A high rate Ṫ−T controller for thermally stimulated currents

A hot stage temperature controller with automatic transition between linear sweep and constant temperature modes, using only two operational amplifiers, is described. The derivative of temperature is nulled against a fixed reference signal instead of the usual approach in which temperature is compared with a time−dependent reference. This allows an increased loop gain at high speeds, while allowing the Ṫ−T crossover to be achieved without additional amplifiers. Ramp speeds of 0.1−10 K/sec in the range 300−820 K are obtained with accuracy of 1%−3%. Noise− and transient−free dc heater power simplifies shielding requirements in the 10−15 A range. A new approximation of the type K thermocouple characteristic is used to estimate its influence on linearity.