Method and apparatus for controlling the current through a magnetic coil

Apparatus and circuitry for maintaining current in a coil near a desired non-zero value, the coil coupled to an external electrical source to energize the coil, said apparatus and circuitry comprising:

• a) a power switch connected in series with the coil, the power switch being switchable between an open and closed position, in the closed position permitting energization of the coil by the external electrical source and in the open position preventing energization of the coil by the external electrical source;
• b) a resistor connected in series with the coil and said power switch;
• c) a first measurement device for measuring a value of a voltage drop across said resistor;
• d) a second measurement device for measuring a value of a voltage drop across said coil;
• and
• e) a control device coupled to the first and second measurement devices and the switch and including means for determining a current flow through the coil based on the measured value of the resistor voltage drop and a resistance of the resistor and for determining a resistance of the coil based on the measured value of the coil voltage drop and the current flow through the coil and further including means for opening and closing said switch such that the switch is closed during a portion of a predetermined cycle time period to provide a pulsed signal to maintain the current flow through the coil near the desired current flow value.

A system for maintaining current flow through a coil near a desired non-zero value, said system comprising:

• a) a switch connected in series with the coil;
• b) a resistor connected in series with the coil and said switch;
• c) a source for providing an electrical potential tending to produce current flow through said series connected coil, switch and resistor;
• d) a first measurement device coupled across the coil for producing an output signal whose value is a function of a voltage drop across the coil;
• e) a second measurement device coupled across the resistor for producing an output signal whose value is a function of a voltage drop across said resistor;
• and
• f) control circuitry comprising:
• i) circuitry responsive to said output signals of said first and second measurement devices for determining a current flow through the coil based on the second measurement device output signal and a resistance of the resistor and for determining a resistance of the coil based on the first measurement device output signal and the determined current flow through the coil;
• and
• ii) circuitry for opening and closing said switch for causing pulsed current to flow through said coil, the leading edges of said pulses occurring at intervals uniformly spaced in time, and the duration of said pulses being a function of both the determined current flow through the coil and the resistance of the coil to maintain coil current near the desired value.