Phase Control

Author: J.B. Hoag

We next consider an operating condition much used in practice with gas-filled triodes. We propose, first, to apply a small alternating voltage to the grid and a large alternating voltage to the plate by means of two transformers as in Fig. 19 C. Now, by some hook or crook, we propose to vary the relative phase of these two voltages, that is, the relative time at which they reach their peak values.

Fig. 19 F. Phase control of the plate current of a thyratron

As shown in Fig. 19 F(a), if the grid voltage reaches its peak value ahead of that of the plate voltage at a time corresponding to 170 electrical degrees, then the plate current will flow for the duration of the positive half-cycles, as indicated b the shaded areas. If the grid voltage reaches its peak value one-quarter of a period or 90 electrical degrees behind the peak value of the plate voltage, the plate current will flow for only that portion of the positive half-cycles indicated by the shaded areas in Fig. 19 F(d). As we adjust the phase of the grid voltage to lag more and more behind the plate voltage, the plate current flows for shorter and shorter intervals of time until, for a half-cycle or 180° lag, there will be no plate current at all.

Fig. 19 G. Effect of resistive and inductive loads in the plate circuit of a phase controlled thyratron

Figure 19 G shows the average value of the plate current or load voltage for various phases of the grid voltage (with respect to the phase of the plate voltage), in the case where the load in the plate circuit is resistive, and where it is inductive.