US2310054A - Protection for bias filter condensers - Google Patents

Description

Feb. 2, 1943. J. E. BAUDINO 2,310,054

PROTECTION FOR BIAS FILTER CONDENSERS Filed July 15, 1941 l pm wr 4 7b AGupp/g/ Peczz'fz'ad i kizar'ed Power Supply WITNESSES: INVENTOR Jbsep/z EBaz/dma Patented Feb. 2, 1943 PROTECTION FOR BIAS FILTER CONDENSERS Joseph E. Baudino, Merion, Pa., assignor to Westinghouse Electric & Manuiacturing Company,

a corporation of Pennsyl- East Pittsburgh, Pa.,

vania Application July 15, 1941, Serial No. 402,486 6 Claims. (Cl. 179-171) This invention relates to protective systems and, more particularly, to protective circuits preventing overvoltage in certain portions of electrical transmission networks.

The invention finds particular application in transmission systems, such as amplifiers, for preventing the damage which overvoltages may cause to components endangered by surges far in excess to the normal operating potentials to which they are intended to be exposed.

In the operation of a high power vacuum tube as in the power amplifier or high level class B modulation amplifier of a radio transmitter, the grid circuit is sometimes momentarily subjected to excessively high voltage due to fiashovers from the anode. These fiashovers may be due to several causes, such as overmodulation or the operating characteristics of the tubes themselves. The grid circuit is usually not designed to withstand the high voltage so produced and a breakdown of the low voltage large capacity condensers in the grid bias filter usually follows. A breakdown of this type seriously interferes with the normal operation of the transmitter.

A particular feature of this invention is that conditions of overvoltage to which an impedance element, such, for example, as the filter condenser above mentioned may be subjected, is prevented automatically and to this end means are provided for forming a current path of high conductivity in shunt with the impedance whenever the voltage rises beyond a certain predetermined value.

A particular advantage of this invention is that the current path above referred to has practically infinite impedance at the normal operating voltage appearing across the terminals of the impedance, whereas when the voltage exceeds a certain predetermined magnitude the protective current path has a very low resistance.

Another feature of this invention is that aside from the very high resistance of the shunt circuit at the normal operating voltage, the current conductivity therein depends also on the polarity of the operating voltage. In other words, conductivity depends on two factors, namely, the magnitude of the voltage and the direction of the current flow.

In its broader aspect, the invention comprises a shunt path across an impedance comprising two elements in series, one of which has a relatively high resistance with respect to said impedance at the normal operating voltage and a relatively low resistance therebeyond and the other having a relatively high resistance in the direction of current flow under normal operating conditions and a relatively low resistance in the direction of current fiow opposite to that of normal operating conditions.

Other features and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims and taken in connection with the accompanying drawing, in which the single figure illustrates the application of the invention to a high power vacuum tube amplifier.

While the invention is applicable to all types of electrical systems in which it is desired to prevent overvoltage, it is illustrated here by way of example in connection with a bias voltage circuit for an amplifier stage which may be a portion of a radio transmitter.

The protective control system shown in the figure is associated with the input circuit of an amplifier system of the push-pull type which finds wide application in radio transmitters for the amplification of modulation frequency energy. Certain stages of an amplifier of this type, particularly the final output stage, operates as a class B amplifier, requiring a compartively large negative bias voltage for the grid circuit of the tubes. When the output of the amplifier utilizes high power tubes, the bias voltage for the grid circuit may reach several hundred volts. This voltage is generally derived from a separate power supply circuit of the alternating current rectifier type including a filter for eliminating the pulsating form of the rectified current. The filter includes generally an inductance and a capacity, the latter shunting the load resistance of the rectifier, which is usually in the grid circuit of the amplifying stage. In practice, it was often experienced that the filter condenser mentioned shorted due to sudden high voltage caused by a fiashover from the plate of the amplifier tube. While such condensers are designed within safe limits for the articular service intended, they will not withstand such voltage surges. The protective circuit of this invention for preventing the application of excessive voltages includes a series network of a gaseous discharge tube and a rectifier effectively in parallel with the load resistance of the bias supply rectifier.

Referring to the figure, the push-pull amplifier stage comprises vacuum tubes i and 2, each including, respectively, anodes 3 and 3', cathodes 4 and 4' and grid electrodes 5 and 5'. The input circuit between grid and cathode of each tube includes the coupling condenser 8 and 8', the input reactors I and I, and the load resistances 8 and 8'. The reactor 7. and the load resistor 8 are in series between grid 5 and cathode 4 of tube l. The elements marked with primary indices indicate similar connections with respect to tube 2. A bias supply rectifier for the grid circuit of tube l comprises the rectifier tube IS in a full wave rectifying circuit arrangement whereby the anodes II and I! are connected across the terminals of the secondary winding l8 of the power transformer is indicated to be connected to the alternating current supply line. The center tap l6 of the winding l8 connects to the junction point of the resistor 8 and the reactor 1. The resistor 8 is by-passed by condenser l8. The circuit of the rectifier is completed to the cathode I!) through conductor 28 which is at ground or cathode potential with respect to the amplifier tubes. The bias supply for the tube 2 is similar in all its details to the one just described, the components thereof being indicated with identical reference characters bearing primary indices. The signal input to the amplifier tubes may comprise any suitable source of signal voltage not shown here, which may be connected to the terminals 25, 25' and the common terminal 26. The connection of these terminals is marked as being to the exclter voltage input.

The output circuit between anode 3 of tube i and anode 3' of tube 2, comprises the center tapped primary winding 28 of the output transformer 29. The secondary winding 38 serves as the output source of the amplifier.

The anode voltage source for the amplifier is indicated here by a portion of the rectified power supply comprising the reactor 32 in the positive side of the voltage source, and filter condensers 33 and 34 connected between terminals of the reactor 32 and ground.

The protective circuit comprises two elements, namely, a gaseous discharge device 48 and a rectifier 4| connected in series across the terminals of condenser l8 and resistor 8. The discharge device 48 has a plurality of electrodes of which 42 and 43 may be connected in parallel and joined by means of conductor 44 with the junction point of the reactor I, the resistor 8 and the condenser 18. Another electrode 45 of the discharge device 48 connects to the anode 46 of the rectifier 4|. The cathode 41 thereof is at ground potential being connected also to the junction point of resistors 8 and 8'. In the other half of the push-pull input circuit, a second discharge device 48' is connected in a manner similar to the one aforementioned. Electrodes 42 and 43 are connected by means of conductor 44' to the junction point of reactor 1', resistor 8', and condenser l8' and the third electrode 45' of the device 48' is connected by means of conductor 48 to the electrode 45 and also the anode 46.

The protective circuit includes, with respect to each filter condenser, current conductive elements which form a series parallel circuit with the two condensers l8 and I8, namely, the discharge devices 48 and 48 and a common current conductive circuit between each series branch comprising the rectifier 4|. The discharge devices 48 and 48' are of the type known in the art as protector tubes being non-directional as to current conductivity and exhibit infinitely high impedance below a certain threshold value of potential impressed between any of the electrodes. Above a critical value, the gas dis- I4, the primary I5 of which 7 asiaooa charge reduces the internal resistance between electrodes to a very low value in comparison with the infinitely high value below the critical potential. Gaseous discharge tubes of this type are described in Patent 1,649,035 to McCoy and are known as, for example, the Westinghouse type KX642 tubes.

Referring to the operation of the system, normally no current is passing through the discharge device 48 or 48' nor the rectifier 4! due to the fact that the voltage across each of the resistors 8 and 8' is normally of such value which is below the breakdown value of the discharge tubes 48 or 48. Furthermore, the rectifier 4| is connected in such polarity as to be non-conductive in the direction of current fiow under normal operating voltage conditions. The useful function of the rectifier 4i is to prevent current conductivity in the shunt path when the normal operating voltage across the condenser l8 or I8 is of such value which would be sumcient to cause a breakdown between the electrodes of the discharge device 48 or 48. 'In other words, current conductivity is assured only, in this particular case, when the breakdown is due not only to excessive voltage but also to a voltage of opposite polarity to the one which each bias source produces. The polarity of the voltage on the grid circuit due to a fiashover is positive and is opposite to the negative bias voltage appearing across the resistors 8 or 8. Whenever such a fiashover occurs, the voltage across the bias filter condenser i8 or l8 increases and when it reaches the breakdown voltage characteristics of the discharge tubes 48 or 48, a breakdown occurs. The polarity is now correct for conduction through the rectifier since the anode 46 will be positive with respect to the cathode 41 and the rectifier 41 as well as the tubes 48 and 48' now form a conductive path of low resistance for the current due to excess voltage. In this manner, the fiashover current has a low impedance current path to ground. When the fiashover voltage has been removed from the grid circuit, that is, when normal operation is established, the protective circuit clears itself and then remains at high resistance until the next fiashover.

I claim as my invention:

1. In an overvoltage protective system, an impedance subjected to a unidirectional potential difierence at predetermined polarity and normal operating value, a circuit associated with said impedance from which excessive voltage magnitudes at reverse polarity are likely to be impressed thereacross, means for preventing a rise of voltage across said impedance beyond said normal operating value comprising a current path shunting said impedance including in series an element having infinite resistance below a threshold value of voltage and relatively low resistance therebeyond, and another element having infinite resistance at said operating polarity and relatively low resistance at the reverse of said polarity.

2. In an overvoltage protective system, an impedance subjected to a unidirectional potential difierence at predetermined polarity and normal operating value, a circuit associated with said impedance from which excessive voltage magnitudes at reverse polarity are likely to be impressed thereacross, means for preventing a rise of voltage across said impedance beyond said normal operating value comprising a current path shunting saidimpedance including in series an element having a relatively high resistance with respect to said impedance at said operating voltage and a resistance much lower than said impedance above said voltage, and a second element having a relatively high resistance with respect to said impedance at said operating polarity and a resistance much lower than said impedance at the reverse of said polarity.

3. In an overvoltage protective system, an impedance subjected to a unidirectional potential difference at predetermined polarity and normal operating value, a circuit associated with said impedance from which excessive voltage magnitudes at reverse polarity are likely to be impressed thereacross, means for preventing a rise of voltage across said impedance beyond said normal operating value comprising a current path shunting said impedance including in series a gaseous discharge device having infinite resistance below a threshold value 01' voltage and relatively low resistance therebeyond and a rectifier connected in conductive relation at the reverse of said polarity having infinite resistance at said operating polarity and relatively low resistance at the reverse of said polarity.

4. In an overvoltage protective system, a pair of impedances in series each subjected to a unidirectional potential difierence at predetermined polarity and opposing with respect to each other at a normal operating value, circuits associated with each of said impedances from which excessive voltage magnitudes at reverse polarity with respect to the normal operating polarity for each of said impedances are likely to be impressed across said impedances, respectively, means for preventing a rise of voltage across said impedances comprising a current path interconnecting said series impedances and including as elements a pair of gaseous discharge devices in series, said impedances in said devices forming a series-parallel circuit and a rectifier forming a current conductive path common to both said impedances connected between junction points of said elements and said impedances.

5. In an overvoltage protective system, a pair of impedances in series each subjected to a unidirectional potential diflerence at predetermined polarity and opposing with respect to each other at a normal operating value, circuits associated with each of said impedances from which excessive voltage magnitudes at reverse polarity with respect to the normal operating polarity for each of said impedances arelikely to be impressed across said impedances, respectively, means for preventing a rise of voltage across said impedances comprising a current path interconnecting said seriesimpedances and including as elements a pair of gaseous discharge devices in series, said impedances in said devices forming a series-parallel circuit and a rectifier forming a current conductive path common to both said impedances connected between junction points of said elements and said impedances, comprising a current path shunting each of said impedances including at least one gaseous discharge device for each path and a rectifier common to both of said current paths connected in conductive relation with respect to the polarity of operating voltage across one of said impedances.

6. In an amplifier system, an amplifier stage of the push-pull type including a pair of electron discharge devices each having anode, cathode and grid electrodes, respectively, a divided output circuit between said anodes and cathodes, a divided input circuit between said grid and cathodes, said last-mentioned circuit including in series between grid electrodes and cathodes of each of said devices an impedance and a resistance, respectively, a source of bias voltage connected across the terminals of each of said resistances, a filter circuit therebetween including an impedance in shunt with each of said resistances in the form of a filter condenser, means forv preventing excessive voltage magnitudes of a reverse polarity to that supplied by said source to be impressed across said condensers due to fiashover between said anodes and grid'electrodes comprising a shunt circuit for each of said condensers including a gaseous discharge device in each of said circuits and a rectifier common to both of such shunt circuits.

JOSEPH E. BAUDINO.

Images