GB1589664A - Starter circuit for power supplies - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO STARTER CIRCUIT FOR POWER SUPPLIES (71) We, GOULD ADVANCE LIMITED, a British company of Roebuck Road, Hainault, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to starter circuits for power supplies.

In power supplies including a rectifier-capacitor combination large switch-on surges may occur if the switch-on occurs at a time when the a.c. supply is at or near its peak value.

Such surges may also occur in transformer input circuits where the magnetic core is left magnetised in one direction at swith-off. At the subsequent switch-on, and in particular if the first half-cylce of the mains is magnetising the core in the same direction as its remanent magnetisation partial saturation of the core occurs and a large surge current flows. Such surges can also occur at switch-on with nonlinear loads such as tungsten filament lamps.

It is an object of the present invention to provide circuit arrangements in which this disadvantage may be overcome or reduced.

The present invention provides a starter circuit for supplying current to a load, the circuit comprising a controlled switching device for controlling flow of current to the load from an a.c. sdurce, means for controlling operation of the switching device so that on initial switch-on current only flows to the load near the end of a half-cycle of current from the a.c. source, and means for advancing the instant of operation of the switching device, the controlling means comprising a time-constant circuit for initially delaying the instant of operation of the switching device within a said half cycle, said time constant circuit being connected to said a.c. source and including an impedance formed of a plurality of individual impedance elements, and said advancing means comprising means for reducing the effective value of said impedance by altering the number of said impedance elements which are effective in said time constant circuit.

Features and advantages of the invention will become apparent from the following description of embodiments thereof given by way of example when taken in conjunction with the accompanying drawing which shows a power supply arrangement including a bidirectional silicon controlled rectifier.

Referring to the drawing, the circuit includes a bidirectional silicon controlled rectifier in series with the supply line, controlled in such a fashion that immediately after the equipment is switched on the bidirectional silicon controlled rectifier is fired very near to the end of a mains half cycle, thus ensuring that initially a reservoir capacitor is charged from a relatively low voltage. As the capacitor charges up, the phase angle at which the silicon controlled rectifier fires is progressively advanced until the capacitor is fully charged. There after the silicon controlled rectifier is fired at or near the beginning of the half cycle. In Figure 1 the a.c. supply source is shown at 1 and supplies a load comprising a rectifier and smoothing capacitor 2. The input and output terminals of the circuit according to the invention are 3 and 4 respectively.A bidirectional silicon controlled rectifier (triac) 11 is connected between terminals 3 and 4 and shunted by the series connection of capacitor 28 and resistor 29. The triac 11 is fired from the voltage derived by an RC circuit comprising resistor 12 and capacitor 13 which is applied through a bidirectional trigger diode (diac) 18 to the control electrode of the triac 11. The control electrode of the triac is shunted to the output terminal 4 through resistor 19. The time constant circuit 12, 13 is supplied with a square wave alternating voltage generated across two series connected back-to-back zener diodes 14 and 15, shunted by a capacitor 16 and supplied through a resistor 17 by the voltage occurring between terminals 3 and 4.Under normal running conditions, the time constant of the resistorcapacitor circuit 12, 13 is arranged to be such that the triac is fired soon after the start of each half cycle of the supply voltage from the source 1.

The capacitor 13 is shunted by a capacitor 20 in series with a rectifier bridge 21, and the rectifier bridge has its output connected to a capacitor 22. Thus the capacitors 20 and 22 are effectively in parallel with capacitor 13. At the moment of switch-on, capacitor 22 in uncharged, and has the effect of connecting capacitor 20 in parallel with 13. If capacitor 20 has a much larger value than capacitor 13, then the capacitive component in the time constant circtuit is provided by capacitor 20 and will cause the triac 22 to fire late in the half cycle of supply voltage. The time constant of resistor 12 and capacitor 20 is selected so that the firing under these circumstances occurs shortly before the end of the half cycle thus achieving that the load 2 is connected to the supply source 1 only towards the end of the half cycles of supply voltage when the voltage is relatively small.

During subsequent half cycles the capacitor 22 gradually charges, and as it does so, the effect of capacitor 20 on the triac triggering time constant will become less and less, thus advancing the triggering point to that defined only by resistor 12 and capacitor 13.

It will be appreciated that the operation as described above depends on the capacitor 22 being uncharged initially. This condition will not hold when the equipment is switched off and then on again rapidly. In these circumstances the circuit will not operate to ensure that only a small voltage is applied to the load initially. For this reason an automatic discharge circuit consisting of a transistor 23 connected in shunt with capacitor 22, a resistor 24 connecting the collector to the base of transistor 23 a capacitor 27 connecting the base to the emitter of transistor 23, and diodes 25 and 26 connecting the junction of resistor 24 and capacitor 27 to the emitter of transistor 23 is provided.

The rectifier 21 is connected to the junction of capacitor 22 with the collector of transistor 23, and to the junction of the two diodes 25 and 26. In operation, whilst a supply voltage is being fed to the discharge circuit via the rectifier 21, transistor 23 is held non-conducting by the forward biasing of the diodes 25 and 26, and accordingly capacitor 22 retains its charge. Widen the discharge circuit is not supplied from the rectifier 21, as. will happen when the supply is switched off, the transistor 23 is rendered conducting by resistor 24 after a time determined by the time constant of resistor 24 and capacitor 27, and in doing so discharges the capacitor 22. Accordingly the circuit is ready for any subsequent switch-on to provide the required initial low voltage supply at switch-on.

WHAT WE CLAIM IS: 1. A starter circuit for supplying current to a load, the circuit comprising a controlled switching device for controlling flow of current to the load from an a.c. source, means for controlling operation of the switching device so that on initial switch-on current only flows to the load near the end of a half cycle of current from the a.c. source, and means for advancing the instant of operation of the switching device, the controlling means comprising a time constant circuit for initially delaying the instant of operation of the switching device within a said half cycle, said time constant circuit being connected to said a.c. source and including an impedance formed of a plurality of individual impedance elements, and said advancing means comprising means for reducing the effective value of said impedance by altering the number of said impedance elements which are effective in said time constant circuit.

2. A starter circuit according to Claim 1, wherein the impedance elements are capacitors and the reducing means comprises a further capacitor, charging of which effectively removes one of said impedance element capacitors from the time constant circuit.

3. A starter circuit according to Claim 2, wherein the further capacitor is provided with discharge means for discharging the further capacitor on disconnecting the starter circuit from the a.c. source, 4. A starter circuit according to any one of the preceding claims, wherein the switching device is a silicon controlled rectifier device.

5. A starter circuit according to Claim 4, wherein the silicon controlled rectifier device is a triac.

6. A starter circuit substantially as hereinbefore described with reference to the accompanying drawing.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. the source 1. The capacitor 13 is shunted by a capacitor 20 in series with a rectifier bridge 21, and the rectifier bridge has its output connected to a capacitor 22. Thus the capacitors 20 and 22 are effectively in parallel with capacitor 13. At the moment of switch-on, capacitor 22 in uncharged, and has the effect of connecting capacitor 20 in parallel with 13. If capacitor 20 has a much larger value than capacitor 13, then the capacitive component in the time constant circtuit is provided by capacitor 20 and will cause the triac 22 to fire late in the half cycle of supply voltage. The time constant of resistor 12 and capacitor 20 is selected so that the firing under these circumstances occurs shortly before the end of the half cycle thus achieving that the load 2 is connected to the supply source 1 only towards the end of the half cycles of supply voltage when the voltage is relatively small. During subsequent half cycles the capacitor 22 gradually charges, and as it does so, the effect of capacitor 20 on the triac triggering time constant will become less and less, thus advancing the triggering point to that defined only by resistor 12 and capacitor 13. It will be appreciated that the operation as described above depends on the capacitor 22 being uncharged initially. This condition will not hold when the equipment is switched off and then on again rapidly. In these circumstances the circuit will not operate to ensure that only a small voltage is applied to the load initially. For this reason an automatic discharge circuit consisting of a transistor 23 connected in shunt with capacitor 22, a resistor 24 connecting the collector to the base of transistor 23 a capacitor 27 connecting the base to the emitter of transistor 23, and diodes 25 and 26 connecting the junction of resistor 24 and capacitor 27 to the emitter of transistor 23 is provided. The rectifier 21 is connected to the junction of capacitor 22 with the collector of transistor 23, and to the junction of the two diodes 25 and 26. In operation, whilst a supply voltage is being fed to the discharge circuit via the rectifier 21, transistor 23 is held non-conducting by the forward biasing of the diodes 25 and 26, and accordingly capacitor 22 retains its charge. Widen the discharge circuit is not supplied from the rectifier 21, as. will happen when the supply is switched off, the transistor 23 is rendered conducting by resistor 24 after a time determined by the time constant of resistor 24 and capacitor 27, and in doing so discharges the capacitor 22. Accordingly the circuit is ready for any subsequent switch-on to provide the required initial low voltage supply at switch-on. WHAT WE CLAIM IS:

1. A starter circuit for supplying current to a load, the circuit comprising a controlled switching device for controlling flow of current to the load from an a.c. source, means for controlling operation of the switching device so that on initial switch-on current only flows to the load near the end of a half cycle of current from the a.c. source, and means for advancing the instant of operation of the switching device, the controlling means comprising a time constant circuit for initially delaying the instant of operation of the switching device within a said half cycle, said time constant circuit being connected to said a.c. source and including an impedance formed of a plurality of individual impedance elements, and said advancing means comprising means for reducing the effective value of said impedance by altering the number of said impedance elements which are effective in said time constant circuit.

2. A starter circuit according to Claim 1, wherein the impedance elements are capacitors and the reducing means comprises a further capacitor, charging of which effectively removes one of said impedance element capacitors from the time constant circuit.

3. A starter circuit according to Claim 2, wherein the further capacitor is provided with discharge means for discharging the further capacitor on disconnecting the starter circuit from the a.c. source,

4. A starter circuit according to any one of the preceding claims, wherein the switching device is a silicon controlled rectifier device.

5. A starter circuit according to Claim 4, wherein the silicon controlled rectifier device is a triac.

6. A starter circuit substantially as hereinbefore described with reference to the accompanying drawing.