GB2432268A

GB2432268A - Discharge lamp trigger circuit

Info

Publication number: GB2432268A
Application number: GB0523140A
Authority: GB
Inventors: Jeffrey Dennis Evemy; Vince Johnson
Original assignee: INTUNNEL Ltd
Current assignee: INTUNNEL Ltd
Priority date: 2005-11-12
Filing date: 2005-11-12
Publication date: 2007-05-16
Also published as: GB0523140D0; WO2007054734A1

Abstract

A trigger circuit for a xenon gas discharge lamp 10, comprises a generator 15 for generating an alternating voltage Vac, the output of the generator 15 being connected to a step-up transformer T. A capacitor C3 forms a tuned circuit with the secondary winding of the transformer T, the output of the tuned circuit being applied to the trigger terminal of the lamp 10. The generator 15 induces voltage oscillations in tuned circuit and means 16 are provided for varying the frequency of the alternating voltage Vac to vary the amplitude of the trigger output signal from the tuned circuit. The waveform envelope of the high voltage trigger output signal can thus be modulated to avoid sudden changes and to control its amplitude and duration.

Description

DISCHARGE LAMP TRIGGER CIRCUIT

This invention relates to a trigger circuit for a gas discharge lamp.

Gas discharge lamps containing an inert gas such as xenon are widely used to generate high intensity and short duration pulses of light. One known use of lamps of this kind is to sequentially illuminate successive images mounted along an underground train tunnel, so that passengers in a passing underground train experience the visual effect of viewing a moving picture whilst looking out of the window of the train.

Referring to Figure 1 of the drawings, there is shown a conventional circuit for firing a xenon lamp 10, the lamp 10 being connected in parallel with a capacitor C2. The cathode of the lamp 10 is connected to ground and the anode is connected to the positive rail of the dc supply Vs by a resistor R2. The trigger electrode of the lamp 10 is connected to ground via the secondary winding of a transformer 1. The primary winding of the transformer T is connected between ground and one terminal of a capacitor Cl. The other terminal of the capacitor Cl is connected to the positive rail of the dc supply Vs by a resistor Rl and to ground via a switch Sl.

In use, the capacitor C2 stores the energy to be discharged through the lamp 10: typically, for a low-medium power lamp, a couple of hundred volts are held across the capacitor C2. In order to fire the lamp 10, a high voltage pulse needs to be applied to the trigger electrode of the lamp 10 to ionise the xenon atoms inside the lamp. The ionised atoms create a current path between the anode and cathode allowing the capacitor C2 to rapidly discharge. The electrons flowing along the current path collide with other atoms causing ionisation and light is emitted as the ionised atoms recombine. As the voltage across capacitor C2 falls, the current weakens and the lamp turns off. The capacitor C2 is then free to recharge via resistor R2, ready for the next discharge.

In order to trigger the lamp, a high voltage pulse is generated in the primary portion of the circuit by closing the switch Si to cause capacitor Cl to rapidly discharge through the primary of the transformer T. The turns ratio of the transformer T creates an increased voltage across its secondary winding, which is sufficient to trigger the lamp 10. Once the lamp is triggered, the switch Si is opened and the capacitor Cl recharges through resistor Ri.

The capacitor Cl and the primary winding of the transformer T in the primary side of the circuit in fact form a tuned circuit and accordingly, the high-voltage pulse produced by the tuned circuit has a very sharp initial positive rise followed by a rapidly decaying sinusoidal oscillation at the resonant frequency of the tuned circuit, as shown in Figure 2 of the drawings. Whilst the initial portion of the waveform is sufficiently large to trigger the lamp 10, the further oscillation which continues for tens of microseconds is of no use and energy is lost as heat within the circuit and as radiated electromagnetic interference.

Another problem of the conventional circuit is that the very fast initial rise time of the trigger voltage (several kV per microsecond) generates a substantial amount of broadband electromagnetic interference. In order to minimise the radiated energy at radio frequencies, the connection from the transformer T to the lamp 10 has to be minimised, which in turn requires that the transformer TI, the capacitor Cl and the switch Si are mounted close to the lamp 10.

in practice a solid state device, such as a triac, thyristor or FET is used in the place of the switch Si. Triacs and thyristors have good conduction characteristics but additional circuitry or operational compromises may be required to ensure that the devices turn off. Unlike the triac, the thyristor will only pass current in one direction, which hinders resonance on the primary-side of the transformer T and reduces the effectiveness of the circuit as a high voltage generator. In order to overcome this problem with thyristors, a higher supply voltage Vdc and a larger capacitance is employed, requiring a more robust switching device and increasing the cost of the circuit. Modern high power FETs often have built-in diodes which permit conduction in the other direction. This reduces the loss in efficiency, however, FETs do not switch as fast as triacs or thyristors and thus the initial current surge is slowed, which results in a lower initial voltage spike on the transformer secondary.

We have now devised a trigger circuit for a gas discharge lamp which alleviates the above-mentioned problems.

In accordance with this invention, there is provided a trigger circuit for a gas discharge lamp, the circuit comprising a step-up transformer having a primary circuit portion and a secondary circuit portion, the primary circuit portion comprising means for generating an alternating voltage to induce voltage oscillations in a tuned circuit of the secondary portion, the output of the tuned circuit being connected to the trigger terminal of the lamp, wherein the primary circuit portion comprises means for varying a parameter of said alternating voltage to vary the amplitude of the output signal from said tuned circuit.

In use, a parameter of the alternating voltage generated in the primary circuit portion is preferably varied to gradually increase the amplitude of the output signal towards a value at which the lamp is triggered. In this manner, a sharp rise in the amplitude of the high voltage signal can be avoided, thereby significantly reducing radiated electromagnetic interference.

Furthermore, the duration of the high voltage output signal can be minimised by varying the parameter of the alternating voltage generated in the primary circuit portion, so as to avoid wasting energy and to further minimise the risk of generating interference. Also, the duration of the time for which the output signal is at amplitude sufficient to trigger the lamp can be controlled by varying the parameter of the alternating voltage generated in the primary circuit portion. In this manner any risk of generating an insufficient trigger voltage can be avoided.

The controlled shape of the envelope of the output signal to the lamp allows the trigger circuit to be placed some distance from the lamp without excessively compromising flash reliability or electromagnetic interference performances.

It will be appreciated that there is lower stress on the components in the primary portion of the circuit, since there are no sharp current discharges of the kind present in a conventional circuit. The components used in the primary portion of the circuit are no longer required to have good pulse characteristics and thus less expensive components can be selected. Furthermore, the reliability of the circuit is improved.

Preferably said means for generating the alternating voltage is arranged to generate a voltage having a frequency at or near to the resonant frequency of the tuned circuit.

An advantage of this is that a lower alternating voltage in the primary portion of the circuit is needed to produce the same voltage output signal in the secondary portion of the circuit.

The primary circuit portion may comprise means for varying the amplitude of said alternating voltage to vary the amplitude of the signal output from said tuned circuit.

However, such a circuit would be complicated in construction and would utilise costly components. In order to minimise costs, particularly in cost sensitive products, the primary circuit portion preferably comprises means for varying the frequency of said alternating voltage to vary the amplitude of the signal output from said tuned circuit.

Preferably the means for varying the frequency of said alternating voltage is arranged to vary the frequency towards the resonant frequency of the tuned circuit, in order to increase the amplitude of the signal output from the tuned circuit.

Preferably the means for varying the frequency of said alternating voltage is arranged to vary the frequency away from the resonant frequency of the tuned circuit, in order to decrease the amplitude of the signal output from the tuned circuit.

A circuit for adjusting the frequency of the alternating voltage generated in the primary circuit portion can be realised using relatively inexpensive components.

Preferably, said means for generating an alternating voltage in the primary portion of the circuit is arranged to generate a square wave. It will be appreciated that a circuit for generating a square wave is relatively simple in construction.

Preferably, said means for generating an alternating voltage comprises a microprocessor. An advantage of using a microprocessor is that it can be configured to generate a square wave. The microprocessor can also be configured to vary the frequency of the square wave to control the amplitude of the signal output from the tuned circuit.

As gas discharge lamps age, the voltage needed to trigger them increases.

Accordingly, there is a risk that the signal output from the tuned circuit will eventually be insufficient to trigger the lamp. In order to overcome this problem, the microprocessor or other means is preferably arranged to determine whether the lamp has failed to trigger, for example by sensing the voltage across a charge capacitor connected across the lamp, and to vary a parameter of said alternating voltage to increase the amplitude of the output signal from said tuned circuit, for example by varying the frequency of the alternating voltage towards the resonant frequency of the tuned circuit.

Another problem of trigger circuits for gas discharge lamps is that the charge capacitor, which is connected across the lamp, can degrade and fail. In order to overcome this problem, the microprocessor may be arranged to analyse the voltage waveform across the capacitor during discharge and to compare the waveform with a known waveform to provide an indication if the capacitor has substantially degraded.

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a conventional trigger circuit for a gas discharge lamp; Figure 2 is a diagram of the trigger voltage waveform generated by the circuit of Figure 1; Figure 3 is a schematic diagram of a first embodiment of trigger circuit in accordance with the present invention for a gas discharge lamp; Figure 4 is a graph of trigger voltage vs. excitation frequency for the circuit of Figure 3; Figure 5 is a diagram of the trigger voltage waveform generated by the circuit of Figure 3, when operating at three different excitation frequencies; Figure 6 is a schematic diagram of a second embodiment of trigger circuit in accordance with the present invention for a gas discharge lamp; Figure 7 is a schematic diagram of a third embodiment of trigger circuit in accordance with the present invention for a gas discharge lamp; and Figure 8 is graph of trigger voltage vs. excitation frequency for the circuit of Figure 7.

Referring to Figure 3 of the drawings, there is shown a circuit in accordance with the present invention for triggering a gas discharge lamp 10. The circuit is similar to the conventional circuit of Figure 1 and like parts are given like reference numerals.

The lamp 10 is again connected in parallel with a capacitor C2, the cathode of the lamp 10 being connected to ground and the anode being connected to the positive rail of the dc supply Vdc by a resistor R2. The trigger electrode of the lamp 10 is connected to ground via a capacitor C3, which is connected in parallel with the secondary winding of the transformer T. The primary winding of the transformer T is connected between ground and one terminal of a capacitor Cl. The other terminal of the capacitor Cl is connected to the output of an amplifier 10, which is arranged to amplify an ac signal Vac generated by a signal generator 15.

The capacitor C3 and the secondary of the transformer T now form the dominant resonant circuit, having a resonant frequency of between one and two hundred kilohertz. The primary of the transformer T is driven by the ac signal Vac from the signal generator 15 and a control circuit 16 is connected to the signal generator 15 to vary the amplitude or frequency of the generated signal Vac. The generator 15 is controlled to produce an ac signal Vac having a frequency at or near to the close to the resonant frequency of the tuned circuit of the secondary circuit. This arrangement generates a large ac voltage on the trigger electrode of the lamp 10, which is sufficient to trigger the lamp.

An advantage of driving the transformer T with an ac signal at the resonant frequency of the secondary circuit is that a lower input voltage is needed to produce the same lamp trigger voltage. Another advantage is that there is lower stress on the driving components since there are no sharp current discharges. The capacitor Cl is no longer required to have good pulse characteristics and thus a less expensive component can be selected. Furthermore, the reliability of the circuit is improved.

Another advantage of driving the transformer T with an ac signal at the resonant frequency of the secondary circuit is that the drive to the transformer T can be sustained, extending the time during which the trigger voltage is above the firing threshold. Also, the shape of the envelope of the trigger waveform can be controlled, thus avoiding the sharp rise times of the conventional firing circuit and significantly reducing radiated interference. The controlled shape of the envelope of the trigger waveform allows the trigger circuit to be placed some distance from the lamp without excessively compromising flash reliability or electromagnetic interference performances.

Referring to Figure 4 of the drawings, it can be seen that the trigger voltage varies according to the drive frequency of the ac signal Vac, which is applied to the primary of the transformer T. The trigger voltage is maximised when the drive frequency is equal to the resonant frequency Fc of the secondary circuit.

The above-mentioned controllable amplitude ac signal source needs to deliver a sine wave to the transformer T, which is variable between 100 to 200 Volts. Such circuits require buffer amplifiers and are complicated and costly, particularly in a cost sensitive product.

In order to overcome this problem, the amplitude of the ac signal Vac can be fixed, the shape of the trigger voltage waveform being varied by adjusting the frequency of the ac signal Vac. Referring to Figure 5 of the drawings, it can be seen how the amplitude of the trigger voltage waveform varies when three different ac signal Vac frequencies Fl, F2 and F3 of the same amplitude are applied to the primary of the transformer T. Thus, if the frequency of the ac signal Vac is swept from a low frequency towards the resonant frequency Fc and then back down again, the resultant trigger waveform will rise and fall, thereby avoiding the undesirable very sharp initial pulse of the typical circuit which causes RF interference.

Instead of the generator of the ac signal Vac producing a pure sine wave, the highly resonant nature of the circuit configuration permits the transformer primary to be driven by a square wave. In this manner, the generator of the ac signal Vac can be a switch, thereby significantly simplifying the circuit.

Referring to Figure 6 of the drawings, there is shown an alternative circuit in accordance with the present invention for triggering a gas discharge lamp 10. The circuit is similar to the circuit of Figure 3 and like parts are given like reference numerals. In this embodiment the primary winding of the transformer T is connected between ground and the collector of a pnp transistor Q2. The collector of the transistor Q2 is also connected to the cathode of a diode Dl, the anode of the diode Dl being connected to ground. The emitter of the transistor Q2 is connected to the positive rail of the dc supply Vdc.

The base and emitter of the transistor Q2 are interconnected by a resistor R4, the base also being connected to the collector of an npn transistor QI by a resistor R5. The emitter of the transistor Qi is connected to ground and the base is connected to the output 11 of a microprocessor or CPU (not shown) by a resistor R3.

The transistor Q2 is the switching device which controls the power to the transformer T. Transistor Qi permits the switching device to be controlled via a low voltage pulse, which is applied to the circuit from the microprocessor or CPU. The resonant frequency of the trigger circuit formed by the secondary of the transformer T and the capacitor C3 is low enough to permit the generation of the pulse train using a small low cost microprocessor. If the microprocessor has analogue capabilities, such as an analogue-to-digital processor or a comparator, additional functionality can be gained for very little cost.

Referring to Figure 7 of the drawings, there is shown an alternative circuit in accordance with the present invention for triggering a gas discharge lamp 10. The circuit is similar to the circuit of Figure 6 and like parts are given like reference numerals. In this embodiment one side of the windings of both the primary and secondary of the transformer T are connected to the cathode of a diode Dl, the anode of the diode Dl being connected to ground. The cathode of the diode Dl is also connected to the drain of a field effect transistor or so-called FET Q3. The source of the FET Q3 is connected to ground. The gate of the FET Q3 is connected to the output of a microprocessor or CPU (not shown) by a resistor R3.

The other side of the primary winding of the transformer T is connected to ground via a resistor R5 and to the positive rail of the dc supply Vdc by a resistor R4. A capacitor C2 is connected in parallel with the resistor R5.

The drive circuit is thus simplified to a low-side configuration and the drive voltage is reduced via the potential divider circuit of resistors R4 and R5. The transformer T has a common connection between the windings of the primary and secondary, which compromises the low-side circuit operation somewhat, but the principles of secondary resonance and using drive frequency to shape the envelope of the trigger waveform can still be utilised to good effect.

As gas discharge lamps age they require progressively higher trigger voltages in order to initiate firing. In order to detect whether the lamp has failed to fire, a potential divider network comprising resistors R6 and R7 is connected across the main charge capacitor C2. The output 13 of the potential divider circuit can be connected to the microprocessor, which can sense the voltage and detect whether the lamp 10 has failed to fire. The microprocessor may then be arranged to adjust the frequency of its output, in order to produce a higher trigger voltage for subsequent flashes.

Referring to Figure 8 of the drawings, when a lamp is new, the microprocessor uses drive frequency F! which is spectrally spaced apart from the resonant frequency of the trigger circuit formed by the secondary of the transformer T and the capacitor C3.

As time passes, the trigger threshold voltage Vth for the lamp rises, and the lamp will eventually fail to fire. The microprocessor detects this and adjusts the drive frequency to F2, thereby increasing the trigger voltage for future flashes. Towards the end of the life of the lamp, the trigger voltage at frequency to F2 will also fail to fire the lamp.

Again, the microprocessor detects this and increases the drive to F3. At this point there is not much life left in the lamp and the microprocessor can provide an alert to warn that the lamp needs changing.

Another potential failure point in trigger circuits for gas discharge lamps is the main charge capacitor C2. The lamp discharges this capacitor C2 within a couple of hundred microseconds and pulse currents exceeding 500 Amperes are not uncommon.

Such high pulse currents and repetition rates can cause the capacitor to overheat, reducing its reliability and life. In order to provide an indication of the condition of the capacitor, the voltage across the lamp may be fed to a medium speed analogue-to-digital converter, so that the discharge curve can be analysed by a microprocessor and compared with known or historical values.

The provision of a microprocessor in the circuit enables a plurality of self-monitoring features. In addition to failure prediction and detection many other environmental conditions may be monitored andlor logged, such as temperature, ambient light, noise, barometric pressure, tamper detection, etc. The failure monitoring and prediction feature is especially useful if the lamp circuit is located in areas of poor or difficult access, such as within an underground train tunnel.

In such situations the microprocessor could report the condition of the lamp and main capacitor to a remote monitoring station. Lamp or circuit replacement can thus be actioned before the circuit completely fails thus avoiding potential safety issues and improving the quality of service.

The presence invention utilises the frequency response of a tuned secondary circuit to control the rise time, amplitude and duration of the trigger pulse, whereas conventional trigger circuits are unable to control the trigger pulse in this manner.

Conventional circuits are designed to generate very large fast pulses, which emit large levels of broadband electromagnetic interference. Tests have shown that the electromagnetic interference radiated by the present invention is more than 10dB less than that radiated by conventional trigger circuits. The present invention also simplifies the physical design and construction of the circuit and allows the lamp to mounted further from the trigger generator without affecting operation or substantially increasing electromagnetic interference. The present invention also has the advantage of permitting the prediction of failure of lamps and main charge capacitors.

Claims

CLAIMS

1. A trigger circuit for a gas discharge lamp, the circuit comprising a step-up transformer having a primary circuit portion and a secondary circuit portion, the primary circuit portion comprising means for generating an alternating voltage to induce voltage oscillations in a tuned circuit of the secondary portion, the output of the tuned circuit being connected to the trigger terminal of the lamp, wherein the primary circuit portion comprises means for varying a parameter of said alternating voltage to vary the amplitude of the output signal from said tuned circuit.

2. A trigger circuit as claimed in claim 1, in which a parameter of the alternating voltage generated in the primary circuit portion is varied to gradually increase the amplitude of the output signal towards a value at which the lamp is triggered.

3. A trigger circuit as claimed in claims 1 or 2, in which said means for generating the alternating voltage is arranged to generate a voltage having a frequency at or near to the resonant frequency of the tuned circuit.

4. A trigger circuit as claimed in any preceding claim, in which said primary circuit portion comprises means for varying the amplitude of said alternating voltage to vary the amplitude of the signal output from said tuned circuit.

5. A trigger circuit as claimed in any of claims 1 to 3, in which said primary circuit portion comprises means for varying the frequency of said alternating voltage to vary the amplitude of the signal output from said tuned circuit.

6. A trigger circuit as claimed in claim 5, in which said means for varying the frequency of said alternating voltage is arranged to vary the frequency towards the resonant frequency of the tuned circuit, in order to increase the amplitude of the signal output from the tuned circuit.

7. A trigger circuit as claimed in claims 5 or 6, in which the means for varying the frequency of said alternating voltage is arranged to vary the frequency away from the resonant frequency of the tuned circuit, in order to decrease the amplitude of the signal output from the tuned circuit.

8. A trigger circuit as claimed in any preceding claim, in which said means for generating an alternating voltage in the primary portion of the circuit is arranged to generate a square wave.

9. A trigger circuit as claimed in any preceding claim, in which said means for generating an alternating voltage comprises a microprocessor.

10. A trigger circuit as claimed in any preceding claim, arranged to determine whether the lamp has failed to trigger and to vary a parameter of said alternating voltage to increase the amplitude of the output signal from said tuned circuit.

11. A trigger circuit as claimed in any preceding claim, arranged to monitor the voltage waveform values across a lamp charge capacitor of the circuit during discharge and to provide an indication if the monitored values deviate from stored values.

12. A trigger circuit substantially as herein described with reference to Figures 3 to 5, Figure 6, or Figures 7 and 8 of the accompanying drawings.