Apparatus for producing positive or negative ions, especially for neutralizing charged workpieces

A pulse generator operating at a frequency preferably somewhat higher than 100 Hz charges a capacitor through a diode and the trailing edge of the pulse switches-in a discharge circuit to discharge the capacitor through the low-voltage winding of a pulse transformer, the high-voltage winding of which charges, through another diode, the high-voltage insulated conductor of an ionizer that cooperates with a point-bearing electrode that is grounded. The high-voltage electrode discharges through a resistor between pulses and the diode assures that ions of the same polarity will be produced. The ionizer is suited for being located inside a spray gun of an electrostatic coating apparatus for charging the coating material particles with the ions without producing an electric field between the spray gun and the workpiece that would interfere with the coating of hollow places of the workpiece because of the so-called cage effect.

This invention concerns an apparatus for producing positive and/or negative 
ions by means of a high-voltage ionizer operating with an electrostatic 
induction discharge. The conductor of the apparatus which is at high 
voltage with respect to the environment is insulated, whereas the point 
discharge electrodes are grounded. 
In electrostatic coating with liquid paint droplets or with powder or in 
electrostatic flocking to provide a napped coating, spraying apparatus 
with a protruding high-voltage electrode has heretofore been used. Such a 
spray apparatus produces an electric field, along the field lines of which 
the coating material flies through the intervening space to the workpiece 
by electrostatic forces. 
This electric field, however, gives rise to a so-called cage effect in the 
coating of hollow bodies. The electric field lines or force going out of 
the spray gun run predominantly to the outside of the hollow body and 
accordingly the coating material flies only poorly into the hollows. 
It is an object of the present invention to avoid this disadvantage. 
Furthermore, in countless practical applications there is a need to 
maintain workpieces so far as possible in a state devoid of electrical 
charge, or to lead away a charge present on a workpiece so far as possible 
in order that the handling of the workpiece by factory or shop personnel 
should be trouble-free and especially without danger, so far as possible, 
of electrical shock. This need exists for example in electrostatic 
spray-coating whenever possible spark discharges of the workpiece could 
lead to explosive ignition of the powder and air mixture contained in the 
spraying cabinet. Furthermore, the quality of coating is impaired since 
then there is a change in the effective potential drop between spray 
apparatus and workpiece. 
High-voltage ionizers of the general kind above mentioned which operate 
without contact are known from U.S. Pat. No. 3,369,152 and their use for 
electrostatic spray coating is known from U.S. Pat. No. 4,042,971. In each 
case the ionizers described are ionizers driven by alternating voltage. 
That feature leads to the result that the ions produced at the points of 
the grounded electrodes are, in alternation, positively and negatively 
charged. Many of the positive ions are neutralized by recombination with 
negative ions, and vice versa. The ion yield is thereby impaired and 
likewise the spatial extent of the ionization. 
There are also known direct current ionizers by which ions all charged in 
the same polarity are produced. In these the point electrodes which are 
freely accessible from the outside must stand at high voltage, however, so 
that there are problems of protection against contact and risk of 
spark-over. These ionizers are quite out of the question for installations 
where there is risk of explosion. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a high-voltage ionizer 
of the general kind above described, namely one with grounded point 
electrodes, improved in such a way that the above described recombination 
of the ions produced does not take place. At the same time the ionizer 
should lend itself well to charging of coating material without also 
building up an electric field between ionizer and workpiece. 
Briefly, the insulated conductor of the ionizer is connected, on one hand, 
through a diode over to a switchable discharge circuit of its capacitor 
which can be charged up in a predetermined polarity and, on the other 
hand, the said insulated conductor of the ionizer is grounded through a 
discharge resistance. 
As a result of the capacitor which can be charged in a polarity that 
remains the same, the result is obtained that the discharge current in the 
ionizer continuously produces ions of the same polarity. In order that the 
discharge of the capacitor does not complete itself in the form of a more 
or less damped oscillation, the insulated conductor of the ionizer is 
connected through a diode to the discharge circuit of the capacitor. In 
that way, the expectable resonant circuit behavior of the ionizer together 
with the discharge circuit is excluded, which is to say that in the 
negative phase (back flow of the charge) no ions of opposite sign can be 
produced in the ionizer which would contribute to recombination. The 
stored charge can thus not flow back into the discharge circuit of the 
capacitor, but instead is led away by the discharge resistance of the 
insulated conductor in order to make possible a new charge current pulse 
for the ionizer. 
The ionizer is exposed to a high value of charging up current and a low 
value of discharge current. The discharge accordingly takes place tenfold, 
hundredfold or thousandfold slower than the charging up, so that 
practically no ions of opposite polarity are produced. For each charging 
pulse a certain quantity of ions of the same polarity are produced in 
front of the insulated conductor and the succeeding ion boost of the same 
polarity drives these existing ion clouds off and thus away from the point 
electrodes. Accordingly, it is desirable to operate with a pulse frequency 
that is as high as possible, especially a frequency above 100 Hz. The 
charge pulse is then present only for very little time and therefore a 
resistor of low power rating is sufficient for the discharge resistance. 
Because of the insulation of the high-voltage conductor the ionizer can be 
built into a spray apparatus without danger, so that the charging up of 
the coating material takes place in the interior of the spray apparatus 
and the heretofore necessary electrodes disposed at the spray gun exit are 
dispensed with. An electrical field over to the workpiece with the 
undesired cage effect accordingly cannot develop. 
In a further elaboration of the invention it has been found particularly 
useful for the discharge circuit to contain one of the windings of a 
high-voltage transformer of which the other winding is interposed in the 
connecting conductor of the ionizer. The advantage then results that the 
discharge circuit can be operated at low voltage and only the conductor 
connected to the ionizer itself is at high voltage. The cabling of the 
installation is thereby made substantially more economical. A 
thyristor-controlled pulse generator provided with an oscillator is 
desirable for energizing the capacitor, which interrupts the connection of 
the capacitor to the generator and closes an interrupter in the discharge 
circuit after a predetermined desired voltage is reached at the capacitor. 
Furthermore, a monitoring and regulating unit can be connected to the 
pulse generator so that chance of short circuits or impermissibly high 
currents or voltages can be recognized at once and result in shutting down 
the apparatus.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The ionizer 1 consists of a conductor 2 which is insulated against the 
environment and surrounds a row of upwardly projecting point electrodes 3. 
The conductor 2 is connected to a high voltage of between 50 and 150 kV, 
while the point electrodes 3 are grounded. It is of course possible to 
provide the construction of the ionizer in some other way going back to 
other kinds of construction as shown for example in German Patent No. 25 
36 091. 
The conductor 2 is on the one hand connected through a discharge resistance 
4 to ground and on the other hand connected over a diode 5 and a 
high-voltage transformer 6 to the discharge circuit of a capacitor 7. The 
high-voltage winding of the transformer 6 is in series with the already 
mentioned connection of the conductor 2 and the low-voltage winding is in 
series with the discharge circuit. An interrupter switch 8 is also built 
into the discharge circuit. It is operated in response to the downward or 
trailing flank of the discharge pulse of the capacitor 7. 
Since the capacitor 7 is in general connected to alternating voltage, a 
diode 9 is interposed in circuit in front of it. It assures that the 
charging of the capacitor takes place always at constant polarity and the 
ionizer 1 accordingly produces only ions of the same polarity. 
It is within the scope of the invention to integrate the discharge 
resistance 4, the diode 5 and in certain cases also the transformer 6 in 
the casing of the ionizer 1. 
The circuit block diagram shown in FIG. 2 shows how the pulse generator for 
energizing the capacitor 7 is constituted. The circuit comprises a 
transformer 10 equipped with a rectifier not shown, an oscillator 11, a 
control unit 12 for controlling current and voltage, an work switching 
circuit 13, a pause control switch 14 and a current and voltage measuring 
device 15. 
The oscillator 11 that operates at a frequency from 10 to 300 Hz, 
preferably at a frequency above 100 Hz, closes the switch of the work 
switching circuit 13. The capacitor is thereby charged. As soon as the 
measuring circuit 15 determines that the desired voltage has been reached 
at the capacitor, the current supply to the switching circuit 13 is 
interrupted, and the pause switch 14 is closed. The voltage between the 
conductors A and B then falls to zero and this voltage drop in front of 
the diode 9 controls the switch 8, (FIG. 1) which then closes when the 
voltage in front of the diode 9 lies near zero. The capacitor can then 
discharge, as a result of which the discharge current in the high-voltage 
transformer 6 and thereby in the conductor 2 of the ionizer 1 produces the 
desired high-voltage pulse. When the pulse voltage has receded to a 
prescribed value, the switch 8 and the pause switch 14 are opened by known 
electronic means that need not be further described, the working switch 13 
is closed, and the process then repeats itself. 
Of course, the current supply of the circuit shown in FIG. 1 can also be 
arranged in some other way. 
If it is necessary to expose the workpieces to be discharged both with 
positive and with negative ions, it is desirable to utilize two 
correspondingly polarized ionizers, at a distance from each other, 
however, such that their ion clouds will not intersect each other but will 
meet only in the region of the workpiece. 
If the apparatus of the invention, instead of being used for discharging 
workpieces, should be used for charging with the same polarity of charge, 
particularly the charging of coating material of electrostatic coating 
installations, the point electrodes 3 in the flow channel of the coating 
material are so disposed that the coating material is positively or 
negatively charged in passing the ionizer. The coating material in the 
form of powder or droplets then flies through the air driven by 
electrostatic attraction forces over to the workpiece and may in its path 
be supported by compressed air. Since the point electrodes are disposed 
inside the spray gun and thus can build up no electric field with the 
workpiece to be coated, the operation cannot get involved with the 
occurrence of the above-mentioned cage effect. It makes no difference, 
then, whether external surfaces or hollows are to be coated. 
Because the electrodes at high voltage are insulated there is also no 
danger that sparking-over, explosions or the like will take place within 
the spray apparatus. 
The invention also has the advantage that the operation no longer can 
involved mutual neutralization of oppositely charged coating particles. 
The elimination of neutralized coating material that no longer reaches the 
workpiece but merely loads the surrounding air is thereby substantially 
reduced. 
As already mentioned in several respects, variations and modifications are 
possible within the inventive concept.