Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-07-09
2003-07-08
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000, C315S224000
Reexamination Certificate
active
06590350
ABSTRACT:
TECHNICAL FIELD
The present invention relates to lighting systems including high intensity discharge lamps and electrical controls therefor.
BACKGROUND
High intensity discharge lamps in this specification are characterised in that they have a short arc length contained within an envelope, the envelope being filled with fill materials that may not be fully evaporated and hence have a low pressure when the lamp is cold and before the lamp has started conducting and when the lamp is operating and is hot, the said fill materials have a high pressure. High intensity discharge lamps are further characterised in that as a result of this increase in pressure of the fill material an ignition voltage required to start such lamps may increase sharply as the lamp becomes hot. For example a lamp with a cold ignition voltage of 2,000 volts may when hot require an ignition voltage of 30,000 volts to restart the lamp. High intensity discharge lamps frequently suffer from acoustic resonance, a phenomen whereby a frequency of the supply voltage excites a standing pressure wave within the lamp envelope.
Such electrical high intensity discharge lamps are constructed with a sealed envelope containing at least two electrodes for an electrical discharge, and are arranged to be used for lighting when an arc is established across the electrodes.
Such lamps have a high impedance before they are lit, and a low impedance while they are lit.
This characteristic means that in order to use such lamps in lighting systems it is necessary to combine the lamp with electrical controls matched to the lamp characteristic. Historically, the controls comprise a wound choke or ballast, which produces a back emf, which limits the current flow through the lamp while it is lit. To enable the lamp to become lit a starter is provided which produces a very high voltage across the lamp, which is normally sufficient to break down the resistance of the lamp and cause it to enter a conductive state. If a light fails to start on the first attempt, the starter will provide a further high voltage pulse across the lamp. This process continues until the lamp lights or the electrical power is removed from the circuit. It is this process that gives rise to the characteristic flicker of a discharge lamp that has failed to start when pulsed.
Although wound chokes are reliable, it is more energy efficient to provide controls comprising an electronic ballast. These electronic ballasts limit the current by generating a high frequency wave form which may provide for an intermittent power supply to the lamp.
As with a wound choke, when the controls comprise an electronic ballast to limit the current flowing through the lamp when it is lit, it is necessary to provide means for starting the lamp. While when a wound choke is used the starter can generate a high voltage by interrupting the current flow through the choke, there is insufficient stored energy in an electronic ballast to utilise it in this way. Hence it is necessary to incorporate means specifically to generate the high voltage required for starting the lamp. Known means to generate high voltage includes resonant circuits and suddenly discharged capacitor circuits. Known electronic controls having a self oscillating circuit operate at a frequency determined by the resonance of power handling components in the control circuit. A benefit of these self oscillating circuits is simplicity and low cost, however a disadvantage is that it is difficult to vary the operating frequency of such a control circuit as the operating frequency is determined solely by the values of fixed components, the values of which are determined by the power the circuit is arranged to control. Also known are electronic controls where the operating frequency is determined solely by a frequency generator such that the operating frequency can be arranged to be independent of the characteristics of power handling components in the circuit.
An oscillating control circuit where the frequency is determined by the characteristics of the power handling components is described in U.S. Pat. No. 5,341,067 to Nilssen. FIG. 1 of Nilssen's Patent shows that the oscillation of the circuit depends on the characteristics of the capacitor 52, the inductor 51, and the saturable inductors 47 and 49. To prevent the destruction of the control circuit, Nilssen arranges for the capacitor 52 to be removed from the circuit with the lamp 26. Such removal of the capacitor interrupts the resonant circuit important to self-oscillation, and as a consequence the oscillation stops completely. Nilssen's circuit is arranged to control and power a low pressure discharge lamp, such as fluorescent lamp, and would not be readily adaptable to operate a high intensity discharge lamp because of the very different starting and running requirements of high intensity discharge lamps, which require higher ignition voltages and operate with higher arc currents. Additionally the acoustic resonance phenomena exhibited by high intensity discharge lamps precludes operation in the frequency range specified by Nilssen.
UK Patent Application No. GB 2,226,463 to Yazdanian shows a high frequency electronic control utilising a frequency generator for use with a fluorescent lamp. Yazdanian discloses on page 9 to 11 the use of a series resonant circuit for lamp ignition, whereby there is a starting sequence in which the frequency sweeps from a frequency above the resonant frequency, through the resonant frequency to a lower frequency which is held for a dwell period, after which the frequency is increased again. Yazdanian states, on page 10 line 5, that a benefit of this is to assure that striking does take place for a variety of fluorescent lights irrespective of tolerances in the lamps and the control. This application does not disclose or teach the use of a limited frequency range as a means of discriminating between lamp power ratings.
A high intensity discharge lamp including a temperature dependent capacitor connected in series or in parallel with the lamp is shown in U.S. Pat. No. 4,134,042 to Philips. The Philips' Patent describes in column 3, beginning at line 45, how on starting the lamp, an initial high impedance of the capacitor ensures the lamp rapidly reaches its operating condition, when the heat generated within the lamp heats the capacitor causing its capacitance value to decrease (and hence its impedance increases) causing a reduced current to flow through the lamp. As a result the lamp warms up quickly, but once hot is not over-powered. Philips does not teach the use of a thermally coupled capacitor to modulate the ignition of a lamp.
Once started discharge lamps generate heat and as a result discharge lamps can be more difficult to start after a period of use when the lamp is hot, than they are when they are first started from cold.
Since the controls tend to have a longer life than the lamp it is normal to incorporate the controls into a lighting system and to have the lamp easily replaceable. The lamp is normally provided with male contacts which when it is fitted in the lighting system make electrical connection with the electrical supply through female contacts in a lampholder. A hazard with the female contacts in the lampholder is that when the lamp is removed the controls will detect a very high impedance across the contacts, which will be similar to the condition which pertains before the lamp is lit. Hence the controls will try to start a lamp, by producing the necessary high voltages across the contacts in the lampholder. This means that the hazard from the exposed contacts is increased, since the supply voltage will typically be 240 v or 100 v, but the voltage produced by the starter circuit will be in the order of several thousand volts. Hence the hazard from accidental personal contact with the terminals is greatly increased by the starter. A further hazard arising from cost and space considerations means that the insulating parts in an electrical circuit comprising the controls, the lamp holder, lead wires interconnecting
Alemu Ephrem
Microlights Limited
Wong Don
LandOfFree
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