US2690524A - Protective circuit against cathodeto-heater breakdown - Google Patents

Description

Sept 28, 1954 J. l.. MARsEY :TAL 2,690,524

PROTECTIVE CIRCUIT AGAINST CATHODE-TO-HEATER BREKDOWN Filed Oct. 30. 1951 M55 vanaaf Regnum/e @qumrfo 1 B+ /4 l El (Haar) IZ S /2 7V 27K 0% j a, k 33 F/q 1 g .sz

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F] 2 BY 7710/77/45 J.' Ry/qf) 9T' @wwuw v Patented Sept. 28, 1954 PROTECTIVE CIRCUIT AGAINST CATHODE- 'IO-HEATER BREAKDOWN .lohn L. Marsey, Philadelphia, and rlhomas J.

Ryan, Langhorne, Pa., assignors to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application ctoller 30, 1951, Serial No. 254,784

6 Claims. l

The invention herein described and claimed relates to electronic voltage regulators. More specifically, the present invention relates to an improvement in electronic voltage regulators whereby the tubes of the regulator circuit, particularly though not necessarily miniature type tubes, are protected against failure due to voltage breakdown between cathode and heater.

As is well known, the conventional electronic voltage regulator usually comprises three tubes, one a cold-cathode gas-lled tube and the other two hot-cathode vacuum tubes. One of the hotcathode tubes functions as a variable resistance in series with the load current. This tube is sometimes called the series tube but will be referred to in this specification as the pass tube. The other hot-cathode tube, the control tube, functions to reverse the phase of, and to amplify, small changes in the D.C. output voltage. The output of the control tube is applied to the grid of the pass tube to change the plate resistance of the pass tube in response to changes in the D.C. output voltage, thereby to maintain the output voltage substantially constant. The coldcathode gas-filled tube is used to hold the cathode of the control tube at a constant reference potential.

Within recent years, the voltage regulator has become a virtually indispensible feature of many electronic equipments. By means of the voltage regulator, oscillators are kept from straying over the spectrum, reference voltages are held constant Within rigid limits, and grid biases are given a high degree of stability. Unfortunately, the conventional electronic voltage regulator is bulky, and in many instances requires more space than can be allotted to it. The use of miniature tubes would, of course, conserve space, but unless separate windings be provided on the heater transformer for supplying separately the heater of each tube, the miniature tubes will burn out very quickly, if not instantly, due to voltage breakdown between cathode and heater, the cathode-to-heater voltage ratings of. miniature tubes being substantially lower than those of the octal type tubes.

The object, therefore, of the present invention is to provide an electronic voltage regulator having protective means for preventing the tubes of the regulator circuit, particularly miniature type tubes, from burning out as a result of voltage breakdown between cathode and heater, though theV heaters be supplied from a common source.

A more specic object of the present invention is to provide a vest-pocket type electronic volt- Cil age regulator employing miniature type tubes whose heaters are supplied from a common source, said regulator to be capable of regulating a voltage of substantial magnitude across a substantial current load, as for example, to be capable of regulating a B+ voltage of several hun- -dred volts across a current load of 50 milliamperes or more.

In accordance with the present invention, the above objects are attained by the provision of means, within the voltage regulator, for maintaining the potential of the heaters of both of the hot-cathode tubes at a value which is intermediate the potentials of the cathodes of the two tubes, thereby to insurethat ythe cathode-to-v vbetween the potentials of the cathodes of the two hot-cathode tubes. During warm-up, the potential of the control-tube cathode is substantially higher than that of the pass-tube cathode, whereas the reverse is true under operating conditions, the potential of the pass-tube cathode during normal. operation being substantially higher than that of the control-tube cathode. The arrangement provided by the present invention is nevertheless effective to place the heater potentials between the-potentials of the two cathodes during both periods, i. e. during both the warm-up and normal-operation periods. During the warm-up period, the potential of the controltube cathode rises toward the unregulated B+ voltage until the reference tube res and thereafter is maintained. at a voltage determined by the operating voltage of the reference tube. The potential of the pass-tube cathode during the warm-up period is, in accordance with the present invention, caused to follow the potential of the control-tube cathode and to bea fractional part thereof as determined by a voltage-divider network. The potential of the heaters, during the warm-up period, is caused to fall between the potentials of the two oathodes. Following the warm-up period, under operating conditions, the cathode ofthe passv tube rises to, and is held at, the regulated B+ potential, while the cathode of the control tube is held at the reference voltagev `determined. 'by the referencev tube. The differencev between these potentials, i. e., the potential difference between the two cathodes during normal operation is ordinarily quite large and substantially greater than the cathode-to-heater voltage rating of either tube. However, the potential difference between the two cathodes will ordinarily be less than twice the cathode-to-heater voltage ratings. Hence, the present invention, by returning the heaters to a voltage located between, preferably about midway between, the potentials of the two cathodes, holds the cathode-to-heater potential of each tube within the voltage rating of the tube.

The invention will be most readily understood from a consideration of the following detailed description taken together with the accompanying drawing wherein:

Figure 1 is a schematic diagram of a vestpocket type of electronic voltage regulator' employing miniature tubes and embodying the circuit arrangement of the present invention, and

Figure 2 is a graphical illustration which will be helpful in explaining and understanding the invention.

Referring now to Figure 1 there is shown an electronic voltage regulator to which a positive B+ voltage is supplied from power supply i0. The regulator shown is largely conventional except for the employment of miniature type tubes and except for the modifications in circuitry introduced by the present invention and shown in heavy lines in the drawing. The regulator shown includes the cold-cathode gas-filled reference tube I2, the hot-cathode control tube Id, and the hot-cathode pass tube I5. Reference tube I2 may be a miniature type voltage regulator tube, type CB2; control tube Ill may be a miniature type pentode, type GAUS and pass tube I5 may be a miniature type beam power amplifier, type 6AS5. If these tubes were used in a voltage regulator of conventional circuitry (the light lines in Figure 1), the 6AU6 control tube I4 would burn out within a few seconds after the power switch was thrown due to voltage breakdown between cathode and heater. Investigation and analysis of such breakdown indicates the following explanation: It requires about three seconds for the rectiers of power supply I0 to heat up after the switch is thrown. During this time there is no D.C. current flowing in the regulator circuit. As soon as the rectiiiers of the power supply l0 begin to conduct (at the end of about three seconds) the potentials of the plate and cathode of the 6AU6 control tube Ill start to rise, the plate being connected to the unregulated B+ lead I8 by way of the resistor I9, and the cathode being connected thereto by way of resistor 20. The OB2 reference tube I2 remains in non conductive state. The plate and cathode of tube I4 continue to rise, as the unregulated B+ voltage builds up until, by the end of about four and a half seconds, the cathode reaches +135 Volts, the starting voltage of the OB2 reference tube I2. The OB2 tube then fires, and the control-tube cathode drops to +108 volts, the operating voltage of the OB2 tube. During this time, the heaters of the GAUG control tube I4 and of the 6AS5 pass tube I5 are still at ground potential, due to the fact that the center-tap of the heater transformer I1 is connected directly to the cathode of the pass tube I5, and the pass tube does not begin to conduct until at least five seconds have elapsed. Consequently, at an instant approximately four and one-half seconds after the power switch is closed, the cathode of the 6AU6 control tube Ill is at a potential of +135 volts while the heater of the tube I4 4 is still at ground potential. And, since the 6AU6 tube has a cathode-to-heater voltage rating of only volts, the tube burns out.

If, in an effort to correct the above situation, the center tap of the heater transformer I'I be connected directly to the cathode of the 6AU6 control tube I4 instead of to the cathode of the 6.1135 pass tube I5, the BAUS control tube would be safe, when the power switch was thrown, for the cathode and heater elements would then be at substantially the same potential. The 6AS5 pass tube, however, would quickly burn out. The explanation in this case is that the heater of the 6AS5 pass tube I5 would, along with the cathode of the 6AU6 control tube I4, rise in about four and a half seconds to the striking potential of the CB2 reference tube I2 (+135 volts) while the cathode of the 6AS5 pass tube I5 was still at ground potential. And, since the cathode-toheater voltage rating of the 6AS5 tube is +90 volts, the tube would burn out. Even if the 6AS5 tube were fortunate enough to survive the warm-up period, it would quickly burn out under operating conditions, for during normal operation the pass-tube cathode is at the regulated B+ potential (+250 volts in the present example) whereas the heater is at the reference voltage (+108 volts).

To solve the problem hereinabove indicated, the present invention proposes to connect a voltage divider network between the cathode of the control tube I4 and the cathode of the pass tube I5, and to connect the common source of heater potential to a point on the voltage-divider network. In Figure 1, the voltage-divider network is shown to comprise resistors 3I, 32 and 33, with their junction (point a) being connected to the center tap of the secondary winding of heatertransformer I1. The employment of resistor 32 (between point a and ground) is optional this resistor may be omitted, if desired. By properly choosing the values of resistors 3| and 33 (and of 32 where used) the potential of the heaters (point a in Figure l) can be caused to lie between the potentials of the cathodes of the control and pass tubes, both during the warm-up period and under normal operating conditions. Typical (but not limiting) values for resistors 3I, 32 and 33 are shown in Figure 1. It will 'oe understood that if resistor 32 be omitted, the values of resistors SI and 33 will be different than shown in the drawing. In a typical case, with resistor 32 omitted, resistors 3| and 33 are of equal value.

Consider now the operation of the voltage regulator when the particular voltage-divider network shown in Figure 1 is incorporated therein. As the unregulated B+ voltage supplied by the power supply I0 begins to build up (three seconds after the power switch is closed), the potential of the cathode of the 6AU6 control tube I4 rises in corresponding manner, as a result of being connected to the voltage divider at point b, until, at the end of about four and a half seconds, the cathode reaches volts, the ring potential of the CB2 reference tube I2. When the OB2 tube fires, the cathode of the GAUG tube I4 quickly drops to +108 volts, the operating voltage of the OB2 tube. 'I'his is depicted graphically in Figure 2. As the cathode of control tube I 4 rises toward +135 volts, the potential of the heaters (point a) rises in a corresponding and proportional manner, determined by the voltage-divider resistors 3I and 32 and the resistance which is in shunt with resistor 32, namely, the resistance of elements sa, 23, 24 and 25, and the resistance of the output load (not shown). Since the pass tube l is not yet conducting, the potential of its cathode (point c) follows that of the heaters (point a), being a fractional part thereof as a result of the voltage-divider action of resistor 33 in combinaticn'with the output load resistors 23, 24 and 25, and the resistance of the output load (not shown). After the DB2 reference tube l2 fires, the potential of the heaters (point a) and the potential of the pass-tube cathode (point c) are maintained at different fractions of the reference voltage (+108 volts) until the 6AS5 pass tube l5 begins to conduct. This occurs at the expiration of about nine seconds following throwing of the power switch. The potential of the pass-tube cathode (point c) then rises rapidly to the regulated B+ value (+250 volts in the present illustration), but the potential of the cathode of the control tube (point b) remains at the reference voltage (+108 volts). The heaters (point a) as a result of the voltagedivider action of resistors 3l, 32 and 33, then assume a potential between the potentials obtaining at the two cathodes (points b and c). All this is clearly illustrated in Figure 2 of the drawing. As there shown, during the entire warm-up period i. e. both before and after the reference tube res, the potential of the heaters is between the potentials of the two cathodes. Moreover, the cathode-to-heater potential of each tube is well within the tubes voltage rating of +90 volts. For example, just before the CB2 reference tube fires, the control-tube cathode is at +135 volts, the heaters are at +45 volts, and the pass-tube cathode is at +35 volts. After the reference tube fires, the control-tube cathode is at +108 volts, the heaters are at +37 Volts, and the passtube cathode is at +30 volts. Following the warm-up period, the relative positions of the cathode potentials of the pass and control tubes are reversed, i. e. the pass-tube cathode rises to a potential substantially higher than that of the control-tube cathode. Nevertheless, as a result of the voltage-divider action of resistors 3i, 32 and 33, the potential of the heaters changes in such direction and to such extent as to remain between the potential of the pass-tube cathode and that of the control-tube cathode. In the illustration shown, the potential of the heaters, following warm-up and during normal operation, is +175 volts, the potential of the pass-tube cathode is +250 volts, and the potential of the control-tube cathode is +108 volts. It will be seen then that the cathode-to-heater potential of each tube is within the tubes voltage rating of +90 volts during normal operation as well as during the warm-up period.

A vest-pocket type electronic Voltage regulator employing the miniature tubes and circuitry shown in Figure 1 has been successfully built and used. The completed chassis is but a trie larger than a pack of cigarettes. It produces a +250 volt regulated output from a +300 volt supply, the voltage drop across the 6AS5 pass tube being about 50 volts at 50 milliamperes. It regulates a 50 rnilliampere load at 250 volts very well, allowing variations of plus or minus one-tenth volt. If a current in excess of 75 milliamperes is to be regulated, two or more 6AS5 pass tubes can be connected in parallel with small resistors in the plate and grid leads to discourage parasitics and to insure proper division of the load.

The electronic voltage regulator shown in Figure 1 and discussed thus far can also be used as a very effective supressor of hum voltage. Since the superposition of hum voltage upon a D.C. voltage merely changes the amplitude of the output voltage at a regular rate, and since an electronic voltage regulator is designed to suppress changes in the amplitude of the output voltage, it will be seen that a voltage regulator can be used to supplement the action of the powersupply filter in suppressing hum. In order, however, to take full advantage of the hum-suppressing capabilities of the voltage regulator, any hum Voltage present on the regulated B+ lead should be applied to the grid of the control tube with a minimum of attenuation. This is the function of capacitor 26 shown in Figure l of the drawing. A portion of any D.-C. variations in the regulated B+ voltage is applied to the controltube grid by means of the voltage divider consisting of resistors 23, 24 and 25, but a much larger portion of any hum voltage appearing on the regulated B+ lead is applied to the controltube grid by means of the voltage divider comprising capacitor 26, resistor 2l, the lower part of resistor 24, and resistor 25, thus making the voltage regulator very effective as a suppressor of hum. In practice, it has been found that even though the unregulated B+ voltage from the power supply has a peak ripple of as much as 1.5 volts, which is unusually strong, the regulated B+ voltage will have a peak ripple of only J() VOl.

Having described our invention, we claim:

l. In an electrical circuit employing at least two heater-type tubes, the heaters of which are supplied from a common source of heater supply voltage and the cathodes of which are operated at substantially different voltages, the difference between said cathode voltages being greater than the cathode-to-heater voltage ratings of said tubes; a voltage divider connecting said cathodes; and a connection from said common source of said heater supply voltage to an intermediate point on said voltage divider, thereby to maintain said heaters at a potential intermediate the potentials of said cathodes and thereby to prevent cathode-to-heater breakdown.

2. Apparatus as claimed in claim 1 characterized by the fact that said common source of heater supply voltage comprises a transformer having a center-tapped secondary to the ends of which said heaters are connected, and further characterized by the fact that said center tap is connected to said intermediate point on said voltage divider.

3. Apparatus as claimed in claim 2 further characterized by the fact that a resistor is connected from said intermediate point on said voltage divider to a point of fixed reference potential.

4. In a voltage regulator circuit having: a source of positive direct-current voltage, rst and second tubes each having at least anode and cathode electrodes and each having a heater for its cathode, a common source of heater voltage, means connecting said common source of heater voltage to the heaters of both said tubes, a pair of output terminals, means connecting a high potential side of said source to the anode of said first tube, means connecting the cathode of said rst tube to one of said output terminals, a nrst impedance connecting at high potential side of said source to the anode of said second tube, a second impedance connecting a high potential side of said source to the cathode of said second tube, and a gas-filled voltage-regulator tube connected between the cathode of said second tube and a low potential side of said source for maintaining the cathode of said second tube, in the period after said voltage-regulator tube res, at

a fixed potential substantially less positive than the potential of the cathode of said i'irst tube, and in which the potential of said second-tube cathode, in the period prior to the firing of said Voltage-regulator tube, attains a potential substantially more positive than the potential of said iirst-tube cathode, and in which the difference between the potentials of said cathodes in at least one of said periods exceeds the cathode-to-heater voltage ratings of said tubes; the improvement which comprises the provision of a vo1tage divider connected between the cathodes of said rst and second tubes, and means connecting said common source of heater voltage to an intermediate point on said voltage divider to maintain said heaters at a potential intermediate the cathode potentials of said rst and second tubes in the periods both before and after such voltage-regulator tube res.

5. Apparatus as claimed in claim 4, characterized by the fact that said common source of heater supply voltage comprisesa transformer having a center-tapped secondary to the ends of which said heaters are connected, and further characterized by the fact that said center tap is connected to said intermediate point on said voltage divider.

6. Apparatus as claimed in claim 5, further characterized by the fact that a resistor is connected from said intermediate point on said voltage divider to said low-potential side of said source of direct-current voltage.

References Cited in the le 0f this patent UNITED STATES PATENTS Number

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