US3435299A

US3435299A - Circuit for energizing an electromagnet

Info

Publication number: US3435299A
Application number: US551796A
Authority: US
Inventors: Harold S Schwartz
Original assignee: Potter Instrument Co Inc
Current assignee: Potter Instrument Co Inc
Priority date: 1966-05-20
Filing date: 1966-05-20
Publication date: 1969-03-25
Anticipated expiration: 1986-03-25

Description

MarchvZ,l 1969` H. s. SCHWARTZ CIRCUIT FOR ENERGIZING -AN ELECTROMAGNET `Filed May 20, 1966 T Lql.

sheet @f2 1 ATTORNEY United States Patent Ogce U.S. Cl. 317-1485 3 Claims ABSTRACT F THE DISCLOSURE A single-shot multivibrator circuit employing a silicon control rectifier for actuating an electromagnet in whlch a potentiometer is used to vary the time constant in the circuit.

This invention relates to a circuit for energizing an electromagnet and, more particularly, to an improved silicon control rectilier circuit for tiring a solenoid actuated Ihammer in a high speed data printer.

Certain high speed impact printers known in the prior art employ a number of electromagnetically operated print hammers in combination with type faces which are mounted on a continuously circulating belt or drum. A tone Wheel or other similar device produces an output signal consisting of a series of pulses indicative of the position of the type faces relative to the hammers. This output signal is coupled as one input to electronic circuit, the other input to which is 1an input data signal from a computer, for example. The electronic circuit produces an output pulse to ire a hammer when the type face opposite that hammer has the character specified to be printed by the input data.

In printing, each hammer must be rapidly accelerated from a rest position and rapidly returned to its rest position in order that the type face be correctly positioned `and essentially stationary during the period of impact. To achieve the necessary acceleration and impact force necessary for printing, high energy (150 watts, for example) pulses are required for energizing the hammer solenoid. In addition, to achieve printing that is uniform in appearance, the duration and total energy of the drive pulses must be accurately controlled.

A number of circuits have been proposed in the prior art for liring electromagnetically operated printer hammers. However, these circuits have proved to be expensive in their construction or inetlicient in their operation or require elaborately regulated power supplies or a combination of these disadvantages.

IOne object of this invention is the provision of an improved ci-rcuit for -iiring printer hammers, which is less expensive to construct than those of the prior art.

Another object of this invention is to provide an improved circuit which operates efficiently and at a relatively high voltage as compared to printer hammer firing circuits of the prior art.

A further object of this invention is to provide a printer hammer firing circuit, which can operate from relatively inexpensive power supply which supplies the tiring circuits of all the hammers.

3,435,299 Patented Mar. 25, 1969 `One more object of this invention is the provision of an improved printer hammer tiring circuit which provides a current pulse with a short rise time and a predetermined, adjustable magnitude.

Briefly, this invention contemplates the provision of a hammer tiring circuit in which the anodes of a pair of silicon controlled rectiiiers (SCRs) are cross coupled by a commutating capacitor to form a monostable multivibrator. Hammer ring pulses are coupled to the gate electrode of one (first) SCR and a Xed reference potential, which is common to all the hammer firing circuits of the printer, is coupled to the gate of the other (second) SCR.

The hammer solenoid is coupled in series between the positive terminal of a low impedance power supply and the anode of the first SCR; a timing capacitor is coupled in series between the cathode of the second SCR and the negative side ofthe power supply.

In the stable state of the circuit, both SCRs are in a high impedance state. Following a tiring pulse, the rst SCR switches to its low impedance state. The timing capacitor simultaneously commences to discharge at a predetermined, adjustable rate through a resistor and the rst SCR. When the cathode potential of the second SOR drops below the reference potential, it switches to its low impedance state and thereby effectively couples the cornmutating capacitor across the rst SOR, back biasing it and switching it to its high impedance state. Thereafter, the second SCR switches to its high impedance state since the current flow through it rapidly falls below the value necessary to sustain conduction owing to the series coupled timing capacitor. The circuit is thus returned to its stable state.

Having briefly described this invention, it will be described in greater detail along with other objects and advantages in the following detailed description of a preferred embodiment which may be best understood by reference in the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and which like reference numerals are used to indicate like parts in the |various views;

FIG. l is partially a schematic diagram of one embodiment of an improved hammer -iiring monostable multi vibrator constructed in accordance with this invention, and is partially a block diagram showing certain components of a high speed printer in which the monostable multivibrator shown schematically is used; and

FIG. 2 is a timing diagram of typical, idealized wave forms for the firing circuit shown schematically in FIG. 1.

Referring now to FIG. 1, a high speed impact printer typical of those known in the prior art has a row of pivotally mounted hammers 12 disposed in a line adjacent a chain formed of type slugs 14 secured to a belt 16 in a manner known in the art. A pair of drive sprockets or pulleys 22 continuously circulate the belt 16 in a countercloclcwise direction, for example, and a tone Wheel 24, or other suitable apparatus 'known in the art, coupled to one of the pulleys produces a series of output pulses which are indicative of the position of the slugs 14 relative to the hammers 12. These output pulses are coupled as one input toy a decoding and comparing circuit 20. An input from a computer or magnetic tape transport or the like is coupled to another input terminal 21 of the circuit 20.

As will be appreciated readily by those skilled in the art, the three hammers shown are merely representative. A typical printer actually employs from 60 to 120 hammers. Additionally, it will be understood that this invention is equally applic-able to printers employing a continuously rotating drum as to printers employing a chain or belt as shown.

An electromagnet 26 is disposed to attract an armature secured to the shaft of the hammer 12 and to thereby cause the head of the hammer to strike a web of paper (not shown) passing between the hammers and the slugs 14 and to push the web against one of the type slugs. Each electromagnet comprises a U-shaped core of low permeable material such as iron and a coil 32 made of a large num'ber of turns of low resistance wire wrapped around the base of the core.

yIn accordance with this invention, the solenoid 32 for each hammer 12 is energized by a respective SCR monostable multivibrator circuit generally indicated by the reference numeral 34. All of the monostable multivibrators are coupled to a common low impedance direct current (D.C.) power supply 36, and to a common D.C. reference potential source 38. Since all of the multivibrator circuits are identical, only the circuit coupled to the uppermost hammer in FIG. l is shown in detail.

Each multivibrator circuit comprises a pair of silicon controlled

rectiers

42 and 44 whose anodes are interconnected by a commutating capacitor 46. The coil 32 is connected in series between the positive terminal of the power supply 36 and the anode of SCR 44. It should be noted that the D.C. impedance in series with the coil 32 determines -both the rise time of a current pulse in coil 32 (the larger the series resistance the smaller the rise time) and, for a given power supply potential, the current flow through the coil (the larger the resistance, the smaller current ow). It is desirable, of course, that the rise time be short and the current ow large. It has been found advantageous to operate with series resistance of approximately 50 ohms in most applications and a relatively high source voltage. The coil 32 is advantageously made so that it has as low a distributed resistance as is practical in order that the amount of heat which must be dissipated from the coil be as small as possible. For this reason, preferably, an external resistor 48 is coupled in series with the coil to provide the desired operating characteristic.

The gate electrode of SCR 44 is coupled through a pulse differentiating network (resistor 52 and capacitor 54) to a terminal 56 to which is coupled one output terminal of circuit 20. Positive pulses coupled to terminal 56 cause SCR 44 to switch from its high impedance to its low impedance state.

A resistor 58 couples the anode of SCR 42 to the positive terminal of the power supply 36; a timing capacitor 62 couples its cathode to the negative or ground terminal of the power supply 36. A forwardly biased diode 64 couples the gate electrode of SCR 42 to a positive terminal of the D C. reference potential 38, and the gate to cathode bias for SCR 42 develops across a resistor 66 which interconnects these electrodes.

Resistor 68 and potentiometer 72 couple the cathode of SCR 42 to the anode of SCR 44 so that, when SCR 44 is in its low impedance state, resistor 68 and potentiometer 72 form a discharge path lfor the timing capacitor 62. As will be obvious to those skilled in the art, the time constant of the R-C circuit comprising resistor 68 and potentiometer 72 and capacitor 62 may be varied by varying the potentiometer 72.

Referring now to FIG. 2 in addition to FIG. l, in operation SCR 42 and SCR 44 are both initially in their high impedance (non-conducting) state so that the potential at 'the anode of each is approximately equal to the potential at the positive terminal of power supply 36. In response to a trigger pulse a at terminal 56, SCR 44 switches to its low impedance state and its anode rapidly drops to apy proximately ground potential. Current flow through coil 32 rises rapidly, energizing the solenoid and thereby ring the hammer. Current also flows from power supply 36, through resistor 58, commutating capacitor 46 and SCR 44, charging capacitor 46 luntil the potential on the lefthand side of the capacitor equals the potential at the positive terminal of supply 36.

Timing capacitor 62, which had been charged to the potential of power supply 36, commences to discharge through resistor 68, potentiometer 72 and SCR 44. When the potential of the cathode of SCR 42 drops slightly below the reference potential 38, SCR 42 switches to its low impedance state, eifectively coupling commutating capacitor 46 across SCR 44, thereby back biasing SCR 44 and switching it to its high impedance state. Following the termination of conduction through SCR 44, the value of current flow through SCR 42 rapidly falls below the value needed to sustain conduction owing to the series coupled timing capacitor 62, and SCR 42 switches to its high impedance state.

Current flow through SCR 42 prior to its switching to its high impedance state charges capacitor 62 to approximately the potential of power supply 36, thus returning to its initial stable state. The circuit will remain in this state until another positive pulse is applied to terminal 56.

It should be noted that SCR 42 may not switch to its high impedance state if the leakage current through capacitor 62 is larger than that necessary to sustain conduction in SCR 42. However, this does not affect the overall operation of the circuit since SCR 42 will be switched to its high impedance state in any event following the switching of SCR 44 to its low impedance state owing to the commutating action of capacitor 46 following the switching of SCR 44.

It will be understood that certain features` and subcombinations are of utility and may 'be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. It is further obvious that various changes may be made in details within the scope of the claims without departing from the spirit of the invention. It is, therefore, to be understood that this invention is not to be limited to the specific deals shown and described.

Having thus described the invention, what is claimed is:

1. A monostable multivibrator circuit for energizing an electromagnet comprising in combination:

a first silicon control rectifier (SCR) having an anode,

a cathode and a gate electrode, and |having a rst state in which the impedance between said anode and said cathode is a high value and a second state in which the impedance between said anode and said cathode is a low value,

a power supply,

an electromagnet,

means for coupling said electromagnet in a series between one terminal of power supply and the anode `of said iirst silicon control rectiier,

a source of pulses for switching said iirst SCR from said high impedance state Ito said low impedance state,

means for coupling said pulse source to the gate electrode of said rst SCR,

a second SCR having an anode, a cathode and a gate electrode and having a iirst state in which the impedance from said anode to said cathode is a high value and a second state in which the impedance from said anode to said cathode is a low lvalue,

means for coupling said one terminal of said power supply to the anode of said second SCR,

a reference potential source,

means for coupling the gate electrode of said second SCR to said reference potential source,

a timing capacitor coupled in series with the cathode of said second SCR, i

vmeans for charging said timing capacitor to a potential higher than said reference potential,

means including said rst SCR and means for varying the time constant of said timing capacitor for discharging said timing capacitor at a predetermined adjustable rate to a potential below said reference potential, thereby causing said second SCR to switch to its low impedance second state, and f a commutating capacitor coupling the anode of said rst SCR to the anode of said second SCR whereby said rst SCR is back biased and switches to its high impedance state when said second SCR switches to its low impedance state.

2. An electronic circuit as in claim y1 wherein said time constant varying means includes a potentiometer.

UNITED STATES PATENTS 7/1968 Piccione 317-1485 1/ 19617 McCormick.

JOHN F. COUCH, Primary Examiner. W. H. BEHA, JR., Assistant Examiner.

U.S. C1. X.R. lOl-93; 307-265, 273