US3585404A

US3585404A - Solid state electronic ac switching device

Info

Publication number: US3585404A
Application number: US774587A
Authority: US
Inventors: Theodore W Le Vake
Original assignee: United Control Corp
Current assignee: United Control Corp
Priority date: 1968-11-04
Filing date: 1968-11-04
Publication date: 1971-06-15
Anticipated expiration: 1988-06-15

Abstract

A traffic light flashing circuit for periodically supplying power to a lamp load wherein a unijunction multivibrator controls the flashing rate by locking its frequency to that of the power source with the multivibrator output periodically disabling two Silicon Control Rectifiers coupling the power source to the lamp.

Description

United States Patent Theodore W. Le Vake Bellevue, Wash.

Nov. 4, 1968 June 15, 1971 United Control Corporation Redmond, Wash.

Continuation of application Ser. No. 496,486, Oct. 15, 1965, now abandoned.

Inventor Appl. No. Filed Patented Assignee SOLID STATE ELECTRONIC AC SWITCHING DEVICE 17 Claims, 3 Drawing Figs.

US. Cl 307/252, 315/196,315/209,3l5/251 Int. Cl H03k 17/00 Field of Search 307/252;

Primary Examiner- Donald D. Forrer Assistant Examiner-John Zazworsky Attorney-Hofgren, Wegner, Allen, Stelliman and McCord References Cited UNITED STATES PATENTS l0/l962 11/1966 S/l 967 8/1967 6/1968 Rockafellow Carlisle et a1.

Morris, Jr.

Gutzwiller Baker 307/252 X 3071235 X 307/252 307/252 307/252 X ABSTRACT: A traffic light flashing circuit for periodically supplying power to a lamp load wherein a unijunction multivibrator controls the flashing rate by locking its frequency to that of the power source with the multivibrator output periodically disabling two Silicon Control Rectifiers coupling the power source to the lamp.

22 v I If We RECTIFIER TlMER lOb PATENTEU JUN? 5 |9Ti FIG.1

RECTIFIER TIMER THEODORE w.

LE VAKE INVEN'H )R.

ATTORNE Y5 SOLID STATE ELECTRONIC AC SWITCHING DEVICE This application is a continuation of my copending application Ser. No. 496,486, filed Oct. 15, 1965, entitled Solid State Electronic Lamp Flashing Control Device, and now abandoned.

This invention relates to solid state electronic devices operating from alternating current sources. More particularly this invention relates to such a solid-state device for providing power increments to a lamp load in a manner such that the lamp load flashes at a periodic rate.

Known devices for controlling periodically flashing lamps, such as flashing traffic intersection lights and other types of warning lights, are electromechanical devices comprising a motor, a gear train and a switching mechanism. Electromechanical devices such as these suffer from a number of inherent disadvantages. One of these disadvantages is the relatively short switch contact life resulting from the repetitive make-and-break action of the switching contact which'necessitates regular maintenance and replacement of the contacts. For example, a typical maintenance schedule on existing flashing traffic intersection light controllers requires servicing of the controllers every 60 to 90 days. Another disadvantage is that the repetitive making and breaking action of the switching contacts produces an are between the contacts and a concomitant radiated signal. This arcing not only gradually pits the contacts but producesstatic which may be picked up by nearby radio or television sets. A further disadvantage is that, by reason of the size and weight of the electromechanical components, such flasher-controllers are not adaptable to mobile usage.

A primary object of this invention is to provide a solid-state electrical control circuit for controlling flashing lamps that requires no maintenance, produces no radio noise and is sufficiently compact to be adaptable to portable usage. Another object is to provide a solid state electrical circuitry that may be totally encapsulated so that no bulky weatherproof housing is required.

These and other objects and advantages of this invention will become apparent from the following description in conjunction with the accompanying drawing, of which:

FIG. 1 is a circuit diagram of the invention with a firing circuit shown in detail and timing and rectifier circuits shown as blocks.

FIG. 2 is a preferred circuit diagram of the invention; and

FIG. 3 is a circuit diagram depicting additional components added to the FIG. 2 circuit diagram.

The solid-state device of this invention comprises an alternating current-operated circuitry consisting of a firing circuit, a timing circuit for controlling the firing circuit, and a rectifier circuit for converting source alternating current to direct current to operate the timing circuit. The device is provided with two input terminals adapted to be connected to a suitable AC power source and an output terminal adapted to be connected to a lamp load. The components of the firing circuitry comprise a pair of silicon controlled rectifiers (SCR) connected in a full-wave configuration. This connection provides full AC power to a lamp load when the SCRs are turned on by gating circuitry in the timing circuit. The timing circuit includes a gating circuitry which turns the SCRs on at zero phase angle thereby providing longer lamp life and minimal electromagnetic disturbance. These two desirable features are realized because there is no abrupt power application to the load such as would be provided by an electromechanical contactor or a delay-gated SCR configuration. Gating is controlled in a periodic manner by a double-base diode (unijunction) and transistor astable multivibrator. The multivibrator inhibits or enables the gating to the SCRs at a periodic rate determined by the time constants of a charging and discharging resistorcapacitor network. DC power for the multivibrator is derived from an AC source by means of a simplified half wave voltage rectifier regulator which makes the multivibrator frequency independent of line voltage variations.

The firing circuit comprises two oppositely poled silicon controlled rectifiers, each having an input connected to a common input terminal, an output, and a gate or control electrode. With reference to FIG. I, the input of the first SCR 10 is a cathode 10a and that of the second SCR 12 is an anode 12a, the respective outputs being an anode 10b and a cathode 12b. With appropriate modification of the associated circuitry, as would be within the purview of one skilled in the art, the respective SCR polarities could be reversed. The cathode 10a of SCR l0 and the anode 12a of SCR 12 are both connected to input terminal 1. The anode 10b of SCR 10 is connected to the output terminal 3 and, through resistor 14, to the control electrode 12c of SCR 12. The cathode 12b of SCR 12 is connected, through power diode 16, to the output terminal 3 and, through series-connected resistor 18 and capacitor 20, to input terminal 2. Series-connected diode 22 and capacitor 24 are connected to the input terminals 1 and 2 and, through their midpoint 26, to the control electrode of SCR 10. Midpoint 26 is also the gating connection between the timing circuit and the firing circuit through which the firing circuit is controlled.

With reference to FIG. 2, a preferred rectifier circuit comprises a Zener diode 36 with its anode connected to input terminal 1 and its cathode connected to the anode of diode 38 and to a capacitor 40 through junction 42. Capacitor 40 also connects junction 42 to the input terminal 2. A capacitor 43 is connected to input terminal 1 and the cathode of diode 38.

Also with reference to FIG. 2, a preferred timing circuit comprises a double-base diode, or unijunction, transistor 44 with one base 44a connected to the input terminal 1 and the other base 44b connected through resistor 46 to the cathode of diode 38. The emitter 44c of unijunction 44 is connected to base 48a of transistor 48 through series-connected capacitor 50 and diode 52, the anode of diode 52 being connected to capacitor 50. The unijunction emitter side of capacitor 50 is connected to the cathode of diode 38 through resistor 54, and the diode side of capacitor 50 is connected to the cathode of diode 38 through resistor 56. Base 48a and emitter 48b of transistor 48 are connected to the input terminal 1, base 48a being connected through resistor 58. Collector 480 of transistor 48 is connected to midpoint 26 of the firing circuit. Although transistor 44 is the preferred gating device, any other semiconductor device may be substituted therefor. The periodic rate can be controlled by appropriate selection of

resistors

54 and 56 and capacitor 50. Duty cycle, i.e. on-to-off time, can be adjusted by varying the ratios of

resistors

54 and 56.

FIG. 3 depicts the circuit of FIG. 2 with the addition of

resistors

28 and 30, zener diode 32 and capacitor 34 to the firing circuit, and resistors 60 and 62 to the timing circuit.

Resistors

28 and 30 connect the control electrodes to the cathodes of their respective SCRs l0 and 12, resistor 28 connecting the control electrode 100 to the input terminal 1, Zener diode 32 connects control electrode 100 of SCR 10 to midpoint 26 with resistor 28 connecting to the anode of Zener diode 32, and capacitor 34 connects the input terminals 1 and 2. Resistor 60 is connected in parallel with resistor 54 between the unijunction emitter side of capacitor 50 and the cathode of diode 38, and resistor 62 is connected in parallel with resistor 56 between the diode side of capacitor 50 and the cathode of diode 38.

The circuit of FIG. 2 operates in the following manner. Capacitor 40 which couples the basic AC power to the Zener diode 36 provides a limiting impedance to maintain current in Zener diode 36 at a desired level. Zener diode 36 rectifies and clips the AC voltage waveform and establishes a desired operating voltage level. Diode 38 couples the clipped DC voltage to capacitor 43 which stores the DC power to operate the multivibrator circuit. Diode 38 also prevents capacitor 43 from discharging during the Zener nonconducting half-cycle of Zener diode 36.

The multivibrator circuit comprised of unijunction 44,

resistors

54 and 56, capacitor 50, diode 52 and transistor 48 operates as follows. Unijunction 44 is operated alternately between its stable and unstable states and resistor 46 sets the peak discharge operating point of unijunction 44. The network of capacitor 50, diode 52 and the base to emitter junction of transistor 48 is charged through resistor 54. When the voltage on capacitor 50 rises to the peak discharge point of unijunction 44, unijunction 44 conducts lowering the positive side of capacitor 50 to the potential of base 440 of unijunction 44. As a result the base to emitter junction of transistor 48 is back-biased turning off transistor 48. Resistor 56 then conducts current to charge capacitor 50 until diode 52 and the base to emitter junction of transistor 48 are forward-biased turning transistor 48 on. The operation cycle then repeats. Resistor 58 establishes the off-bias of transistor 48.

Transistor 48 during the time it is biased on inhibits the gate current to SCR provided by capacitor 24. Diode 22 protects transistor 48 during the time that the AC power is in the negative direction referenced to emitter 48b of transistor 48. The impedance of capacitor 24 limits the gate current to SCR 10 to the desired level. Capacitor 24 also provides phase leading such that the gate circuitry to SCR 10 is biased on before the AC potential across SCR 10 is in a forward direction. This insures the turning on of SCR 10 at zero crossing.

If permitted by the condition of transistor 48, SCR 10 is turned on during the negative half-cycle of the AC power. This allows capacitor to charge through diode 16 in the case of FIG. 2, or diode 16 and

resistances

14, and 18 in the case of FIG. 3. At the instant SCR 10 ceases to conduct at the completion of the first half-cycle, capacitor 20 discharges through the diode junction of SCR 12 between control electrode 12c and cathode 12b, resistor 14 and the lamp load coupled between

terminals

2 and 3 causing cathode 12b of SCR 12 to drop in potential. This simulates the gate signal to SCR 12 and SCR 12 conducts during the positive half-cycle. Due to the phase leading characteristic of capacitor 20 SCR 12 also turns on at zero crossing. Resistor l4 maintains the operating point of the gate of SCR 12. Resistor l8 limits the discharge current of capacitor 20 to the required operating level. If transistor 48 is turned on, SCR 10 is prevented from conducting which in turn prevents capacitor 20 from charging and thus the SCR switching circuitry remains off.

In the additional circuitry shown in FIG. 3, resistors 60 and 62 provide trimming to establish the desired on-to-ofl' ratio of the multivibrator circuit.

Resistors

28 and 30 establish the gate operating levels and prevent the SCRs from turning on due to thermal instability. Diode 22 provides blocking during the off cycle of transistor 48 and protects the collector to emitter junctions of transistor 48 against excessive reverse voltage. Zener diode 32 establishes the required voltage level to insure that the SCRs will turn on. Capacitor 34 furnishes line voltage transient protection to the circuitry and prevents undesired turn-on due to these transients. Capacitor 34 also reduces radio noise. In this modified circuit, capacitor 24 determines the minimum current required to fire the circuitry and Zener diode 32 determines the minimum voltage required to fire the circuitry.

By properly choosing the capacity value for capacitor 43, a ripple will be produced that will lock the multivibrator circuit into a subharmonic of the 60-cycle 115 volt AC input line. Table l lists a preferred range of capacitive valves for capacitor 43 which will achieve this result and in addition lists preferred values for the other components of the circuit.

TABLE I .1-1 microfarad Resistor 54:

(without trimming resistor 60), 2-20 kilo-ohms (with trimming resistor 60), 5-15 kilo-ohms Resistor 60, 25-100 kilo-ohms Resistor 56:

(without trimming resistor 62), 4-40 kilo-ohms (with resistor 62), 15-25 kilo-ohms Resistor 62, 25-100 kilo-

ohms Resistors

14, 18, 28 and 30, typically about ohms Transistor 44,

VCEO volts SCR

10 and 12, 1-2 kilowatts or greater.

1 claim: 1. A circuit for controlling the incremental alternating current power application to an alternating current load which comprises first and second input terminals adapted to be connected to a suitable alternating current source and an output terminal adapted to be connected to said load;

first and second oppositely poled controlled rectifiers, each of said controlled rectifiers having an input connected to said first terminal, an output and a control electrode;

means connecting the first rectifier output to said output terminal; means connecting a first resistance connecting said first rectifier output to the second rectifier control electrode;

means including a second resistance in series with a first capacitance connecting the second rectifier output to said second input terminal;

means including a first diode connecting said second rectifier output to said output terminal;

means including a second diode in series with a second capacitance connecting said first input terminal to said second input terminal with a midpoint between said second diode and said second capacitance;

means connecting the first rectifier control electrode to said midpoint; and

means connected to said first and second input terminals and to said midpoint for gating said first rectifier.

2. The circuit according to claim 1 wherein the means connecting said first rectifier control electrode to said midpoint includes a voltage breakover device requiring a predetermined voltage thereacross for conduction.

3. The circuit according to claim 1 wherein means including a resistance connects said first input terminal to said first rectifier control electrode; and means including a resistance connects said second rectifier control electrode to said second rectifier output.

4. The circuit according to claim 1 wherein means including a capacitance connects said first and second input terminals.

5. The circuit according to claim 1 wherein the gating means comprises an oscillator circuit including a unijunction transistor having first and second bases and an emitter; means connecting the unijunction first base to said first input terminal; means connecting said second base to a DC input; a semiconductor device; means connecting the unijunction emitter to said semiconductor device; and means connecting said semiconductor device to said midpoint for gating said first rectifier.

6. The circuit according to claim 5 wherein said semiconductor device comprises a transistor having a base connected to the unijunction emitter, an emitter connected to said first input terminal and a collector connected to said midpoint; and means connecting the transistor base to said first input terminal including a resistance.

7. The circuit according to claim 5 including a voltage breakover diode connected to said first input terminal and to a junction, a capacitance connected to said junction and to said second input terminal, a diode connected between said junction and said DC input, and a capacitance connected between said first input terminal and said DC input.

8. A circuit for controlling the energization of a load from a source of alternating current having positive and negative going portions, comprising:

a controllable conduction device gatable from a nonconductive to a conductive state under control of a signal at a gate electrode;

means connecting said device in a circuit between said source and said load for energizing said load when said device is in said conductive state;

means coupled to said source and to said gate electrode for generating said signal only when said alternating current is at substantially the crossover point between voltage portions of alternating current, whereby said load is initially initially energized at a time of minimum power from said source, including means connected across said alternating current source for establishing a current having a different phase relation than said voltage portions, timing means rendering said establishing means effective to cause said establishing means to gate said device to said conductive state only when the voltage portions applied to said device are at substantially the crossover point, said timing means includes a second controllable conduction device gatable between nonconductive and conductive states under control of a timing signal, circuit means connecting said second device and said establishing means in circuit to couple said current to said gate electrode in response to one of said states of said second device, bias means for maintaining said second device in the state opposite said one state except when the timing signal is coupled to said second device. DEVICE.

9. The circuit of claim 8 wherein said circuit means couples said establishing means to said gate electrode, said second device being connected to shunt said current from said gate electrode when said second device is in its conductive state, said bias means maintaining said second device in its conductive state, and said timing signal driving said second device into its nonconductive state.

10. The circuit of claim9 wherein said circuit means connects said second device and said establishing means in series across said alternating current source, the midpoint between said second device and said establishing means being coupled to said gate electrode, said second device comprises a unidirectional conduction device poled to pass one of said portions of alternating current to said load, and diode means in parallel with said second device and poled to shunt said second device whenever the portion of alternating current is opposite to said one portion.

llll. A circuit for controlling the energization of a load from a source of alternating current having positive and negative going portions, comprising:

a controllable conductiondevice gatable from a nonconductive to a conductive state under control of a signal at a gate electrode;

means coupled to said source and to said gate electrode for generating said signal only when said alternating current is at substantially the crossover point between said portions of alternating current, whereby said load is initially energized at a time of minimum power from said source; and

timing means coupled across said source and responsive to a subharmonic of the alternating current for periodically generating said signal at a rate which is less than the rate of occurrence of said alternating current portions.

12. The circuit of claim 11 wherein said timing means includes means oscillating at a subharmonic frequency of the alternating current, semiconductor means responsive to the oscillations of said oscillating means for switching between nonconductive and conductive states, and means connecting said semiconductor means to said gate electrode to couple said current thereto in response to one of the states of said semiconductor means.

13. A circuit for controlling the energization of a load from a source of alternating current having first and second direction going voltage portions, comprising:

a pair of unidirectional conduction devices each being gatable from a nonconductive to a conductive state in response to a trigger signal at a control electrode;

means connecting said pair of devices in a full wave circuit between said source and said load, said devices being poled oppositely to each pass when triggered a different one of said voltage portions of alternating current, includmg capacitor means connected across said alternating current source for establishing a current leading said voltage portions applied to said devices by said full wave circuit connecting means,

means coupled to the control electrode of the first of said pair of devices and to said capacitor means for causing said leading current to form a first trigger signal only at substantially the beginning point of a selected first direction-going voltage portion of alternating current to gate the first device into its conductive state, and

noninductive means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going voltage portion of alternating current which immediately follows said selected first direction-going voltage portion of alternating current, thereby automatically energizing said load for each following voltage portion of alternating current when said load is energized by the corresponding first direction-going voltage portion of alternating current.

14. A circuit for controlling the energization of a load from a source of alternating current having first and second direction-going portions, comprising:

a pair of unidirectional conductive devices each being gatable from a nonconductive to a conductive state in response to a trigger signal at a control electrode;

means connecting said pair of devices in a full wave circuit between said source and said load, said device being poles oppositely to each pass when triggered a different one of said portions of alternating current, including means coupled to the control electrode of the first of said pair of devices for generating a first trigger signal to gate the first device into its conductive state,

means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going portion of alternating current which immediately follows said selected first direction-going portion of alternating current, thereby automatically energizing said load for each following portion of alternating current when said load is ehergized by the corresponding first direction-going portion of alternating current, including capacitor means connected in series between said alternating current source and said first device, said capacitor means being charged by said first direction-going portion of alternating current when said first device is in its conductive state, and means connecting said capacitor means to said second device to cause the stored first directiongoing portion of alternating current to form said second trigger signal.

15. The circuit of claim 14 wherein said capacitor means is connected in series between said second device and said alternating current source to be charged by said second directiongoing portion of alternating current when said second device is in its conductive state, thereby dissipating said second trigger signal after gating said second device.

16. The circuit of claim 15 including diode means coupled between said first device and the series connection of the capacitor means and the second device, said diode means being poled to pass first direction-going portions of alternating current to said first device.

17. A circuit for flashing an alternating current lamp by controlling the energization of the lamp from a source of alter nating current having first and second direction-going portions, comprising:

means connecting said pair of devices in a full wave circuit between said source and said lamp, said devices being poled oppositely to each pass when triggered a different one of said portions of alternating current, including means connecting said lamp between said source and one of said pair of devices for energization when in its conductive state,

means coupled to the control electrode of the first of said pair of devices for generating a first trigger signal to gate the first device into its conductive state,

means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going portion of alternating current which immediately follows said selected first direction-going portion of alternating current, thereby automatically energizing said lamp for each following portion of alternating current when said lamp is energized by the corresponding first direction-going portion of alternating current,

diode means connecting the other of said pair of devices to the midpoint between said lamp and said one device and poled to pass the alternating current portion passed by said other device when in its conductive state, and

means for periodically actuating said first trigger signal generating means.

Claims

1. A circuit for controlling the incremental alternating current power application to an alternating current load which comprises first and second input terminals adapted to be connected to a suitable alternating current source and an output terminal adapted to be connected to said load; first and second oppositely poled controlled rectifiers, each of said controlled rectifiers having an input connected to said first terminal, an output and a control electrode; means connecting the first rectifier output to said output terminal; means connecting a first resistance connecting said first rectifier output to the second rectifier control electrode; means including a second resistance in series with a first capacitance connecting the second rectifier output to said second input terminal; means including a first diode connecting said second rectifier output to said output terminal; means including a second diode in series with a second capacitance connecting said first input terminal to said second input terminal with a midpoint between said second diode and said second capacitance; means connecting the first rectifier control electrode to said midpoint; and means connected to said first and second input terminals and to said midpoint for gating said first rectifier.

8. A circuit for controlling the energization of a load from a source of alternating current having positive and negative going portions, comprising: a controllable conduction device gatable from a nonconductive to a conductive state under control of a signal at a gate electrode; means connecting said device in a circuit between said source and said load for energizing said load when said device is in said conductive state; means coupled to said source and to said gate electrode for generating said signal only when said alternating current is at substantially the crossover point between voltage portions of alternating current, whereby said load is initially initially energized at a time of minimum power from said source, including means connected across said alternating current source for establishing a current having a different phase relation than said voltage portions, timing means rendering said establishing means effective to cause said establishing means to gate said device to said conductive state only when the voltage portions applied to said device are at substantially the crossover point, said timing means includes a second controllable conduction device gatable between nonconductive and conductive states under control of a timing signal, circuit means connecting said second device and said establishing means in circuit to couple said current to said gate electrode in response to one of said states of said second device, bias means for maintaining said second device in the state opposite said one state except when the timing signal is coupled to said second device. DEVICE.

10. The circuit of claim 9 wherein said circuit means connects said second device and said establishing means in series across said alternating current source, the midpoint between said second device and said establishing means being coupled to said gate electrode, said second device comprises a unidirectional conduction device poled to pass one of said portions of alternating current to said load, and diode means in parallel with said second device and poled to shunt said second device whenever the portion of alternating current is opposite to said one portion.

11. A circuit for controlling the energization of a load from a source of alternating current having positive and negative going portions, comprising: a controllable conduction device gatable from a nonconductive to a conductive state under control of a signal at a gate electrode; means connecting said device in a circuit between said source and said load for energizing said load when said device is in said conductive state; means coupled to said source and to said gate electrode for generating said signal only when said alternating current is at substantially the crossover point between said portions of alternating current, whereby said load is initially energized at a time of minimum power from said source; and timing means coupled across said source and responsive to a subharmonic of the alternating current for periodically generating said signal at a rate which is lesS than the rate of occurrence of said alternating current portions.

13. A circuit for controlling the energization of a load from a source of alternating current having first and second direction-going voltage portions, comprising: a pair of unidirectional conduction devices each being gatable from a nonconductive to a conductive state in response to a trigger signal at a control electrode; means connecting said pair of devices in a full wave circuit between said source and said load, said devices being poled oppositely to each pass when triggered a different one of said voltage portions of alternating current, including capacitor means connected across said alternating current source for establishing a current leading said voltage portions applied to said devices by said full wave circuit connecting means, means coupled to the control electrode of the first of said pair of devices and to said capacitor means for causing said leading current to form a first trigger signal only at substantially the beginning point of a selected first direction-going voltage portion of alternating current to gate the first device into its conductive state, and noninductive means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going voltage portion of alternating current which immediately follows said selected first direction-going voltage portion of alternating current, thereby automatically energizing said load for each following voltage portion of alternating current when said load is energized by the corresponding first direction-going voltage portion of alternating current.

14. A circuit for controlling the energization of a load from a source of alternating current having first and second direction-going portions, comprising: a pair of unidirectional conductive devices each being gatable from a nonconductive to a conductive state in response to a trigger signal at a control electrode; means connecting said pair of devices in a full wave circuit between said source and said load, said device being poles oppositely to each pass when triggered a different one of said portions of alternating current, including means coupled to the control electrode of the first of said pair of devices for generating a first trigger signal to gate the first device into its conductive state, means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going portion of alternating current which immediately follows said selected first direction-going portion of alternating current, thereby automatically energizing said load for each following portion of alternating current when said load is energized by the corresponding first direction-going portion of alternating current, including capacitor means connected in series between said alternating current source and said first device, said capacitor means being charged by said first direction-going portion of alternating current when said first device is in its conductive state, and means connecting said capacitor means to said second device to cause the stored first direction-going portion of alternating current to form said second trigger signal.

15. The circuit of claim 14 wherein said capacitor means is connected in series between said second device and said alternating current source to bE charged by said second direction-going portion of alternating current when said second device is in its conductive state, thereby dissipating said second trigger signal after gating said second device.

17. A circuit for flashing an alternating current lamp by controlling the energization of the lamp from a source of alternating current having first and second direction-going portions, comprising: a pair of unidirectional conduction devices each being gatable from a nonconductive to a conductive state in response to a trigger signal at a control electrode; means connecting said pair of devices in a full wave circuit between said source and said lamp, said devices being poled oppositely to each pass when triggered a different one of said portions of alternating current, including means connecting said lamp between said source and one of said pair of devices for energization when in its conductive state, means coupled to the control electrode of the first of said pair of devices for generating a first trigger signal to gate the first device into its conductive state, means coupled between the control electrode of the second of said pair of devices and said first device for generating a second trigger signal which causes said second device to pass the second direction-going portion of alternating current which immediately follows said selected first direction-going portion of alternating current, thereby automatically energizing said lamp for each following portion of alternating current when said lamp is energized by the corresponding first direction-going portion of alternating current, diode means connecting the other of said pair of devices to the midpoint between said lamp and said one device and poled to pass the alternating current portion passed by said other device when in its conductive state, and means for periodically actuating said first trigger signal generating means.