Introduction

Rectifiers are used to convert alternating into direct current. The tubes generally have two electrodes, the cathode and the anode. Both high vacuum and gas-filled tubes are used. Sometimes for control purposes the gas-filled tubes have grids, which are discussed in Electronic Control Circuits.

A high-vacuum rectifier tube characteristic voltage-current curve is shown in Fig. 48.

Fig. 48. High-vacuum rectifier voltage-current curve.

Current flows only when the anode is positive with respect to the cathode. The voltage on this curve is the internal potential drop in the tube when current is drawn through it. This voltage divided by the current gives effective tube resistance at any point. Tube resistance decreases as current increases, up to the emission limit, where all the electrons available from the cathode are used.

Filament voltage governs the emission limit and must be closely controlled. If the filament voltage is too high, the tube life is shortened; if too low, the tube will not deliver rated current at the proper voltage.

Gas-filled rectifier tubes have internal voltage drop which is virtually constant and independent of current. Usually this voltage drop is much lower than that of high vacuum tubes. Consequently, gas-filled tubes are used in high power rectifiers, where high efficiency and low regulation are important. In some rectifiers, silicon or germanium crystals or selenium disks are used as the rectifying elements.

In this chapter, the rectifier circuits are summarized and then rectifier transformers and reactors are discussed.