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.

Topics
Transformer, Thermal instruments, Transition metals, Dialysis
1.
Fine silver was used because sterling silver, which contains 7.5% copper, is considerably less corrosion resistant.
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R. R. Lawrence, Principles of Alternating Current Machinery (McGraw‐Hill, 1940), 3rd ed., pp. 186–190.
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J. H. Galloway, “Using the Triac for Control of ac Power,” Application Note 200. 35, 3/66, General Electric Semiconductor Products Department.
5.
Such coils are designed for a given volt‐second product.
6.
Since the extinction point of the triac, unlike the ignition point, cannot be controlled, there is no possibility of obtaining a transient‐free stop.
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W. L.
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© 1971 The American Institute of Physics.
1971
The American Institute of Physics
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