Temperature Control Circuit: 10uV Precision & 140°C Limits

Circuit Diagram

This design manages a load (specifically, a DC brushless fan) responding to a temperature comparison against a predetermined value. The sensing element utilizes a diode operating in forward bias. Specifically, when subjected to forward current, the voltage drop across a diode exhibits a temperature-dependent characteristic, displaying a negative linear trend. This behavior stems from the Boltzmann distribution, resulting in electrons transitioning into the conduction band due to thermal excitation, which consequently reduces the voltage across the diode.

Essentially, this circuit compares a precise voltage reference (zener type) with the diode's forward voltage drop, which is established with a current of 11mA.

The comparator itself is a simple LM158/258/358 operating in open-loop configuration. The inverting input is connected to the diode sensor, while the non-inverting input is connected to the reference voltage. When the temperature rises above the defined setpoint, the forward voltage drop falls below the voltage reference, triggering the comparator output and activating the transistor, subsequently turning on the fan.

For increased power applications, a larger transistor can be substituted to accommodate greater fans, or alternatively, a relay, IGBT, or MOSFET could be used to manage higher loads and voltages.

The setpoint is adjusted with a potentiometer. A LM3914 LED driver may be incorporated to create a temperature setpoint indicator; however, careful calibration and the utilization of Excel to determine slope and intercept are required.

Numerous modifications are feasible, yet the circuit functions effectively in its basic form.

The comparator is capable of discerning differences of approximately 10uV, corresponding to an approximate 0.01°C variation (through precise potentiometer adjustment, it’s possible to detect body heat from a distance of 1/2 cm or ambient heat, enabling continuous fan activation and deactivation).

Maximum operating temperatures can reach 140°C, although linearity isn't guaranteed.

Potential applications include heatsink cooling, emergency cooling for computers (though a linear device might prove superior to a simple on-off system), and cooling during metal drilling operations.

An important note: this circuit can also be utilized for heating; this necessitates reversing the comparator inputs and replacing the fan with a relay controlling the heating element.