Four state DC motor controller

Claim 1

A drive circuit for controlling the conduction of an H-switch transistor bridge for connecting the positive and negative terminals of a DC source with the winding terminals of a DC motor in accordance with the voltage potential applied to a first and a second control terminals wherein the H-switch includes a first pair and a second pair of bridge transistors, each such pair comprising a PNP bridge transistor connecting one motor terminal to the positive terminal of said DC source and an NPN bridge transistor connecting the other motor terminal to the negative terminal of said DC source, the driver circuit comprising: first and second pairs of NPN driver transistors connected to and controlled by said first and second control terminals respectively, one driver transistor of each such pair of driver transistors being connected to control the mutual conduction of one of said first and second pairs of brige transistors and the other driver transistor of each such pair of driver transistors being connected to hold the NPN bridge transistor of the other of said first and second pairs of bridge transistors in a nonconductive state when the one pair of bridge transistors are conductive, the first and second pairs of NPN driver transistors thereby being effective (1) when a voltage potential substantially corresponding to that at the negative terminal of said DC source is applied to both of said control terminals to render both bridge transistors of said first and second pairs of bridge transistors nonconductive to effectively disconnect the motor from the source, (2) when a voltage substantially corresponding to that at the negative terminal of said DC source is applied to one of said control terminals and a voltage determined in relation to that at the positive terminal of said DC source is applied to the other of said control terminals to effect clockwise or counterclockwise motor rotation, and (3) when a voltage determined in relation to that at the positive terminal of said DC source is applied to both control inputs to render the NPN bridge transistor of each of said first and second pairs of bridge transistors nonconductive and to render the PNP bridge transistor of each of said first and second pairs of bridge transistors conductive to short-circuit the DC motor through a conductive PNP bridge transistor to effect dynamic braking.