US3423641A

US3423641A - Hammer firing circuit for impact printers

Info

Publication number: US3423641A
Application number: US711455A
Authority: US
Inventors: John L Von Feldt
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1968-03-07
Filing date: 1968-03-07
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21

Description

Jan. 21, 1969 J. 1.. VON FELDT HAMMER FIRING CIRCUIT FOR IMPACT PRINTERS Original Filed Nov. 22, 1965 Sheet 4 of 5 HOLD RESET FIG; 1

1 PRINT CYCLE (2.3 MILLISECONDS) Io sEQAJ-L 40o FSEC. 3

MEMORY SCAN (1.5 MILLISEC.)

L1NE137 +fl' M /N VE N 70/? LINE 7 T D L E F N O V L N H 0 J LINE 107 Jan. 21, 1969 J. 1.. VON FELDT 3,423,641

HAMMER FIRING CIRCUIT FOR IMPACT PRINTERS Sheet 2 of 5 I; g SELECTION CIRCUIT g SELECTION cmcun SELECTION cmcun Fee. 2

Jan. 21, 1969 J. L. VON FELDT HAMMER FIRING CIRCUIT FOR IMPACT PRINTERS Original Filed Nov. 22, 1965 Sheet 3 of 5 I I I I I I i I I L.

United States Patent 3,423,641 HAMMER FIRING 'CIRCUIT FOR IMPACT PRINTERS John L. Von Feldt, Rochester, Minn., assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Continuation of application Ser. No. 509,076, Nov. 22, 1965. This application Mar. 7, 1968, Ser. No. 711,455 U.S. Cl. 317-437 5 Claims Int. Cl. H0111 47/32 This application is a continuation of Ser. No. 509,076, filed Nov. 22, 1965, now abandoned.

This invention relates to an improved circuit arrangement for controlling the selective actuation of the hammers in high speed impact printers.

One of the primary output devices of the central processing unit of a data processing system is the high speed printer. These printers are usually of the impact type, that is, a hammer mechanism is caused to move at high speed against a recording medium to cause a selected character (i.e. numeral, letter or special symbol) to 'be imprinted upon the medium.

Typically, there is one hammer in each print position for one line of printing on the medium. Type elements which cause the imprinting of their respective characters on the medium are selectively moved into the print positions. When the desired type element is in a print position, the respective hammer is actuated to cause imprinting. One print cycle is required to move the type elements into a set of print positions, to determine whether each element carries the character which is to be printed in the respective position, and to fire the hammer.

The hammer is usually either of two types, i.e. that in which the hammer is driven directly by electromagnetic means or alternatively, that which is biased by a mechanical spring for movement against the medium incident to a holding means being rendered ineffective by electromagnetic means. In either type, an electromagnetic coil is selectively energized to cause the movement of the hammer.

The circuit of the present application has been particularly adapted for use with a hammer arrangement of the type described in co-pending United States patent application, Ser. No. 473,093, filed by Sieghard Arnold et al. on July 19, 1965, entitled Print Hammer Unit for High Speed Printers, and assigned to the assignee of the present application. Said co-pending application is hereby incorporated in the present application by reference as if it were set forth in its entirety. It will be appreciated however, that the improved circuit may be utilized with other hammer arrangements.

In said co-pending application, a plurality of hammers, one for each print position, are positioned adjacent each other in one row. A type bar including a plurality of type elements is interposed between the hammers and a recording medium. In order to print one line, the type bar is incremented from a start position toward a final position so that each character on the type elements is moved to each print position. The incrementing of the type bar from one position to the next is achieved during one print cycle.

In each said position, a memory scan cycle is initiated to determine whether or not any of the characters on the type elements correspond to the characters which are Patented Jan. 21, 1969 ICC to be imprinted on the medium in the particular print positions. Where there is such correspondence, the respective hammers are actuated. Then the next print cycle is initiated to produce memory scanning and then imprinting. This sequence of operation continues until one complete line of print is completed. The apparatus is then conditioned to start the next line of print.

In said co-pending application, each hammer is carried by a pair of springs. A plurality of magnetic yokes, one for each hammer, are positioned adjacent one end of the hammer. Means are provided for bringing each hammer and-its yoke into contact at the end of each print cycle to lock the hammers in position against the yokes. A hold winding, wound in common around all of the magnetic yokes, is continuously energized to provide the magnetic flux which holds the hammers against the force of their mounting springs. Each yoke is provided with an individual pair of series-connected bucking windings which are selectively energized during the memory scan interval. When the bucking windings or coils of a particular yoke are energized (when the character on the type bar in the respective print position corresponds to the character which it is desired to imprint), their magnetic flux opposes that of the hold winding, thereby releasing the hammer. The force of the springs, which support the hammer, drive the hammer into engagement with the type bar to cause imprinting of the character on the print medium.

It is the bucking coils of the co-pending application which are energized by the improved circuits of the present application.

In known circuits for actuating the hammers, it has been common to provide for each coil a bistable device with set and reset controls (or a monostable device) and several stages of amplification terminated by a power transistor. In these arrangements, timing problems occurred. In adidtion, these arrangements were relatively expensive and particularly so because of the requirement for a relatively high speed power transistor for energizing each coil.

It is therefore the primary object of the present invention to provide an improved hammer drive circuit which is economical yet reliable at high speeds of operation.

This object is achieved in a preferred embodiment of the present invention by providing a silicon controlled rectifier for each hammer coil and one power transistor for all coils. The opposite ends of the coils are connected between the common power transistor and their respective rectifier. The rectifiers are selectively energized during an initial portion of a print cycle and are maintained energized by a hold-reset circuit until the reset interval at the end of a print cycle.

During the interval when the rectifiers are held energized by the hold-reset circuit, the coils are essentially de-energized. After all the selected rectifiers are energized, the power transistor applies a high voltage pulse to the common ends of the coils to produce a high current pulse through each coil which is connected to an energized rectifier. Shortly after the termination of the power pulse, the hold-reset circuit applies a reverse bias potential to the anode of each energized rectifier to turn the rectifiers off.

It is therefore a more specific object of the present invention to provide an improved hammer drive circuit in which a silicon controlled rectifier is provided for each hammer coil, in which the rectifiers are selectively energized and held energized without energizing the respective coils, and in which a power transistor thereafter applies a power pulse to all of the coils to energize the selected coils.

The preferred embodiment of the present application further includes the simple, yet effective circuit for checking the proper operation of the improved hammer drive circuit and for initiating the interruption of power incident to the detection of erroneous operation.

It is therefore another object of the present invention to provide in the improved hammer drive circuit a lowcost, yet reliable checking circuit.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 illustrates the improved hammer actuating circuit arrangement diagrammatically;

FIGS. 2 and 3 are a fragmentary illustration of the improved hammer actuating circuit and its associated control circuit, partially schematic and partially diagrammatic; and

FIG. 4 illustrates the sequence of operation of various portions of the circuit of FIGS. 2 and 3.

The circuit illustrated diagrammatically in FIG. 1 includes a plurality of electromagnetic coils 11 to 1n, each of which is adapted to cause the movement of a respective hammer (not shown) against a recording medium (not shown). A group of silicon controlled rectifiers 5 couple the lower ends of the coils to ground potential. The upper ends of the coils are connected to a positive supply terminal 6 by way of a conductor 2 and a power driver 7. The group of rectifiers 5 is connected to a positive supply terminal 8 by way of a conductor 3 and a holdreset circuit 9. The upper ends of the coils are also connected to an error detect circuit 10 by way of the conductor 2.

Attention is directed to the circuit details illustrated in FIGS. 2 and 3. Each of the coils 1-1 to 1-n is coupled to a respective selection circuit such as 15, 16, 17 and 18. Each of the selection circuits is identical; and only the details of the circuit are shown.

The circuit 15 includes a silicon controlled rectifier 20, having its cathode connected to ground potential and its anode connected to the coil 1-1 by means of an isolating diode 21 and a parallel-connected capacitor 22 and resistor 23. The control electrode of the rectifier is con nected to a level setting voltage

divider comprising resistors

12, 13 and 14 connected in series between

supply terminals

25 and 26.

A printer control circuit is coupled to the control electrode of the rectifier by way of a pair of

diodes

31 and 32 which together with the voltage divider provide a positive AND function.

The anode of the rectifier 20 is coupled to the holdreset circuit 9 by way of a current limiting resistor 33 and the conductor 3.

The hold-reset circuit 9 comprises an output driver including a pair of transistors 40 and 41 connected in a Darlington pair arrangement. A diode 42 is connected across the emitter collector electrodes of the transistor 41 to limit the positive potential at the emitter electrode of the transistor 41 when the coil energizing pulse is applied. A second diode 43 is connected between the base electrode of the transistor 40 and the emitter electrode of the tran sistor 41.

The circuit 9 includes an input section including an input terminal 45 connected to the control circuit 30 by way of conductor 29 and to a voltage

divider including resistors

46, 47 and 48 which are connected in series between positive and negative supply terminals 49 and 50. The junction between the

resistors

47 and 48 is connected to the base electrode of a transistor 51. The emitter electrode of the transistor 51 is connected to ground potential and its collector electrode is connected to the terminal 50 by way of resistors 52 and 53. The junction between the resistors 52 and 53 is connected to the base electrode of a transistor 55. The emitter electrode of the transistor 55 is connected to the supply terminal 50; and its collector electrode is connected to a positive supply terminal 56 by way of

resistors

57 and 58.

A capacitor 59 is connected in parallel with the resistor 58. A second capacitor 60, coupling the base electrode of the transistor 55 to the junction between the

resistors

57 and 58 limits the rate of voltage change at said junction. The junction between the

resistors

57 and 58 is connected to the diode 43 and to the base electrode of the transistor 40.

The power driver circuit 7 includes an input terminal 70 connected to the printer control circuit 30 by way of a conductor 71 and to a voltage

divider including resistors

72, 73 and 74 which are connected in series between positive and negative supply terminals 75 and 76.

The junction between the

resistors

73 and 74 is connected to the base electrode of a transistor 77. The emitter electrode of the transistor 77 is connected to ground potential by means of a resistor 78; and its collector electrode is connected to a positive supply terminal 79 by way of

resistors

80 and 81.

The emitter electrode of the transistor 77 is also connected to the base electrode of a transistor 82. The emitter electrode of the transistor 82 is connected to ground potential; and its collector electrode is connected to the positive supply terminal 79 by way of the resistor 80, a diode 83 and a resistor 84, and also by way of the emitter and collector electrodes of a transistor 85. The baseemitter electrodes of the transistor 85 are connected across the diode 83.

The emitter electrode of the transistor '85 is connected to the base electrode of a power transistor 86. The emitter electrode of the transistor 86 is connected to a positive supply terminal 87; and its collector electrode is connected to ground potential by way of a diode 88 which limits the negative potential swing at the collector electrode when the latter transistor is turned off. The collector electrode of the transistor 86 is connected to each of the hammer actuating coils by way of the drive conductor 2. The conductor 2 is also connected to the input terminal 90 of the error detect circuit 10. The terminal 90 is connected to a first transistor 91 by way of a level setting voltage

divider including resistors

92 and 93 which are connected in series between the terminal 90 and a negative supply terminal 94. The emitter electrode of the transistor 91 is connected to ground potential; and its collector electrode is connected to a positive supply terminal 95 by way of a resistor 96. The collector electrode of the transistor 91 is connected to the base electrode of a grounded emitter transistor 97.

A positive AND circuit includes

input diodes

101, 102 and 103, an output diode 104 and a resistor 105 coupling the diodes to a positive supply terminal 106. The

diodes

101 and 103 are connected to the printer control circuits by way of

conductors

107 and 108. The diode 102 is connected to the collector electrode of the transistor 97.

The output diode 104 is connected to a silicon controlled rectifier 110 by Way of a voltage

divider including resistors

111 and 112 which are connected in series between the diode and a negative supply terminal 113. A resistor 114 is connected across the control electrode and the cathode of the rectifier. The anode of the rectifier is connected to a suitable power interrupt circuit 115.

The input terminal 90 is also connected to a grounded emitter transistor by way of an isolating diode 121 and a voltage

divider including resistors

122, 123 and 124 which are connected in series between positive and

negative supply terminals

125 and 126.

A positive AND circuit includes input diodes 131,

132 and 133, an output diode 134 and a resistor 135 coupling the diodes to a positive supply terminal 136. The output diode 134 and the output diode 104 of the AND circuit 100 perform an OR function input to the rectifier 110. The

input diodes

131 and 133 are connected to the printer control circuit 30 by way of the conductor 108 and a conductor 137, respectively. The input diode 132 is connected to the collector electrode of the transistor 120. The operation of the improved hammer drive circuits of FIGS. 2 and 3 will be described in detail, reference being directed to the voltage waveforms of FIG. 4.

At the beginning of each print cycle, the output of the hold-reset circuit 9 raised to its positive level, will provide a hold current for subsequently energized silicon con-' trolled rectifiers. In order to raise the output of the circuit 9 to its positive level, the printer control circuit 30 applies a positive potential to the line 29, thereby turning the transistor 51 off. This in turn causes the transistor 55 to turn oil? and the transistors 40 and 41 to be turned on. This causes the positive potential at the collector of the transistor 41 to be applied to the hold-reset line 3.

A short time thereafter, the memory scan is initiated; and each rectifier such as 20, associated with a hammer in a position in which a character is to be printed during the cycle, is energized. This energization of the rectifiers is performed in sequence until all of the print positions have been scanned.

When one of the selection circuits such as is selected for energization, positive potentials are applied by the circuit 30 to the

diodes

31 and 32 to turn on the rectifier 20. Once the rectifier is turned on, it is maintained in its energized state by the positive potential on the hold-reset line 3.

After all-of the selected rectifiers are energized, the power driver circuit 7 is energized to produce a positive output pulse. This pulse is produced incident to the application of a positive-going pulse to the conductor 71 by the control circuit 30. This positive-going pulse turns the

transistors

77 and 82 on to saturation. When the

transistors

77 and 82 are turned on, they energize the power transistor 86 to apply the positive potential at the terminal 87 to the drive line 2. This positive pulse energizes each of the coils 1-1 to 1-n which is associated with an energized rectifier such as 20.

The length of the output pulse from the transistor 86 is of sufiicient time duration to assure the release of each of the hammers associated with an energized coil. Hence, after a predetermined time duration, the positive pulse applied to the conductor 71 is removed to turn oil the

transistors

77 and 82. When the

transistors

77 and 82 are turned off, the transistor 85 turns on to provide a low resistance turnotf drive for the power transistor 86.

The

transistors

77 and 82 form a modified Darlington pair. The

resistors

81 and 84 insure current splitting and the saturation of the

transistors

77 and 82 when they are turned on. The diode 83 isolates the base and emitter electrodes of the transistor 85 to insure that the latter transistor is turned oif when the

transistors

77 and 82 are conducting.

When the power transistor 86 turns off, an inductive voltage spike is produced in each of the energized coils. This spike is suppressed by coupling both ends of the coil to ground potential by means of the line 2 and the diode 88 and by the capacitor 22, resistor 23, diode 21 and rectifier associated with the energized coil.

After a time delay sufficient to permit the decay of current in the energized coils, the printer control circuit returns the line 29 to ground potential, thereby turning on the transistors 51 and 55. When the transistor 55 turns on, the negative potential at the terminal 50 is applied to the hold'reset line 3 by way of the transistor 55, the resistor 58, the capacitor 59 and the diode 43. This negative potential resets each energized rectifier such as 20 to its nonconducting state. The potential across the diode 43 turns off the transistors 40 and 41.

It is possible that no rectifier such as 20 will be energized during a memory scan. In such event, the voltage pulse from the transistor 86 is applied to the emitter electrode of the transistor 41 of the hold-reset circuit by way of line 2 and all of the coils 1-1 to 1-n and their associated elements such as capacitor 22,

resistors

23 and 33, diode 21 and line 3. The diode 42 clamps this voltage to its associated supply terminal to prevent damage to the transistors 40 and 41.

The diodes such as 21 prevent current flow from the hold-reset circuit through the coil of an energized rectifier by way of circuits of each de-energized rectifier over paths extending from the conductor 3, resistors such as 33 and 23 and coils such as 1-1 and line 2.

The operation of the error detect circuit 10 of FIG. 3 will now be described, reference being directed to the waveforms of FIG. 4. As will be seen, the circuit 10 is common to all coil energizing circuits and therefore achieves significant economies.

Only the voltage level on the line 2 need be monitored during each print cycle. More particularly, if the holdreset circuit 9 functions properly, two conditions will occur (1) it will apply approximately plus twelve volts to the line 3 at the beginning of each print cycle and (2) the rectifiers will all be reset at the end of the preceding cycle, whereby line 2 will be at some positive voltage at the beginning of the print cycle. The AND circuit 130 checks for this condition.

Further, if the power driver circuit 7 functions properly, the voltage on the conductor 2 will not exceed a predetermined lower value except during the power pulse interval. AND circuit 100 checks for this condition.

As soon as the hold-reset circuit 9 is operated at the beginning of each cycle to produce a positive voltage at its output, the printer control circuit 30 applies a positive pulse of short time duration to the diode 133 by way of the line 137. If the voltage on the line 2 is less than a predetermined level, for example, plus five volts, the transistor 120 will be turned olf. The positive potential at the terminal 136 will be applied to the control electrode of the rectifier 110 to turn the latter on, indicating an error condition.

During the initial powering up of the printer, the line 108 has a negative potential applied thereto to prevent the energization of the rectifier 110, thereby inhibiting the operation of the low and high voltage sections of the error detecting circuit. When the printer has been conditioned for operation, however, a positive potential is applied by the control circuit 30 to the line 10 8, thereby rendering the circuit 10 efiective for the detection of errors.

Thus, we see that the transistor and the diode AND circuit are effective for a very short time interval preceding the memory scan portion of the print cycle to determine whether or not the hold-reset circuit has been effective to apply a positive potential to the holdreset line 3 and whether or not the recifiers have been reset at the end of the preceding print cycle.

The

transistors

91 and 97 and the diode AND circuit 100 are effective during each print cycle to determine whether or not an excessively high positive voltage (e.g. greater than thirty volts) is applied to the line 2 at any time interval other than the power pulse interval.

Except for an interval beginning shortly before the energization of the power pulse and ending shortly after the termination of the power pulse, a positive potential is applied to the line 107 by the control circuit 30. This positive potential renders the AND circuit 100 effective for determining an excessively 'high voltage on the drive line 2. If, while the positive potential is applied to the line 107, the voltage on the drive line 2 exceeds a preselected positive potential level, the transistor 91 will be turned on and the transistor 97 turned off. With the transistor 97 off and positive potentials applied to the

diodes

101 and 103, a positive potential will be applied to the control electrode of the rectifier 110 to turn the latter on. Turning on of the rectifier 110 will initiate suitable alarm and power interrupt circuits 115.

Reliable operation of the circuits of FIGS. 2 and 3 was achieved with the following component values; it will be appreciated, however, that these values are given merely by way of example:

Resistors: Values in ohms Capacitors: Values 22 microfarad 1 59 do .15 '60 picofarads While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an impact printer of the type wherein means including electromagnetically actuate-d hammers are cyclically operated to cause characters to be selectively imprinted upon a recording medium, the combination comprising:

a respective electromagnetic coil effective when energized for causing the actuation of each hammer;

a silicon controlled rectifier corresponding to each coil;

a source of supply potential including a plurality of terminals;

an electronic switch means including a high power transistor connecting a first one of the terminals of the source to a common junction and further including a first diode connecting a second one of the terminals to the common junction;

a plurality of parallel coil energizing current paths connected between the common junction and said second terminal, each path including a respective coil and rectifier connected in series and an additional diode interposed between the coil and rectifier;

a hold-reset circuit means connected to the junctions between each rectifier and additional diode for coupling all of the rectifiers to one or another of the supply terminals to maintain an energized rectifier in its conducting condition or alternatively to reset an energized rectifier to its nonconducting condition; and

cyclically operated control means effective during an initial portion of each print cycle to energize selected ones of the rectifiers and to render the hold-reset circuit means effective to maintain the latter rectifiers energized,

said control means thereafter effective to energize the high power transistor for a predetermined time interval to momentarily complete those coil energizing current paths which include an energize-d rectifier,

said first diode limiting the voltage swing at said common junction produced by the energized coils when the power transistor de-energizes,

said control means thereafter effective to operate the hold-reset circuit means for resetting the energized rectifiers.

2. The combination set forth in claim 1 wherein the electronic switch means further includes amplifier means responsive to the control means for selectively energizing the high power transistor,

said amplifier means comprising first and second interconnected transistor amplifiers operated alternatively at cutoff or in saturation by the control means and having outputs,

an emitter follower transistor amplifier having its base and emitter electrodes connected respectively to the outputs of the first and second transistor amplifiers and operated at cutoff and a high current level when the first and second transistors are at saturation and cutoff respec tively; and

a diode connected across the base-emitter electrodes of the emitter follower transistor amplifier and poled to conduct in its low impedance state when the emitter follower transistor is cut off,

said power transistor being connected to the lastmentioned diode and the emitter follower transistor amplifier and turning on and off respectively in response to turn on of the first and second transistor amplifiers and to turn on of the emitter follower transistor amplifier.

3. The combination set forth in claim 1 together with an error detect circuit connected only to said common junction and the control means and operated by said control means for checking the proper operation of the rectifiers, the electronic switch means and the hold-reset circuit means.

4. The combination set forth in claim 3 wherein the error detect circuit comprises a first coincident circuit rendered effective at the be ginning of each print cycle to determine the reset condition of all rectifiers and the coupling of the maintaining supply terminal to the rectifiers, and

a second coincident circuit effective except during each said predetermined time interval for detecting the presence at said juncition of a potential exceeding a predetermined level.

5. The combination set forth in claim 3 wherein the error detect circuit comprises a first threshold circuit connected to said common junction and producing a first output condition only when the voltage 'at the common junction is less than a predetermined low value indicative of the energized condition of at least one of the rectifiers,

a first coincident circuit rendered effective at the be ginning of each print cycle to detect the presence or absence of said first output condition, thereby checking for the reset condition of all rectifiers and the coupling of the maintaining supply terminal to the rectifiers,

a second threshold circuit connected to said common junction and producing a second output condition only when the voltage at the common junction exceeds a predetermined high value indicative of the energization of the power transistor,

a second coincident circuit effective except during each said predetermined time interval for detecting the presence or absence of said second output condition, and

9 10 means responsive to detection of the presence of either 3,231,786 1/ 1966 Felcheck 317-1485 the first or second output condition for producing an 3,243,665 3/ 1966 Fayer et a1. 317-137 error signal.

LEE T. HIX, Primary Examiner.

References Cited 5 UNITED STATES PATENTS 3,151,311 9/1964 Spector et a1. 3,191,101 6/1965 Reszka 317-1485 X US. Cl. X.R.

Claims

1. IN AN IMPACT PRINTER OF THE TYPE WHEREIN MEANS INCLUDING ELECTROMAGNETICALLY ACTUATED HAMMERS ARE CYCLICALLY OPERATED TO CAUSE CHARACTERS TO BE SELECTIVELY IMPRINTED UPON A RECORDING MEDIUM, THE COMBINATION COMPRISING: A RESPECTIVE ELECTROMAGNETIC COIL EFFECTIVE WHEN ENERGIZED FOR CAUSING THE ACTUATION OF EACH HAMMER; A SILICON CONTROLLED RECTIFIER CORRESPONDING TO EACH COIL; A SOURCE OF SUPPLY POTENTIAL INCLUDING A PLURALITY OF TERMINALS; AN ELECTRONIC SWITCH MEANS INCLUDING A HIGH POWER TRANSISTOR CONNECTING A FIRST ONE OF THE TERMINALS OF THE SOURCE TO A COMMON JUNCTION AND FURTHER INCLUDING A FIRST DIODE CONNECTING A SECOND ONE OF THE TERMINALS TO THE COMMON JUNCTION; A PLURALITY OF PARALLEL COIL ENERGIZING CURRENT PATHSS CONNECTED BETWEEN THE COMMON JUNCTION AND SAID SECOND TERMINAL, EACH PATH INCLUDING A RESPECTIVE COIL AND RECTIFIER CONNECTED IN SERIES AND AN ADDITIONAL DIODE INTERPOSED BETWEEN THE COIL AND RECTIFIER; A HOLD-RESET CIRCUIT MEANS CONNECTED TO THE JUNCTIONS BETWEEN EACH RECTIFIER AND ADDITIONAL DIODE FOR COUPLING ALL OF THE RECTIFIERS TO ONE OR ANOTHER OF THE SUPPLY TERMINALS TO MAINTAIN AN ENERGIZED RECTIFIER IN ITS CONDUCTING CONDITION OR ALTERNATIVELY TO RESET AN ENERGIZED RECTIFIER TO ITS NONCONDUCTING CONDITION; AND CYCLICALLY OPERATED CONTROL MEANS EFFECTIVE DURING AN INITIAL PORTION OF EACH PRINT CYCLE TO ENERGIZE SELECTED ONES OF THE RECTIFIERS AND TO RENDER THE HOLD-RESET CIRCUIT MEANS EFFECTIVE TO MAINTAIN THE LATTER RECTIFIERS ENERGIZED, SAID CONTROL MEANS THEREAFTER EFFECTIVE TO ENERGIZE THE HIGH POWER TRANSISTOR FOR A PREDETERMINED TIME INTERVAL TO MOMENTARILY COMPLETE THOSE COIL ENERGIZING CURRENT PATHS WHICH INCLUDE AN ENERGIZED RECTIFIER, SAID FIRST DIODE LIMITING THE VOLTAGE SWING AT SAID COMMON JUNCTION PRODUCED BY THE ENERGIZED COILS WHEN THE POWER TRANSISTOR DE-ENERGIZES, SAID CONTROL MEANS THEREAFTER EFFECTIVE TO OPERATE THE HOLD-RESET CIRCUIT MEANS FOR RESETTING THE ENERGIZED RECTIFIERS.