Phototube Operated Relay

Frequency,--Dynamic response curve for type 868 gas phototube.

Time

Fig. 13-9.—Lag in response of a gas phototube.

Frequency,--Dynamic response curve for type 868 gas phototube.

The third disadvantage resulting from the use of gas in phototubes is caused by the relatively large mass of the positive ions. Because the time taken for a positive ion to pass from the vicinity of the anode to the cathode is appreciable, there is a perceptible time lag in the response to a change in flux. The manner in which a gas phototube responds to an abrupt increase and decrease of flux is shown in Fig. 13-9. The effect of the time lag is similar to that'of inductance in the phototube circuit, as it tends to prevent changes of anode current and produces a phase difference between sinusoidal periodic light fluctuations and the resulting alternating component of anode current. It also causes the variational sensitivity of the tube to decrease with increase of modulation frequency of the light.1 This is illustrated by the dynamic response curve of Fig. 13-10, which shows the relative variational sensitivity of a type 868 gas phototube as a function of modulation frequency. When a gas phototube is used in conjunction with an amplifier to convert changes of light into

1 Modulation frequency, the frequency at which the illumination varies, should not be confused with the radiation frequency of the incident light.

sound, the effect of a drooping dynamic response curve can be offset by the use of an amplifier that has a rising frequency characteristic.

13-9. Phototube Circuits.—The standard phototube symbol is shown in Fig. 13-11. Phototube currents are so small that a galvanometer is the only current-operated device that they can operate directly. One or more stages of amplification must be used in practical applications of the phototube. Since vacuum-tube amplifiers are voltage-operated devices, the changes of phototube current must be converted into voltage changes by means of impedance in series with the tube. Because the currents are very small, the impedance must be high, usually from 1 to 25 megohms. Although transformers have been designed for use with phototubes, the difficulty of obtaining adequate primary reactance at the lower audio frequencies makes it simpler and cheaper to use resistance or resistance-capacitance coupling between the phototube and the amplifier.

In order to simplify diagrams of circuits discussed in this chapter, all tubes are shown as triodes, and all direct voltage sources as batteries.

Fig. 13-11.—Forward d-c phototube circuit. Fig. 13-12.—Reverse d-c phototube circuit.

Tetrodes and pentodes of proper characteristics (Sec. 13-14) may be used in place of triodes, and batteries may be replaced by other types of power supplies.

When a relay or other current-operated electrical device is to be controlled by changes of steady or average flux, a direct-coupled amplifier must be used. A circuit in which increase of illumination causes an increase in plate current of the amplifier tube is termed a forward circuit; one in which increase of illumination causes a decrease of plate current is called a reverse circuit. Figure 13-11 shows a simple forward circuit in which increase of illumination causes a.relay to be energized. Figure 13-12 shows a similar reverse circuit. It can be seen that, for the same tubes, the forward circuit requires less total supply voltage than the reverse circuit.

By the use of a voltage divider, the various B and C batteries of Figs. 13-11 and 13-12 may be replaced by one voltage source, as in Fig. 13-13. Degenerative feedback resulting from the flow of plate current through Ri reduces the sensitivity of the circuit. Because the amplifier must respond to changes of direct voltage, a by-pass condenser does not remedy this difficulty and R\ and R2 should be no larger than necessary

Fig. 13-11.—Forward d-c phototube circuit. Fig. 13-12.—Reverse d-c phototube circuit.

to limit the dissipation in these resistances to a reasonable value. Reduction of sensitivity as the result of degenerative feedback also makes it inadvisable to use cathode self-biasing resistors in the amplifier stages of phototube circuits designed to respond to changes of steady illumination. If the resistances Ri and are of proper size to give the correct filament

/f~\ _ or heater current, the filament or heater may be

I inserted between these resistances. Only a single power supply is then required. If the relay is shunted by a condenser in order to by-pass the alternating component of plate current, or if a slow-acting relay is employed, the circuit of Fig. 13-13 can also be used on an alternating voltage supply. The phototube and amplifier then pass current during only one-half of the cycle.

Improved forward and reverse a-c-operated circuits are shown in Figs. 13-14 and 13-15.1 The purpose of the grid condensers is to eliminate the difference in phase between grid and anode voltages resulting from the capacitance of the phototube and amplifier electrodes. The circuit of Fig. 13-15 also functions if the polarity of the phototube is reversed. The grid condenser then charges during the half cycle in which the

Fig. 13-13.—Forward phototube circuit using a single source of grid and and plate voltage.

Fig. 13-13.—Forward phototube circuit using a single source of grid and and plate voltage.

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