Electronic Control Circuits

Vacuum-tube control circuits are used for amplification of the input voltage, not always at a single frequency. With thyratrons the input voltage triggers the tube, which then allows current to flow into the controlled circuit, but the output wave may not resemble the input wave, as is described below.

A simple circuit for thyratrons operated with alternating anode supply and resistive load is shown in Fig. 183.⁽¹⁾

Fig. 183. Basic thyratron circuit.

During that part of each cycle when the anode is positive with respect to the cathode, the tube conducts current which passes through the load, provided that the grid is at the right potential.

Fig. 184. Anode and critical voltages in basic thyratron circuit.

In Fig.184 is shown the positive anode voltage for a half-cycle, with the corresponding critical grid voltage. Any value of grid voltage higher than this critical value will permit the tube to conduct. Once tube conduction is started, change of grid voltage to a value less than critical will not stop conduction. Conduction does stop, however, at the end of the half-cycle, or when the anode voltage falls to zero. Three methods of controlling the load current are shown in Fig. 185.

Fig. 185. Grid control of a thyratron tube with (a) variable d-c grid voltage, (b) variable d-c grid voltage with superposed a-c grid voltage, (c) fixed d-c grid voltage with superposed a-c grid voltage of variable phase position.

Figure 185 (a) shows how direct voltage applied to the grid permits conduction through the tube over the shaded portion of the cycle. Minimum controllable current is half that which would flow if the tube were free to conduct over the entire positive half-cycle. This method of control is not precise, especially near the half-power point, because a small difference in d-c input control voltage produces a comparatively large change in conduction angle or may cause the tube to fail to fire altogether.

Figure 185(b) shows a more satisfactory method of amplitude control. The grid is maintained at a positive d-c potential, and an alternating voltage is superposed on it which lags the anode voltage by 90°. Varying the magnitude of the d-c grid voltage shifts the zero axis of the a-c wave up or down, and intersects the critical a-c grid voltage at different points of the cycle. Tube current can then be varied from zero to maximum. Close control of the tube current can be obtained because the grid voltage wave intersects the critical curve at a large angle.

In Fig. 185 (c) another method is shown. The phase of a superposed alternating voltage is shifted upon a negative d-c bias which is more negative than the critical characteristic. Changing the phase position of the a-c grid voltage varies the tube current from zero to maximum. The phase position of the grid voltage can be shifted by several methods, one of which is discussed in Grid-Controlled Rectifiers.

The anode supply transformer carries load direct current. Core saturation may be prevented by an air gap; heating and regulation in the primary winding due to excitation current govern the length of air gap. Ordinarily, permissible maximum induction may be higher than in a single-side amplifier transformer because impedance or frequency response considerations are irrelevant with a 60-cycle supply line. Excitation current may be comparable in magnitude to load current. However, there is this difference: with a resistive load, current flows only during the positive half-cycle, whereas magnetizing current flows during the whole cycle. Secondary current is a series of pulses, the maximum width of which is 180°. The rms value of these pulses is half the peak amplitude, and this is the current which governs secondary wire size. Rms secondary voltage is 2.22 times maximum d-c load voltage, as in a single-phase half-wave rectifier. Design of the transformer is similar to the anode transformers in Chapter 3, except for the higher induction and current wave form.

Full-wave circuits⁽²⁾ operate with two thyratrons and a center-tapped transformer in which the net d-c flux is zero. The design of the anode transformer is described in Thyratron Transformers.

(1)	See Industrial Electronics, by F. H. Gulliksen and E. H. Vedder, John Wiley & Sons, New York, 1935, p. 45.
(2)	See Gulliksen and Vedder, op. cit., p. 54.