(a) High-Voltage Power Supplies. Solid-state power supplies with excellent characteristics are available for
powering the nuclear detectors Solid-state power supplies reduce the package size, the heat generated, and the maintenance time Many power supplies are capable of supplying power to two or more detectors connected in parallel When power supplies are used to provide for two or more detectors, the detectors must be isolated from ground. Also, note that on a common-failure basis the units are not independent and cannot be used as redundant units in a shutdown circuit. It is desirable that the loss of one power supply will not cause a scram.

The high-voltage power supplies must have a voltage monitoring circuit or an output voltage proportional to the high-voltage output which can be used to monitor the high voltage The high-voltage monitor is used to give an alarm when the high voltage drops below a predetermined value Figure 5.13 shows a high-voltage monitor unit with a variable set-point alarm. The 10-turn potentiometer dial reading corresponds to the trip voltage

The high-voltage power supplies should have short- circuit protection. Protection against voltage surges at the input is also desirable.

(b) Low-Voltage Power Supplies. Power supplies used in the counting equipment must have excellent regulation and stability. The low-voltage supplies should have internal short-circuit protection to prevent supply damage due to overcurrent demands from the unit. Because the counting equipment is of solid-state design, it is important that the
voltage supplied to the printed circuit boards be limited below a value that would cause component damage.

Power supplies with built-m protection are commer­cially available and should be specified when ordering the nuclear equipment. An inexpensive method of providing overvoltage protection is shown in Fig. 5.14. The Zener diodes prevent the voltage from exceeding a given value. Excessive current drawn from the supply through the Zener diodes will cause the fuse to open before circuit damage has occurred.