Abstract: A bi-directional current limiter may be configured to receive an AC signal and bi-directionally limit the loop current to a predetermined current value. A first and second zone may be configured in series with the AC signal and bi-directional current limiter. The first and second zone may include unidirectional current limiters to limit a unidirectional current that is proportionately less than the bi-directionally limited current. A first opto-coupler circuit (ISO1) may be configured to detect both high and low current states in the current loop circuit. Second and third opto-coupler circuits (ISO2 and ISO3) associated with the first and second zones may be configured to detect only high current states in the current loop circuit. The status of the first and second zones may then be determined by an analysis of the current level in the current loop circuit during two half cycles as determined by the various current limiters.

Abstract: An interface circuit configurable in input and output modes comprises a current source; an input/output node; a diode to prevent current from flowing from the input/output node to the current source; a first switch to short-circuit the diode when closed; and a second switch to form a ground path from the input/output node when closed. In input mode, the first switch is closed and the state of a connected sensor circuit may be measured. In the output mode, the first switch is open, and the second switch may be closed to activate an output circuit. In the output mode, power dissipation may be monitored, and the diode may protect the current source and measurement node from current flowing through the output circuit. In the input mode, closing the first switch may prevent the diode from introducing measurement errors due to non-linear attenuation of perturbative AC signals.

Abstract: A cabinet formed of a plurality of panels, includes a tamper detection system that includes an electrical circuit for detecting forming of a hole in a protected region of at least one panels. The electrical circuit includes at least one conductive trace on one of the panels. The conductive trace spans a protected region, wherein the minimum distance from any point in the protected region to the at least one conductive trace does not exceed some defined maximum. An electrical current source provides a current through the conductive trace. A sensor is in electrical communication with the electrical current source to sense a change in current provided to the trace and generate a tamper signal in response thereto.

Abstract: A load limiting circuit includes a transistor switch, for providing current to a load and a latching circuit having a control input, and a latch output driving the transistor switch. A current sensing transistor interconnects with the transistor switch, to sense current in excess of a threshold to the load. The current sensing transistor drives the control input to the latching circuit. An external controller may drive and monitor the control input. The controller may thus turn the transistor switch on and off, and be notified of an over-current condition. The latching circuit may turn the transistor switch off permanently. As well, the controller may attempt to set the load limiting circuit after an over-current condition.

Abstract: A voltage measuring circuit includes a rectifier to receive an alternating current (AC) voltage to be measured and to provide a rectified output; a comparator for comparing the rectified output and producing therefrom a square wave having a pulse width indicative of the rectified output exceeding a threshold; a calculation circuit for converting a measurement of the pulse width into a measurement of the voltage and optionally an opto-isolator interconnecting the comparator to the calculation circuit. The rectifier may provide operating power to the comparator and an input side of the opto-isolator, from the AC voltage signal being measured. The remainder of the measuring circuit may powered by a source isolated from the voltage to be measured.

Abstract: A displacement sensor for sensing displacement of a first surface relative to a second surface includes a coil mounted on the first surface, and encircling a region in plane; and a conductor extending lengthwise from the second surface through the region. The conductor has a volume that varies along its length. An AC source drives the coil to generate a magnetic field that pierces the region and induces eddy currents in the conductor. Eddy current losses resulting from the eddy currents vary in dependence on the volume of the conductor above and beneath the plane. A sensing circuit is coupled to the AC source to detect changes in eddy current losses, and thereby displacement. Detection of displacement may be used to signal tamper/intrusion. The sensor may further include a visual or audible output to signal such displacement.