US3287614A - Portable, self-powered, corona charging apparatus - Google Patents

Description

M- M. SOWIAK Nov. 22, 1966 PORTABLE, SELF- POWERED. CORONA CHARGING APPARATUS Filed May 27, 1963 \N \NN INVEN TOR. M/zmv M Jaw/4K /1 Mix United States Patent M 3,287,614 PORTABLE, SELF-POWERED, CORONA CHARGING APPARATUS Milton M. Sowiak, Mercerville, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed May 27, 1963, Ser. No. 283,163 9 Claims. (Cl. 317-262) This invention relates to improved apparatus for generating corona, and more specifically to improved corona generating apparatus for electrostatically charging insulating surfaces. I

In electrostatic printing, it is common to apply a uniform electrostatic charge to the surface of an insulating or photoconductive insulating layer. The charge in selected areas is then changed ordissipated to provide a latent image consisting of a pattern of charged areas on the surface. In the case of insulating layers, charge dissipation may be accomplished electrically, and, in the case of photoconductive layers, by exposure of the layer surface to a light image. In either case, the resulting charge pattern can be rendered visible by applying thereto finelydivided developer particles which adhere to the surface by electrostatic attraction.

The procedure for producing visible images on photoconductive layers is often carried out by hand. Electrostatic charging, in a hand process, is carried out, for example, by positioning a sheet of electrophotographic paper on a grounded metal plate and thereafter passing a corona charging device over the paper one or more times. Such a device commonly consists of 2 or 3 fine wires supported on an insulating frame and backed by a grounded shield. Corona is produced when high voltage is applied to the wires from a power supply which converts alternating line voltage to a direct current potential of about 5000 to 10,000 volts.

Production of corona by a fine wire can be observed visually. When such a wire is connected to a source of constant positive high voltage, the corona appears as a substantially uniform sheath surrounding the entire length of the wire. When the wire is connected to a source of constant negative high voltage, the corona is not uniform but rather appears as a series of bright spots at discrete sites along the length of the wire. When the negative wire is used to charge an insulating surface, that surface does not become uniformly charged. Some areas on the sheet are charged to maximum potential whereas other areas have little or no charge. Since the production of useful visible images requires substantially uniform charging, steps have been taken to overcome the deficiencies inherent in the use of corona wires for negative charging. One step has been to make several passes over the surface while charging it. Sometimes a circular motion of the charging unit is also employed. Charging in this manner helps to provide a more uniform charge on a surface by filling in areas which would not be charged during a single pass of the corona unit. Sometimes a single charging unit is provided with two or three wires. This also helps to fill in areas which might not be charged with a single wire. facilitated by providing a metal shield adjacent to and parallel with the corona wires. Further difficulties exist in hand processing because most corona devices require a bulky power supply and a source of line voltage as well Generation of corona by wires has also been 3,287,614 Patented Nov. 22, 1966 apparatus capable of uniformly charging an insulating surface with corona generated from a single wire.

Yet another object is to provide an improved portable hand-operated corona charging apparatus.

These and other objects and advantages are achieved,

in accordance with this invention, with an improved com pact corona generating apparatus which is battery powered. A unitized structure is provided which includes circuit means for producing high voltage corona generating pulses which are applied to one or more corona generating electrodes preferably consisting of one or more fine wires. The circuit elements of the structure are preferably mounted in a housing which provides means for gripping the apparatus during manual operation as well 25 as providing a protective enclosure for the circuit elements. In a preferred embodiment, a single corona generating wire is mounted externally of the enclosed circuit elements for applying charge to a surface when moved thereover.

The invention will be described in greater detail by reference to the accompanying drawings wherein:

FIGURE 1 is a schematic wiring diagram of improved corona generating apparatus in accordance with this invention;

FIGURE 2 is a perspective view illustrating the mounting, with cover removed, of the principal circuit elements of FIGURE 1; and

FIGURE 3 is a perspective view of the improved corona apparatus of this invention as it is used in charging an insulating layer on a conductive surface.

Similar reference characters are applied to similar elements throughout the drawings.

for corona generation. The circuit is battery powered, two batteries 21 and 22 being shown connected in series for purposes of illustration. The negative terminal of battery 22 is connected to the cathode of a semiconductive diode 24, the anode of which is connected to the collector c of a transistor 26. The base b of the transistor 26 is connected through a resistor 27 to one end of the primary winding 28 of atransformer 29. The emitter e of the transistor 26 is connected to a tap on the primary 28 dividing the primary winding 28 into two windings 28 and 28". The other end of the primary winding 28 is connected to the movable contact 30a of a single-pole, double-throw switch 32 having a first, fixed contact 3012 connected to a first roller contact 34. A second, fixed contact 300 of the switch 32 is normally in engagement with the movable contact 30a, as, for example, in a pushbutton switch where a movable contact is spring biased normally against one of two contacts. Another roller contact 36 is connected to the positive side of the battery 21 so that when both the roller contacts 34 and 36 rest on a conductive surface 13 at the same time that the 6 double throw switch is actuated by pressing a push-button 30 attached to the movable contact 30a, a closed current path is completed from the batteries 21 and 22, through the transistor 26, the primary winding 28, and back to the battery 21.

High voltage pulses are produced in another portion of the circuit of FIGURE 1 which includes a secondary winding 38 on the transformer 29. One end of this winding 38 is connected to the same tap on the primary 28 as is the emitter of the transistor 26. The other end of the secondary winding is coupled through a capacitor 40 and a spring connector 45 to a corona wire 44 positioned parallel to and above the conductive surface 13. A rectifier 42, shunts the high voltage output circuit between the corona wire terminal and the movable contact 30a of the switch 32. A bleeder resistor 31 is provided as an optional component connected to the second, fixed contacts 300 of the switch 32 for selective connection in parallel with the rectifier 42 when the push bottom 30 is released.

The following table lists circuit elements actually employed to provide a highly eflicient corona charging device:

Batteries 21 and 22 volt mercury cells (Mallory TR-l34-R).

Transistor 26 2N1905.

Diode 24 1N1764.

Resistor 27 100-l80 ohms.

Resistor 31 100 megohms.

Capacitor 40 500 pf.12.5 kv.

Rectifier 42 5642.

Corona wire 44 2 mil stainless steel 8 /2 or 11 inches long. Transformer primary 28 150 turns #29 HF wire. Transformer secondary 38 6000 turns #37 HF wire. Transformer core 2 ferrite U cores (Allen- Bradley U1620C160B).

In assembling a circuit including the elements tabulated above, care was exercised to obtain as high a performance factor as was feasible and to minimize battery current drain for a desired corona current output. To obtain this output, acei'tain number of ampere-turns are used for the primary winding 28 and the number of primary turns should be high if battery current is to be a minimum. However, if the primary 28 has too many turns, the transistor 26 may fail as a result of being driven into its avalanche region during the first half cycle of oscillation. A compromise was made resulting in winding speeifications producing excellent results. The primary winding 28 was constructed of 150 turns of #29 HF wire tapped at 135 turns and connected to the transistor 26 so that there were 15 turns between the emitter e and base 11 of the transistor 26. The primary winding 28 was wound in two layers on a coil form consisting of a single layer of 0.005 inch kraft paper. The high voltage secondary winding 38 consisted of 6000 turns of #37 HF wire raudom wound on a plastic (polystyrene) coil form. Another secondary winding 39 (FIGURE 1) consisted of 3 turns of #22 wire at a random position on the core of transformer 29 and served as a heater winding for the rectifier 42.

Operation of the circuit of FIGURE 1 is initiated, when both of the roller contacts 34 and 36 rest on the conductive surface 13, by depressing the push button 30 to actuate the switch 32, thereby connecting the center pole 30a to the fixed contact 3012. This completes a current path from the positive terminal of the battery 21, through the roller contact 36, the plate 13, the roller contact 34, the closed contacts 30b and 30a of switch 32, the winding 28", to the collector c of the transistor 26 through two parallel paths. The first of the parallel paths is through the winding 28, the resistor 27, and the basecollector junction of the transistor 26. The second of the parallel paths is through the emitter-collector junction of the transistor 26. After combining at the collec tor c, the current path continues through the diode 24 to the negative terminal of battery 22.

When the switch 32 is first actuated, as stated above, a small current surge is introduced into the winding 28" due to (1) the base-to-collector leakage current and the current to charge the base-collector junction capacitance in the first of the two parallel paths mentioned above, and (2) the emitter-to-collector leakage current (with zero voltage across the emitter-base junction) in the second path. This small current surge through the winding 28" induces a voltage in the winding 28 that forward biases the emitter-base junction of transistor 26, further increasing the current in the winding 28". The current build-up in the winding 28" is regenerative and continues to increase until the magnetic flux in the ferrite core of transformer 29 approaches a maximum, whereupon the voltage induced in winding 28' (and, therefore, the baseemitter current) decreases to a value at which the transistor 26 can no longer sustain a current increase in winding 28". The flux in the ferrite core now begins to decrease, inducing a voltage in winding 28' that reverse biases the base-emitter junction of transistor 26, whereby to cut off conduction of the transistor 26. The magnetic flux in the core now collapses and excites the transformer winding 28 and 38 into oscillation at a resonant frequency determined chiefly by the inductance of winding 38 and a combination of its distributed winding capacitance and external circuit wiring capacitances. After one complete cycle of sinusoidal oscillation, the transistor 26 conducts again and current begins to increase in the winding 28" as described above to begin a repetitive sequence of events.

In the resonant period, during the first half cycle of oscillation, the voltage induced in winding 28 is of the polarity whereby the emitter e of the PNP transistor 26 is positive with respect to collector c. The voltage induced in winding 28', however, reverse biases the transistor 26 during this half cycle and keeps it cut off. During the second half cycle of oscillation, the voltages in both windings, 28" and 28' reverse polarity. If the diode 24 were omitted, that is, shorted, the voltage induced in the winding 28' would attempt to forward bias the emitter-base junction. The latter voltage would, however, be opposed by the much larger voltage induced in the winding 28" which would forward bias the collectorbase junction and attempt to reverse bias the emitter-base junction with a high voltage. However, the emitter-base junction can withstand only a few volts reverse bias before it breaks down and conducts due to an avalanche phenomenon. This would result in a very low impedance shunting the winding 28" through the batteries 21, 22, the roller contacts 34 and 36, and the closed switch '32. The diode 24 is'inserted to prevent current flow in this direction. If the diode 24 were omitted, the low impedance of the transistor 26 during avalanche breakdown would load the winding 28", heavily, and the second half cycle of resonant oscillation would be eliminated, resulting in a substantial reduction in the peak-to-peak output voltage. Near the end of the second half cycle, when the induced voltage in the winding 28" has decreased to less than the sum of the battery voltages 21 and 22, the diode 24 becomes forward biased, the emitter-collector voltage reverts to the normal direction for the PNP transistor 26, and the emitter-base junction becomes forward biased. Therefore, the transistor 26 conducts, and the flux in the ferrite core begins to increase as described previously. Using components of the type described above for the circuit of FIGURE 1, pulses of about 6 kilovolts peak-topeak and of a frequency of about 5 kilocycles are obtained. Pulses of an amplitude of at least 5 kilovolts and a frequency of at least 1 kilocycle are necessary to produce a uniform corona discharge. The winding 38 is wound so that its induced voltage is in phase with the voltage induced in winding 28". Since the wave-forms of both induced voltages are identical, the voltage appearing at the upper end of winding 38 and applied to the capacitor 40 is the sum of the voltages across the windings 28" and 38. Using the component values mentioned above, the first half resonant voltage cycle is positive, having a peak amplitude of 3.75 kilovolts, and the second half cycle is negative, having a peak value of 2.25 kilovolts.

The high voltage output of the secondary'winding 38 is applied to the clamping means consisting of the capacitor 40 and the rectifier 42 which clamp the positive peaks to essentially circuit ground, or reference potential (taken as the voltage at the surface 13). This results in a negativevoltage applied to the corona wire 44, having peak excursions of about 6 kilovolts with respect to the grounded plate, the conductive surface 13. The magnitude of the corona current, from the corona wire 44 to the ground plate 13, is determined by the amplitude of the negative, clamped voltage. The peak-to-peak amplitude of this voltage is a function of the peak magnetic flux attained in the ferrite core, and the peak flux is a function of the peak current conducted through the winding 28" by the transistor 26.

For any given Voltage induced in the winding 28', the resistor 27 determines the value of emitter-base forward bias current which, in turn, determines the peak current that can be sustained through the winding 28". Therefore, the value of the resistor 27 determines the magnitude of the corona current. Preferably, the value of the resistor 27 is selected so that a corona current of approximately 50 microamperes flows between the corona wire 44 and the ground plate 13. In general, the value of the resistor 27 is selected as a function of the beta-parameter of the transistor 26 and ditfers from transistor to transistor. Typical values of the resistor 27 fall within a range of from 100 to 180 ohms. v

Power to operate the circuit is preferably obtained from two -volt mercury cells or batteries 21 and 22 in series. Their constant voltage characteristic is desirable for maintaining constant corona current and, therefore, consistent charging results when the resistor 27 is a fixed resistor. The circuit of FIGURE 1 draws an average of approximately 100 milliamps from the batteries 21 and 22 for a corona current of 50 microamps. Since mercury cells, such as Mallory TR-l34R, are rated at 1000 milliampere hours, more than 12,000 ordinary sheets of electrophotographic paper can be charged with a corona unit, having the circuit shown in FIGURE 1, before battery replacement is needed. Ordinary penlight dry cells may be substituted but then only about 500 sheets can be charged using a single set of 6 dry cells. In addition, such dry cells usually have a drooping discharge characteristic and periodic adjustment should be made in the value of the resistor 27 to maintain the desired corona current. If desired, rechargeable batteries, such as the nickel-cadmium types, may be employed but then provi sion of means for recharging them from a source of line voltage is desirable.

The bleeder resistor 31, when included in the circuit, provides a discharge path for the capacitor 40 upon release of the push-button 30, that is, when the movable contact 30a engages the fixed contact 300. Thus, this resistor 31 is a safety device minimizing shock hazard from charge stored in the capacitor 40, which charge might cause shock if contact were accidentally made between the corona wire 44 and one of the roller contacts 34 and 36 after using the corona apparatus in a charging operation.

The corona apparatus of this invention is also shown in FIGURE 2 which illustrates the manner in which the major components of the circuit of FIGURE 1 may be mounted. The apparatus includes a base plate 51 made of nonconductive material. Attached to one end of the base plate is a pair of rollers 34 and 35, a similar other pair being attached to the other end of the base plate 51 (only one roller 36 of the other pair showing in the figure). Suspended under the base plate 51 is the 6 corona wire 44, one end of the wire being attached to an insulating support 53 and the other end to a spring connector 45. The batteries 21 and 22 rest on the base plate 51 separated by an insulating spacer 55. The positive side of one of the batteries is connected to the negative side of the other by a metal bus 57. The diode 24 is connected to the negative side of battery 22 through vertical insulating plate 59 on which the diode 24 is mounted. Also mounted on the vertical plate 59 is a block 61 which supports the transistor 26 and the switch 32. The vertical plate 59, and end plate 63, and side plates 65 and 67, all of insulating material, rest on the base plate 51 to form an open box structure in which the major circuit components are mounted. In addition to those mentioned, the transformer 29 is mounted in the box with its lower U shaped core on the base plate 51. The capacitor 40 is mounted with one end on the base plate 51. Both the capacitor 40 and the transformer 29 are held in position by an insulating strap 69 fastened to the side plate 65 and the other end of the capacitor 40. The rectifier 42 rests on the base plate 51 adjacent the side wall 67. Two metal covers, better shown in FIG- URE 3, are provided. One cover 56 encloses the space wherein the batteries 21 and 22 are mounted. The other cover 58 fits over the vertical plate 59 and the end plate 63 to enclose the major circuit components and provide a hand grip for the corona apparatus.

FIGURE 3 illustrates the manner in which the corona apparatus of this invention is employed to charge a sheet of electrophotographic paper 11 resting on the metal plate 13. The corona apparatus is positioned with its- roller contacts 34, 35 and 36 resting directly on the metal plate 13 on either side of the electrophotographic paper 11. Corona voltage is applied to the corona wire 44 by pressing the push button 30. At the same time, the corona apparatus is moved alternately in the direction of the arrows 15 and 17 to make one or more passes over the paper 11 and thereby produce a substantially uniform electrostatic charge on the surface of the paper 11.

When the movable contact 30a of the switch 32 is engaged with a fixed contact 30b, negative corona generating pulses are applied to the wire 44 and the corona generated thereby has the appearance of a uniform sheath surrounding the wire 44. As a result, an ordinary sheet of electrophotographic paper can be substantially uniformly charged in one second or less with a single corona wire 44 in one pass over the electrophotographic sheet. Thus, the apparatus of this invention obviates the need for multiple wires and/ or multiple passes as normally required when negative corona is generated with a uniform D.C. potential. The apparatus is compact and self-powered.

What is claimed is: 1. A battery-powered corona charging apparatus comprising:

elongated electrode means; battery-powered circuit means for producing pulses of an amplitude of at least 5 kilovolts and of a frequency of at least one kilocycle; means for applying said pulses to said electrode means. 2. A battery-powered corona charging apparatus comprising:

circuit means for producing high voltage pulses of an amplitude of at least 5 kilovolts and of a frequency of at least one kilocycle; housing means providing a protective enclosure for said circuit means and providing means for supporting at least one battery to provide electrical power for said circuit; at least one elongated electrode mounted externally of said housing; and means for applying said pulses to said electrode. 3. A battery-powered corona charging apparatus comprising:

a transistor oscillator circuit for producing pulses of a frequency of at least one kilocycle;

said circuit comprising means for transforming said pulses to an amplitude of at least kilovolts;

housing means providing a protective enclosure for said circuit;

means associated with said housing means for supporting at least one battery;

connection means for supplying battery power to said circuit;

at least one corona generating wire mounted externally of said protective enclosure; and

means for applying said pulses of at least 5 kilovolts to said wire.

4. A battery-powered corona charging apparatus comprising:

a circuit for producing pulses of a frequency of at least one kilocycle;

said circuit comprising means for transforming said pulses to an amplitude of at least 5 kilovolts;

a housing providing a protective enclosure for said circuit;

battery supporting means associated with said housing;

connection means for supplying battery power to said circuit;

a single corona-generating wire mounted externally of said enclosure; and

means for applying said pulses of at least 5 kilovolts to said wire.

5. Battery-powered apparatus for applying a surface charge to a chargeable member resting on a conductive surface, said apparatus comprising:

circuit means for producing high voltage pulses of an amplitude of at least 5 kilovolts and of a frequency of at least one kilocycle;

housing means providing a protective enclosure for said circuit means;

at least one corona generating wire mounted externally of said enclosure;

means for applying said corona-generating pulses to said wire;

battery mounting means associated with said housing means;

at least two electrical contacts externally mounted on said housing adapted to contact said conductive surface; and

connection means coupling said contacts and said battery to said circuit means to supply battery power thereto when said electrical contacts are in contact with said conductive surface.

6. Battery-powered apparatus for applying a surface charge to a chargeable member resting on a conductive surface, said apparatus comprising:

circuit means for producing pulses of an amplitude of at least 5 kilovolts and of a frequency of at least one kilocycle;

housing means providing a protective enclosure for said circuit means;

at least one corona generating wire mounted externally of said enclosure;

means for applying said pulses to said wire;

battery mounting means associated with said housing means;

a pair of electrically conductive support members adapted to support said housing means and said wire over a chargeable member on said conductive surface and to provide electrical contact to said conductive surface adjacent opposite sides of said chargeable member; and

means coupling said support members in said circuit means for producing said pulses when said support members contact said conductive surface.

7. Battery-powered apparatus for applying a surface charge to a chargeable member resting on a conductive surface, said apparatus comprising:

circuit means for producing high voltage pulses of an amplitude of at least 5 kilovolts and of a frequency of at least one kilocycle;

housing means providing a protective enclosure for said circuit means;

battery mounting means associated with said housing means;

electrical coupling means for supplying battery power to said circuit means including on-off switching means operable from the exterior of said housing means and electrical interlock means having a pair of electrical contacts adapted to provide electrical connection to said conductive surface in series with said switching means;

at least one corona generating wire mounted externally of said enclosure; and

means for applying said pulses to said wire.

8. Battery-powered apparatus for corona charging a chargeable member resting on a conductive surface, said apparatus comprising: I

a circuit for producing pulses of a frequency of at least one kilocycle;

said circuit comprising means for transforming said pulses to an amplitude of at least 5 kilovolts;

a housing providing a protective enclosure for said circuit;

means within said housing for supporting at least one battery;

connection means, including on-off switching means,

for supplying battery power to said circuit;

a single corona generating wire mounted externally of said enclosure;

means for applying said pulses of at least 5 kilovolts to said wire; and

means for supporting said housing for movement over a chargeable member on said conductive surface with said wire spaced from and substantially parallel with said member.

9. Battery-powered corona generating apparatus for producing a surface charge on a chargeable member resting on a conductive surface, said apparatus comprising, in combination:

a housing providing a protective enclosure,

means within said housing for supporting at least one battery,

a wire, adapted to emit a corona when energized by pulses of about 6 kilovolts, mounted externally of said enclosure and electrically insulated from said housing,

a pair of electrically conductive support members mounted on said housing adjacent to, and electrically insulated from, opposite ends of said wire and disposed to contact said conductive surface and to support said wire above, and substantially parallel to, said chargeable member when said housing is moved thereover,

a transformer within said enclosure having a primary winding and secondary winding, said secondary winding having one end connected to a tap intermediate the opposite ends of said primary winding,

a circuit for generating pulses of a frequency of about 5 kilocycles and an amplitude of about 6 kilovolts including a transistor having a base, a collector, and an emitter,

means connecting one of said ends of said primary winding to said base,

means connecting said emitter to said tap on said primary winding,

means, including a diode, connecting said collector to one terminal of said battery, said diode being poled -to prevent current from flowing through said transistor in a reverse direction,

means connecting another terminal of said battery to one of said support members,

10 WlIldlHg, sa1d capacitor and sa1d rectifier comprising clamping means, whereby to cause said circuit to produce, when energized, said pulses for application to said wire.

10 References Cited by the Examiner UNITED STATES PATENTS 3/1952 Carlson 317262 X 7/1954 Mayo 317262 X 1/1957 Walkup 250-49.5 X 11/ 1962 Codichini 250-495 X FOREIGN PATENTS 4/1960 Germany.

MILTON O. HIRSHFIELD, Primary Examiner.

SAMUEL BERNSTEIN, Examiner.

I. A. SILVERMAN, Assistant Examiner.

Claims (1)

1. A BATTERY-POWERED CORONA CHARGING APPARATUS COMPRISING: ELONGATED ELECTRODE MEANS; BATTERY-POWERED CIRCUIT MEANS FOR PRODUCING PULSES OF AN AMPLITUDE OF AT LEAST 5 KILOVOLTS AND OF A FREQUENCY OF AT LEAST ONE KILOCYCLE; MEANS FOR APPLYING SAID PULSES TO SAID ELECTRODE MEANS.

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