Patent ID: 7024864

Claim:
An apparatus for applying an adjustable bipolar current to a load comprising: a voltage-controlled power supply having a unipolar output; an H-bridge with four active elements having a first side with two active conductive elements, connected at a first node in series on each side between said unipolar output of the power supply and ground, and having a second side with two active conductive elements, connected at a second node, in series on each side between said unipolar output of the power supply and ground, wherein said first node and said second node are adapted to be coupled to said load so that current flows between said first and second nodes through the load, the current having a polarity determined by a direction of said flow; at least one of the two active conductive elements on each of the first and second sides of the H-bridge comprising active conductive elements responsive to control signals which set a magnitude of current flow through the conductive elements; control logic that provides the control signals to said active conductive elements on the first arid second sides to enable one conductive element on each of said first and second sides to set the polarity of the current from said unipolar output of the power supply through said load, and to control the magnitude of the current flow through said load; and logic, coupled to said voltage-controlled power supply and to said H-bridge and coordinated with the control logic, to supply a control voltage to said voltage-controlled power supply such that that voltage drop across at least one of said active conductive elements remains near a lower limit of said range unless this condition results in said voltage-controlled power supply operating below some threshold voltage, in which case the output of said voltage-controlled power supply is fixed to said threshold voltage.