Stabilized power supplies for instrument applications

IN SOME instrument applications, batteries are used as sources of d-c power because they are portable, and free of internal noise (except for slow changes with temperature and time), and because they can be completely shielded from external noise. These advantages are frequently offset, however, by problems of size, weight, and replacement, and as a result increasing use is being made of a-c powered electronic d-c power supplies for a-c amplifiers, d-c amplifiers, and d-c potentiometers for voltage measurement. Although such electronic d-c supplies require a-c input power, this requirement is generally not objectionable because a-c power outlets are conveniently available. Means for suppressing the effects of line voltage fluctuation are often required for a-c amplifiers, generally required for d-c amplifiers, and always required for d-c potentiometers for voltage measurement.1 The use of a series (rheostat) tube with an appropriate d-c control amplifier and voltage reference has been generally accepted as a preferred way to accomplish this,2 an example of which is shown in Fig. 1. The heavy lines indicate the path of the power to the load RL. The series rheostat tube, under the control of the components indicated by light lines, stabilizes the d-c load voltage VL.