A ±15 microdegree temperature controller

Abstract

We have constructed a temperature controller utilizing a thermistor in a high impedance ac Wheatstone bridge. It maintains the temperature of a 1.3 liter volume constant to within ±15 μ°C, over a 40 h period. The controller employs a ratio transformer, a simple homemade preamplifier, a commercial lock‐in amplifier, a small bipolar power amplifier, and a thermoelectric device. Access to the controlled region can be obtained in a few seconds.