A Self-Stopping Circuit

Author: J.B. Hoag

In order to stop the plate current of a thyratron, a relay may be used as described in the preceding section or a combination of resistors and capacitors may be used, as in Fig. 19 B.

Fig. 19 B. A self-stopping circuit

In order to understand the operation of this circuit, imagine that at the start there is no plate current and that the grid is more negative than the critical or striking potential. The condenser C, of considerable capacitance, has been charged by the battery, E_b, through the resistance R₁ (of fairly high value, say 50,000 ohms). Because there is no potential drop across the load resistance R₂ (of comparatively small value, say 10,000 ohms), the full voltage of the condenser is across the tube. Assume, now, that for a very short time interval, a positive potential is applied to the grid, causing it to exceed the striking-value. Then the plate current is turned on. This results in the sudden discharge of C through R₂, and considerable potential drop occurs across R₂. This reduces the plate voltage to zero and shuts off the plate current. Between successive pulses on the grid, the condenser is refilled from the plate battery.