GB2448560A - Drive circuit for discharge tube - Google Patents

Abstract

The drive circuit 100 comprises a primary circuit 101 and a secondary circuit 102, both arranged to deliver power to the discharge tube 103. The primary circuit 101 comprises a first power supply 104 and a first controller 108 for controlling the delivery of power from the first power supply 104 to the discharge tube 103. The secondary circuit 102 comprises a power storage arrangement in the form of a capacitor bank 112, a second power supply 111 for charging the power storage arrangement 112, and a second controller 113 for controlling the discharge of the power storage arrangement 112. The second controller 113 is arranged to selectively discharge the power storage arrangement 112 at intervals determined by the first controller 108, such that the selective discharge supplements the first power supply 104 from the primary circuit 101 at times when the first power supply 104 is below a threshold value. This allows a discharge arc of any desired period to be created, and may smooth the lamp supply further than a normal rectified and filtered dc supply would, and prevents uneven optical output, or flickering, from the lamp 103.

Description

Drive Circuit for a Discharge Tube The present invention relates to a

drive circuit and particularly, but not exclusively, to a drive circuit for a discharge tube (also known as a discharge lamp).

Discharge tubes typically comprise an arrangement of electrodes in a gas, housed within an insulating, temperature resistant glass or ceramic envelope. Discharge tubes operate by ionising the gas with an applied voltage across the electrodes, to create a conduction path within the gas between the electrodes. The electrical breakdown of the gas produces a plasma discharge with the result that upon passing a current through the plasma, an intense optical pulse is generated as the free electrons within the plasma combine with the ionised gas atoms.

Discharge tubes are typically powered using a circuit such as that illustrated in Figure I of the accompanying drawings. In such a circuit, a capacitor 11 is charged by a direct current (dc) source 10. The dc supply 10 and capacitor 11 are electrically connected to a discharge tube 12 by a first series of windings arranged upon the core of a transformer (not shown). However, the discharge tube 12 does not produce any output in this initial state, since there is no conductive path through the gas 13 between electrodes 14.

The conductive path is created by ionising the gas within the tube 12, and this is performed using a trigger circuit 15. The trigger circuit 15 induces a high voltage supply on the first series of windings (not shown) causing the gas 13 within the discharge tube 12 to break down. The trigger circuit is typically controlled by a controller 16 and comprises a second series of fewer windings on the same transformer core as the first series of windings, to create a step up in voltage. This high voltage produced across the tube creates a conduction path between the tube electrodes, thereby allowing the capacitor 11 to discharge and thus produce an intense arc.

It is also known to power discharge lamps using alternating current (ac) sources.

However, in such systems it is first necessary to rectify the ac signal, for example with a diode bridge. The well-known waveform of a full wave rectified sinusoidal voltage is shown in Figure 2 of the accompanying drawings. The figure further provides an indication of the time interval (illustrated as the interval t1-t2) over which a discharge arc would be possible. In this manner, when the voltage of the rectified sinusoidal waveform reaches, for example, 100 volts, an optical output (that is, an arc) will be produced that will continue until the rectified waveform voltage has dropped to, for example, 50 volts.

US Patent 6965203 discloses a method and circuitry for repetitively firing a flash lamp that is powered with a periodic voltage signal. A problem with powering discharge tubes using a rectified mains supply, however, is that the optical output is limited to a flash rate of twice the mains frequency (that is 100 Hz for a 50 Hz mains supply and 120 Hz for a 60 Hz mains supply) and the optical output from the discharge tube will only continue when the mains voltage exceeds a threshold voltage. Once the mains voltage drops below the threshold voltage, the optical output from the discharge tube will terminate.

A known solution to this problem is to incorporate a capacitor bank after the diode bridge in order to hold the voltage above the threshold voltage. Such a capacitor bank is found to smooth the voltage supply 17 to the discharge tube. However, such an arrangement is prone to fluctuations in the voltage supply as shown in Figure 3 of the accompanying drawings and this manifests as a fluctuation in the optical output of the discharge arc.

We have now devised a circuit which alleviates these problems.

In accordance with this invention as seen from a first aspect, there is provided a drive circuit for a discharge tube, the drive circuit comprising a primary circuit and a secondary circuit both arranged to deliver power to the discharge tube, the primary circuit comprising a first power supply and a first controller for controlling the delivery of power from the first power supply to the discharge tube, the secondary circuit comprising power storage means, a second power supply for charging the power storage means, and at least one second controller for controlling the discharge of the power storage means, the second controller being arranged to selectively discharge the power storage means at intervals determined by the first controller, such that the selective discharge supplements the first power supply from the primary circuit at times when the first power supply is below a threshold value.

By this means, the drive circuit according to the invention is enabled to supply a substantially constant power supply to the discharge tube.

The primary circuit preferably includes a diode bridge for rectifying ac mains supply.

Such a diode bridge preferably provides full wave rectification of the ac mains.

Preferably, the primary circuit includes a trigger mechanism for inducing a high voltage spike across the discharge tube to ionise the gas within the discharge tube.

The first controller preferably comprises a timing circuit to time the triggering of the trigger mechanism with the rectified ac mains supply. Such a timing circuit preferably enables the trigger mechanism to be triggered only in those intervals when the rectified ac mains voltage is above a threshold voltage.

Preferably the threshold voltage is the minimum voltage necessary to sustain a discharge arc in the discharge tube.

The second power supply preferably comprises a direct current (dc) supply for charging the power storage means. Preferably the power storage means comprises a capacitor bank.

The at least one second controller preferably causes the capacitor bank to discharge in order to compensate for the drop in voltage from the rectified ac mains supply and to maintain the voltage across the discharge tube above the threshold voltage.

Preferably, the discharge arc is arranged to be terminated by a switch within the primary circuit.

Preferably, the first controller and the at least one second controller are arranged in electronic communication to enable synchronous discharge of the capacitor bank with the rectified voltage.

An exemplary embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings. In the drawings: Figure 1 is a circuit diagram of a conventional drive circuit for a discharge tube, as described above; Figure 2 is a graphical representation of the voltage waveform of the rectified ac mains supply using such a conventional circuit, as described above; Figure 3 is a graphical representation of the voltage waveform of a rectified ac mains supply, following smoothing by a capacitor bank as described above; and, Figure 4 is a circuit diagram of an exemplary drive circuit according to the present invention.

Referring to Figure 4, there is shown a drive circuit 100 comprising a primary circuit 101 and a secondary circuit 102, for powering a discharge tube 103. The primary circuit 101 powers the discharge tube 103 and the secondary circuit 102 compensates for the variation in the rectified ac power supply in the primary circuit 101 to provide a substantially constant voltage.

The primary circuit 101 comprises a diode bridge 105, which rectifies the incoming ac mains power supply 104, to provide a full wave rectified voltage signal. The output of the diode bridge 105 is electrically connected to the discharge tube 103 via a single diode 106 which ensures unidirectional flow of current toward the discharge tube 103 from the power supply 104. However, the rectified voltage is insufficient to initiate a discharge within the tube 103, since there is no conduction path between the electrodes 107 of the discharge tube.

The primary circuit 101 includes a controller 108 for controlling a trigger circuit 109.

The controller 108 receives an input signal from the ac mains supply 104 and monitors the variation in waveform via digital signal processing means (not shown).

The controller 108 includes an output for outputhng a voltage as input to a trigger circuit 109. The trigger circuit 109 typically includes a step up transformer (not shown) which produces a high voltage spike across the tube electrodes 107.

The high potential difference created across the electrodes 107 by the trigger circuit 109, causes the gas 110 within the tube to ionise thereby reducing the impedance of the tube 103. However, in order to ensure that the trigger circuit 109 applies the high voltage spike at the correct time, that is, when the rectified voltage is sufficient to create a discharge arc, the controller 108 monitors the rectified voltage and outputs a signal to the trigger circuit 109 at a time determined by the digital signal processing means (not shown) to cause the trigger circuit 109 to produce the voltage spike.

However, with such a system, there is no control over the duration or frequency of the discharge arc.

Accordingly, the circuit according to the present invention further comprises a secondary drive circuit 102 which includes a second (dc) power supply 111 arranged in series with the rectified voltage supply, and used to charge a capacitor bank 112.

The secondary circuit further includes a second controller 113 arranged in electronic communication with the first controller 108 to control selective discharge of the capacitor bank 112.

In use, the first controller 108 causes the trigger circuit 109 to produce a high voltage across the electrodes 107, causing the gas 110 within the tubes 103 to ionise, when the rectified voltage across the tube electrodes 107 reaches the minimum value necessary to maintain a discharge arc.

As the rectified voltage begins to fall below the threshold voltage, the second controller 113 causes the capacitor bank 112 to progressively discharge, so as to compensate for the fall in voltage on the rectified voltage input. This maintains a constant voltage across the tube electrodes 107 and therefore a uniform discharge arc.

Initially, the discharge of the capacitor bank 112 is quite small. However, as the rectified voltage is reduced further from the threshold voltage, the capacitor bank 112 discharge increases, to offset the fall in rectified voltage. As the rectified voltage begins to rise again on the next cycle, the capacitor bank 112 begins to progressively recharge from the second power supply 111, so that it is ready to discharge again in accordance with the fall in rectified voltage.

The second controller 113 monitors the first controller 108 to time the selective charge and discharge of the capacitor bank 112. In this manner, the cyclic charge and discharge of the capacitor bank 112 can be used to maintain the discharge arc for a desired time period.

When it is desired to terminate the arc, a switch (not shown) provided within the primary circuit is switched to the OFF state to remove the ac power supply 104 from the discharge tube 103.

From the foregoing therefore1 it is evident that the drive circuit of the present invention enables a discharge tube, powered from a mains supply, to produce a discharge arc of a desired period of time.

Claims (12)

1. A drive circuit for a discharge tube, said drive circuit comprising a primary circuit and a secondary circuit both arranged to deliver power to the discharge tube, said primary circuit comprising a first power supply and a first controller for controlling the delivery of power from said first power supply to said discharge tube, said secondary circuit comprising power storage means, a second power supply for charging the power storage means, and at least one second controller for controlling the discharge of said power storage means, said second controller being arranged to selectively discharge said power storage means at intervals determined by said first controller, such that the selective discharge supplements said first power supply from said primary circuit at times when said first power supply is below a threshold value.

2. A drive circuit according to claim 1, wherein said first power supply is an ac mains supply and the primary circuit includes a diode bridge for full wave rectification of the ac mains supply.

3. A drive circuit according to claim I or 2, wherein the primary circuit includes a trigger mechanism for inducing a high voltage spike across the discharge tube to ionise gas within said tube.

4. A drive circuit according to claim 3, which includes a timing circuit to time the triggering of the trigger mechanism with the rectified ac mains supply.

5. A drive circuit according to claim 4, wherein said timing circuit enables the trigger mechanism to be triggered only in those intervals when the rectified ac mains voltage is above a threshold voltage.

6. A drive circuit according to claim 5, wherein the threshold voltage is the minimum voltage necessary to sustain a discharge arc in the discharge tube.

7. A drive circuit according to claim 6, wherein the at least one second controller causes the power storage means to discharge in order to compensate for the drop in voltage from the rectified ac mains supply and to maintain the voltage across the discharge tube above the threshold voltage.

8. A drive circuit according to any preceding claim, wherein the second power supply comprises a direct current supply for charging the power storage means.

9. A drive circuit according to any preceding claim, wherein the first controller and the at least one second controller are arranged in electronic communication to enable synchronous discharge of the power storage means with the rectified ac voltage.

10. A drive circuit according to any preceding claim, wherein at least one of said first and second controllers comprise digital signal processing means.

11. A drive circuit according to any preceding claim, wherein said power storage means comprises a capacitor bank.

12. A drive circuit according to any preceding claim wherein the discharge arc is terminated by a switch arranged within the primary circuit is arranged to terminate a discharge arc within the discharge tube..