Abstract: A high magnitude potential supply comprises a first circuit for generating a first signal related to a desired output high magnitude potential across a pair of output terminals of the supply, a second circuit for generating a second signal related to an output current from the high magnitude potential supply, and a third circuit for supplying an operating potential to the high magnitude potential supply so that it can produce the high magnitude operating potential. The third circuit has a control terminal. A fourth circuit is coupled to the first and second circuits and to the control terminal. The fourth circuit receives the first and second signals from the first and second circuits and controls the operating potential supplied to the high magnitude potential supply by the third circuit. A fifth circuit is provided for disabling the supply of operating potential to the high magnitude potential supply in certain conditions so that no high magnitude operating potential can be supplied by it.