Abstract: A safety circuit according to the invention controls operation of an equipment load. A manual control switch of the safety circuit is connected in series with the equipment load for controlling the flow of electrical current to the equipment load. The equipment load and control switch series combination are further in communication with a relay contact switch of the safety circuit, which, like the manual control switch controls the flow of electrical current to the equipment load. Arranged in this configuration the equipment load operates only if both the mechanical switch and relay contact are closed. If either the manual control switch or the relay contact are open, the equipment load does not operate.

Abstract: The present invention is a power monitoring circuit for sensing line conditions in a three phase power line. In the circuit a plurality of series connected power supplies for powering various components of the circuit are connected in series with a current sensing element for sensing current supplied to the power supplies. The power supply and current sensing element arrangement, in turn, is connected across a three phase bridge rectifier, which is in communication with the three phase power supply line. A circuit analyzer in communication with the current sensing element senses unbalanced line conditions based at least in part on a signal developed by the current sensing element. Preferably, the circuit further includes a three voltage reducing resistors interposed between the three phase power line and the bridge rectifier for reducing the voltage stresses encountered by the circuit's components.

Abstract: The present invention is a current sensing method and apparatus wherein a current transformer producing a secondary current is disposed in communication with a conductor having a primary current desired to be sensed. A current analyzer in communication with the current transformer is adapted to estimate the primary current in the conductor based on the secondary current output waveform, even in the case where the secondary current is in saturation. In one embodiment, the current analyzer estimates the primary current in the conductor by capturing the secondary current waveform output from the current transformer, selecting a predetermined number of data points in an undistorted, region of the secondary current waveform, and estimating the primary current based on the selected data points.

Abstract: The invention is an improved timer circuit for operating a heating or air conditioning unit which utilizes a single time delay generating circuit for delaying operation of both a compressor and a unit fan under certain operating conditions. A logic circuit for operating a fan relay is made responsive to the thermostat output and a time delay generating circuit output so that the fan relay is de-energized only in the case that the compressor has been turned off for at least the duration of a time delay generator timing period, and is in a state commensurate with a satisfactory building temperature. When the timer circuit is used in large-scale heating or air conditioning systems, the timing period of each timing circuit can be adjusted to a different duration so that a voltage lag problem on start up is avoided.

Abstract: The duration of the timing cycle of a selector adjusted timer controlling intermittant operation of a heating element in an electrical appliance, is reduced as an inverse function of the duration of a preceding off-time interval. A manually operable switch which turns on the appliance, simultaneously triggers operation of the timer to determine the duration of the off-time intervals affecting the compensating action reducing the duration of the timing cycle.