Phototube Relays

Since as already described the current change accomplished in photoelectric cells is very small, it is necessary to employ an amplifier in order to receive a current of sufficient strength to operate a relay.

When selecting a relay for operation in a photoelectric circuit, it is of course necessary to be informed about the characteristics of the circuit, speed of operation, etc.

It is also important that the relay employed is able to carry the load to be operated across its contacts without damage.

A relay generally is defined as a form of electro-magnet; when a current is passed through the wire surrounding the iron core, this latter becomes magnetized and will close or open a contact placed close to the core.

If the relay is designed to close a circuit, it is often referred to as a "circuit closing relay" or when designed to merely open the circuit as a "circuit opening relay."

With respect to its function a relay may be considered as,a form of electrical multiplier in that a weak current in one of the circuits controls a strong current in the other.

The type of relays most commonly used in photoelectric circuits for commercial purposes are usually referred to as the communication type or simply "telephone relays." This type will operate at approximately 5 milliamperes, and the maximum current the spring contacts will carry is about 250 milliampere, i.e. the current flowing in the circuit it is desired to control, should not exceed of an ampere.


Fig. 1.—Circuit illustrating approximate maximum current to be carried over contacts of a communication type of relay.


Fig. 1.—Circuit illustrating approximate maximum current to be carried over contacts of a communication type of relay.

In many cases this amount of current is insufficient and the telephone relay is used to energize a larger relay carrying 10 to 15, ampere. The limitation of a telephone type relay and method of connection when an additional relay is used, is shown in figs. 1 and 2.

There is at present a great multiplicity of special purpose relays developed suitable for photoelectric circuits, some of which have very desirable characteristics. Sometimes a combination of a photocell and amplifier is referred to as a light relay, or photomatic light relay. In this type the amount of light on the cell can be varied by inserting plates of various apertures. Other types are known as controlled rectifier relays, photoroller relays, etc.

Fig. 2.—Circuit showing typical connection when it is necessary to operate a larger load than is possible when using only a communication type relay.

A relay of the type described in fig. 1 is capable of operating on a few milliamperes.output of the amplifier, however, it cannot control any large amount of power directly, nor even the operating coil circuits of medium size contactors or solenoids.

A High-Speed Light Belay.—Photographic illustration of a typical and compact photoelectric relay unit is shown in fig. 3 and its circuit diagram in fig. 4.

This relay is able to carry approximately one ampere across its contact tips at 115 volts (non-inductive). It operates with a minimum illumination of 5 foot candles on the phototube and upon a 50% change in light which lasts for not less than 0.066 second when operated from a 60 cycle source, and will operate up to 400 times per minute.

Figs. 3 and 4.—An A. C. operated photo-electric relay and connection diagram. A relay of this type may be used in automatic weighing processes; in machines for grading ball bearings; for the automatic inspection of vent holes in battery caps, and in many other applications.

Figs. 3 and 4.—An A. C. operated photo-electric relay and connection diagram. A relay of this type may be used in automatic weighing processes; in machines for grading ball bearings; for the automatic inspection of vent holes in battery caps, and in many other applications.

The operation of this relay is as follows: When terminal A of the transformer secondary is positive, both the triode amplifier tube and phototube anodes are negative, hence no current flows in either circuit inasmuch as electron tubes are inherently rectifiers.

However, if C\ be considered to have zero charge the grid of the amplifier tube will be positive when terminal A is positive and grid current, limited by grid resistor will flow to charge grid capacitor Ci in the sense indicated on the diagram. Upon reversal of the a.c. voltage, the amplifier tube and photoelectric tube anodes become positive, but the amplifier tube grid is negative by an amount equal to the charge of the capacitor plus the voltage from terminal A to the slider of the potentiometer and, therefore, no current flows in the amplifier tube anode circuit.


If there be light on the photo-electric tube, current passes through it to the capacitor in the direction which tends to discharge the capacitor and charge it in the opposite sense. As the voltage across the capacitor decreases and finally reverses, the amplifier tube grid is made less and less negative with respect to the cathode and finally reaches a potential which permits current flow in the anode circuit.

The current of the amplifier tube at first flows largely into the smoothing capacitor C2 inasmuch as the inductive effect of the magnetic circuit of the sensitive relay tends to maintain zero magnetic flux linkages in the relay coil at the beginning of current flow in the anode circuit. Shortly after anode current of the amplifier tube starts to flow the anode voltage of this tube again reverses and anode current stops.

However, current flows in the local circuit composed of the capacitor C2, and the coil of the sensitive relay until the energy stored in that circuit is dissipated in the coil resistance. C2 is made sufficiently large that it is enabled to maintain continuous current through the coil of the relay between pulses of the amplifier tube anode current before the average value of the current in the relay coil reaches the value necessary to operate the relay.

Hence, the relay has but small tendency to chatter at the critical average current value for pick-up. This complete series of events is repeated each cycle as long as there is light on the photo-electric tube.

With no light on the photo-electric tube, the action during the half-cycle when terminal A is positive is the same as before. However, in this case, when the a.c. voltage reverses, practically no current flows in the photo-electric tube, and therefore capacitor Ci, is not discharged but maintains its negative charge and keeps the grid negative throughout the positive half-cycle, thereby prohibiting flow of current in the amplifier tube, anode circuit.

Adjustment of the circuit is accomplished by means of the potentiometer, the position of which determines how far capacitor Ci must be discharged by the photo-electric tube current to permit current to flow in the anode circuit of the amplifier tube.

The average value of anode current of the amplifier tube is therefore determined by 2 factors: (1) The value of light flux impinging upon the photo-electric tube (the magnitude of photo-electric tube current is a function of the magnitude of the light flux); and (2) the potentiometer adjustment.

The Thyratron Electronic Relay.—In a number of applications the type of relay previously .described works perfectly satisfactory, but frequently applications are encountered in which the speed of response as well as the amount of current required is too great and hence make the described methods unsatisfactory.

In such cases it has been found advantageous to substitute the relay or relays for a grid controlled glow discharge tube, which is also known as the Thyratron tube or the Thyratron electronic relay.

These tubes are extensively used in a wide variety of electronic circuits where the grid is used for various control purposes. They may thus be used in a modified form of rectifier circuit to provide power to perform relay operations as well as to secure control of a switching circuit for a definite number of alternating current cycles.

These tubes resemble the conventional three electrode vacuum tubes in construction inasmuch as they contain an anode, a cathode, and a grid, spaced relatively to one another and sealed in an air tight glass or metal container. However, the resemblance ceases here as these tubes are filled with mercury-vapor at a suitably low pressure, and the conduction of current from the cathode to the anode is through a glow or arc-discharge occurring in the gas or vapor. In the conduction of current through a gas or vapor, the electrons ionize the neutral atoms and thus produce positive ions. These positive ions neutralize the negative space charge caused by the electrons, and thereby

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