A Simple Sweep-Circuit

Author: J.B. Hoag

In order to deflect the spot of light across the screen at a uniform rate, special "sweep-circuits" have been developed. A crude but simple circuit is shown in Fig. 22 A.

Fig. 22 A. A simple sweep circuit

Here, a battery whose voltage is E sends a current through the resistor R into the condenser C. The rate at which the current flows into the condenser depends upon the value of R and C.

Fig. 22 B. Current flow into the condenser of Fig. 22 A

As shown in Fig. 22 B, it occurs rapidly when the battery is first connected, then more and more slowly as the condenser becomes charged. Across the condenser in Fig. 22 A is a glow-tube, which will start to conduct electricity only when the voltage applied to its terminals has risen to a definite value, called the striking potential (E_s). When the voltage across the condenser terminals has reached this critical value, the glow-tube becomes conductive and the electricity in the condenser suddenly empties out through the low resistance glow-tube's path. The glow-tube becomes non-conductive or "shuts off" at a low voltage called the extinction potential E_x. When the condenser has suddenly discharged its electricity, and the glow-tube has become non-conductive, electricity again comes from the battery slowly through the resistance R, to refill the condenser and repeat the cycle of events. The voltage across the condenser rises and falls in the manner shown in Fig. 22 C.

Fig. 22 C. Voltage across the condenser of Fig. 22 A

The frequency of the relaxation oscillator, described in the preceding paragraph, depends upon the size of R and C, and upon the comparative voltage at which the glow-tube strikes, and the battery voltage. Suppose we keep everything the same except the resistance R. As R is decreased, the time required to fill the condenser becomes less and less and the frequency of oscillations is increased, as shown in Fig. 22 D.

Fig. 22 D. The effect of changing R and C in Fig. 22 A

Similarly if C is the only variable, and its value is made smaller and smaller, the time needed to raise the voltage across its terminals to the striking-potential of the glow-tube becomes less and less and the frequency of oscillations again is increased. In other words, the time constant RC determines the frequency of the oscillations.

We now suppose that all parts of the circuit remain constant except the glow-tube. Imagine that we have at hand a series of glow-tubes so constructed and with internal gas pressures such that they strike at different potentials. If we use the tube with highest striking-potential, then it will take a longer time before the voltage across the condenser has risen sufficiently to strike the tube, whereas if we use a tube with a low striking-potential, the condenser's voltage will be sufficient to strike the tube after a comparatively short time interval. These conditions are shown in Fig. 22 E.

Fig. 22 E. The effect of changing the striking potential (Es) of the glow-tube in Fig. 22 A

The voltage across the condenser (which we intend to apply as a sweep-voltage for a cathode-ray oscilloscope) is dependent on the frequency of the oscillations, as can be readily seen in the preceding figure.