An Inexpensive, Large Bandwidth, Precision D. C. Current Transformer for Stabilised Magnet Power Supplies

Precision D. C. current transformers for magnet power supplies are generally based on:-either (a) the "zero flux", 2nd harmonic modulator principle, or (b) the series connected transductor (Kraemer) circuit. Solution (a), while capable of the highest precision, is relatively costly and characterised by large time constants. Solution (b) while much cheaper by contrast, suffers from a high ripple voltage level and from ratio errors which stem from the magnetic uncoupling of primary and secondary windings when the cores saturate simultaneously twice per cycle of the excitation supply. The Brentford/Hingorani circuit, described in the paper, by a simple circuit change eliminates the uncoupling error and considerably reduces the ripple. A further small component addition reduces the residual ripple to negligible amplitude. Very large bandwidths (for pulsed supplies) are easily obtainable without detriment to stability. The paper describes some second and third order error sources and their elimination and concludes with a brief description of the design of D. C.C.T. used for the 101 stabilised power supplies employed in the beam transfer lines of the Intersecting Storage Ring installation at C.E.R.N., Geneva, Switzerland. Measured performances on this installation show many of the power supplies capable of half-hour stabilities of one or two p.p.m. in the presence of environmental deviations much greater than those usually experienced in similar installations.