## Phototube Operated Relay

ZQO v.r-m-s-i a-IIO i/.-> Fig. 13-36.—Circuit diagram for Prob. 13-1.

13-2. The following data apply to a photoelectrically controlled relay:

Phototube sensitivity. . .

Phototube cathode area.

Amplifier tube

Coupling resistor

Relay operating currents

### Relay resistance

a. Draw the diagram of a circuit in which increase of illumination will cause the relay to be opened. All phototube and amplifier voltages are to be derived from a single 220-volt d-c supply.

b. Specify values of resistance that will provide the correct filament current and suitable values of plate voltage and adjustable biasing voltage.

c. Specify the value and polarity of the C-supply voltage that will just open the relay at a phototube illumination of 72 ft-candles. The source of light is a tungsten filament operated at 2870°K.

d. How much must the illumination be decreased to cause the relay to close?

8 megohms

{8 ma, closing 7 ma, opening 1000 ohms

13-3. The following data apply to a phototube-amplifier-relay circuit:

Phototube cathode area 0.9 sq in.

Amplifier tube Type 6J5

Coupling resistance 1 megohm

Relay resistance 1000 ohms

Relay closing current 6 ma

Relay opening current 4 ma

Phototube anode supply voltage (including biasing voltage) 90 volts

Amplifier plate supply voltage 90 volts

Light source Tungsten filament at 2870°K

a. Show the circuit diagram for a circuit in which increase of illumination causes the relay to close.

b. From the plate characteristics of the type 6J5 tube find the grid voltages at which the relay will open and close.

c. From the characteristic curves of the type 868 phototube (Fig. 13-7), plot a curve of voltage across the coupling resistor as a function of light flux.

d. From the curve of (c) determine the grid supply voltage necessary to open the relay when the incident flux is 0.2 lumen.

e. Determine the changa in illumination, in foot-candles, necessary to close the relay after the circuit is adjusted as in (d).

13-4. a. Plot curves of per cent second and third harmonic in the voltage output of a type 918 phototube operated from an 80-volt supply with a 7-megohm load, as a function of incident light flux for 80 per cent and 100 per cent modulation of incident light.

6. Repeat for a load having a d-c resistance of 10 megohms and an a-c resistance of 7 megohms at signal frequency.

13-5. Determine the factor by which the current scale of the current-voltage characteristics of a type 868 gas phototube must be multiplied in order that the characteristics shall hold for a light source whose energy distribuion is given by the following table:

 Wave length, Relative Wave length, Relative Wave length, Relative angstroms energy angstroms energy angstroms energy