Patent ID: 7408274

Claim:
A method for turning on at least two power switches that are series connected and interposed between a source of electrical power and an electrical load, for energizing the load in response to a signal at a control input, the method comprising: (a) applying a known voltage to a node between the switches, when both switches are open, through a resistance connected to a voltage source; (b) sensing the voltage between the switches with respect to a reference to provide a sensed first value; (c) comparing the sensed first value to an expected first value that exists if both switches are turned off, the expected first value being said known voltage; (d) aborting turning on of the switches and discontinuing the method if the sensed first value is unequal to the expected first value and turning on a first one of the two switches if the sensed first value is equal to the expected first value; (e) if the first switch was turned on, sensing the voltage between the switches with respect to a reference or the current through the switches to provide a sensed second value; (f) comparing the sensed second value to an expected second value that exists if the first switch is turned on and the second one of the two switches is turned off; and (g) aborting turning on of the switches if the sensed second value is unequal to the expected second value and turning on the second one of the two switches if the sensed second value is equal to the expected second value.