A 20 ampere shunt regulator for controlling individual magnets in a seriesed string

At the CEBAF accelerator, groups of dipole magnets are often connected together in series and powered from a single large constant-current regulated DC supply, referred to locally as a "box" supply. Besides the economy realized, this configuration promotes correct field tracking of all elements of a beam transport channel. However, it is often desirable to vary the current in one element of a string independently of the others, particularly at the entrance and exits of transport channels. A 20 ampere programmable current shunt is described. Installed in parallel with the desired magnet, it permits shunting (bypassing) up to 5% of the string current around that magnet. The shunt regulator consists of a bank of MOSFET power transistors operated in linear mode. Regulation of current through the passbank is obtained by feedback from a manganin shunt maintained at a constant temperature for stability. The module is designed for remote operation and provides readbacks of various parameters useful for maintenance. To achieve complete electrical isolation, an onboard microprocessor provides remote communications via an opto-isolated serial data link. Constructional details and initial operating experience with the plug-in shunt are described.