GB2029134A - Variable frequency dimming for high intensity gaseous discharge lamps - Google Patents

Abstract

A dimming network for an HID lamp 10 includes a variable frequency generator 14 for increasing the effective impedance of a ballast 12 connected to the lamp and thereby reducing the current therethrough for the same applied voltage level. Preferably, the ballast 12 is primarily inductive so that increasing the frequency reduces the lamp current. The generator 14 may include a switching circuit producing a square wave output. <IMAGE>

Description

SPECIFICATION Variable low frequency dimming for high intensity gaseous discharge lamp This invention relates to lamp dimming and more particularly to providing High Intensity Gaseous Discharge (HID) lamps with lamp dimming control without having to provide existing or conventional lamp-and-ballast networks with additional wiring or components.

It has been discovered that by providing HID lamps with less than rated current, but at the same voltage level, such lamps can operate very efficiently at lower intensity than their rated value. It is desirable, for instance, to be able to turn down HID lamps in areas that are little used. Turning off the lamps in such areas is often undesirable because having more than ambient light is frequently preferred to having no light at all. Furthermore, if the lamps were completely turned off, a relatively long warm-up time is required to bring the lamps back to full illumination.

Examples of such use include a warehouse installation, a vehicle park installation, a tennis court installation after hours of regular use, and a great lighting installation at late hours when there are virtually no automobiles on the road.

Heretofore, it has been common to provide dimming by having a ballast capable of at least partial current bypass operation. When less than full current goes through the lamp, because some of the current is bypassed around a portion of the ballast reactor, then the lamp dims. The amount of current bypass controls the amount of dimming.

The usual method or technique of providing this bypass it to utilise a ballast having at least two separate inductors and to connect a gated semiconductor device, such as a gated triac, around one of them. The gating of the triac determines the degree of bypass, and hence the amount of lamp current.

Many devices may be used to gate the bypass semiconductor. Some of these devices use separate wiring to the semiconductor.

Others use superimposed signals to perform the gating. But, in both instances there is a requirement to add to the existing installation, either in the form of additional wiring, additional electronic components, or both.

According to the invention, there is provided a dimmer circuit for controlling the amount of current through an HID lamp, comprising: variable frequency means for producing an ac voltage of substantially constant amplitude and of variable frequency; and ballast means connected to said variable frequency means and to the lamp, said ballast means having a non-resistive component, a voltage from said variable frequency means at a frequency different from the frequency for operating the lamp under quiescent condition increasing the effective impedance of said ballast means, thereby reducing the effective current through the lamp compared with the application of the same voltage level applied at the frequency for operating the lamp under quiescent condition.

In a preferred embodiment of the invention a variable frequency generator is used to provide power at a constant voltage level and over a frequency range from about 60 Hz to 1 80 Hz. One convenient generator includes a switching network for selectively providing first a positive voltage value and then a comparable negative voltage value, the resulting waveform being a square wave.

Following amplification, the variable frequency voltage is applied to a lamp-and-ballast network having a primarily inductive component. As the frequency is increased, the current is decreased, for the same applied voltage level, and, therefore, the lamp of the network is dimmed.

In order that the invention may be readily understood, embodiments thereof will now be described by way of example, with reference to the accompanying drawing, in which: Figure 1 is a simplified block diagram of a preferred embodiment of the present invention.

Figure 2 is a simplified schematic diagram of one inductive ballast-and-lamp network for a circuit embodying the invention.

Figure 3 is a simplified schematic diagram of a second inductive ballast-and-lamp network for a circuit embodying the invention.

Figure 4 is a simplified schematic diagram of a variable frequency, square-wave generator for a circuit embodying the invention.

Figure 5 is a simplified biock and schematic diagram of a switching network useful in the network shown in Fig. 4.

Fig. 1 shows a dimming circuit for a lamp 10. Lamp 10 is connected to a ballast network 1 2 comprising both an inductive and a capacitive component for well known operational purposes not directly involved with this invention. The drive voltage which is normally applied to the network is the power line voltage at a nominal 60 Hz. In the circuit shown, power is provided from variable frequency generator 14 including a square-wave voltage forming network 1 6 for reasons hereafter explained. The output from generator 1 4 is applied to a voltage amplifier 1 8 to produce an output level suitable for powering the lamp-and-ballast network.

In operation, the output from variable frequency generator 1 4 can be best understood as being first at a predetermined level applied to the lamp-and-ballast network at a nominal 60 Hz. The lamp and ballast components present a complex inductive load under quiescent operating conditions that establish a nominal full brightness current. When the variable frequency generator is adjusted to increase the frequency, the voltage level remaining the same, then the inductive load becomes greater and, hence, the current becomes smaller. In notational language, E = IZ, wherein E equals the applied voltage level; I equals the current through the lamp and inductive ballast; and Z equals the impedance of the total load.The inductive component of impedance can be further represented by j2sfL, wherein j indicates that this component is 90 degrees out of phase from the resistance component; 277 and L are constants for a given inductor; and f equals frequency.

Hence, when E remains constant and f is increased, I then decreases. decreasing current through lamp 10 reduced the light intensity therefrom. Hence, an increase in frequency results in a dimming of the lamp.

Two possible arrangements of an inductive load in combination with lamp 10 are shown in Figs. 2 and 3. In each case inductor 20 is in series with the lamp. In Fig. 2, capacitor 22 is in series with inductor 20 and lamp 10.

In Fig. 3, capacitor 24 is connected across lamp 10.

Fig. 4 is a simplified schematic diagram of a network suitable for development of a square-wave signal for operating a dimmer circuit embodying the present invention. The line voltage at 60 Hz is applied to bridge 32, which is illustrated in simplified form. The output therefrom for the operation of a typical HID lamp network is a resultant 400-600 dc voltage, capacitor 34 acting as a filter component for the bridge output.

A capacitor divider comprising identical capacitors 36 and 38 (which alternatively can be equalized or balanced) establishes a midpoint 37 therebetween at zero volts. These capacitors have low impedance values for all operating frequencies of interest and provide low impedance buffering with regard to voltage changes. Switches 40 and 42 are electronic switches which open and close in alternate fashion first to present an output of first polarity at terminal 39 with respect to the zero-volt terminal 37 and then an output of second polarity with respect thereto. If the total dc level is 600 volts, then the peak of each polarity would be 300 volts. The switching rate of switches 40 and 42 determines the frequency of the square wave output.

Additional low pass filters can be provided to filter out high frequency harmonics, switching transients and the like, since the circuit is frequency sensitive. The production of a square wave in the manner described eliminates the effect of line distortions and other imposed high frequencies that may be included on the power line. An alternative to the two-transistor switch network is a full bridge.

Fig. 5 illustrates a network suitable for operating as the electronic switching network of Fig. 4 just described. It is understood, of course, that the Fig. 4 network illustrates a preferred embodiment only; however, there are many alternative circuits for establishing a square wave voltage. This network includes a multivibrator 50 having two alternately pm duced output pulses, one of which is connected to the base of npn transistor 52 and the other of which is connected to the base of npn transistor 54. These two transistors are connected so that the emitter of transistor 52 is connected to the collector of transistor 54.

Diode 56 is connected across the collectoremitter of transistor 52 and diode 58 is connected across the collector-emitter of transistor 54. The collector of transistor 52 is connected to a positive bias voltage and the emitter of transistor 54 is connected to a negative bias. The output is connected to the junction between the diodes. Alternate conduction to saturation of transistors 52 and 54 causes the creation of the square-wave output previously described.

It is readily apparent that although the description of the exemplary network has been with respect to the development of a square-wave voltage, a variable frequency voltage of any other configuration maintaining a predetermined rms level would satisfactorily operate in the manner just described. A square wave is readily generated and variable and hence its selection in the preferred embodiment. For example, a pulse width modulator may readily be used to change the nominal sine wave of the power line to a suitable operating from a full bright 60 Hz frequency to a higher frequency, where where "full dim" operation occurs. The 60 Hz frequency would be a frequency above the resonant frequency for the LC circuit (where the lamp current would be greatest) and a relatively large capacitor would be employed.As the frequency is increased, operation would be further away from the resonant peak current. A full dim current is typically about onethird of a full dim frequency would be about 180 Hz.

However, the same principle would apply to any frequency range of operation, 60 hz only being selected for discussion purposes because of convenience. When the circuit operates with respect of higher frequencies, however, the components are smaller in size. If operating in the kHz range, it is better to operate above the acoustic frequency range for the lamp(s) to ensure stable, and apparent noise-free operation.

N autotransformer connection can also be connected in conjunction with capacitor 24 and provides ready connection for higher open circuit voltage for starting purposes or for operating metal halide HID lamps.

The same principles discussed above would also apply to a primarily capacitive ballast connection. This would be because operation would be with respect to operating frequen cies below the resonant frequency for the LC circuit. A 60 Hz frequency would be farther away than a 1 80 Hz frequency in this case.

Or, as the frequency increases, operation is more and more toward the peak, hence achieving brighter operation. A relatively small capacitor is used in this case.

It is not practical to have the operating current below 60 Hz, since to go much lower than that would cause a visible flicker condition and even shut-off of the lamp. Therefore, for a primarily capacitive ballast, it would be advisable to select the component values such that full bright operation is at a voltage operation applied at a frequency of 1 80 Hz and full dim operation is at a voltage operation applied at a frequency of 60 Hz, assuming that the resonant frequency selections can be made operating in accordance with the principles just discussed.

Although inductive and capacitive ballast arrangements have been discussed above, it is apparent that any type of reactive type ballast can be used. For example, a lead peak regulating ballast is a popular type of ballast with which the present invention is useful.

One primary advantage of the present dimming system just described over dimming systems in the prior art is that dimming control can be provided without additional wiring to the lamp-and-ballast or lamp structure for the sole purpose of accommodating to providing lamp dimming. This means that existing lamps can be readily provided lamp dimming by merely disconnecting the power factor capacitors to prevent a load from being connected across the switching transistors and by modifying the voltage development network applying the power to the lamp or lamps in the manner discussed.

While particular embodiments of the invention have been shown, it will be understood that the invention is not limited thereto, since many modifications may be made. One example is that variable frequency generator 1 4 does not have to be set manually to achieve dimming, but may be part of an automated system. For example, it may be desirable to dim street lights when no automobiles are being sensed along a particular stretch of road. A sensor network could provide the variable frequency voltage for effecting dimming and a return to full brightness when an approaching vehicle is sensed. Moreover, the above discussion has been with regard HID lighting. Similar operation of fluorescent lamps, incandescent lamps and LPS lamps can be provided. Other modifications also may be made and will become apparent to those skilled in the art.

Claims (9)

1. A dimmer circuit for controlling the amount of current through an HID lamp comprising: variable frequency means for producing an ac voltage of substantially constant amplitude and of variable frequency; and ballast means connected to said variable frequency means and to the lamp, said ballast means having a non-resistive component, a voltage from said variable frequency means at a frequency different from the frequency for operating the lamp under quiescent condition increasing the effective impedance of said ballast means, thereby reducing the effective current through the lamp compared with the application of the same voltage level applied at the frequency for operating the lamp under quiescent condition.

2. A dimmer circuit for controlling the amount of current through an HID lamp, comprising: variable frequency means for producing an ac voltage at substantially constant amplitude and variable in frequency; and ballast means connected to said variable frequency means and to the lamp, said ballast means being primarily inductive, a voltage from said variable frequency means at a frequency greater than power line frequency increasing the effective impedance of said ballast means, thereby reducing the effective current through the lamp compared with the application of the same voltage level at power line frequency.

3. A dimmer circuit according to claim 2, wherein said variable frequency means includes a network for producing a squarewave, ac voltage.

4. A dimmer circuit according to claim 3, wherein said square-wave producing network includes: a rectifier for changing applied ac line voltage to a dc voltage; and electronic switch means connected to said rectifier for switching the output therefrom between a negative and positive level to produce said square-wave, ac voltage.

5. A dimmer circuit according to any one of claims 2 to 4, wherein said ballast means includes a low impedance in series with the lamp.

6. A dimmer circuit according to claim 5, wherein said ballast means includes a large capacitor connected in series with said lamp.

7. A dimmer circuit according to claim 5, wherein said ballast means includes a small capacitor connected across said lamp.

8. A lamp circuit substantially as hereinbefore described with reference to the accompanying drawing.

9. Any novel feature or combination of feature herein described.