Circuit producing candle-flicker light output from lamp

[57] ABSTRACT

An incandescent lamp and a controllable switch such as a thyristor are connected in series across an alternating-current power line. An oscillator supplies gate pulses to the switch to energize the lamp for substantially randomly varying portions of half cycles of the line current. The lamp produces a constantly varying light output which closely simulates flickering light of a candle.

Claims, 3 Drawing Figures

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PATENTEDJAN 91973

3,710,182

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3,710,182

CIRCUIT PRODUCING CANDLE-FLICKER LIGHT OUTPUT FROM LAMP

BACKGROUND OF THE INVENTION

Candles produce an aesthetically pleasing light which constantly flickers or varies in intensity above some minimum level. Candles are thus often used as a sole light source in dining rooms or other areas where a soft and relaxed lighting mood is desired. Conventional candles, however, present a fire hazard, are relatively expensive, and require frequent replacement.

Electric lights having envelopes shaped to simulate a candle flame are well known, but they produce a steady light output which lacks the appealing flicker quality of true candlelight. Conventional on-off flasher circuits can be used with these lights, but the result is a harsh on-off light output which is not acceptable as a substitute for the randomly varying intensity variations of candlelight.

The circuit of this invention controls current flow through an electric lamp to produce a light output which constantly varies around an average level to simulate candlelight. Modular construction permits the entire circuit to be installed in a candelabra-style lighting fixture immediately below a light bulb which preferably has an envelope shaped to simulate a candle flame. A number of circuits and lamps can be grouped in a single fixture to provide an adequate average light level for the illuminated space, while still preserving the pleasant constantly varying or flickering effect of a number of candle flames.

SUMMARY OF THE INVENTION

Briefly stated, the invention relates to a lighting apparatus which includes a lamp, and a control means connected in series with the lamp across a pair of power input terminals. The control means is adapted to deliver substantially randomly varying amounts of electrical energy to the lamp whereby candlelight-simulating light of constantly varying intensity is produced. Preferably, the control means and a socket for the lamp are mounted in a frame which is fitted within a housing shell. The lamp is preferably of a type having an en- 45 velope or outer surface in the general shape of a candle flame to enhance the candle-simulating properties of the system.

In a presently preferred form, the lighting apparatus includes an electronic oscillator and power supply con- 50 nected to an A-C power line, and adapted to generate a train of gating signals which constantly vary in phase with respect to the phase of the input power. The gating signals are delivered to a switch means such as a thyristor which is connected in series with the lamp 55 across the input power terminals. The thyristor acts as a half-wave rectifier which delivers varying portions of input-power half cycles to the lamp in response to the phase-varying control-gating signals. Irregular phasing of leading edges of the gating signals is in part produced 60 by loading of the oscillator circuit by the thyristor, and by using an oscillator power supply which has an unregulated output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS 65

FIG. 1 is a schematic diagram of a circuit according to the invention;

FIG. 2 is a view corresponding to a portion of the diagram of FIG. 1, but showing an alternate method of coupling the circuit components; and

FIG. 3 is a sectional elevation of a lamp and as⁵ sociated circuit according to the invention mounted in a housing shell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A schematic wiring diagram of a lighting circuit 10 according to the invention is shown in FIG. 1. The circuit includes a pair of power input terminals 11 and 12 for connection to a conventional A-C power line. Con15 ^ne&lt;-t^ed in series between the power terminals are a lamp 14 and a switching means such as a thyristor or silicon controlled rectifier (SCR) Q3. The SCR has an anode electrode 15 connected to lamp 14, a cathode electrode 16 connected to terminal 11, and a gate or 20 control electrode 17. Lamp 14 is normally selected to produce a relatively low level of light, and a lamp rating in the range of 3 to 25 watts is a typical choice.

A D-C power supply 20 includes a diode DI, and a pair of resistors R5 and R6, the three components 25 being connected in series across power terminals 11 and 12. Resistors R5 and R6 act as a voltage divider, and the resistances are selected to provide a nominal no-load output voltage of about 6 volts D-C between a positive output terminal 21 (at the junction of resistors ³⁰ R5 and R6) and a common or negative output terminal 22 (at the junction of resistor R5 with terminal 11 and cathode electrode 16). A capacitor C3 is connected across resistor R5 to filter and smooth the power25 supply output-voltage ripple.

Circuit 10 further includes an oscillator circuit 25 which controls SCR Q3. The oscillator is essentially a two-stage transistor amplifier which is intentionally unstable to produce an oscillatory output. A first stage of the oscillator includes an NPN small-signal silicon transistor QI having a base electrode 26, an emitter electrode 27 and a collector electrode 28. A resistor R1 is connected between base electrode 26 and positive output terminal 21 of the power supply. Resistor R1 is the bias resistor for transistor QI, and it is also selected to produce a desired no-load pulse repetition rate or frequency from the oscillator. A load resistor R2 is connected between collector electrode 28 of transistor QI and positive terminal 21 of the power supply.

A second stage of the oscillator includes a PNP small-signal silicon transistor Q2 having a base electrode 30, emitter electrode 31 and collector electrode 32. A bias resistor R3 is connected between base electrode 30 and negative output terminal 22 of power supply 20. An output signal from the first stage of the oscillator is coupled to transistor Q2 by a coupling capacitor C2 connected between collector electrode 28 and base electrode 30. Emitter electrode 31 is connected to positive output terminal 21 of the power supply, and collector electrode 32 is connected to an oscillator output terminal 33.

A resistor R4 is connected between oscillator output terminal 33 and negative output terminal 22 of the power supply. Resistor R4 is a load resistor for transistor Q2, and also acts as a biasing resistance for SCR Q3. A feedback capacitor Cl is connected between output terminal 33 and base electrode 26 of

3,710,182 transistor QI to couple a portion of the oscillator output signal back to the input of the first stage of the circuit.

Typical component values for the circuit just described are shown in the following table:

<td> Transistor QI</td><td> 2N2711</td>

<td> Transistor Q2</td><td> 2N3638</td>

<td> Thyristor Q3</td><td> 2N5063</td>

<td> Resistor Rl</td><td> 100K to 500K ohms</td>

<td> Resistor R2</td><td> 15K ohms</td>

<td> Resistor R3</td><td> 390K ohms</td>

<td> Resistor R4</td><td> 4.7K ohms</td>

<td> Resistor R5</td><td> 6.8K ohms</td>

<td> Resistor R6</td><td> 56K ohms</td>

<td> Capacitor Cl</td><td> 0.05 microfarad</td>

<td> Capacitor C2</td><td> 5 microfarad</td>

<td> Capacitor C3</td><td> 50 microfarad</td>

The no-load output frequency of the oscillator is typically in the range of 60 to 240 pulses per second, depending upon the value of Rl. The value of this resistor may be varied from one circuit to another to provide a distinctly different twinkling quality in several lamp circuits used together. Typically, Rl is selected to provide a no-load (lamp 14 out of the circuit) output frequency of about 150 pulses per second.

In operation, the amount of lamp current flowing through lamp 14 is controlled by SCR Q3. The SCR is nonconductive during negative half cycles of the input voltage across terminals 11 and 12, and is conductive during varying portions of the positive half cycle depending upon the time during the cycle at which a gate or output pulse from oscillator 25 is delivered to control electrode 17. Once the SCR is gated on by the oscillator circuit, current conduction through the lamp continues until the input voltage becomes negative at which point the SCR ceases to conduct until again turned on by another gating signal during a positive half cycle of the input voltage.

The SCR presents a significant load to oscillator 25, and this circuit loading decreases the output frequency or repetition rate of the gating pulses, and also provides a semi-random phasing of the leading edges of the gating pulses with respect to the input voltage from the power line. The waveform at control electrode 17 is a ₄₅train of substantially rectangular pulses of constantly varying width corresponding to the conduction period of the SCR. The trailing edge of the pulse occurs when the SCR is quenched upon polarity reversal of the input voltage. The leading edge of the pulse varies constantly 50 with respect to the phase of the input voltage from the power line, and hence SCR Q3 is gated on for constantly varying portions of the positive half cycle of the input voltage.

In a typical circuit, the duration or pulse width of the 55 gating pulses which trigger the SCR varies constantly in a general range of about 1 millisecond to about 6 milliseconds as the circuit is operated. This variation in the gating pulse is produced by variations in the phasing of the pulse leading edge with respect to line-voltage ⁶⁰phase, which is in turn produced by the instability of the driving circuit when operating in conjunction with the SCR and lamp. The pulse amplitude is essentially clamped at a positive level by the gate electrode after the SCR begins to conduct, and remains at this level ⁶⁵until the line voltage becomes negative and the SCR ceases to conduct. Gating pulses delivered to the SCR while the line voltage is negative are ineffective, and the SCR does not conduct again until arrival of a gating pulse during the next positive half cycle of the line voltage.

Another source of random variation in the phasing of the gating pulse is the intentionally poor regulation of power supply 20. Variations in circuit load on the power supply result in a significant change in powersupply output voltage, contributing to the substantially ¹⁰ randomly varying phasing of the leading edge of the gating pulse. The essentially unregulated power supply and oscillator loading by the SCR appear to work together in a way which varies the recovery time of the 6 oscillator and thus the generally random appearance of the first gating pulse when line-voltage polarity permits conduction by the SCR.

The light output of lamp 14 constantly twinkles or flickers in a random manner, producing an average 2Q level of illumination with a superimposed random brightness modulation above and below the average level. This light output closely simulates the desired flame-like quality of candlelight, while completely avoiding any flashing or blinking on-off lighting which 25 would be produced by a flasher-type circuit. Resistor Rl is the primary control over the light output from lamp 14, and it is selected to provide a sufficiently high repetition rate of gating pulses so the light output is effectively integrated by the human eye to appear as a ³⁰ steady average light output with the twinkling modulation described above.

FIG. 2 shows an alternative way of coupling the oscillator output signal to SCR Q3. Portions of the circuit not shown in FIG. 2 are identical to the circuit in FIG. ³⁵ 1. The circuit in FIG. 2 uses a capacitor C4 (typically 0.05 microfarad to couple the gate signal from output terminal 33 to control electrode 17. A resistor R7 (typically 1.5K ohms) biases the SCR and provides a con₄₀ duction path for the gate signal. Capacitor C4 blocks the flow of direct current between the oscillator and gate electrode, and permits the use of a wider range of SCR’s in the lighting circuit.

The capacitive-coupling technique shown in FIG. 2 adds several components to the circuit, but is normally preferred as it adapts the circuit for use with a wide range of SCR types. The direct-coupling technique shown in FIG. 1 is simple and involves the minimum number of circuit components, and has been found satisfactory with sensitive-gate thyristors such as types 2N5063, MCR-115, GE-MR-5 and GE-C106B.

FIG. 3 shows a typical mechanical arrangement of the components of the lighting circuit. A U-shaped frame or bracket 35 is of a conventional type used in lighting fixtures, and includes a base with a threaded opening 36 for connection to a lamp base. A circuit board 37 carries most of the components of the lighting circuit. The sides of the circuit board extend through slots in the sides of bracket 35 so the board is supported within the U-shaped bracket. Input power terminals 11 and 12 are mounted on the bottom of the circuit board for connection to a power line. All of the components of oscillator 25 are supported in a potted module 39 secured to the circuit board, and the components (not shown) of power supply 20 are positioned on the opposite side of the board. SCR Q3 is mounted at an upper end of the board immediately below a conven3,710,182 tional lamp socket 40 secured at an upper end of bracket 35. A tubular housing or shell 41 is fitted against socket 40, and encloses the bracket and electronic components to simulate a candle body.

FIG. 3 also shows a lamp assembly 42 which is par- 5 ticularly suited for use in the invention as it enhances the candle-like appearance of the unit. Assembly 42 includes a conventional threaded lamp base 43, and a small lamp bulb 44 supported above the base by a lampcord pair of connecting wires 45 which simulate a ¹⁰candle wick. Lamp bulb 44 is preferably of the small pilot-light type such as a T-2 bulb. A body of translucent plastic 47 is molded around bulb 44, and is shaped to simulate a candle flame. The plastic conceals the lamp bulb from direct view and tends to diffuse light ¹⁵emitted by the lamp.

The candlelight effect of the lighting circuit is enhanced when a number of separate circuits and associated light bulbs are used together in a multiplelamp candelabra-style lighting fixture. Alternatively, a single oscillator, power supply and SCR can be used to drive a number of light bulbs in series or parallel connection, with due care being given to the power rating of the SCR. The use of separate circuits is preferred as it provides a different flickering light output from each of the bulbs in the group.

The light bulb or bulbs need not be immediately adjacent the control circuit, and operation with remote lamps is entirely practical. For example, the control circuit can be separately packaged in a small housing for in-line insertion in a power line leading to one or more lamps on a Christmas tree. Similarly, the separately packaged circuit can be mounted in a base or junction box of an existing lighting fixture which is not suited to the mechanical housing arrangement shown in FIG. 3.

The term candlelight as used herein refers to light having the twinkling, constantly changing intensity variations which characterize the flickering light of a fireplace or candle flame. Such natural-flame light sources provide an average level of illumination with a superimposed constant intensity variation above and below the average level, but are entirely distinct from on-off light sources which are controlled by flasher circuits. The circuit of this invention permits close simulation of the pleasing, softly flickering qualities of firelight when used with a conventional incandescent lamp.

There has been described a simple, compact and economical circuit which operates a conventional light bulb to produce a candlelight-simulating light output. The circuit is assembled from readily available commercial components, and is sufficiently small that it may be housed in a tubular barrel which simulates the shank of a conventional candle. The lamp is mounted at the top of this tubular barrel, providing a close visual resemblance to a candle.