Precision temperature control circuits for small thermal loads like oscillator crystals and voltage references tend to be fairly easy to design. This is because simple, well-behaved, linear-output drivers running from regulated dc supplies are often used for straightforward heater control. While handy for low-wattage heaters, linear drive is an unappealing solution for satisfying the demands of bigger loads and their bigger heaters. Those thirsty power consumers call for more efficient switch-mode power-handling circuits.

This pursuit of efficiency frequently leads to the use of power-control circuits based on thyristors with 60-Hz phase-angle triggering. Cheap and robust, thyristor phase-angle circuits can easily drive multikilowatt heaters while achieving greater than 90% efficiency. Unfortunately, a serious obstacle hinders the use of these devices in precision control applications.

The problem can be seen in this typical thyristor phase-control response plot *(Fig. 1, curve A). *Here, 80% of the thyristor rms output range (0.1 to 0.9 of full scale) is spanned by only 55% of the control-input range (0.12 to 0.67). This severe nonlinearity complicates the simultaneous achievement of adequate system feedback gain and non-oscillatory stability over the full range of heater control inputs.

Figure 2's thermostat dodges this bullet by incorporating a control-voltage-to-trigger-angle converter circuit (A4, D1, C3, Q5, and R2). The circuit achieves an essentially linear relationship between the control voltage and the rms heater drive *(Fig. 1, curve B)*. On each positive half-cycle of the silicon controlled rectifier (SCR) anode voltage, V_{AC}, timing capacitor C3 begins charging through D1 and R2. A4 compares the accumulating voltage (V_{C3}) to the heater control voltage on C2. A trigger pulse is issued to the SCR gate when V_{C3} = V_{C2}. This, in turn, generates the V_{C2}/SCR phase-conversion function and also resets C3 via Q5.

It's the sigmoidal (rather than triangular) shape of the V_{C3} waveform that produces the excellent linearity of the V_{C2}/rms-heater voltage conversion. This wave shape results in an inverse-sigmoidal relationship between V_{C2 }and the SCR trigger timing. Consequently, the sigmoidal functionality of firing-angle-to-thyristor/heater current is accurately compensated and linearized. Although as illustrated the converter implements a 0° to 180° half-wave ac-control function, modification for full-wave 360° operation would be straightforward.

Figure 2's circuit was de-signed to serve an Air Curtain Incubator application that satisfies a requirement for accurate thermostasis of biological samples and culture dishes when transferred from a cabinet incubator to a microscope viewing stage. This application requires the generation of a flow of temperature-controlled air. In this case, the airflow is conveniently and cheaply produced by an ordinary, unmodified household hair dryer.

The airstream temperature is sensed using Q3's V_{BE} temperature coefficient (−2 mV/°C). Q3's V_{BE} (V_{T}) is compared to the 0.43- to 0.65-V setpoint voltage (V_{S}). After being integrated, the V_{S }− V_{T} error signal is applied to A4's voltage-to-trigger-angle converter circuit.

Due to the finite velocity of the heat-transporting airstream, there's a thermal time lag between heater H and sensor Q3. This interval complicates servo stability issues and requires the use of a robust, integrating, convergence-by-bisection feedback control algorithm *(see "Take Back Half: A Novel Integrating Temperature-Control Algorithm,"* Electronic Design*, Dec. 4, 2000, p. 132)*.

Take Back Half (TBH) damps oscillations and stabilizes the servo loop by revising the estimate of the optimum heater input at each setpoint (V_{T} = V_{S}) crossing. Comparator A2 detects these setpoint crossings by going high when V_{T} is less than V_{S} and low when V_{T} is greater than V_{S}. The positive feedback network around A2 keeps these logic transitions snappy.

Meanwhile, the role of TBH variables H_{O} and H are served by the sample-and-hold capacitor C1 and integrator cap C2, respectively. Further details of the workings of the analog TBH controller circuitry can be found in, "*Circuit Enables Precision Control In Radiant Heating Systems," *Electronic Design*, Jan. 8, 2001, p. 131*.