| 摘要 |
A device protects an electric load, such as a motor, from excessive current levels. The current flows from a source to the load through a switch and a primary winding of a current sensing transformer (21, 22, 23). The voltage across a secondary winding of the current sensing transformer is rectified (30) to produce a voltage which is measured to determine the level of current through the load. The voltage across the secondary winding also powers components of the device (16). In a first mode of operation, current from the secondary winding is applied to charge a power supply capacitor (35). In a second mode of operation the level of the rectified voltage is sensed (60, 73, 74, 80, 89) to produce a sample of the level of the current flowing through the load. From this sample the change in temperature of the load since a previous sample is calculated and used to derive the present load temperature. When the present load temperature exceeds a defined threshold level, the switch is opened to disconnect the load from the electricity source. <IMAGE>
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