Switching filter producing a low impedance control input on a high

Abstract

A switching filter is disclosed for discriminating false control signals from actual control signals. The switching filter is particularly useful on a high impedance input, such as that of a motor controller. The filter is designed to compensate for capacitive, inductive, and/or resistive coupling between extended control lines that are run in parallel with other power lines. A low impedance is switched into the otherwise high impedance control line input for a brief instant to dissipate and phase shift false control signals to prevent the controller from interpreting the false signals as actual control signals. The filter includes a first resistor connectable to sense a presence of a possible false control signal, and a second resistor, having much lower resistance than the first resistor, connected in a path between the control line input and circuit common. A switch is provided in series with the second resistor to open and close the path between the control line input and circuit common in response to the possible false control signal. A timer is activated by a presence of a possible false control signal on the first resistor and activates the switch to provide a low shunting resistance across the control line to common for the brief period. The low shunting resistance not only dissipates the amplitude of any false signals, but also phase shifts the signals such that the controller cannot recognize multiple, successive false signals as a true control signal.