GB2436402A

GB2436402A - Electronic starter for gas discharge lamp

Info

Publication number: GB2436402A
Application number: GB0605507A
Authority: GB
Inventors: Charles Forster Edward
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-03-20
Filing date: 2006-03-20
Publication date: 2007-09-26
Anticipated expiration: 2026-03-20
Also published as: GB2436402B; GB0605507D0

Abstract

A starter S for a gas discharge lamp LP1 comprises an electronic switching means BDS to connect a capacitor C2 between the terminals of the lamp. This creates a resonant circuit with the power supply inductor L which raises the applied lamp voltage and the cathode heating current during starting. A bidirectional voltage limiter BDL and resistor R1 are connected in series between the lamp cathodes to discontinuously damp resonance and control the peak voltage rise across the cathodes to prevent the lamp from igniting at an undesirable high voltage before the cathodes have heated sufficiently. The electronic switching means connects the capacitor by reference to a non-zero current flowing in the resistor, and disconnects the capacitor by reference to zero currents flowing in the resistor and the capacitor. The electronic switching means may be a triac (Q, figure 2) and the bidirectional voltage limiter may be a cold cathode gas discharge lamp (LP2, figure 2).

Description

Electronic Starter for Gas Discharge Lamp

Field of the invention and background

This invention relates to a method of igniting gas discharge lamps by means of an electronic starter. Such lamps include fluorescent lamps that are widely used in commerce, industry and the home. Discharge lamps often have preheated cathodes to aid starting and, as is known, the function of a starter is to momentarily preheat the lamp cathodes, provide an elevated voltage to the lamp to initiate discharge and finally to return the lamp to its normal operating conditions. This invention relates to a method of starting discharge lamps that are operated at alternating current (ac) mains frequency with conventional series inductive ballast.

Starters are conventionally two terminal devices connected in parallel with the lamp and a high voltage ignition impulse has usually been generated by shorting out the lamp through its cathodes so that a high current flows through the series ballast inductor storing a high magnetic field within it. The current flowing through the cathodes also provides their necessary preheating.

Breaking the current collapses the magnetic field to produce a high voltage impulse across the inductor and the lamp, which starts the lamp discharge.

The delayed action and erratic behaviour of the traditional fluorescent lamp electromechanical starter is a considerable disincentive to the domestic use of fluorescent lamp fixtures subject to frequent on off switching where near instant start is preferable.

Modern electronic versions of the traditional starter continue to rely on interrupting current in the series inductor to ignite the lamp. There is some difficulty in ensuring that the interruption occurs at an instant of high current.

Some types generate periodic impulses until ignition occurs providing more opportunitieS to generate unwanted electromagnetic interference.

Whereas miniature high frequency electronic ballasts have been developed for small wattage, incandescent lamp replacements, their application to higher wattage conventional discharge lamp lighting is not so clearly advantageous on economic grounds.

It is known that a high ignition voltage reduces cathode electron emissivity over time and that lamps with preheated cathodes ignite at a lower voltage.

Thus cathode preheating extends lamp lifetime, but conventional systems operated at ac mains frequency often apply a fixed period of preheating irrespective of cathode temperature at the start.

As is known, resonating the series ballast inductor with a capacitor across the lamp will raise the applied sinusoidal voltage to initiate ignition and this is a method also used in modern high frequency ballasts, but it has not been applied very successfully with conventional mains frequency inductive ballast driven discharge lamps. Sinusoidal voltage starters do not generate transient interference to the same extent as those that rely on impulses.

Advantages and statements of the invention

It is the object of the invention to considerably speed up the starting process for an ac mains frequency operated gas discharge lamp fed by conventional series ballast inductor by using an electronic switching means and capacitor to apply controlled resonance of the ballast inductor such that said resonance accelerates cathode preheating and provides a controlled ignition voltage. It is a feature of the invention that, once commenced, resonance is continuous under the control of said electronic switching means until ignition of the lamp occurs. Another feature of the invention is that said resonance is discontinuously damped to provide a modestly augmented and continuously applied sinusoidal ignition voltage in the starting period. By limiting the ignition voltage, cathode preheating is automatically applied as necessary for the required cathode ignition temperature to be reached. A further feature of the invention is that cathode preheating current is about double the conventional value to provide accelerated heating.

Advantages claimed for the invention include: a) Quicker starting of cold lamps than by conventional means.

b) Near instantaneous starting for warm lamps. No need to go through an unnecessary fixed length cathode preheating process as this is curtailed automatically.

C) May be implemented with one triac as an only semiconductor.

d) May use a standard cold cathode lamp with bimetallic switch to provide several functions as voltage reference, limiter, and thermal switch to promote fail safe disconnection.

e) May be implemented as a plug-in unit to replace existing starters.

f) Fails safe with faulty lamps.

Whereas it is known that the operating circuit of a gas discharge lamp with self heating cathodes comprises a series ballast inductor that connects a first wire of a single phase two wire ac mains voltage supply to a first terminal of said lamp first cathode and that a first terminal of said lamp second cathode connects to a second wire of said ac mains voltage supply, and second terminals of said lamp cathodes connect to a starter.

According to the invention there is provided a starter comprising first an electronic switching means operative to connect a capacitor substantially between a gas discharge lamp cathode second terminals, and second a bidirectional voltage limiter and resistor substantially connected in series between said lamp cathode second terminals such that said electronic switching means connects said capacitor by reference to substantially non zero current flowing in said resistor and only disconnects said capacitor subsequently by reference to currents flowing in said resistor and said capacitor being substantially both zero; conditions which both obtain at the onset of settled discharge in said lamp.

Said voltage limiter is provided to conduct above its breakdown voltage chosen to be between the peak operating conduction voltage of said lamp and the peak voltage of said ac mains voltage supply. Said capacitor is provided to resonate or to be near resonance with said inductor at ac mains voltage supply frequency so that both applied lamp voltage and cathode heating current are raised. Said resistor and said voltage limiter are provided to discontinuously damp resonance and substantially control peak voltage rise across said lamp.

Optionally, to reduce the number of capacitors, subsequent to ignition of said lamp said capacitor may be connected by said electronic switching means in parallel with said ac mains voltage supply to perform power factor correction.

Methods of further electromagnetic interference suppression to meet specific international standards, are not described herein as such methods are well known in the art if found to be necessary.

Dra wings Three examples of the invention with simple electronic switching means are shown in Figures 1,2 and 3. Certain common components are included in the figures to allow a clear understanding of the operation of the invention. These components are found in conventional discharge lamp lighting circuits and are well known.

Descriptions

An embodiment of the invention is shown in Figure 1. Reference is first made to common components of these examples. Discharge lamp LP1, includes self heating cathode elements K1 and K2. Series ballast inductor L connects a first wire of a single phase two wire ac mains voltage supply VAC to a first terminal of cathode Ki and a first terminal of cathode K2 is connected to the return second wire of ac mains voltage supply VAC. Capacitor Cl is a power factor correcting capacitor connected in parallel with ac mains voltage supply VAC.

Second terminals of cathodes Ki and K2 connect to circuit points P1 and P2 which are the points by which a starter is connected to the lamp LP1. It should be appreciated that the indicated ac mains voltage supply VAC may have been increased by a transformer before being applied as shown but that has no impact on the operation of the invention as described herein. So much is known for typical discharge lamp supply circuits.

An embodiment of the invention is the starter circuit S shown enclosed by dashed lines in Figure 1. Starter S connects to the lamp circuit comprising L, LP1 and Cl at circuit points P1 and P2. Bidirectional voltage limiter BDL conducts when the voltage applied across it exceeds its breakdown voltage.

The breakdown voltage of bidirectional voltage limiter BDL is the main reference to initiate starting and it is provided to be below ac mains VAC peak voltage but above the running breakdown voltage of discharge lamp LP1.

Initially, application of the ac mains voltage supply VAC causes a similar voltage to appear across the lamp LP1 and across circuit points P1 and P2 but is insufficient to ignite said lamp LP1. Bidirectional voltage limiter BDL breaks down before reaching the peak of the voltage cycle appearing between circuit points P1 and P2 and current flows through series resistor Ri into the current sensitive control terminal 1 of bidirectional semiconductor switching device BDS which turns on and via its main terminals 2 and common terminal 3 connects capacitor C2 across circuit points P1 and P2. Capacitor C2, by substantially resonating with inductor L at ac mains VAC frequency, causes the peak voltage appearing between circuit points P1 and P2 to be raised but such resonance is damped by resistor Ri when bidirectional semiconductor switching device BDL conducts at its limiting voltage and current flows into bidirectional semiconductor switching device BDS control terminal 1 that has a low resistance to its terminal 3 relative to the resistance of resistor Ri. Thus said voltage rise is reduced by current flowing in resistor Ri above a threshold voltage set by the breakdown voltage of bidirectional voltage limiter BDL and is restrained to prevent undesirable ignition occurring in lamp LP1 at a too high voltage before its cathodes have heated sufficiently. Resonance also raises the current flowing in series connected cathode elements Ki and K2 so that more rapid heating occurs than in conventional circuits.

Once bidirectional semiconductor switching device BDS is switched on, the current flowing in resistor Ri is out of phase with the current flowing in capacitor C2 and thus currents flowing through control terminal 1 of bidirectional semiconductor switching device BDS and its load terminal 2 are never both simultaneously zero and bidirectional semiconductor switching device BDS remains switched on continuously throughout the resonant voltage cycle until both currents are brought to zero by the onset of settled discharge in lamp LP1.

Bidirectional semiconductor switching device BDS presents a substantially low impedance path between terminals 1 and 3. While bidirectional semiconductor switching device BDS is shown as current sensitive having a low internal resistance it could be configured internally or externally to have significant resistance approaching the value of resistor Ri between its terminals 1 and 3 whereupon resistor Ri could be correspondingly reduced in value, but it is to be understood that the overall operation of the circuit would be substantially unaffected by such a modification.

It is to be understood that bidirectional semiconductor switching device BDS may be comprised of any combination of semiconductor devices such that the functionality of the invention described herein is obtained. Bidirectional voltage limiter device BDL may be provided by a semiconductor device or network of semiconductor devices or a cold cathode gas discharge lamp having equivalent functionality.

When at some point the cathodes K1 and K2 have heated and the lamp LP1 ignites, the voltage limiting effect of lamp LP1 on the occurrence of discharge limits the voltage across points P1 and P2 so that bidirectional semiconductor switching device BDL ceases to conduct causing the current in resistor Ri to fall to zero. Also, the capacitor G2 discharges to the lower voltage across lamp LP1 through terminals 2 and 3 of bidirectional semiconductor switching device BDS until current flowing in BDS falls to zero. This discharge of capacitor C2 gives a modest further boost to cathode heating of lamp LP1. Once turned on bidirectional semiconductor switching device BDS turns off only when its control current at terminal 1 and load current at terminal 2 both fall substantially to zero and now this occurs. At this point the starter disconnects allowing the lamp LP1 to continue running normally.

When the lamp LP1 is already warm after recent use it can be ignited almost instantaneously as the conditions for ignition are reached automatically much faster.

Another embodiment of the invention is the starter circuit S shown in Figure 2.

Cold cathode gas discharge lamp LP2 functions as bidirectional voltage limiter BDL in Figure 1. The bidirectional semiconductor switching device BDS of Figure 1 is replaced by triac 0 with control gate G which turns on a conducting path between its main terminals MT1 and MT2. It is a feature of the invention that the required functionality of the bidirectional semiconductor switching device BDS may be supplied by one standard semiconductor triac in this simple embodiment. The two terminal impedance of control gate G and main terminal MT1 for practical types of triac may be low with respect to practical values of resistor Ri.

A preferred embodiment of the invention is the starter circuit S shown in Figure 3. Lamp LP2 is a conventional cold cathode gas discharge lamp with an internal thermal bimetallic switch SW normally open and a further element, fusible link F, is added in series. The purpose of this modification is to make the starter inoperative after a certain short period of unsuccessful operation when the lamp LP1 may not ignite due to loss of gas for example. After prolonged heating from discharge in lamp LP2 the switch SW will close.

Although it may be possible that fuse F may blow after a short period without switch SW, by closing, it significantly increases voltage across resistor Ri and the current in fuse F to make it certain that fuse F will blow. In the domestic situation it is unlikely that faulty lamps will not be noticed immediately and replaced making it unlikely that fuse F would actually blow in normal use. In commercial applications where lighting fixtures are unattended, a fail safe mechanism is a useful protection measure. As an alternative to a separate fuse resistor Ri may be constructed to be fusible.

For a 65 watt fluorescent lamp operated from a 220 volt, 50 Hz ac supply, component values would be typically R = 300 ohms 3W, L = 1.6H, Cl = 5.6uF, C2 = 4.OuF and F = 160 mA quick blow. Cold lamps typically ignite within 0.5 seconds.

Claims

Claims 1. A starter for a gas discharge lamp comprising first an

electronic switching means operative to connect a capacitor substantially between cathodes of a gas discharge lamp so that said capacitor operates to resonate or to be near resonance with said lamp's power supply inductor at ac mains voltage supply frequency to raise both applied lamp voltage and cathode heating current during starting, and second a bidirectional voltage limiter and resistor substantially connected in series between said lamp cathodes operative to discontinuously damp resonance and substantially control peak voltage rise across said lamp cathodes to prevent lamp ignition occurring at an undesirable high voltage before its cathodes have heated sufficiently, such that said electronic switching means is operative to connect said capacitor by reference to substantially non zero current flowing in said resistor and only operative to disconnect said capacitor subsequently by reference to currents flowing in said resistor and said capacitor being substantially both zero.

2. A starter as claimed in Claim 1 wherein said electronic switching means is a bidirectional semiconductor device or network of devices such that the functionality of Claim 1 is obtained.

3. A starter as claimed in Claim 1 wherein said electronic switching means is a triac.

4. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is bidirectional semiconducting device or network of devices such that the functionality of Claim 1 is obtained.

5. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is a cold cathode gas discharge lamp.

6. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is a cold cathode gas discharge lamp with a parallel internal thermal bimetallic switch operative to close after a period of heating by said cold cathode lamp gas discharge.

7. A starter as claimed in Claim 1 wherein said resistor is connected in series with a fusible link.

8. A starter as claimed in Claim 1 wherein said resistor is fusible.

9. A starter as claimed in Claim 1 implemented as a plug-in unit to replace existing starters.

10. A starter as claimed in Claim 1 wherein subsequent to ignition of said lamp said electronic switching means is operative to connect said capacitor in parallel with said ac mains voltage supply to perform power factor correction.

11. A starter as claimed in Claim 1 substantially as herein described and with reference to figures 1, 2 and 3.

Amendments to the claims have been tiled as follows lLj-Claims 1. A starter for a gas discharge lamp comprising first an electronic switching means operative to connect a capacitor between cathodes of a gas discharge lamp so that said capacitor operates to resonate or to be near resonance with said lamp's power supply inductor at ac mains voltage supply frequency to raise both applied lamp voltage and cathode heating I,..

current during starting, and second a bidirectional voltage limiter and resistor connected in series between said lamp cathodes operative to discontinuously damp resonance and control peak voltage rise across said lamp cathodes to prevent lamp ignition occurring at an undesirable high *SS*..

* voltage before its cathodes have heated sufficiently, such that said electronic switching means is operative to connect said capacitor by reference to non zero current flowing in said resistor and only operative to disconnect said capacitor subsequently by reference to currents flowing in said resistor and said capacitor being both zero.

2. A starter as claimed in Claim 1 wherein said electronic switching means is a bidirectional semiconductor device or network of devices such that the functionality of Claim 1 is obtained.

3. A starter as claimed in Claim 1 wherein said electronic switching means is a triac. is

4. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is bidirectional semiconducting device or network of devices such that the functionality of Claim 1 is obtained.

5. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is a cold cathode gas discharge lamp.

6. A starter as claimed in Claim 1 wherein said bidirectional voltage limiter is a cold cathode gas discharge lamp with a parallel internal thermal bimetallic switch operative to close after a period of heating by said cold cathode lamp gas discharge.

7. A starter as claimed in Claim 1 wherein said resistor is connected in series with a fusible link.

8. A starter as claimed in Claim 1 wherein said resistor is fusible.

9. A starter as claimed in Claim 1 implemented as a plug-in unit to replace existing starters. ((7

10. A starter as claimed in Claim 1 wherein subsequent to ignition of said lamp said electronic Switching means is operative to connect said capacitor in paralJel with said ac mains voltage supply to perform power factor correction.

11. A starter as claimed in Claim 1 substantially as herein described and with reference to figures 1, 2 and 3.