TABLE OF CONTENTS Early DC generators employed an excitation system that was controlled by a "tick-tock" regulator. If the generator voltage fell below the regulated value, the excitation was switched on until it was back to normal. Then, the excitation was switched off again, and the field current free-wheeled through a shorting contact. The field time constant was long enough that this simple regulator was entirely satisfactory to maintain generator voltage within a narrow band. Until transistors came along, such a simple system regulated the voltage for the instrument cluster in an automobile, and one could occasionally see a slight flicker in the dash lights of old cars. The DC generators in autos also were current limited by an on-off regulator. The modern world expects more than most simple on-off regulators can deliver, and the basic principles have segued into sophisticated pulse width modulated power electronics systems where the on-off operation is completely transparent to the user.
If the DC supply shown in Fig. 13.1 can be switched rapidly on and off, the load waveforms will be those shown. A continuously variable average voltage from (nearly) zero to (nearly) the full supply voltage is available by changing the duty cycle of the switching. The "nearly" has to be added, because switching devices in the real world have minimum on times that produce a small output and minimum off times that prevent them from reaching full output voltage in a controlled manner. They can be switched completely on or completely...
