature in 5°C increments from 20°C to 60°C using a controlled hot-air source and a calibrated thermocouple reference. Fan speed, measured by a tachometer sensing a blade-interrupted IR beam, tracked temperature with a nearly linear characteristic (R² = 0.97 over the test range), with no observable oscillation or hunting at any steady-state temperature. The smooth proportional response contrasts sharply with the bang-bang switching behavior that an on/off thermostat would produce, demonstrating the value of proportional control for noise-sensitive or mechanically fragile loads.
This project established the Wheatstone bridge as a practical transducer interface — a configuration that reappears in precision measurement systems, strain-gauge instrumentation, and current sensing in power electronics. The MOSFET switching stage introduced the fundamentals of PWM motor drive that underpin variable-frequency drive (VFD) technology at utility scale, drawing a direct conceptual line from this undergraduate circuit to industrial motor control in power systems.
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