US3754166A

US3754166A - A driver circuit for actuating print wire solenoids

Info

Publication number: US3754166A
Application number: US00252616A
Authority: US
Inventors: R Howard
Original assignee: Centronics Data Computer Corp
Current assignee: Genicom Corp
Priority date: 1971-06-14
Filing date: 1972-05-12
Publication date: 1973-08-21
Anticipated expiration: 1990-08-21

Abstract

A driver circuit for energizing solenoids to actuate print wires wherein the solenoid operation is such as to rapidly move the print wire into impact direction and permit rapid return thereof. The drive circuit provides for an external determination of the solenoid drive time. A transistor having a current limiting resistor coupled thereto drives the solenoid. Current amplification means is coupled to the imput of the transistor and its input, in turn is controlled by a voltage regulating semiconductor device for maintaining constant current through the solenoid. Capacitance means coupled to the inductor through diode means provides a discharge circuit for the solenoid. A capacitor cooperates with the solenoid inductance to form a resonant circuit enabling the current through the solenoid to decrease in a short time period and prevent generation of high voltages during reverse conduction of the inductance. Control circuit means preventing energization of the solenoids during cutoff of the solenoid power source.

Description

Howard DRIVER CIRCUIT FOR ACTUATING PRINT WIRE SOLENOIDS Aug. 21, 1973 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney0strenk, Faber, Gerb & Soffen [75] Inventor: Robert Howard, Roslyn, N.Y.

[73] Assignee: Centronlcs Data Computer 1 Corporation, Hudson, NH. [57] ABSTRACT Filedl, y 1972 A drivercircuit for energizing solenoids to actuate [21] AppL 252 616 print wires wherein the solenoid operation is such as to rapidly move the print wire into impact direction and Rellted 15- Appli a ion 8 permit rapid return thereof. The drive circuit provides [60] Division of Ser. No. l52,598,Junel6,1971,Pat.No. for an external determination of the solenoid drive 3,670,431, which is a continuation-in-part of Ser. No. time. A transistor having a current limiting resistor couy 1970- pled thereto drives the solenoid. Current amplification means is coupled to the imput of the transistor and its [52] US. Cl. 317/1485 B, 3 l7/DlG. 6 i t, i turn is controlled by a voltage regulating semi- [51] Int. Cl. H0lh 47/32 conductor device for maintaining constant current F1616 131G- through the solenoid. Capacitance means coupled to 148-5 R the inductor through diode means provides a discharge circuit for the solenoid. A capacitor cooperates with Reference! C1161 the solenoid inductance to form a resonant circuit en- UNITED STATES PATENTS abling the current through the solenoid to decrease in 3 049 650 8/l962 Greenblatt 3l7/DIG. 6 a time P?riod and Prevent generatim high 2:980:369 4/1961 Ruof 317/1485 R ages during reverse conduction of the inductance 3,141,530 7/1964 Morley 317/1485 R rol ircuit means preventing energization of the sole- 3,343,046 9/1967 Ladd, .Ir 317/1485 R noids during cutoff of the solenoid power source. 2,997,632 8/l96l Shepard, Jr 3l7/DlG. 4

3 Claims, 11 Drawing Figures 7 /2 6 20 6 WWW/M 7 H i L-Jl I ////l1///// JZT/Z/AA Z F ll 12 /2( l T \\\\\\\\\\\\Y\\\\ 1\ r Patented Aug. 21, 1973 3,754,166

5 Sheets-She t 1 v i f: -//c- Y //a //6 [/e I v 1 Patented Aug. 21, 197 3 3 Sheets-Sheet 11 Patented Aug. 21, 1973 3,754,166

3 Sheets-Sheet 5 DRIVER CIRCUIT FOR ACTUATING PRINT WIRE SOLENOIDS This is a division of parent application Ser. No. 152, 598, filed June 16, 1971 now U.S. Pat. No. 3, 670, 431, which is a continuation-in-part of, and an improvement upon copending application Ser. No. 37, 815 filed May 15, I970. The present invention relates to solenoid drive circuits, and more particularly to drive circuits useful for driving print wire solenoids employed in dot matrix type printing devices wherein the solenoids are driving by high constant current and abruptly turned off during deenergization thereof without producing high voltages upon deenergization of the solenoids.

The solenoid assembly to be described hereinbelow in greater detail, is extremely advantageous for use in the high speed dot matrix printer described in the inventor's copending application Ser. No. 35,405 filed May 7, 1970. The printer described therein is capable of forming characters by selectively impacting one or more of seven vertically aligned slender print wires against an inking ribbon so as to print dots upon the surface of the paper document. A plurality of the columns printed are related so as to form any characteror symbol, typically within a five by seven dot matrix. Obviously, another size matrix may be employed, if desired. Since printing speed is of the essence in such devices it is therefore very important to provide an apparatus for rapidly impacting the print wires and rapidly returning the print wires to the rest position with a minimum of bounce in order that the entire printing assembly may operate at such high speeds. The printing rate in the above mentioned printing apparatus is of the order of 165 characters per second with 60 lines of characters per minute being printed (with each line containing up to 132 characters). To obtain these speeds it is important to provide solenoid assemblies which are capable of moving the slender print wires from the rest position to the impact position and return to the rest position within the time interval of one millisecond or less while at the same time providing an impact of sufficient force to print a clearly legible dot upon the paper document or other printing surface.

The present invention is characterized by providing driving circuit for solenoid assemblies. l

The solenoid assembly of the present invention comprises a case for housing the coil and armature. A stem portion threadedly engages the forward end of the housing and is provided with an axially aligned elongatedopening for receiving and reciprocally mounting a slender elongated print wires. The rearward end of the print wire is fixedly secured to the solenoid annature at the forward end of the armature. The rearward end of the armature is fixedly secured to a spring member of' the wagon wheel type at its central portion. The peripheral portions of the spring rest against an an- 1 gular shaped spacer which abuts a shoulder provided therefor within the solenoid housing. The rearward portion of the armature housing abuts a cap member which threadedly engages and seals the rearward end of the housing. The cap member is adjustable to regulate the amount of stress or tension (i.e., preloading) imposed upon the spring member. A half spool memberpositioned within the solenoid housing cooperates with the stern member'to define an annular shaped hollow interior space for positioning and receiving the solenoid coil. The exposed forward end of the stem portion is threaded so as to threadedly engage an appropriate tapped aperture provided in the solenoid mounting portion of the print head assembly. A tubular wire guide is positioned within the solenoid assembly stem portion and is appropriately lubricated so as to reduce wearing of the reciprocating print wire extending therethrough.

The print head assembly is provided with a plurality of guide means for guiding and directing each of the print wires toward the forward end of the print head housing which is further provided at its forward end with a multi-apertured jeweled member which ultimately aligns the forwardmost ends of the slender print wires along a vertical straight line. I

The wagon wheel spring means is formed from a flat sheet of prehardened tempered spring steel or any other suitable material exhibiting similar characteristics. The spring has a centrally located hub portion having radially aligned spoke portions extending outwardly therefrom and integrally joined therewith. Each of these spoke portions are provided with arcuate shaped portions integrally joined to their free ends so as to define a substantially circular shaped periphery wherein gaps are provided at discrete intervals about the periphery of the spring, which gaps define the spacing between the outer ends of adjacent arcuate portions. The arcuate shaped portions rest against the spacer member referred to hereinabove while the hub portion of the spring rest against the forward end of the threaded cap member which is adjustably positioned relative to the rearward end of the solenoid housing so as to prestress or preload the spring member and thereby provide the precise amount of loading required to assure appropriate high speed reciprocating operation of the print wires under the control of the solenoid.

The springs maintain the wires of their associated solenoids in the non-printing position when the solenoid coil is deenergized. Upon energization of the solenoid coil, the electromagnetic field tends to urge or pull the armature towards the forward end of the solenoid assembly. This force is counteracted by the spring member which sequentially develops two counteracting spring forces in the following manner:

The initial forward movement of the solenoid armature causes each of the radially aligned spokes to flex, which flexing establishes a counteracting spring force which is substantially linearly related to the movement of the annature. Subseuqently thereto, as the armature moves still further in the forward direction, the arcuate portions of the spring-provided at the free ends of the spoke portions experience a twisting. The resultant counteracting force developed by the spring thus follows a logarithmic curve whereby the magnitude of the counteracting force increases more rapidly than the otherwise linear force developed by the radial portions of the spring alone. The initial counteracting force is both small and relatively linear in nature to enable the solenoid annature to be rapidly accelerated. The counteracting force of the spring reaches its maximum magnitude as the slender print wires impact the paper document at which time the dots of the character are printed. The deenergization of the solenoid coil places the print wires, which are now in the impact position, under the exclusive control of the counteracting force of the spring which is substantially at its maximum mag- Y nitude at this time causing the print wire to rapidly return to the rest position. The flexed spokes, in returning to their normal undeflected position, tend to overshoot their normal rest position. However, the "twisted arcuate portions tend to counteract the overshooting and thereby more rapidly return the annature and print wire to the rest position with a minimum of overshoot and bounce.

The threadedly engaging members of the solenoid assembly permit simple rapid assembly of the component parts after which precise adjustments may be made whereupon the adjustable members are then epoxied into position so as to maintain the accurate positioning of the components thereby providing a solenoid assembly which is formed of components whose tolerances may be somewhat imprecise while at the same time are capable of being readily and easily adjusted to produce a solenoid assembly whose operating characteristics lie within highly precise and controlled ranges. The print head housing supports sensing means for precisely registering the lines of dots printed.

The driver circuit for actuating the print wire solenoids utilizes a transistor coupled in circuit with the solenoid coil, a resistor and a DC source. A second transistor coupled as a current source drives the base of the first transistor. The input of the second transistor is provided with voltage regulating diode means coupled to its input. The first and second transistors together with the voltage regulating diode means function to maintain a constant current through the solenoid coil during energization thereof. Series connected second diode means and a capacitor are coupled between the solenoid coil and ground potential for rapidly dissipating the current in the solenoid in the solenoid coil during deenergization thereof and serve to prevent the generation of high voltages upon deenergization. The capacitance means has a capacity value which functions to form a tuned circuit with the inductance of the solenoid coil to permitdecrease of current through the solenoid coil.

It is therefore one object of the present invention to provide a novel driver circuit for solenoid assemblies employed in high speed dot matrix printers and the like wherein the driver circuit is designed to provide a large constant current to the solenoid coil during energization thereof and is further provided with means for providing rapid deenergization of the energizing currents.

Another object of the present invention is to provide a novel solenoid assembly for use in dot matrix printers and the like in which the components are made adjustable so as to facilitate ease of assembly and subsequent precise adjustments thereof.

Another object of the present invention is to provide a spring member of wagon wheel" design for use in solenoid assemblies. 1

These as well as other objects of the present invention will become apparent when reading the accompanying description and drawings in which:

FIG. 1 is a sectional view of a solenoid assembly designed in accordance with the principals of the present invention;

FIG. la is a sectional view of the solenoid case of FIG. 1;

FIG. lb is a sectional view of the solenoid armature of FIG. 1;

FIG. 10 is an end view of the cap member of FIG. 1;

FIG. 2 is a plan view of the solenoid spring employed in the assembly of FIG. 1;

FIG. 2a is a plot of curves and FIGS. 2b and 2c are top end views of portions of the spring of FIG. 2 useful in describing the operation of the spring;

FIG. 3 is a perspective view of a print head assembly employing solenoid assemblies of the type shown in FIG. 1; and

FIG. 4 is a schematic diagram of the solenoid driver circuit.

Referring now to the drawings, the solenoid assembly 10, shown .best in FIG. 1, is comprised of a cylindrical shaped case 11 which is also shown in sectional fashion in FIG. la. The case is a substantially hollow cylindrical member provided with a recessed shoulder aspaced inwardly from its left-hand end and a recessed shoulder 1 1b spaced inwardly from its right-hand end. The righthand or rearward end is threaded at 11c for receiving an adjustable threaded closing cap, to be more fully described. An annular groove 11d is positioned immediately adjacent the innermost end of tapped portion He. The forward or left-hand end of casing 11 is further provided with a slot lle through which the leads of the solenoid (to be more fully described) extend so as to couple the solenoid to the peripheral driving circuitry, not shown herein for purposes of simplicity.

The forward (left-hand) end of case 11 receives a stem member 12 having a threaded portion for threadedly engaging a tapped mounting hole provided in the print head assembly, which will be more fully described hereinbelow. A lock nut 13 threadedly engages threaded portion 12a to firmly secure the solenoid assembly to the print head assembly. 7

The stem is provided with a circular shaped flange portion 12b which is received within the forward end of case 11 whereby the right-hand peripheral edge of flange 12b may nominally abut against shoulder 11a. The rearwardmost portion 12c of stem 12 has a diameter which is less than both flange 12b and threaded portion 12a with the extreme right-hand end portion 12d being of still further reduced diameter so as to form a shoulder 12c positioned between sections 120 and 12d.

Stem 12 is provided with an axially aligned opening which is comprised of a portion 12f of a first diameter and a portion 12g of slightly enlarged diameter extending therethrough. The extreme left-hand end portion 12h is tapered to form a conical entrance portion to facilitate the insertion of a hollow tubular elongated wire guide 14 which is positioned within opening portion 12g so that its right edge abuts against the shoulder l2j positioned between portions 12f and 12g of the axially aligned opening. The wire guide is preferably forcefitted within opening portion 12g so as to experience no linear movement relative to stem 12. The interior of wire guide 14 is preferably coated with a lubricant such as, for example, Molycote" to reduce wearing of the print wire 15 which is mounted for reciprocating movement therethrough.

A solenoid core member l6 having a tubular shaped portion 16a and an outwardly extending flange portion 16b, is telescoped upon the right-hand end of stem 12 whereby the left-hand end of hollow cylindrical portion abuts against shoulder He in the manner shown. The hollow cylindrical portion [6a is preferably forcefitted upon the right-hand portion 12d of stem 12. The flanges 12!: and 16b, the stem portion 12c, the tubular portion 160 and the interior surface of case 11 define a hollow interior space which is provided for receiving the solenoid coil 17 whose turns or windings are wound about stem portion 120 and tubular portion 160 with the axial length of the windings being defined and physically limited by flanges 12b and 16b. The two end terminals of solenoid coil 17 are wound so as to extend through the slot He provided in case 11 (see FIG. la). The coil end terminals 17a and 17b are shown as extending outwardly through slot e. A predetermined length of each of the leads 17a and 17b is mounted within an associated insulating sleeve 17c and 17d, respectively.

As was described hereinabove, the slender elongated print wire is slidably received by the interior of wire guide 14 and extends still further to the right so as to be slidably received by the portion 12f of the stem axial opening and to protrude therebeyond by a predetermined distance. The right-hand end portion 15a of print wire 15 is mounted within an axially aligned opening 18a provided within the solenoid armature 18. Armature 18 is a substantially cylindrical shaped member which is further provided with a right-hand portion 18b of reduced diameter relative to the main body portion, forming an annular shoulder 18c, portion 18b which is further provided with an axially aligned opening 18d. A solenoid spring 19 is mounted upon armature 18 so that its central opening '(tobe more fully described in connection with FIG. 2) receives reduced diameter portion 18b and rests against shoulder 18c. A washer 20 is positioncd upon spring 19 so that its central opening receives reduced diameter portion 18b. A fastening member 21 has a shaft portion (not shown)'passing through I the openings in washer 20, spring 19 and armature opening 18d so as to firmly secure washer 20 and spring 19 to the right-hand end of armature 18.

Spring member 19 has a substantially circular shaped periphery as will be described in detail herinbelowJA ring shaped spacer member 22 has its left-hand surface whose left-hand surface bears against the head of fastener 21. An axially aligned opening 230 is provided for adjusting the preloading of spring 19 in a manner to be more fully described. A linear groove 23d is provided on the left-hand surface of cap 23 to facilitate the insertion of an adjusting tool such as, for example, a screw driver head. By rotating cap 23 within casing 11 the amount of preloading of spring 19 may be easily and accurately controlled.

After assembly of the solenoid, the stem 12 may be accurately positioned relative to casing 11 and maint'ained in this position by application of a suitable epoxy shown at 24 which, in addition to securing stem 12 to casing 11, serves to provide a moisture seal over slot l'le'. If desired, a shim in the form of a washer 25 may be positioned between shoulder 11a of case 11 and the right-hand peripheral edge of flange 12b. As another obvious alternative, the annular periphery of flange 12b may be threaded and the interior surface of the extreme left-hand portion of casing 11 may be tapped so as to provide for adjustable threaded engagement between these two members. Deposition of epoxy upon the members in the manner shown at 24 may be provided to rigidly secure stem 12 to case 1 l in the desired position.

As was described hereinabove, the amount of preloading imposed upon spring 19 may be controlled by rotation of cap 23 within casing 11. The amount of preloading may be measured by the insertion of a probe through axial opening 23. After precise adjustment of the preloading, epoxy may bedeposited at the position 25 shown in FIG. 1c in order to rigidly maintain cap 23 in the desired position. From the foregoing it can be seen that the assembly of the solenoid can be performed in a simple straightforward fashion since relatively little concern need be given to final adjustment thereof. For example, the assembly steps may be comprised of mounting core 18 upon stem 12; winding coil 17 about this assembly and inserting the assembly into casing 11. Spacer 22 may then be positioned within casing l 1 so as to abut against shoulder 11b. The armature assembly comprised of print wire 15, armature l8, spring 19, washer 20 and fastening member 21 may then be inserted into the casing whereby wire guide 15 is passed through the axial opening in stem 12 (which may be fitted with wire guide 14 either before or after insertion of wire 15 therethrough). Finally, cap 23 may be threaded into casing 11 thereby completing the asassembly may then be performed, at which time epoxy is deposited at the forward and rearward portions ofthe housing in the manner shown in FIG. 1 to retain the components in their desired alignment.

FIG. 2 is a detailed view of spring 19 which is comprised of a central portion having a central opening 19b for receiving the reduced diameter portion 18b of armature 18. A plurality of radially aligned spoke portions 19c are integrally formed with the central portion 19a and extend outwardly therefrom. the free ends of spoke portions 190 are each provided with arcuate shaped portions 19d whose outer edges define a sub stantially circular shaped periphery. The free ends of adjacent arcuate shaped portions define gaps [9e therebetween. As was described hereinabove, the central portion 19a engages armature 18 while the marginal portions, arcuate sections 19d rest upon the righthand surface of spacer 22 (relative to FIG. 1). It has been found that the provision of gaps 19c between the adjacent free ends of arcuate sections 19d significantly increase the operating life of the spring.

In one preferred embodiment of the present invention, the print wires are moved a distance of 0.015 inches from the rest position to the impact position. FIG. 2a is a plot which is useful in explaining the operation of spring 19 wherein distance D is plotted against spring force F. The nature of a flexing member, such as, for example, the spoke portions 19c, is such that the spring force varies linearly with distance, i.e., F kx, where F is the spring force, x is the flexing or deflection distance, and k is the spring constant. Curve A represents the spring force of a flexing member. Due to the design of the spring 19, however, it has been found that the relationship of force against distance is a logarithmic relationship as represented by curve B of FIG. 2a. Although it is not desired to predicate patentability on the theory of operation of the spring, it is believed that the logarithmic relationship is the result of the combined characteristics of the flexing and twisting of the spoke and arcuate shaped portions, respectively, which appear to operate as follows:

Considering FIGS. 2b and 2c, which are top and end views respectively of one spoke 19c and one arcuate shaped portion 19d the energization of the solenoid coil causes the armature to move toward the left from the rest posiion (relative to FIG. 1). The initial movement of armature l8 imparts a force represented by arrow F, as shown in FIG. 2c, to be imparted to the spring. The force F initially causes the spokes to flex downwardly as shown by dotted lines 190". After the spokes undergo a predetermined amount of flexure it has been found that the arcuate shaped portions, such as, for example, arcuate shaped portion 19d, reacts in such a manner that its end portions 19f and 19g remain stationary while its central portion intermediate the ends experiences a twisting relative thereto, as shown by arrow R, which is the apparent reason for the significant departure from the linear force versus distance relationship to the logarithmic relationship as shown by the respective curves A and B.

When the solenoid coil is deenergized, the magnitude of the spring force is substantially at its maximum value (represented by the value of curve B when the print wires have moved a distance of the order of 0.015 inches) thereby placing the spring wire exclusively under control of spring 19. The large force imposed upon the armature at this time causes a rapid return of the armature toward the rest position. The spoke portions tend to return to their normal undeflected position. the arcuate portions tend to significantly reduce the amount of overshooting which may occur as a result of the movement of the spoke portions toward their normal undeflected state thereby significantly reducing the amount of bounce and overshooting which would otherwise occur in springs of conventional de- In the high speed printer described in the above mentioned copending application Ser. No. 35,405, the print wires are designed to move from the initial rest position through a distance of 0.015 inches to the impact position and return to the rest position within the span of l millisecond. Since a typicalline may contain 132 characters, any given print wire may experience as many as 660 deflections per line, all of said deflections occurring within the span of less than 1 second. Taking the above high speed operation into-consideration with the fact that the printer is intended for use over extremely long periods of time with a minimum of down time, it can be seen that the spring members must be quite rugged while at the same time providing highly uniform operating characteristics for any given individual deflection. This necessitates the provision of a spring member which is dimensionally precise.

FIG. 3 is an exploded perspective view showing a printing head assembly 30 which incorporates sole- I noids 19 of the type described hereinabove. The assembly 30 is comprised of a body portion 31 having a rear wall 32 provided with a plurality of tapped openings 33 which threadedly engage the threaded portions 12a of each of the solenoid stems 12.

Body portion 31 is further provided with first and second pairs of vertically aligned guide slots 34 and 35 for receiving

positioning members

36 and 37 which are respectively positioned within the guide slots to align the slender print wires therethrough. A plurality of wire guides 38 extend through appropriate openings provided in positioning members 36 and 37m guide the print wires therethrough. The interiors of th hollow elongated tubular members 38 are preferably lubricated to reduce wear.

The forward end of housing 31 is provided with an opening 39 and a pair of

guide slots

43a and 43b for receiving and positioning a jeweled member 44 which, in turn, is provided with a plurality of vertically aligned openings for receiving each of the print wires.

Housing 31 is further provided with a pair of outwardly extending flanges 44a and 44 b each having tapped openings 45 for threadedly engaging fastening members to secure the print head housing to mounting

plates

46a and 46b which form a part of the printer and serve to move the print head along a horizontal line to successively print a plurality of vertical columns of dots, with the total number of dots per column being selectively controlled in accordance with the particular character or symbol to be printed.

The rear end of housing 31 further supports a registration device 60 shown in FIG. 3a and includes a pair of supporting members 61,61 for securing assembly 60 to housing 32. A plate 63 is secured to members 61,61 and supports a lamp 64 and a photo-head 65 suspended from the underside of plate 63. A stationary mylar strip 66 having a plurality of narrow transparent slits 67 is mounted in spaced parallel fashion relative to the printer platen (not shown). As the print head assembly moves in the direction of arrow A during a printing operation, light passes through a transparent slit and inpinges on the photo-head 65. The light is carried by a fiber optic bundle 68 to a photocell 69 to activate the photocell only when passing a transparent slit. the photocell generates a pulse which enables the solesolenoids to be energized only at the positions of the narrow transparent slits thus assuring accuracy in the locations of the dots printed regardless of the speed of movement (i.e., at a constant or a non-constant speed) of the print head assembly.

FIG. 4 is a schematic diagram showing the driver circuitry employed for selectively driving solenoids of the type shown in FIG. 1. Since the driving circuits for each solenoid are all identical in both design and operation,

' only one of the circuits will be described herein for purposes of simplicity.

As shown in FIG. 4, the driving circuit 50 is provided with an input terminal 51 normally coupled to one of the seven outputs of a character generator of the type described in the aforementioned application Ser. No. 35,405. when the output of the character generator coupled to input terminal 51 is low, input terminal 51 is held at ground potential and no current flows through resistor R4. When the output of the character generator goes high, then current flows through resistor R4. If diode D1 is back-biased, the current flows into the base of transistor Q3. This current, in turn, causes current to flow through resistor R3 and transistor Q3, thereby establishing a voltage across Zener diode ZDI. This voltage causes transistors Q2 and O1 to act as emitter followers, thereby developing a voltage drop across resistor R1. Resistor R2 limits the power dissipation in transistor Q2. The voltage drop across resistor R1 causes a collector current of the order of 2.5 amps to flow through transistor Q1. When the stage first turns ON, the inductance of the solenoid prohibits the current from flowing through transistor Q1 and at this point transistor O1 is saturated. When the current flowing through the solenoid reaches approximately 2.5

amps, transistor Q1 goes into an active region and limits the current at this value.

When transistor 01 shuts OFF, the current flowing through the solenoid passes through diode D2 and causes capacitor C1 to charge. The value of capacitor C1 is chosen to act as a parallel resonant circuit with the inductance in the solenoid. Diode D allows only one-quarter wave of this characteristic fr quency to be developed thereacross.

After the capacitors of each charging circuit (one charging circuit of the type 50 shown in FIG. 4 is provided in each solenoid) have charged to approximately 60 volts due to the discharge of the printer solenoids, resistor R5 serves to bleed ofi' this charge so that the capacitors will be at a 30 volt bias at the time of the next discharge. The value of the resistor is chosen so that a time constant of about 275microseconds results.

Each diode D1 of each of the driver circuits of the type 50 shown in FIG. 4 have their cathodes tied together and connected to the collector of transistor Q4 in the manner shown. When the printer is normally running, a voltage is developed across relay S1 to close its relay contact Cs to prohibit current from flowing through diode D3 and into the base of transistor Q4, thereby insuring that transistor Q4 is shut off. In this condition, each diode D1 of each driver circuit cannot shunt current away from the solenoid drivers. When the printer is shut off, however, the power supply which energizes relay Rl will drop before the power supply employed to operate the solenoid driver circuits 50. When this happens, relay R1 opens, thereby allowing the current to flow through resistor R6 and diode D3 into the base of transistor Q4, thereby saturating 04. This causes all the bases of the solenoid drivers to be tied to ground through their diodes D1, thus preventing any of the solenoids from firing on shut off. As shown in FIG. 4, each solenoid and solenoid driver circuit is protected against overload current by a fuse F1.

Although not shown for purposes of simplicity, it should be understood that a cover merlnber is positioned upon housing 31 so as to keep the interior of housing member 31 free from contamination.

lt can be seen from the foregoing description that the present invention described a solenoid assembly especially adapted for use in printers of the dot matrix type in whch high speed of th print wires comparable with present day printers are obtained due to the nature of the design of the solenoid assembly and of the springs employed therein which are further designed to provide a unique operation not heretofore obtained in the performance of such components.

Although there has been described a preferred 'embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

What is claimed is:

I l. A driver circuit for energizing the coil of a solenoid comprising:

a d.c. source;

a capacitor, a first diode and said coil being connected in series across said do. source with one terminal of said first diode connected to one terminal of said capacitor,

a resistor being connected in parallel across the series connected coil and diode;

first transistor means having base, emitter and collector electrodes;

a second resistor coupled to said emitter; said electrode and said second resistor being connected in parallel across the series connected first diode and capacitor;

second transistor means having an input and at least one output terminal;

said one output terminal being connected to the base of said first transistor means;

voltage regulating diode means connected to the input of said second transistor means for controlling the voltage applied to the base electrode of said first transistor means when said second transistor means is rendered conductive to thereby supply constant current to said coil; said diode coupling said coil to said capacitor when said first transistor means is rendered non-inductive.

2. The driver circuit of claim 1 wherein said capacitor and the solenoid coil form a resonant circuit; said first diode being adapted to pass only one-quarter wave of the characteristic frequency to said capacitor when said first transistor means is turned off.

3. The driver circuit of claim 1 further comprising second diode means coupled to the input of said second transistor means;

third transistor means for back-biasing said second diode means when said third transistor means is not conducting;

a second d.c. source;'

means responsive to turn off said second d.c. source for turning on said third transistor means to backbias said second diode means to prevent said coil from being erroneously energized by said capacitOl'.

Claims

1. A driver circuit for energizing the coil of a solenoid comprising: a d.c. source; a capacitor, a first diode and said coil being connected in series across said d.c. source with one terminal of said first diode connected to one terminal of said capacitor, a resistor being connected in parallel across the series connected coil and diode; first transistor means having base, emitter and collector electrodes; a second resistor coupled to said emitter; said electrode and said second resistor being connected in parallel across the series connected first diode and capacitor; second transistor means having an input and at least one output terminal; said one output terminal being connected to the base of said first transistor means; voltage regulating diode means connected to the input of said second transistor means for controlling the voltage applied to the base electrode of said first transistor means when said second transistor means is rendered conductive to thereby supply constant current to said coil; said diode coupling said coil to said capacitor when said first transistor means is rendered non-inductive.

3. The driver circuit of claim 1 further comprising second diode means coupled to the input of said second transistor means; third transistor means for back-biasing said second diode means when said third transistor means is not conducting; a second d.c. source; means responsive to turn off said second d.c. source for turning on said third transistor means to back-bias said second diode means to prevent said coil from being erroneously energized by said capacitor.