Low-Power-Operated DC Instrument Transformers

In view of the various applications of nonlinear-magnetic control devices in earth-satellite equipment with solar-battery supply, a low-power-operated prototype design of a miniaturized dc instrument transformer with single-turn or multiturn control has been recently developed. This design combines a saturable-reactor push-pull circuit with a battery-supplied switching-transistor magnetic-core multivibrator. When measuring a direct current in the range 0 . . . 1 a, the four toroidal cores of the push-pull circuit link the wire, which carries this current and acts as a single-turn control winding. The corresponding dc output voltage of this circuit (0 . . . 5 v) is polarity-reversible and linearly proportional to the current to be measured. In this case, application of the single-turn-controlled dc instrument transformer eliminates the considerable power dissipation (up to 5 w) in a 5-ohm shunt resistor carrying the current to be measured. Smaller currents are measured with multiturn control where the wire carrying the control current can be threaded through the window opening for 2, 5, or 10 turns to give 5-v rated output-voltage value for 0.5-, 0.2- or 0.1-a control current, respectively. DC voltages of the order of 1 to 100 volts can be measured indirectly by employing the current range 0 . . . 100 or 0 . . . 200 μa, control windings with 10,000 or 5000 turns, and a series resistor corresponding to the multiplier resistor of an ordinary dc voltmeter.