Title:
Arc generator for an emission spectrometer
United States Patent 3906291


Abstract:
The invention relates to an arc generator for an emission spectrometer. The generator notably comprises an ignition circuit which renders an ignition electrode conductive, and a low-voltage circuit which produces a discharge in a discharge electrode and which notably comprises a capacitor and a coil, but no resistor.



Inventors:
Schayes, Raymond Georges (Brussels, BE)
Gustin, Pol Ambroise Ghislain Joseph (Brussels, BE)
Schmitz, Jean Fernand Raymond (Ganshoren, BE)
Application Number:
05/402181
Publication Date:
09/16/1975
Filing Date:
10/01/1973
Assignee:
U.S. PHILIPS CORPORATION
Primary Class:
Other Classes:
307/75, 307/86, 315/172, 315/173, 315/176, 356/313
International Classes:
B23K9/067; G01N21/67; H01T15/00; H02M3/135; H02M7/523; H05B31/30; H05B41/30; (IPC1-7): H05B41/22; H05B41/30
Field of Search:
315/171,173,176,160,167,170,172 356
View Patent Images:
US Patent References:
3749975ADJUSTABLE WAVEFORM SPARK SOURCE1973-07-31Walters
3376470Capacitor discharge circuit for starting and sustaining a welding arc1968-04-02Stone et al.
3309567Pulse discharge lamp circuit1967-03-14Flieder et al.



Primary Examiner:
Lawrence, James W.
Assistant Examiner:
La Roche E. R.
Attorney, Agent or Firm:
Trifari, Frank R.
Claims:
What is claimed is

1. An arc generator for an emission spectrometer comprising, a pair of spaced electrodes defining an arc gap therebetween, an ignition circuit for generating a voltage capable of igniting an arc across said pair of electrodes, said ignition circuit comprising a capacitor, a charge circuit for the capacitor that includes a first source of DC voltage and a capacitor discharge circuit that includes a controlled rectifier device for selectively coupling the capacitor to said electrodes, and a low-voltage circuit including only reactive circuit elements for maintaining the discharge across the electrodes and comprising, a second capacitor and an inductor coupling the second capacitor to the pair of electrodes to provide a discharge path for the second capacitor across the electrodes and via said inductor, and a control circuit for controlling an energy current for the discharge across said pair of electrodes, said control circuit comprising a thyristor and a coil in series with the second capacitor across a source of DC voltage, and a further thyristor controlled by a level-detection circuit with input terminals coupled across the second capacitor.

2. An arc generator as claimed in claim 1 wherein the low-voltage circuit further comprises a supervision circuit coupled to the second capacitor for checking the end of a one-pole discharge, a variable coil, a diode, and means connecting the variable coil and the diode in series with the second capacitor to allow selective passage of half current periods.

3. An arc generator as claimed in claim 1 wherein said low-voltage circuit includes a diode coupling the second capacitor to the pair of electrodes and means for selectively providing one-pole or oscillating discharges by means of a selection system provided with a contactor for short-circuiting said diode so as to obtain an oscillating discharge.

4. Apparatus for generating an arc discharge across a pair of spaced electrodes comprising, an ignition circuit for initiating an arc discharge and a low-voltage circuit for maintaining said arc discharge and including only reactive passive components, said ignition circuit comprising, a first source of DC voltage, a first capacitor, means for selectively charging said first capacitor from said first DC voltage source and discharging said first capacitor across said pair of electrodes, and means controlled by the first capacitor voltage for terminating the capacitor charge cycle at a given capacitor voltage level and for initiating the start of the discharge cycle, and said low-voltage circuit comprises a second capacitor, a second DC voltage source, a charge circuit for selectively coupling the second capacitor to the second DC voltage source and including a first controlled rectifier device, a discharge circuit for selectively coupling the second capacitor to said pair of electrodes and including an inductor, and a second means controlled by the second capacitor voltage for terminating the second capacitor charge cycle at a given capacitor voltage level and for initiating the start of the second capacitor discharge cycle.

5. Apparatus as claimed in claim 4 further comprising a diode connected in series with said inductor in the second capacitor discharge circuit, and means for selectively short-circuiting said diode to provide an oscillatory mode of discharge cycle.

6. Apparatus as claimed in claim 4 wherein said low-voltage circuit further comprises a second controlled rectifier device connected to form a closed loop circuit with said second capacitor and said inductor and being triggered into conduction at a different time than said first controlled rectifier.

7. Apparatus as claimed in claim 6 wherein the discharge circuit of said low-voltage circuit further comprises a diode connected in series with the second capacitor, a part of said inductor and said part of spaced electrodes.

8. Apparatus as claimed in claim 4 wherein said ignition circuit further comprises, an auxiliary DC voltage source, a second controlled rectifier for selectively coupling the first capacitor to said auxiliary voltage source for charging the first capacitor with a voltage of reverse polarity to that provided by the first voltage source.

9. Apparatus as claimed in claim 8 wherein said means for selectively charging and discharging the first capacitor comprises third and fourth controlled rectifiers, and means for triggering said second, third and fourth controlled rectifiers into conduction at different times.

Description:
The invention relates to an arc generator for an emission spectrometer which can be used for various applications. This generator, referred to hereinafter in general as "source", serves to generate an electric arc discharge on a discharge electrode such that the sample evaporates and is excited.

The said source comprises two principal circuits: on the one hand, the control circuit which serves to render the ignition electrode conductive, and on the other hand the low-voltage circuit which serves to maintain the energetic discharge on said electrode.

All known sources comprise a low-voltage circuit in which use is made of a capacitor, a coil and a resistor in a given combination, it being possible to adjust the said circuit elements to the different types of discharge required by the analyses.

The arc generator (source) according to the invention comprises a low-voltage circuit in which only a capacitor and a coil are used. The absence of the dissipative circuit element (the resistor) ensures high efficiency of the low-voltage circuit, with the result that the required power is substantially smaller than the power required for spectrum emission measurements by contemporary sources for the same dissipated energy quantity in the discharge electrode.

The invention will now be described in greater detail with reference to the accompanying drawing in which:

FIG. 1 shows a circuit diagram of a source (arc generator) according to the invention.

FIG. 2 shows diagrams of the control circuit and the low-voltage circuit thereof, and

FIGS. 3, 4 and 5 show diagrams of circuit waveforms for a further explanation of the operation.

FIG. 1 shows the circuit diagram of an arc generator according to the invention which is of the modular type, comprising the combination of an ignition unit or ignition circuit 1, a control unit or control circuit 2, and a low-voltage unit or low-voltage circuit 3.

The ignition circuit is used for rendering the ignition electrode conductive and is supplied, via a charging circuit 4, from a source which supplies a direct voltage Vtrig which is derived by rectification of the supply voltage.

Control circuit 2 regroups all external control signals of the generator and provides correct selection and switching of the various analytic parameters. The said control circuit 2 supplies the control pulses in a sequence as required for the operation of the other circuits. In addition, the said control circuit 2 also supervises the operation of the generator. The low-voltage unit 3 comprises the low-voltage discharge circuits and the supply sections of these circuits.

FIG. 2 shows the diagrams of the two principal circuits, the ignition circuit (at the top in FIG. 2) serving to render the ignition electrode conductive, and the low-voltage circuit (at the bottom in FIG. 2) which serves to generate the energetic discharge on said electrode.

The power supply of the ignition circuit is provided by a source which supplies a direct voltage Vtrig which is obtained by rectification of the supply voltage. When the arc generator is switched on, an auxiliary voltage source is used which supplies an auxiliary voltage Vaux which is obtained in the same manner. The ignition circuit operates as follows. A capacitor 5 is initially discharged. A first control pulse φ1 is applied to thyristor 6. This pulse φ1 renders the thyristor 6 conductive, with the result that the capacitor 5, the coil 7 and the auxiliary voltage source are connected in series. A sinusoidal current pulse passes through the circuit and charges the capacitor 5 to a voltage which is equal to -2 Vaux if the effect of the losses occurring in the circuit is ignored (see FIG. 3II).

The capacitor 5 is disconnected from the circuit by switching off thyristor as soon as the current tends to flow in the other direction; in this case the capacitor 5 is then no longer connected to the auxiliary voltage source. As soon as this has taken place, a control pulse φ2 can be applied to thyristor 8. Because the thyristor 8 is now conductive, a series connection is obtained which is formed by the circuit elements 5, 9 and the trigger voltage source, the capacitor 5 having a negative charge corresponding to -2 Vaux. This circuit then produces a sinusoidal current pulse by which the capacitor 5 is charged to a voltage which is equal to 2Vtrig + 2Vaux (see FIG. 3II) if the circuit losses are ignored.

The voltage across the capacitor 5 is continuously measured by a level-detection circuit 10. This circuit supplies a control pulse as soon as a variable threshold is reached. If the control system is designed such that this control pulse is obtained at V5 = V5F, a thyristor 11 becomes conductive, with the result that the thyristor 8 is cut off. The energy still present in coil 9 is recovered by the source which supplies the direct voltage Vtrig.

At the instant at which thyristor 8 is cut off, the voltage at the electrodes of this thyristor has the value: (VAK)8 = V5F - (1 + 1/m) Vtrig, in which m is the ratio between the numbers of turns of the coil 9.

As of this instant, capacitor 5 is charged to a predetermined voltage V5F and is no longer connected to the charging circuit. A control pulse φ3 can then be applied to the control electrode of thyristor 12. The capacitor 5 is then discharged via thyristor 12 and the primary winding of a high-voltage pulse transformer 13.

The variation of the current through the capacitor 5 flows in accordance with FIG. 4-I. The voltage across a capacitor 14 increases consinusoidally until an auxiliary spark gap 15 breaks down. This corresponds to the first part of the discharging of the capacitor 5 (oscillation circuit formed by the series connection of the dispersion inductance of transformer 13 and the capacitors 5 and 14 with respect to the primary transformer side). After the breakdown of the auxiliary spark gap 15, a high-voltage wave having a steep edge is thus applied to an ignition electrode 16 which is also rendered conductive. Before the discharge of the capacitor 5 terminates, the procedure is as if the secondary side of transformer 13 were short-circuited and a half-period oscillation develops which corresponds to the series connection of the dispersion inductance of the transformer 13 and the capacitor 5. This phenomenon ceases as soon as the current tends to change its direction via the thyristor 12 (disconnection from circuit by way of normal changing-over). At this instant the polarity of the voltage on the ends of the capacitor 5 is inverted with respect to the polarity of the charging voltage V5F at the beginning, so it becomes -V5R.

The parameters of the circuit are such that in the case of normal operation the voltage V5-R is larger than Vaux.

The cycle can be repeated by first applying a control pulse φ1 to the control electrode of thyristor 6. This pulse has no effect because said thyristor 6 has the inverted polarisation voltage at this instant. The operation is subsequently continued in the described manner. However, a difference exists in that the initial voltage of the capacitor 5 is no longer equal to -2 Vaux, but is equal to -V5R.

In accordance with the desired type of discharge, the low-voltage circuit can function in two different ways, i.e. one-pole operation or oscillating operation. The change-over is effected by means of a contactor 28. One-pole discharging is based on a direct voltage VBT which is derived from the supply voltage by rectification. Using a control pulse φ2, a thyristor 17 is rendered conductive at the same instant as the thyristor 8 of the ignition circuit. It is assumed that at the beginning a capacitor 18 has a negative voltage -Ein which is the case, as will be established hereinafter, after the first cycle. Because the thyristor 17 becomes conductive, the elements 18, 19 and the source supplying the voltage VBT are connected in series. This is an oscillating circuit in which a sinusoidal current pulse will tend to develop; the duration of this pulse is π√L19 C18 and the amplitude thereof is given by the expression ##EQU1## (see FIG. 5).

The voltage at the terminals of the capacitor 18 increases according to a cosine function and tends to reach the value 2 VBT + Ein if the losses in the circuits are ignored. A voltage VF is desired on the capacitor 18 for discharging.

To this end it is ensured that the level-detection circuit 25 which continuously measures the voltage at the terminals of the capacitor 18 supplies a control pulse as soon as V18 = VF. This is possible in as far as VF < 2 VBT + Ein.

This condition can be satisfied by a suitable choice of the circuit elements. The control pulse renders a thyristor 20 conductive, which corresponds to the application of a voltage VBT to the ends of the auxiliary winding of a coil 19. The thyristor 17 is thus cut off if VF is larger than VBT (1 + 1/m), m being the ratio of the number of turns of the main winding of the coil 19 to the number of turns of the auxiliary winding of said coil. The energy still present in coil 19 is recovered by a thyristor 20 in the source for the voltage VBT at the instant of disconnection. The capacitor 18 is thus charged to a voltage VF, and the charging circuit is no longer connected to said capacitor.

Discharging can now commence. Discharging is controlled by the ignition circuit which renders the ignition electrode 16 conductive. The high-voltage pulse having a steep edge is converted into a low-voltage by a low-pass filter (21-22).

The low-voltage ignition circuit is formed by the series combination of a capacitor 18, initially charged to a voltage VF, a coil 23 and a diode 24. The capacitance of the capacitor 18 and the inductance of the coil 23 are chosen in accordance with the type of discharge required (proper choice of the current peak value and the duration thereof).

The discharge realized is of the one-pole type, while the current is sinusoidal (FIG. 5). The amplitude of this current is equal to ##EQU2## and the duration thereof is given by the expression π√(L23 + L21)C18.

The end of discharging is determined by the circuit elements which are not directly coupled to the discharge electrode 16, and hence it is reproducible from one discharge to the other. If the current in the diode 24 tends to change its direction, the discharge current is switched off after one half current period has passed. The polarity of the voltage at the ends of capacitor 18 then changes, and the energy which remains, ignoring the losses in the circuits, is equal to the initial energy 1/2 C18 VF2 minus the energy dissipated in the ignition electrode 16. This is the previously referred to voltage Ein.

The charging cycle can be resumed as of this negative residual voltage. After the very first discharge the value of Ein is equal to zero, but after a few discharges a residual voltage will always remain.

In order to achieve operation according to the oscillating mode, the change-overs are realized by three sets of contacts 28. The diode 24 is short-circuited, the cathode of thyristor 20 is connected to ground, and the initial direct voltage is increased to the value VBT + V'BT.

The initial voltage at the ends of the capacitor 18 can then be equal to zero, and charging itself will be performed as previously by rendering thyristor 17 conductive. The voltage at the ends of the capacitor 18 tends to reach the value 2(VBT + V'BT), but the charging is interrupted by means of a level-detection circuit 25 if V18 = VF < 2(VBT + V'BT).

As soon as the charging circuit is no longer connected to the capacitor 5, discharging can take place; this discharging will have an oscillating character with a period of 2 π √ (L23 + L21)C18. The damping obtained is dependent on the losses incurred in the circuits and of the energy dissipated in the ignition electrode 16. At a given instant, the circumstances prevailing on the terminals of the ignition electrode will be such that after the current has been reduced to zero no further ignition can take place. The said circumstances are dependent on the type of ignition electrode used, of the analysed material, of the atmosphere used, etc.

Consequently, a given uncertainty exists as regards the polarity of the residual voltage which will remain on the ends of capacitor 18 after the last half period of the discharging low-voltage. In order to eliminate this uncertainty, a control pulse is applied to a thyristor 26 before the next charge is started (φ1 in synchronism with the pulse applied to thyristor 6 of the ignition circuit).

Thyristor 26 is connected in series with a coil 27, a part of the low-voltage coil 23, and capacitor 18. If the residual voltage of capacitor 18 is positive, the thyristor 26 becomes conductive, and this conductive state allows passage of one half period of a current whose amplitude is given by ##EQU3## the duration thereof being equal to

π √ C18 (L27 +L23 +L23')

As a result, the polarisation of the voltage of the capacitor 18 is inverted. If the residual voltage is negative before application of a pulse φ1, nothing happens because the thyristor 26 then has a polarisation voltage of opposite sign. It is to be noted that this is also so in the case of operation according to the non-oscillating mode (mono half period): even though the thyristor 26 receives control pulses, it does not become conductive due to the fact that at this instant the voltage on the thyristor terminals has opposite polarity.

The charging cycle can then be completed as usual. The low-voltage capacitor 18 then already has an initial voltage which is equal to zero which is negative. The choice of the type of discharge is also effected by a suitable choice of the capacitance of the capacitor 18 and of the inductance of the coil 23.