PTC water level sensor and control

摘要

A PTC resistive strip is mounted on a wall of a washer tub. Water in the tub carries away heat from the energized strip thereby reducing its resistance. The water level in the tub is measured by measuring the voltage drop across the strip. The resistance of the strip can be compared to the resistance across a variable resistance or resistor to indicate that the water has reached a desired level. A comparator output can be used to control the water level and washer motor operation via a control circuit which operates the water supply valves and motor switch. A second PTC resistive element is also provided for sensing overfill.