Design Ideas: October 26,, 1995

Using a negative-temperature-coefficient thermistor, the circuit in Fig 1a controls temperature within 1°C or better with a minimum of components, cost, and complexity. The circuit includes protection against a short or open temperature sensor. You can purchase all of the key components, except the actual heating element, at the local Radio Shack.

The controller is a PWM type that has an exponential, rather than linear, transfer characteristic. The design is based on a single LM339 quad comparator and includes temperature compensation. Changes in V_OS due to temperature drift cause the oscillator's output to change. However, these changes occur in the same direction in the duty-cycle-generating comparator and cause a cancellation of this temperature drift in the controller.

The heart of the controller is the oscillator comprising IC_1A, IC_1B, and associated components. The peak and minimum voltages of the oscillator (Fig 1b) mainly determine the controller's precision. Simplified equations for the oscillator are:

The oscillator's output connects directly to the input of the duty-cycle-generating comparator, IC_1C. R₈ determines the temperature setpoint. The R₈-R_THERM voltage divider presents a voltage to the duty-cycle-generating comparator, the output of which drives IC₂, an optoisolated triac driver.

The temperature range of the component values in Fig 1 is approximately 25 to 115°C. D₁ and D₂ are optional temperature-sensor-fault and duty-cycle-indication LEDs. R₉ and R₁₀ set the level at which IC_1D detects the absence of the thermistor. R₂ primarily sets the width of the cycling band and temperature-regulation tolerance. Note that instability may result if too much thermal lag exists between the heating element and the sense element, in this case, the thermistor. (DI #1778)