diode. The voltage drop across a gaseous tube, designed for voltage regulation, is nearlyconstant. Tubes are available for regulation at moderately high voltages.Surge currentAt the instant a power supply is switched on, a sudden current surge occurs, even withno load at the output. This is because the filter capacitor(s) need an initial charge, andthey draw a lot of current for a short time. The surge current is far greater than the op-erating current. This can destroy the rectifier diodes. The phenomenon is worst inhigh-voltage supplies and voltage-multiplier circuits. Diode failure can be prevented inat least four different ways.The first method uses “brute force.” You can simply use diodes with a current rat-ing of many times the operating level. The main disadvantage is cost. High-voltage,high-current diodes can get expensive.A second method involves connecting several units in parallel wherever a diode iscalled for in the circuit. This is actually a variation on the first method. The overall costmight be less. Current-equalizing resistors are necessary.A third scheme for surge protection is to apply the input voltage little by little. Avariable transformer, called a Variac, is useful for this. You start at zero input and turna knob to get up to the full voltage. This can completely get rid of the current surge.A fourth way to limit the current surge is to use an automatic switching circuit inthe transformer primary. This applies a reduced ac voltage for a second or two, and thenswitches in the full input voltage.Which of these methods is best? It depends on the overall cost, the operating con-venience, and the whim of the design engineer.Surge current39321-12A voltage regulator circuit using a Zener diode and apower transistor.