US2850676A - Regulated filament supply - Google Patents

Description

Sept. 2, 1958 e. s. KAN EIAL REGULATED FILAMENT SUPPLY Filed NOV. 5, 1954 I 54 g I I ZNVENTORS WWI Ii V ZTOPA/EK? l flmiard United States Patent Cfiice 2,85ll,15?5 Patented Sept. 2, 1958 REGULATED FILAIVZENT SUPPLY George S. Kan and Bernard M. Oliver, Palo Alto, Calif., assignors to Hewlett=Packard Company, Pain Alto, Calif a corporation of California Application November 5, 1954, Serial No. 467,122

7 Claims. (Cl. 315-97) This invention relates to an electronic network for supplying regulated filament power.

It is well known that variations in a filament power vary the electron emission, and therefore, the current conducted by a vacuum tube. This affects the performance and characteristics of various electronic networks which incorporate a filament power supply for the vacuum tubes. For example, in an oscillator network, variations in the filament supply afiect the frequency of operation by influencing the alternating current flow through the tank circuit by modifying the plate cathode impedance of the vacuum tube or tubes employed. In electronic amplifying networks such changes vary the amplification for a given plate voltage.

It is conventional practice for electronic networks to include ballast tubes for regulating the filament power, such tubes being placed in series with the filament supply. As the current tends to vary, the filament temperature and resistance vary to effect current regulation. Such ballast tubes are subject to certain disadvantages. Normally the filament is operated within a temperature range at which it tends to become brittle. Therefore, the filament is susceptible to breakage by mechanical shock or vibration. The efiiciency is relatively low because such tubes insert a resistance in series with the current supply. Also the regulation is not as accurate or complete as is frequently desired. Special current regulating transformers have been used in place of ballast tubes, but they are not satisfactory for many types of equipments, due to their weight, size and frequency limitations.

In general, it is an object of the present invention to provide an electronic network having a novel regulated filament current supply incorporated in the same. In particular, the present invention is characterized by freedom from injury due to mechanical shock or vibration, good efficiency and good regulation.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been described in detail in conjunction with the accompanying drawings.

Referring to the drawings:

Figure l is a circuit diagram illustrating an electronic network incorporating one embodiment of the present invention;

Figure 2 is a circuit diagram illustrating an electronic network incorporating another embodiment of the present invention;

Figure 3 is a circuit diagram illustrating still another embodiment of the present invention.

The particular network illustrated in Figure 1 is a filament supply means for supplying a regulated current to the filaments (i. e. cathode heater) of vacuum tubes of associated circuits. The network illustrated includes two parts, namely, the part 11 which comprises the filament supply means, and the part 12 which shows a plurality 1 of tubes being supplied with regulated filament current. The current supply means 11 is a multivibrator of the vacuum tube type, making use of two vacuum tubes 13 and 14, each having plate, cathode and control grid element. By way of example, these tubes may consist of a single twin triode. A transformer 16 serves to couple the multivibrator to the filament circuit of the associated circuit 12. One terminal of the primary of transformer 16 is connected to the plate of tube 13, and the other terminal to the plate of tube 14. The remaining elements of the multivibrator circuit are such as to cause it to function as a square wave generator. Thus the cathodes of tubes 13 and 14 are grounded. The control grid of tube 13 is connected to ground through the resistor 22. Similarly, the control grid of tube 14 connects to ground through resistor 24. Condenser 26 con nects between the plate of tube 14 and the grid of tube 13. Condenser 27 is similarly connected between the plate of tube 13 and the grid of tube 14. The resistors 21 and 23 connect the plates to the positive terminal of the plate supply through line 19. The plate current supply unit can be of conventional construction, and includes conventional regulating means to maintain the plate voltage substantially constant.

As is well known to those familiar with construction and operation of multivibrators the two tubes 13 and 14 are alternately conducting and nonconducting, whereby the output of the transformer 16 is of square wave form, except for modification of the waveforms by the inductive character of the coupling. The frequency of operation of the multivibrator is determined by the time constants of the grid resistor-condenser arrangement employed. For the purpose of the present invention it is satisfactory to select a frequency of operation within the range of say, 50 to 5,000 C. P. S. It is apparent, of course, that by proper selection of the circuit elements, it is possible to operate the network at any desired frequency.

The part 12 shows the tubes 31 and 32 having filaments or cathodes heaters 33 and 34. These tubes might be, for example, in an oscillator network, where variations in the filament supply affect the frequency of oscillation, or in an electronic amplifying network where such changes vary the amplification for a given plate voltage. The filaments or cathode heaters are connected to the current supply means through the circuit which include the transformer 16. Thus one side of the filaments is grounded while the other is connected to one terminal of the secondary of the transformer 16, in series with the adjustable resistor 36. The other terminal of the transformer secondary is grounded.

Operation of the network illustrated in Figure 1 is as follows: The multivibrator circuit 11 operates to produce a square waveform of substantially constant amplitude, and this is employed to heat the filament of tube 31. The amount of filament power supplied can be adjusted by varying the setting of resistor 36. During the conduction period of either tube 13 or 14, only a small portion of the B-supply voltage appears across the tube, the remainder is supplied to the external load. As a consequence, a given. percentage change in the Voltage drop across the tubes, due to changes in the tube parameters, results in a greatly reduced percentage change in the output voltage. Thus the regulation of the filament supply is inherently that of the regulated B-supply which supplies the plate voltage to the multivibrator. During conduction the tube drop is low and the tube losses are at a minimum.

The operation and efficiency of the filament supply 11 is enhanced by employing the circuit 11a of Figure 2. The plate resistors 21 and 23 are replaced by transformer 46 having a center tapped primary 47. One end of the primary is connected to the plate of tube 13, the other to the plate of tube 14. I The center tap 47 is connected to the positive side of the plate supply voltage lead 19. The transformer 46 acts to transform the impedance of the filaments 33 and 34 et al. of the associatedrtubes to be suitable as a load in the plate circuit of thefilament supply-11a. In this manner only the power dissipated by the plate resistanceof the tube and'the gridiresisto'rs is wasted, whereas in the circuit of Figure 1 thegpower dissipated in resistors 21 and 23Was also wasted. The remaining elements of the rnultivibrator circuit are such that they cause it to function as a square Wave generator. The cathode of tubes 13 and '14 are grounded. The control grid of tube 1 3 is connected to groundthrough the resistors 48511. 49. Similarly,- the control grid of tube 14 is connected to ground through resistors 51 and 52. Condenser 37 connects the plate of tube '14 to the common uncuanf'brresistbrs 48 and 49. Condenser 38 connects between the plate 'oftub e 13 and the common junction or resistors. 51 and sz.

Resistors 48" ahd 51-opefate as follows: Assume that tube 14"has just" gene into conduction and tube 13 just out of conduction. The plate of tube 14 must drop from2E3 E to E' j wh'ei'e E, is the B su'pply voltage and E is the tube dro Thplate' o'f tube 13 must risefrom E to 2E ;'E, To permit this rise, condenser 38 must be charged. If resistor 'srvr 're'finrpie'sentthe charging would" benea'rly instantaneous. A high current surge would flow 'from'the grid to cathode of tube 14 since the grid to cathoder'esistance of this tube is'low. As' a result, by transformer action, a high plate current will flow in tube' 14. To supply this current a higher plate voltage would be required. Because, of the higher plate current and voltage, ahigher dissipation loss would result. By the addition of resistor 51, which limits the charging current, the charging action can take plate gradually over the firs t part of the half cycle. The peak plate voltage and current requirements are reduced, therefore, the plate dissipation is reduced.

Further, if the resistors 48 and 51 were not present the filament supply would have a tendency to act as a blocking oscillator. The capacitor 38' charging instantaneously would cause a higher instantaneous plate current to flow in'tube '14 which would rapidly drop ofi as the capacitor 33 became charged. Thus the input to the filaments 33 and 34' would be a pulse having relatively high amplitude and short duration. Resistor 51 prevents the capacitor from charging"instantaneously. Consequently'the input to f'the filaments is in creased because the inputis substantially of the square wave type. Resistor 48 serves the's ame purpose'as res'i'stor' 51' when the tube 13 has gone" into conduction.

As has beer'fdescribed' previously, the tubes 13 and 14 i R48R49 l 52 v R48+R49 R51+Rsz line-re s'If ss 4s= 51 and 49= 52- It an be shown that the energy stored in' the capacitors 37 and 38 is dissipated in the resistors and may be expressed as where E'is the voltage on' the capacitor, C the capacitance and f the frequency of the niultivibrator. Thus if C can be reduced and the time constant maintained the efliciency of the filament supply will be further increased.

The circuit of Figure 3 achieves this result. The time constant for charging the capacitors 37 and 38 is now expressed as vibrator 11 can be-selected such that the transformers 16 or 46 arerelatively light in weight and'of small; size.

'Further, thefrequency may be selected as not to affect.

the circuit for which the filament power is supplied.

The entire filament supply unit can be made relatively compact and light-weight, and can'be incorporated as an integral part of'the equipment.

As previously explained the invention is applicable to varioustypesof electronic networks. Thus, it 'caube used with amplifiers, counting circuits and the like.

In one particular instance, the multivibrator circuit 11 shown in Figure 3"was constructed as follows:

Tubes '13"a'nd"14 5687 Condenser '37 af 0.01 condens r; 38""; ,uf 0 .01 Resistori49 kn '33 Resistor 52 kSZ I 33 Resistor 53 :kS2 f Resistor 54 kQ- 68 The transformer 46 had a turns ratio of 2320:48. The

frequency'of the multivibrator 11 was 500 C. P; S.

For normal operation the average power supplied to the filament of tube 31 was 3 watts. The plate eificiency of this apparatus was about 50 percent. A variation of 30 percent in the voltage on the current supply lines supplying the filament of the multivibrator resultedin a filament voltage variation of the tube 31 of less than l percent.

We claim: 4

1. In an electronicnetwork of the type including a vacuum tube having aheated cathode filament, and having the plate of a tube connected to a direct current source, a square wave-generator of the multivibrator type,

said'square wave generator including vacuum tubes having their plates supplied with direct current from said source, and means for coupling the filament of the first named tube to the output of said multivibrator.

2. In an electronic network including at least one vacuum tube having a cathode and having a plate connected to a source of direct current, a square wave generator of the multivibrator type, said generator including two vacuum tubes having their plates connected to said source through plate impedances, and means for coupling the output of the multivibratorto the filament of the first named tube.

3. Apparatus as in claim 2 in which the coupling means comprises a transformer having its primary connected between the plates of said vacuum tubes, and the secondary connected to the filament of the first named tube.

4. Apparatus as in claim 2 in which the coupling means and plate impedances comprises a transformer having a center tapped primary connected between theplates of said two vacuum tubes with the center tap connected to the said direct current source and providing plate current to said tubes through the transformer impedance, and

7 thesecondary of said transformer connected to supply the filament power for said first named tube.

5 In an electronic network including at least one vacuum tube having a cathode and having a plate connected to a source of direct current, a square wave generator of the multivibrator type, said generator including two vacuum tubes having their plates connected to said source through plate impedances and having current limiting resistors connected in series with the plate to grid coupling, and means for coupling the output of the multivibrator to the filament of the first named tube.

6. Apparatus as in claim 5 in which the said coupling means and plate impedances consists of a transformer having a divided primary, one-half of the primary being in the plate circuit of one of the multivibrator tubes and the other being in the plate circuit of the other tube.

7. In an electronic network including at least one vacuum tube having a cathode and plate connected to a source of direct current, a square Wave generator of the multi-vibrator type, said generator including a pair of vacuum tubes having at least plate, grid and cathode elements, a non-resonant transformer having a center tapped primary connected between the plates, the center tap of said primary being connected to said source, a capacitor and current limiting resistor connected in series and in the plate-to-grid path of each of said tubes, and the secondary of said transformer connected to supply the filament power to said first named tube.

References Cited in the tile of this patent UNITED STATES PATENTS 2,128,117 Braband Aug. 23, 1938 2,130,441 Wohlfarth et al Sept. 20, 1938 2,140,707 Lee Dec. 20, 1938 2,149,090 Wolfi Feb. 28, 1939 2,262,044 Philpott Nov. 11, 1941 2,302,900 Vance Nov. 24, 1942

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