GB1068477A - Direct current measuring circuits - Google Patents

Abstract

1,068,477. D. C. current measurement. W. B. ZELINA and J. L. BUCHANAN. Aug. 4, 1964 [July 29, 1963), No. 31152/64. Heading G1U. [Also in Division H3] A circuit for measuring a large D. C. current Ig incorporates a saturable transformer having bias, feedback and control windings wound on square loop (ferrite) material in addition to a one turn primary winding carrying the D. C. current Ig. The control winding Nc is connected to an npn transistor T 1 and a resistor R o , the voltage drop across which is determined to provide a measure of Ig, and a diode D 2 is connected across the winding and resistor. The feedback winding N F is connected in series with a diode D 1 to the transistor base, and the bias winding N B is connected across the circuit D. C. supply Voc in series with a potential divider R 1 , R 2 . A similar alternative circuit incorporating a pnp transistor is also described Fig. 4 (not shown). In operation, with zero current Ig, current flows as shown through the bias winding N B to bias the flux of the transformer to negative saturation. The transistor T1 is biased into conduction by its base potential, so that a large current flows through the winding Nc to cause the flux in the core to move towards positive saturation. The reduced voltage in the feedback winding N F drives the transistor base more positive, and the transformer core is driven firmly into positive saturation. When this occurs, a large spike of current flows because the reactive impedance of the transformer disappears, driving the transformer core far into saturation. The loss of induced voltage in the feedback winding causes the transistor to conduct less, and the flux in the transformer starts back negatively. This induces a small voltage in the winding NF which tends to turn off the transistor. Regenerative action cause the transistor to turn off completely, and the negative ampere turns of the winding N B drive the flux of the transformer toward negative saturation. When this occurs, the induced voltage in Ny disappears, the transistor becomes partially conductive and the cycle starts again. It is shown in the Specification that when the bias current I b in the bias winding NB is supported by a current to be measured I g , then (approximately). where I c is the current in the control winding. Since I b is fixed, I c will be proportional to I g , and is measured as the voltage appearing across the load resistor R o