Thermistor Temperature Control

A temperature controller using small bead thermistors as temperature sensing elements has been developed. The controller has an over-all range of +20 to +300°C, or −60 to +190°C, using 51A2 or 32A12 thermistors, respectively, in an ac bridge. Extension of the operating temperature range to other limits is possible using other sensing elements. Linear proportional action is achieved with power relay output by comparing the amplified and rectified bridge signal with a stable and linear sawtooth wave form. This comparison controls the fractional ``on'' time of the power relay during a periodic ``on-off'' cycle. The controller offers stable control under all conditions of unbalance, and is stable to 0.1°C or better over an eight-day period.