An Inductive Water Thermostat Using On‐Off Triac Control and Platinum Sensing

An on‐off thermostat is described using novel means for heating, sensing, and triac control. Heating is performed by sending the water through a coil of silver tubing which forms the short‐circuited secondary winding of a transformer. This arrangement permits extremely good insulation, which was essential in the medical application (a dialysis water thermostat) for which it was designed; its quick response also contributes to the excellent regulation achieved with simple on‐off control. Sensing is provided by a very low resistance platinum coil in direct contact with the water, thus providing quick response, stability, and, because of the transformer‐coupled circuit used with this probe, the possibility of very good insulation. Because on‐off control entails continual switching of a large (1.7 kW) transformer, a triac control circuit was designed which avoids the usual starting (inrush current) transients. © 1971 The American Institute of Physics