| 摘要 |
A device for controlling a switching relay to deliver power to a load. The switching relay selectively couples a hot leg of a utility power supply to the load. The load is coupled to a neutral wire of the utility power supply. The device includes a sensor for detecting whether an area is occupied. If the sensor indicates that the area is occupied and the load is powered off, the device monitors the line voltage for a zero crossing. Upon detecting a zero crossing, an amount of time to a subsequent zero crossing is measured and stored. Then, the device waits the amount of time between zero crossings, less a delay time for closing the contacts of the relay, and initiates a closing of the contacts of the relay. Similarly, when the sensor detects that the area has not been occupied for a predetermined amount of time and the load is powered on, the line voltage is monitored for a zero crossing and a time to a subsequent zero crossing is measured and stored. Then, the device waits the amount of time between zero crossings, less a delay time for opening the contacts of the relay, and initiates an opening of the contacts of the relay. Accordingly, when the relay contacts actually close or open, the line voltage is at the ground level. Therefore, even if the load has a high in-rush current or high kick-back power, the relay contacts are not likely to be damaged.
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