GB2064891A

GB2064891A - Thyristor converter circuit for welding apparatus

Info

Publication number: GB2064891A
Application number: GB7940103A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-11-20
Filing date: 1979-11-20
Publication date: 1981-06-17
Also published as: GB2064891B

Abstract

The useful control range of a thyristor circuit, in particular for welding plant, is extended by connecting in parallel with the thyristor anode and cathode a diode in series with a transistor, preferably of the unipolar type. At low currents, the parallel connected transistors have a significant effect on the circuit by giving an improved drooping characteristic, assisting the turn off of the thyristors and extending the useful control range to nearly zero. <IMAGE>

Description

SPECIFICATION Thyristor control for welding rectifiers The invention refers to electric circuits including thryistors, in particular to circuits with thyristors used as means for setting and controlling the current supply to welding plant.

It is known to use thyristors in welding rectifiers, either for changing the control current for saturated reactors which are connected in series with the input, or in the opened branches of bridge rectifiers.

With circuits of this type it is usually found difficult with rectifiers intended for use at high current to obtain a satisfactory stable arc at low current settings. It is also difficult to produce a steeply drooping, nearly constant current characteristic at low and medium currents, if the circuit has been designed to cope with high currents. Object of the invention is the improvement of the characteristic of circuits, in particular those of welding rectifiers, containing thyristors. Another object of the invention is the combination of the economical use of thyristors, which are able to cope with high currents, with the ease of control of the plant characteristics achieved with some other semi-conductor devices.

According to the invention a circuit for the control of the output from an alternating current supply, with a thyristor on which the phase of a voltage supplied to its gate is variable, contains a transistor in series with a diode connected in parallel with the thyristor anode and cathode. The transistor or the transistors used according to the invention should preferably be of the unipolartype, like the so called power Mosfet devices which are field effect transistors.

Examples of the invention are shown in the drawing. In Figure 1, a three phase bridge rectifier supplied with a.c. from a source not shown in the drawing contains the three diodes 1, 2 and 3 as well as the three thyristors 4, 5 and 6, which controls the d.c. output into the load 7 which may be a welding arc. The unipolar field effect transistors 11, 12 and 13 are connected in series with diodes 14, 15 and 16, parallel to the thyristors 4, 5 and 6.

The thyristors 4, 5 and 6 are controlled by known conventional means not shown in the drawing. The current through the load 7 produces a voltage drop across the resistor 8 which is amplified in the differential amplifier 9 and compared with a reference voltage by the amplifier 10, the output of which feeds the gates of transistors 11, 12 and 13. The diodes 14, 15 and 16 prevent a reverse current flowing through the transistors 11, 12 and 13.

At low currents the parallel connected transistors have a significant effect on the circuit, by giving an improved drooping characteristic, assisting the turn off of the thyristors and extending the useful current control to very nearly zero.

Figure 2 shows a single phase bridge rectifier 17, the a.c. output of which is controlled by the thyristor 18 connected in the known manner across its d.c.

terminals. According to the invention a unipolar transistor 19, in series with the diode 20 is connected across the thyristor 18. The a.c. output might be used for the current supply to a load 21, like an a.c.

welding arc, where it will improve the arc characteristics in the area of voltage reversal. As an alternative, the a.c. output may be rectified and used for the control of saturated reactors. With a unipolar transistor the current flowing through it should be limited to a fraction of the current supplied in the most favourable phase of its voltage by the thyristor, thus making use of the high current rating of thyristors readily available; a current limiting resistor should be connected in series with a bipolar transistor.

1. A circuit for the control of the output from an alternating current supply with a thyristor, on which the phase of a voltage supplied to its gate is variable, characterised by a transistor in series with a diode connected in parallel with the thyristor anode and cathode.

2. A circuit according to claim 1, with a unipolar transistor.

3. A circuit according to claim 1 with a bipolar transistor connected in series with a current limiting resistor.

4. A circuit according to claim 2, with a unipolar transistor, the current through which is limited to a fraction of the current supplied in the most favourable phase of its gate voltage by the thyristor.

5. A power source for arc welding including a circuit according to claim 1 to 4.

6. A power source according to claim 5 for feeding an a.c. welding arc, including a single phase bridge rectifier with a circuit according to claim 1 to 4 connected across its d.c. terminals.

7. A power source according to claim 5 for feeding a d.c. welding arc, including a bridge rectifier with at least half of its diodes replaced by a circuit according to claim 1 to 4.

New claims or amendments to claims filed on 13/2/81 Superseded claims 1 New or amended claims: 1. A circuit for the control of the output from an alternating current supply with a thyristor, on which the phase of a voltage supplied to its gate is variable, characterised by a transistor in series with a diode connected in parallel with the thyristor anode and cathode, and means for varying the current through said transistor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

SPECIFICATION Thyristor control for welding rectifiers The invention refers to electric circuits including thryistors, in particular to circuits with thyristors used as means for setting and controlling the current supply to welding plant.

It is known to use thyristors in welding rectifiers, either for changing the control current for saturated reactors which are connected in series with the input, or in the opened branches of bridge rectifiers.

With circuits of this type it is usually found difficult with rectifiers intended for use at high current to obtain a satisfactory stable arc at low current settings. It is also difficult to produce a steeply drooping, nearly constant current characteristic at low and medium currents, if the circuit has been designed to cope with high currents. Object of the invention is the improvement of the characteristic of circuits, in particular those of welding rectifiers, containing thyristors. Another object of the invention is the combination of the economical use of thyristors, which are able to cope with high currents, with the ease of control of the plant characteristics achieved with some other semi-conductor devices.

According to the invention a circuit for the control of the output from an alternating current supply, with a thyristor on which the phase of a voltage supplied to its gate is variable, contains a transistor in series with a diode connected in parallel with the thyristor anode and cathode. The transistor or the transistors used according to the invention should preferably be of the unipolartype, like the so called power Mosfet devices which are field effect transistors.

Examples of the invention are shown in the drawing. In Figure 1, a three phase bridge rectifier supplied with a.c. from a source not shown in the drawing contains the three diodes 1, 2 and 3 as well as the three thyristors 4, 5 and 6, which controls the d.c. output into the load 7 which may be a welding arc. The unipolar field effect transistors 11, 12 and 13 are connected in series with diodes 14, 15 and 16, parallel to the thyristors 4, 5 and 6.

The thyristors 4, 5 and 6 are controlled by known conventional means not shown in the drawing. The current through the load 7 produces a voltage drop across the resistor 8 which is amplified in the differential amplifier 9 and compared with a reference voltage by the amplifier 10, the output of which feeds the gates of transistors 11, 12 and 13. The diodes 14, 15 and 16 prevent a reverse current flowing through the transistors 11, 12 and 13.

At low currents the parallel connected transistors have a significant effect on the circuit, by giving an improved drooping characteristic, assisting the turn off of the thyristors and extending the useful current control to very nearly zero.

Figure 2 shows a single phase bridge rectifier 17, the a.c. output of which is controlled by the thyristor 18 connected in the known manner across its d.c.

terminals. According to the invention a unipolar transistor 19, in series with the diode 20 is connected across the thyristor 18. The a.c. output might be used for the current supply to a load 21, like an a.c.

welding arc, where it will improve the arc characteristics in the area of voltage reversal. As an alternative, the a.c. output may be rectified and used for the control of saturated reactors. With a unipolar transistor the current flowing through it should be limited to a fraction of the current supplied in the most favourable phase of its voltage by the thyristor, thus making use of the high current rating of thyristors readily available; a current limiting resistor should be connected in series with a bipolar transistor.

1. A circuit for the control of the output from an alternating current supply with a thyristor, on which the phase of a voltage supplied to its gate is variable, characterised by a transistor in series with a diode connected in parallel with the thyristor anode and cathode.

2. A circuit according to claim 1, with a unipolar transistor.

3. A circuit according to claim 1 with a bipolar transistor connected in series with a current limiting resistor.

4. A circuit according to claim 2, with a unipolar transistor, the current through which is limited to a fraction of the current supplied in the most favourable phase of its gate voltage by the thyristor.

5. A power source for arc welding including a circuit according to claim 1 to 4.

6. A power source according to claim 5 for feeding an a.c. welding arc, including a single phase bridge rectifier with a circuit according to claim 1 to 4 connected across its d.c. terminals.

7. A power source according to claim 5 for feeding a d.c. welding arc, including a bridge rectifier with at least half of its diodes replaced by a circuit according to claim 1 to 4.

New claims or amendments to claims filed on 13/2/81 Superseded claims 1 New or amended claims:

1. A circuit for the control of the output from an alternating current supply with a thyristor, on which the phase of a voltage supplied to its gate is variable, characterised by a transistor in series with a diode connected in parallel with the thyristor anode and cathode, and means for varying the current through said transistor.