US2548887A - Filtered thyratron control circuits - Google Patents

Description

April 1951 c. IZENOUR 2,548,887

' FILTERED THYRATRON CONTROL cmbun's Filed Oct. 27, 1949 V INVENTOR GEORGE C. IZENOUR ATTORNEY I Patented Apr. 17, 1951 UNITED STATES PATENT OFFICE FILTERED THYRATRON CONTROL CIRCUITS '5 Claims. 1 The present invention relates to alternatingcurrent lighting circuits including phase-responsive grid-controlled gaseous electron-discharge tubes, or thyratrons, as in my Patent 2,463,463

trol the saturation of an iron-core reactance that is in series with a lamp or lamp bank; or in another circuit the thyratron is connected in series with the lamp or lamp bank. In both types of lamp circuits an adjustable control circuit is provided for the thyratron grid, for firing the thyratron at a time in each successive cycle that depends on the desired lamp intensity. Early firing produces bright illumination, whereas tardy firing results in lamp dimming.

A plurality of circuits for difierently controlled lamps are commonly required in theatre lighting. The early firing of thyratrons in one lighting circuit erratically tends to trip other thyratrons set for later firing. This premature, sporadic discharge results from transient pulses produced by the abrupt current-change in earlyfired thyratrons, from non-conducting to conducting state. The efiect is found in both types of lighting circuit, although it is more prominent in the direct series connection of thyratron and lamp than in the indirect control circuit that utilizes a saturable reactor.

' In an effort to meet the transient-flicker problem of such systems, filtering devicesv have been considered to suppress the transients, such as low-resistance chokes in series with the lamps.

This has various disadvantages, including the high cost, and the great weight and bulk of such devices which must pass the heavy current of the lamps. Furthermore, a lamp of onlyone power level is suitable to any given transient filter, smaller lamps being inadequately filtered and larger lamps causing excessive voltage drop in the filter.

Rather than to attempt the suppression of the transients, the present invention achieves stability of control by preventing transients from affecting the control circuits. A low-pass filter is. interposed, in the illustrative embodiment of the invention described in detail below, in each control circuit to attenute high-frequency transients produced in the power line by earlier-fired thyratrons of other lighting circuits; and the filter is subdivided to include a choke in series. with the A.C. leads of thephase-control circuit, and

a broad-band low-pass resistance-capacitance filter at the output of the phase-control circuit. The choke is of such inductance as not to shift the phase of the control-circuit supply excessively relative to the lamp-circuit supply, but because of the low power requirements of the control circuit, its impedance can nevertheless be substantial. A broad-band, low-pass filter of the resistance-capacitance type is interposed between the control circuit and the thyratron grids to increase the filtering effect, the subdivision of filters achieving the desired result without appreciably restricting the range of the control circuit. Both forms of filter are in the low current-level part of the system, rather than in the high current part, and hence are of much smaller size and greater eificiency, .and can be of more elaborate, effective design Without excessive cost. Both forms of filter are connected in that circuit which is energized by the power line but is connected between the load circuit and the control electrode of the thyratron. And by using an inductance as the filtering impedance in the A.-C. supply leads of the adjustable phase-shifter of the resistance-capacitance type having a resistive voltage divider, the necessary broad latitude of control is preserved for varying the lighting from full brilliance to extinction.

The invention will be more fully appreciated from the following detailed description of an illustrative embodiment in which reference is made to accompanying drawings.

Fig. 1 is the wiring diagram of an illustrative two-lamp system embodying features of the invention, and v Fig. 2 is a diagram of an alternating-current wave-form of line voltage, including a typical transient.

In Fig. 1, a pair of terminals Ill, l2 represent an alternating current source that may include transmission lines, transformers, etc., typical of commercial power mains. To this a series circuit is connected including thyratron l4 and lamp load IS. The. term thyratron is here used generically to include. gaseous electron-discharge tubes havin control electrodes, which electrodes are herein termed grids. A second thyratron I8; is reversely connected in parallel with thyratron l4 for potentially full-cycle conductivity, the cathode of each thyratron to the plate of the other in an arrangement conveniently termed back-to-back connection.

An alternating-current voltage is impressed between the thyratron grids; and cathode... having a controlled, adjustable phase in relation to the phase of the thyratron-lamp circuit, to adjust the light intensity. Any inst-ability of this circuit may cause flickering or even unintentional sustained lighting. Transient spikes (Fig. 2) resulting in the power line from the abrupt firing of the thyratrons in an associated lighting circuit of the system, tend to produce such effects.

Rather than to insert bulky and costly chokes in series with the lamp loads where they must necessarily lose power, and where they are limited to lighting loads of a certain rating, I have isolated the thyratron input electrodes from the effects of the transients by a filter of such character as not to limit the range of the adjustable phase-shifter. Choke 2D is interposed between the input of the adjustable phase-control circuit 22 and the connections of the thyratron-lamp circuit to the supply line. The choke should not be so large as to limit the necessary range of the phase-shifter, but it can nevertheless be quite large. Additionally low-pass filter 24 is interposed between the output of the phase-control circuit 22 and the thyratron input electrodes. This filter is of special importance because of the unfiltered input that controls the phase-shift circuit, as will be seen. This filter is of the pi resistance-capacitance type, including resistor 25 and capacitors 28 and 30 for tube Hi. The resistor serves the further purpose of limiting the thyratron grid current in the intervals when the grid of tube I4 is positive in respect to its cathode. Tube l8 similarly has a pi-type low-pass filter 2411, including resistor 26a and condensers 28a'and 38a.

The particular phase-shifter shown is that disclosed in greater detail and claimed in my issued patent mentioned above. It includes a "resistive voltage divider 32, 3B in one circuit and a condenser 36 and a grid-controlled back-toback pair of vacuum tubes 38 in a second circuit paralleling the voltage divider. An output transformer 39 has its primary winding connected between intermediate points of these two circuits and has a separate secondary winding for each thyratron. An adjustable alternating-current source of grid-control voltage is provided for the back-to-back vacuum tubes, including potentiometer 40 and transformer 42 that has a separate secondary winding for each vacuum-tube grid. Varied input voltage from potentiometer it to the vacuum tube grids changes the plate resistances, and shifts the phase of the voltage to transformer 39. The vacuum tubes have their grids driven negative during half-cycles when the respective plates are positive, by proper phasing of the secondary windings of transformer 42.

The order of magnitude of the components in relation to each other is given in the following example: Choke 30, 7 henries; resistors 32 and 34, 5,000 ohms; resistors 26 and 26a, 50,000 ohms; condensers 28 and 28a, .004 mid; condenser 30 and 30a, .002 mfd; tube 38, 6SL7 or 5691; condenser 36, 0.25 mfd; resistor M, 50,000. The latter is a trimmer, to adjust the range of the phaseshift network.

A second lighting and control circuit, the dup'licate of that described, is shown in Fig. 1. This circuit has parts with primed numerals correspending to the parts in the circuit described, and

is connected to the same line terminals Ill, I2. The two lighting circuits will normally be adjusted differently (for different intensities of the two lamps l6 and I6) as may be required to designated master.

light a set on a theatre stage. Transients produced by one lighting circuit are not suppressed, but are permitted to reach the line. The transients are not allowed to affect the ultimate control because of the filters described that isolate the thyraton grid circuits from the thyratron load circuits. I have discovered that transients of this nature are damped in comparatively short feeder lines so that they do not noticeably affeet equipment outside the immediate lighting installation.

The two phase-shift networks are controlled separately by" individual potentiometers 40 and 40. These are to be pre-set in uses as described in my aforementioned patent, and are proportionally dimmed by a common potentiometer 46 It is notable that the master is energized directly by the power line where transients are to be expected to be troublesome; but such transients are attenuated in the phaseshift network, and whatever the extent that they may be transmitted through the phase-shift network, they are suppressed by the pi resistancecapacitance filters 24 and 24a.

What is claimed is:

1. An alternating-current lighting circuit including a lamp, a thyratron connected in control relation to said lamp and to power lines, a widerange phase-shift circuit connected to said lines, said phase-shift network including a tapped resistive voltage divider, an adjustable series resistive-capacitive phase-shift circuit in parallel with aid voltage divider, and a transformer having its primary winding connected between said resistive voltage divider and said phaseshift circuit, and a secondary winding connected to the control grid of said thyratron, a choke in the connection between said power line and said phase-shift network, and a resistor-capacitor low-pass filter in the connections between said secondary winding and said thyratron grid.

2. An alternating-current lighting system including two lamps to be separately controlled, a thyratron connected in control relation to each said lamp and to a power line, separate widerange phase-shift circuits connected to said power line and connected in control relation to said thyratrons, respectively, whereby transients resulting from the early firing of one thyratron appear in the power line, and a low-pass filter in the connection of each phase-shift network to said power line, and an additional low-pass filter in the connection of each said thyratron to said phase-shift network, for preventing transmission of the transients to the thyratron that is set for later firing.

3. An alternating-current lighting circuit having a lamp, a thyratron connected in control relation to said lamp, said circuit being energized by a power line, a grid-controlled vacuum tube and a condenser connected as a resistancecapacitance circuit in control relation to the thyratron grid and having energizing connections to said power line, an adjustable-amplitude input circuit for the grid of said vacuum tube energized directly by said power line, and separate low-pass filters in the connection of the power line to said phase-shift circuit and in the connection 'of 'said phase-shift circuit to said thyratron, respectively, whereby transients in the power line are prevented from reaching the phase-shift network from the power line, and are prevented from reaching the thyratron input electrodes through the phase-shift circuit.

4. An alternating-current lighting system including separate lighting circuits and lightingcontrol networks connected to a common power line, each lighting circuit including a thyratron connected in control relation to a lamp, whereby early firing of one thyratron produces transients in the power line to disturb the waveform available to the control network of the other thyratron, and low-pass filters between each control network and the power line and between each thyratron and its related control network, respectively.

5. An alternating-current lighting system including separate thyratron-controlled lighting circuits and lighting-control networks each including a resistive voltage-divider and an adjustable resistance-capacitance phase-shift circuit, said lighting circuits and said phase-shift networks having energizing connections to a common power line, whereby early firing of a thyratron in one lighting circuit produces transients in the power line, an input-filtering choke and an output-filtering resistancecapacitance network connected to each phaseshift network for preventing transients in the power line produced by one lighting circuit from tripping the thyratron in the other lighting circuit prematurely.

GEORGE C. IZENOUR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,819,105 Machlett et a1. Aug. 18, 1931 1,937,369 Willis Nov. 28, 1933 2,445,549 Wittenberg July 20, 1948 Izenour Mar. 1, 1949

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