GROUP DIMMING SYSTEM FOR GAS DISCHARGE LAMPS

Abstract: The invention relates to dimming a plurality of gas discharge lamps without using high current switching components, thereby reducing electromagnetic interference commonly associated with the switching of high currents.

Description: GROUP DIMMING SYSTEM FOR GAS DISCHARGE LAMPS RELATED APPLICATION This patent application claims priority of United States provisional patent application number 60/059,201, filed on September 18, 1997. FIELD OF THE INVENTION The present invention relates to dimming groups of lamps. In particular, the present invention is designed for dimming a plurality of gas discharge lamps at minimal cost. A plurality of lamps may be dimmed without using high current switching components, thereby reducing electromagnetic interference commonly associated with the switching of high currents. BACKGROUND OF THE INVENTION A problem in prior dimming control systems is the difficulty in having sufficient fine control at the low end of dimming, i.e. low-level light output. In prior art designs of group dimming control, some of the lamps turn off and some would remain on at the low end of dimming. A desirable characteristic is that at the low end of the dimming control, the lamps should shut off together similar to a switch. Another difficulty with prior dimming control systems is at maximum output, i.e. high-level light output, there can be a noticeable, audible buzzing sound. Such an audible buzz is an annoyance and not a desirable characteristic for a lighting system. SUMMARY OF THE INVENTION The invention provides a dimming apparatus and method designed for dimming a plurality of gas discharge lamps. The lamps can be dimmed without using high current switching components, thereby reducing electromagnetic interference commonly associated with the switching of high currents. The invention includes a master dimmer circuit or module which creates a low frequency signal that has a variable duty cycle that is used to control the lamps. It is one object of the present invention to provide an improved method and apparatus to dim a plurality of gas discharge lamps. It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps without the use of high current switching components. It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps at minimal cost. It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps that reduces the electromagnetic interference commonly associated with such dimming methods . It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps that includes sufficient fine control at the low end of dimming. It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps so that at the low end of the dimming control, the lamps shut off together. It is another object of the present invention to provide a method and apparatus to dim a plurality of gas discharge lamps such that at the high end of the dimming control, audible buzzing is reduced or eliminated. It is another object of the present invention to provide a master dimmer module that includes the use of two MOSFETS. It is another object of the present invention to provide a master dimmer module that includes the use of a charge pump. Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic of a first embodiment of the master dimmer module embodying the invention; and Fig. 2 is a schematic of a second embodiment of the master dimmer module embodying the invention. Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. DETAILED DESCRIPTION OF THE INVENTION In the present invention, a master dimmer is provided for a plurality of lamps, such as gas discharge lamps, whereby a control signal is provided to each of the power supplies or ballasts of the lamps which are to be dimmed. Since the control signal has a relatively low current, only low current devices are required to dim a plurality of lamps with the present invention. The control signal is sent via a third control wire to each lamp's power supply. If the wiring in the application is a two wire system (e.g. an older home, vehicle or boat), the control wire of each power supply/ballast is connected to the twelve volt ("12V") supply. This disables the dimming feature, but retains normal on/off operation without re-wiring the building, vehicle or boat. Thus, lamps including the invention may be used in both three wire and two wire applications without re-wiring. Typically, lamps of this type are operated at a frequency that is relatively higher than 100 Hz. Such operation can be thought of as a continuous series of power pulses occurring at the aforementioned relatively higher frequency, and these pulses cause the lamp to light. The master dimmer according to the present invention operates based upon pulse width modulation where the master dimmer control sends out control pulses. The master dimmer controls the drive circuit that applies power to each of the lamps by gating the power which feeds each lamp ballast or lamp at a low frequency of under about 100 hertz, or preferably about 70-80 hertz. That is, the power to the lamp is pulse group modulated by changing the number of pulses within a set period, with the number of pulses in each group determining how bright the lamps are. More particularly, in an installation comprising several ballasts and lamps, the CT (control) pins of each ballast may be connected together with a common switch to 12V which will turn all lamps on and off simultaneously or to a common low frequency pulse width modulated (PWM) source to provide substantially equal dimming of all lamps without the running of individual unsightly and expensive power wires for each ballast and dimmer or switch. Conversely, since all ballast/lamp assemblies are connected to the input power supply at all times, local control of each lamp on a dimmer circuit can be had merely by connecting the CT terminal to 12V for full light output and opening the CT terminal by the same switch to turn the lamp desired completely off, or even locally controlled in brightness with a local pulse width modulated source. When this local ballast/lamp switch is thrown to the "master dim" position, thereby connecting the CT pin to the master dim line, the master dimmer will control the brightness of that lamp. Referring now to Figs . 1 and 2 , two embodiments of the master dimmer circuit or module 6 and 8, respectively, are illustrated. The purpose of the dimmer modules 6 and 8 is to create a low frequency signal that has a variable duty cycle that is used to control the lamps. Referring now to the first embodiment as illustrated in Fig. 1, the master dimmer module 6 is shown. A 12 VDC signal is applied to power a 555 timer 10. Other integrated timing circuits may be used, as well as circuits composed of discrete components. A potentiometer 12 is used to select the desired lamp brightness. The signal at the slide of potentiometer 12 is applied to pin 2 to set the duty cycle of the output of timer 10. Diodes Dl and D2 ensure that the operating frequency does not vary for different duty cycle settings. The output signal Q as generated by pin 3 of timer 10 is applied to an output control wire 14. The control signal on wire 14 is applied to the power supply 16 of each individual lamp via the ballast's control pin CT to pulse width modulate the power to each lamp's power supply when switch 18 is in the "Master Dim" position. When switch 18 is in the "Local Dim" position, each lamp may be dimmed locally using a local potentiometer 20 and a local pulse width modulation circuit 22. In either case, ballast 16 receives a pulse width modulated signal to dim lamp 24. Resistor R3 and capacitor C4 slow the leading and trailing edges of the timer's output signal to prevent the output control wire 14 from radiating electromagnetic or radio interference. Resistor R3 also serves as a current limiter in the event of a short circuit fault on the output of control wire 14. Referring now to Fig. 2 and the second embodiment of the master dimmer module 8, a pulse width modulation system is also utilized where the dimmer control sends out control pulses at a low frequency of under 100 hertz, or preferably about 80 hertz. As the pulses get wider, the lamps are in an ON state for longer periods of time. The module 8 includes an EMI filtering portion which includes fuse FI; capacitors Cl, C2, and C4; inductor LI; resistor Rl; and diodes Dl (which provides reverse polarity protection) and D4 (which functions as a voltage regulator). The EMI filter portion is connected to positive voltage supply at fuse FI. The voltage supply is preferably a 12 or 24 volt DC supply. An output drive portion of the module 8 includes two power MOSFETS QIA and Q1B. The common practice is to use a half bridge configured circuit in this situation. Such a circuit however requires special circuitry for efficient activation of the high side MOSFET switch. In a preferred configuration of this second embodiment, the power MOSFETS are both N-channel devices. Thus, there is no longer dependence on external logic to guarantee that the switches cannot cross conduct. It is left to the drive circuitry to supply logic that prevents the N-channel devices from being on at the same time. If both of the N-channel devices are on at the same time, a direct short circuit path shorting out the power supply may occur, and allow high currents to flow which may potentially destroy one of the two or both switch devices. A standard half- bridge is changed slightly so that the high side switch and the low side switch are separated by diode D2. Instead of external logic, the low side MOSFET Q1B is electrically connected directly to the gate of the high side MOSFET QIA. The diode D2 is connected to the source of the high side MOSFET QIA and ultimately electrically to the load. When the low side MOSFET Q1B is on and the high side MOSFET QIA is off, the gate of the high side MOSFET QIA is essentially at a ground potential because the low side MOSFET Q1B is at a very low resistance, for example 1 ohm. The gate-to-source voltage on the high side MOSFET QIA is reversed (negative) due to the load current being drawn through diode D2. Thus, the gate will be lower than the source by a voltage equivalent to the forward drop of diode D2. Thus, since the gate of high side MOSFET QIA is at a lower potential than the source, the MOSFET QIA is guaranteed to be off. The output portion of the second embodiment of the master dimmer module 8 eliminates the need for an intelligent drive circuit to decide how to sequence the two MOSFETS properly and requires only the control of the low side MOSFET Q1B. A specialized drive chip is eliminated and replaced with a single-ended drive controlling just the low side MOSFET Q1B with the high side MOSFET QIA acting as a slave. In this second embodiment, the high side MOSFET QIA is fully enhanced when the low side MOSFET Q1B is turned off. Traditionally, one can use an isolated power supply that floats relative to the source, thus allowing control of the high side MOSFET QIA. The use of another power supply can add significant cost to the module. Also, traditionally used is a boot strap power source which may charge a capacitor when the low side MOSFET Q1B is on. When the capacitor is charged, voltage is supplied proportionately higher to fully enhance the MOSFET QIA. However, in order for the boot strap power source to function, there has to be some off time where the capacitor is recharged. The standard boot strap therefore cannot accomplish the desired non-audible full-on condition. A further traditional approach is to have a bias generator as part of the circuitry that supplies an actuation signal capable of fully enhancing the MOSFET QIA. In the second embodiment of the dimmer module 8 , a gated charge pump technique is utilized to fully enhance the high side MOSFET QIA. With reference to Fig. 2, a charge pump portion is illustrated which generally includes a comparator section and associated resistors and capacitors. More specifically, the charge pump portion includes resistors R6, R7 , R8 , R9 , RIO, Rll, R17; capacitors C5 and CIO; diodes D3 and D6; and comparator U1D. The charge pump portion is connected to the EMI filtering portion at the common node (the anode of diode D3 ) and at the node between resistor Rl and supply voltage V cc . In the charge pump portion of the module 8, the series capacitor C5 is utilized so that the drive side can be referenced to one supply and the load side can have some other reference. The difference in the potential of the two references is blocked by the series capacitor C5. For example, to the left of series capacitor C5 with reference to Fig. 2, everything in the drive circuit is referenced to a common circuit. If one were to look at the waveform at that point and if a nominal 12-volt supply is used, a 12-volt square wave bearing from 0 to 12 volts would be seen. To the right of capacitor C5 with reference to Fig. 2, one would also see a square wave with a 12-volt transition but its reference is not necessarily 0 volts. In fact, the reference voltage is the same voltage as the source of the MOSFET QIA. As the source of MOSFET QIA moves up or down in voltage, the 12-volt wave form references up and down. Thus, the 12-volt ratio is maintained with simply a DC offset equivalent to the reference voltage. The difference in the DC potential is the charge on the series capacitor C5. Therefore, the charge pump portion of the module 8 enables a 12 volt peak-to-peak square wave on the left side that is ground referenced and, on the right side, a 12 volt peak-to-peak square wave that is referenced to the source voltage of MOSFET QIA. Thus, the high side MOSFET QIA is enhanced 100% of the time. However, the charge pump technique typically requires the smoothing out of the pulses. If the same square wave is fed directly to the MOSFET QIA, the output would be modulated at the charge pump frequency, approximately 8 kilohertz. Because the MOSFET QIA is very fast, if a method were not utilized to take the pulses and convert them to DC, the MOSFET QIA would correspondingly be turning on and off at an 8 kilohertz rate, which is undesirable. Typically, to convert the pulsing signal into a pure DC signal, a full wave rectifier and storage capacitor are required. Diode D6 is a dual diode configured as a full wave rectifier. However, in the preferred configuration of module 8, no such storage capacitor is used. Instead, the gate of the MOSFET QIA is utilized as a reservoir capacitor. At 8 kilohertz for example, the gate can be charged very quickly and it stays charged because there is nothing here to drain it thus providing continuous enhancement of the high-side MOSFET QIA. The only connection to the gate of MOSFET QIA that is not through diode D6 is the connection to the low side MOSFET QIB. If QIA is to be turned off, the low side MOSFET QIB is turned on and the charge pump current is fed down through the low side MOSFET QIB. The charge pump could be allowed to continue operating with additional current being pumped through the low side MOSFET QIB. Optionally, a means to turn the charge pump off when the low side MOSFET QIB is turned on can be employed. This means includes the use of diode D7. Diode D7 is configured such that whenever the low side MOSFET QIB is turned on, diode D7 supplies current which forces the charge pump to shut off. This forces the comparator U1D to think that it is at a certain state all the time so that it does stop oscillating. Conversely, when the output of comparator U1A is switched from a "0" to a "1" and the low side MOSFET QIB is off, this releases the current to diode D7 which allows the charge pump to start up again. The start up time determines how quickly the high side MOSFET QIA will turn on, which also guarantees that there will be no cross conduction of the pair of MOSFETS. MOSFET QIA cannot turn on right away because it takes time for the charge pump to get going, for example about 20 microseconds. Still referring to Fig. 2, the circuit 8 includes a master oscillator for driving MOSFET QIB, i.e., the low-side switch. The master oscillator includes resistors R2, R3, R4 and R5; capacitor C7; diode D5; and comparator UIB connected as shown to form an eighty (80) hertz "triangle" wave generator. The amplitude of the triangle wave output is approximately one-third of the supply voltage (V cc ) , and the d.c. offset of the triangle waveform is approximately one-half of the supply voltage (V cc ), so that the triangle waveform has a minimum value of approximately one-third of the supply voltage (V cc ) on the low end, and a maximum value of approximately two-thirds of the supply voltage (V cc ) on the high end. The output of the master oscillator is connected to the positive input terminal of comparator UlA. The output of comparator U1A generates a waveform for controlling MOSFET QIB which in turn generates a dimmer control output at connections J2-2 and J2-3. The output includes varistor RVl and capacitor C8 which provides EMI filtering and short circuit protection. The circuit 8 also includes a voltage divider network including resistors R13, R14, R15 and R18; and potentiometer R16. The voltage divider network generates a voltage reference on the wiper of R16. The wiper of R16 is connected to the negative terminal of comparator UlA so that the output of the master oscillator is referenced against the voltage reference generated by the voltage divider network. Potentiometer R16 is user adjustable to set the reference. Comparator UlA generates the rectangular wave output to drive MOSFET QIB. Adjustment of potentiometer R16 changes the duty cycle of the output of comparator UlA while keeping the frequency at a constant eighty hertz. In the preferred embodiment, the voltage divider network is designed so that the lower limit of the voltage reference is slightly less than one-third of the supply voltage (V cc ) and so that the upper limit is slightly greater than two-thirds of the supply voltage (V cc ). Designing the limits of the voltage reference to be broader than the amplitude of the triangle wave output guarantees that the lights will be completely off when the potentiometer is adjusted to the low end, and eliminates the generation of audible noise when the potentiometer is adjusted to the high end. The circuit 8 also includes a comparator UlC connected to the voltage divider. The negative input terminal of comparator UlC is connected to the wiper of potentiometer R16 and the positive input terminal of comparator UlC is connected to the node between R14 and R18. Resistors R13, R14, R15 and R18 are selected so that the voltage at the node between R14 and R18 is approximately five percent of the supply voltage (V cc ) . The output of comparator UlC is connected to the master oscillator through diode D5. The provision of comparator UlC in this configuration is a redundant measure to guarantee that all lights in the system are "off" when the potentiometer R16 is adjusted to the low end. Notwithstanding the ideal component values designed into the circuit 8, it is possible that variations in specific values of the components within the tolerances for those components would result in the lower or higher references generated by the potentiometer R16 being either higher or lower than one-third V cc or two- thirds V cc , respectively. When dimming, the change in specific component values (even though still within tolerances) may result in some lights in the system being "off" while some other lights in the system are on but at a low intensity level. If the wiper voltage (i.e., the output of the voltage divider) is less than five percent of the total supply voltage, then comparator UlC will turn "off" thereby turning "off" the master oscillator. Though not shown, this type of control could also be included to guarantee circuit performance at maximum brightness. However, the difference between maximum (100%) brightness and less- than-maximum (95%) brightness is nearly indiscernible, and this condition is not viewed as being problematic. Though other component values may be appropriate, the preferred component values for the second embodiment of the master dimmer module 8 are set forth in Fig. 2. Other features and advantages of the invention are set forth in the following claims.

Claims: CLAIMS We claim: 1. A dimmer for a plurality of lamps, each of said lamps having a power supply associated therewith, the dimmer comprising: an input device that inputs a selected lamp brightness level; and a pulse width modulator that outputs a pulse width modulated control signal to each of said power supplies in response to an input signal from the input device, and that outputs a pulsed DC control signal to each of the power supplies. 2. The dimmer of claim 1, wherein said pulse width modulator comprises a 555 timer. 3. The dimmer of claim 1, wherein said input device includes a potentiometer. 4. The dimmer of claim 1, wherein said control signal has a frequency of less than about 100 cycles per second. 5. The dimmer of claim 1, wherein said pulse width modulator includes a comparator.

Inventors: MONDLOCH MICHAEL, PACHOLOK DAVID, PYNE HARRY, MONDLOCH, MICHAEL, PACHOLOK, DAVID, PYNE, HARRY
Applicants: EVERBRITE INC, MONDLOCH MICHAEL, PACHOLOK DAVID, PYNE HARRY, EVERBRITE, INC., MONDLOCH, MICHAEL, PACHOLOK, DAVID, PYNE, HARRY
Application year: 1998
Main classification: H05B37
IPC codes: H05B37, H05B41