GB2117192A

GB2117192A - Lamp control circuit

Info

Publication number: GB2117192A
Application number: GB08205698A
Authority: GB
Inventors: Peter Strassheim
Original assignee: TRANSTAR Ltd
Current assignee: TRANSTAR Ltd
Priority date: 1982-02-26
Filing date: 1982-02-26
Publication date: 1983-10-05
Also published as: GB2117192B

Abstract

A discharge lamp control circuit comprises a switching circuit 12-17 which is controlled by a control circuit (32-70, Fig. 2) to switch an electrical supply to the lamp on and off, the ratio of on-time to off-time being controlled digitally. The control circuit includes manually-operable switches (32-34, Fig. 2) for setting up binary code elements representing a required brightness level, and an overriding circuit (53-59, Fig. 2) which overrides the manual switch setting during pre-heat and soft start phases of a starting sequence. During the pre-heat phase, the switching frequency is higher than the resonance frequency of the lamp drive circuit 19-22 but the ratio of on-time to off-time is at a maximum. During a first part of the soft start phase, the switching occurs at the resonance frequency so that a high voltage is applied to the lamp, but the ratio is kept low, so that the energy is insufficient to cause the lamp to fire. During a second part of that phase, the ratio is increased, so that the high voltage causes the lamp to fire. The control circuit then steps to a running phase, in which the ratio is controlled by the manual switches. The stepping through the preheat, soft start and running phases is controlled by a low- frequency clock (53, Fig. 2) and a register (54, Fig. 2) which is stepped by the clock. Alternatively both transistors 12, 13 may be driven via opto-couplers. Various protection arrangements are used, e.g. overcurrent, lamp failure, supply voltage failure, etc. <IMAGE>

Description

SPECIFICATION Lamp control circuit This invention relates to a circuit for controlling the operation of a discharge lamp, such as a fluorescent lamp.

Various lamp starting circuits and brightness control circuits which operate with analogue signals are well known. However, such circuits suffer from certain disadvantages. Firstly, they are very susceptible to electrical noise generated within the circuitry or coming from outside sources. Secondly, the setting of analogue controls for a given brightness level can vary considerably for different lamp units, and it may be necessary to use a special setting-up procedure for each individual unit.

It is an object of the present invention to provide an improved control circuit.

According to the present invention, a circuit for controlling the operation of a discharge lamp comprises a switching circuit for cyclically switching on and off an electrical supply to the iamp; and a control circuit for digitally controlling the ratio of on-time to off-time of the switching circuit.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a power supply circuit for a discharge lamp, and Fig. 2 is a block schematic diagram of a logic circuit for controlling the power supply circuit.

Referring to Fig. 1 of the drawings, a circuit for supplying power to lamps 1 and 2 comprises a half-bridge inverter 3 which is energised by d.c.

power from a bridge recifier circuit 4. The rectifier circuit 4 is connected to an a.c. supply 5 via an interference suppression circuit 6 comprising inductors 7 and 8 and capacitors 9, 10 and 11.

The inverter 3 comprises field-effect transistors 12 and 1 3 connected in series across the d.c.

supply, and capacitors 14 and 15, also connected in series across the supply. The transistors 12 and 13 are interconnected at a junction 16, and the capacitors are interconnected at a junction 1 7.

Alternatively, bipolar transistors may be used in place of the FETs, by suitable modification of the drive circuitry. However, FETs are preferred because of their high switching speed.

Each of the lamps 1 and 2 has two heater electrodes 1 9 and 20, which are connected in a series circuit comprising the electrode 19, a capacitor 21, the electrode 20 and the inductor 22. The series circuit is connected between the junctions 16 and 1 7.

The capacitors 14 and 1 5 are charged from the d.c. supply, and are alternately discharged through the lamp circuit via the transistors 1 3 and 12, respectively, so that an alternating current flows through the lamp circuit. The capacitor 21 and the inductor 22 form a series resonant circuit at the operating frequency of the inverter. This resonant circuit provides a high voltage (e.g.

1.5KV) between the lamp electrodes to cause the lamp to strike. When the lamp has struck, the voltage across it falls to the normal lamp running voltage, and the tuned circuit is damped by the relatively low-impedance path through the lamp, which shunts the capacitor 21. In this condition the lamp current is limited by the inductor 22. The current is approximately sinusoidal, due to the tuned circuit. Freewheel diodes 23 and 24 provide a path for energy from the tuned circuit when the transistors 12 and 13 are off.

One half-cycle of the lamp current is controlled by the transistor 1 3 and its associated digital drive circuitry (Fig. 2). The other half-cycle is provided by the transistor 12, which has a gating circuit comprising a capacitor 25 and resistors 26 and 27. However, the gating circuit becomes operative to gate the transistor 12 on, only when the transistor 1 3 is off. This control is effected by interconnection of the junction of the resistors 26 and 27 with the source of the transistor 1 3.

Referring to Fig. 2, a clock pulse generator 30 produces clock pulses at a repetition rate of approximately 500KHz. The pulses are fed to a binary counter 31, having a capacity of 4 bits, which acts as a divide-by-sixteen circuit. Its output frequency is, therefore, 500/16 6 31 KHz, which is made equal to the resonant frequency of the tuned circuit 21,22 of Fig. 1.

The required lamp brightness level is set up on switches 32-34, which correspond to binary code elements of 4, 2 and 1 significance, respectively. Closure of these switches puts a logic "0" signal on respective output lines 3537. Each of the lines 35 and 36 is connected to an input of a respective NAND gate 38 and 39, and the line 37 is connected to a NOT gate 40.

The outputs of the gates 38-40 are connected to respective exclusive-Or gates 41-43. The other inputs of the gates 41 43 are connected to "4", "2" and "1" count outputs, respectively, of the counter 31. The gates 41-43 therefore compare the count from the counter 31 with the demand set up on the switches 32-34. The outputs of the gates 38 40 will be at the logic 1 level if the gates receive 0 signals from the switches 32-34. In the case of the gates 38 and 39, their other inputs must also receive a 0 from a line 44. When the signal on this line is at the 1 level the gates 38 and 39 are disabled for "soft starting", as will be explained later.If both inputs of any of the gates 41-43 are at the 1 level, showing correspondence between the particular counter output and the switch setting, the output of that gate will be 0. When all of the outputs are 0, the output of a NOR gate 45 will be 1, which output acts as a "total count equals demand" signal.

The transistor 1 3 is driven into conduction by a drive pulse applied to a line 46. An "ON" flip-flop 47, controlled by a "zero count" signal from the counter 31 on a line 48, sets the begining of the drive pulse. An "OFF" flip-flop 49, controlled by the signal from the gate 45, via a flip-flop 50 and an inverter 51, sets the end of the drive pulse. The flip-flop 49 is reset by the "zero-count" signal.

Hence, the duration of the drive pulse is equal to n/1 6 x the period of one half-cycle of the lamp current waveform, where n is the number, from 0 to 7, set up on the switches 32-34.

The maximum amount of energy is fed to the lamps 1 and 2, and hence the maximum brightness is obtained, when each of the transistors 12 and 13 conducts for its full halfcycie of the output resonant frequency. Hence, the lamps may be dimmed by shortening the drive pulse to the transistor 1 3 so that this transistor turns off before the end of its half-cycle. As previously mentioned, the drive pulse length can be adjusted, by means of the switches 32-34, in sixteenths of a half-cycle period, from 1/16 to 7/1 6.If the whole half-cycle period (i.e. 8/16) is required, thus giving maximum brightness, nonclosure of all the switches 32-34 causes the gate 45 to produce its terminate-drive-pulse signal in response to the "8" count from the counter 31, which is fed to one input of an exclusive-OR gate 52, the other input of which is permanently connected to the positive supply line.

Circuitry is also provided for automatically stepping through a lamp starting and running sequence. This circuitry comprises a slow clock pulse generator 53 operating at a repetition rate of, say 1 7Hz, and an 8-bit shift register 54 which is stepped by the clock pulses.

On closure of an ON/OFF switch 55, a "start or reset" circuit 56 feeds a "reset" pulse to the shift register 54, so that the register starts at the beginning of its cycle of operation. During a "preheat" phase the drive signal on the line 46 operates on a full half-cycle basis (i.e. as for maximum brightness) but at twice the resonant frequency of the tuned circuit 21,22, Hence, the tuned circuit is operating well away from resonance, and only a relatively low voltage is applied between the electrodes 1 9 and 20 of the lamp-insufficient to cause the lamp to strike.

This phase continues for two steps of the register 54. The control of the drive signal is effected by an OR gate 57 which is enabled by the first two register stages, and an output gate 58 which lets through the "4" output of the counter 31. The gate 58 is controlled by a NOT gate 59 connected to the output of the gate 57.

During the next step of the register 54 the circuit is in a "soft start" phase in which the output frequency is equal to the tuned frequency of the circuit 21, 22, but the conduction time of the transistor 13 is only 1/1 6th of the full half cycle (i.e. equivalent to setting up the number "1" on the switches 32-34). This is controlled by a NOT gate 60, which feeds a "O" to the gate 40.

Since the lamp circuit is resonant, a firing voltage appears between the electrodes 1 9 and 20, but the energy level is low. The line 44 is held at the "1" level.

During the fourth, fifth and sixth steps of the register 54, the conduction time of the transistor 1 3 is increased to 2/1 6ths of the half-cycle period, and the energy and applied voltage are sufficient to fire the lamp. This control is effected via a NOR gate 61, a NOT gate 62 and a NOR gate 63 which latter applies a "O" to the line 44.

The seventh and eighth stages of the register 54 are not used, but when the register is stepped to these stages the running phase is reached. In this phase, the conduction time of the transistor 13 will be the full half-cycle period, or whatever fraction of the half-cycle is demanded by the setting of the switches 32-34. The voltage across the lamp will be its normal running voltage.

In order to obtain satisfactory running of the lamp it may be found that 1/16th of the half-cycle period is an insufficient transistor conduction time, but a small increase in this time (say to 3/32nds of the period) may readily be effected by the logic. All previous references to 1/16th of the period would then actually relate to this longer time.

The "start-or-reset" circuit 56 is operative to reset the register whenever the action of closing the switch 55 or switching on the power is effected.

A number of protective circuits are provided.

The current in the transistors, and hence the lamp current, is monitored by monitoring the voltage drop across resistors 64 connected in the drain circuit of the transistor 13. This voltage is fed, via a line 65, to an amplification and gating circuit 66. In the event of an excessive lamp current being detected, the output of the gating circuit operates a flip-flop 67 which appiies a "O" to an output gate 68, thereby inhibiting the outputting of the drive pulse to the line 46. The existence of a de-energised lamp is also detected by the circuit, because the lack of damping on the tuned circuit will also produce an excessive voltage on the line 65. The output is latched off until the switch 55 or the mains switching are recycled.

If the voltage of the d.c. supply to the logic circuit falls so that the circuit operation is in danger of becoming unreliable, a circuit comprising a zenner diode 69 and a resistor 70, connected across the d.c. supply, operates the protection flip-flop 67 to shut down the output.

A thermal trip (not shown) may also be provided to cause operation of the flip-flop 67 if an excessive temperature occurs in the circuitry, or if a lamp fails to operate at the minimum brightness level.

If it is desired to increase the resolution (i.e. the number of steps) of the brightness demand switching, this can readily be done by adding one or more further switches, each one doubling the resolution. The frequency of the fast clock generator 30 must then be increased accordingly.

However, such an increase in the number of switches may, in some applications, be objectionable because each switch requires an extra control line between the switch and the logic circuitry. Even the use of four lines required for the three switches 32-34 of the above described embodiment may be objectionable. It is therefore proposed that the demand input bits could be fed to the logic circuitry in a serial fashion over a two-wire data link. By the use of sufficient bits, the steps couid become so small that the brightness control appears to be as smooth as an analogue control, whilst still retaining the advantages of the digital system.

Such advantages are:- a) Repeatable and digitally-defined levels of brightness, which are exactly reproduced in all units driven in "parallel" and in all production units without involved setting up procedures.

b) Digital signals are used between the system controller and all electronic ballasts.

c) A custom l.C. or U.L.A. is easily produced in digital circuitry-preferably in low-power logic (e.g. CMOS) so that production costs are minimal and circuit confidentially ensured. Low-power logic enables lower power drain from the mains and hence efficiency at low cost.

d) Digital circuitry is more immune than analogue circuitry to the electrical noise generated by the switching devices and/or outside sources.

The high-frequency operation of the circuit gives more light output from fluorescent tubes than is obtainable at mains frequency. It also gives silent operation.

In the above embodiment, the interconnection of the drive for the transistor 1 2 with the circuit of the transistor 1 3 gives an economical circuit.

However, an alternative, but more expensive, technique would be to drive the transistor 12 independently of the transistor 13 by use of a transformer or an opto-coupler.

Although the circuit operation has been described with reference to a single lamp, two lamps 1 and 2 and associated capacitors and inductors may be connected as shown, or any other suitable number of lamps and associated components may be similarly connected.

Claims (filed on 25 Feb. 83) 1. A lamp control circuit for controlling the operation of a discharge lamp, comprising a switching circuit for cyclically switching on and off an electrical supply to the lamp; and a control circuit for digitally controlling the ratio of on-time to off-time of the switching circuit.

2. A circuit as claimed in Claim 1, wherein the control circuit includes means for generating a digital command representing a required lamp brightness; a clock pulse generator; a counter for counting the clock pulses; comparison means for detecting correspondence between the clock pulse count and the digital command; and means to cause the switching circuit to switch on at the beginning of the count and to switch off when said correspondence is detected.

3. A circuit as claimed in Claim 2, wherein the means for generating a digital command includes manually-operable switch means which produces binary code elements for controlling the lamp brightness.

4. A circuit as claimed in Claim 3, wherein the means for generating a digital command includes means for over-riding the setting of the manuallyoperable switch means during a lamp starting sequence.

5. A circuit as claimed in Claim 4, wherein during a running phase of the lamp the switching of the electrical supply by the switching circuit is controlled to occur at a frequency substantially equal to the resonance frequency of a drive circuit in which the lamp is connected, and during a "pre-heat" phase of the starting sequence the switching of the electrical supply is controlled to occur at a frequency which does not cause resonance of the drive circuit, the ratio of on-time to off-time then being substantially at a maximum value.

6. A circuit as claimed in Claim 5, wherein during a first part of a "soft start" phase of the starting sequence the switching of the electrical supply is controlled to occur substantially at said resonance frequency but with a ratio of on-time to off-time less than that required to cause firing of the lamp.

7. A circuit as claimed in Claim 6, wherein during a second part of said soft start phase the ratio is increased so that the lamp fires.

8. A circuit as claimed in any one of Claims 47, wherein the overriding means includes a second clock pulse generator; a register which is stepped by the second clock pulses; and means responsive to the stepping of the register to cause the control circuit to step through the pre-heat, soft start and running phases in turn.

9. A circuit as claimed in any preceding claim wherein the switching circuit includes switching devices comprising field effect transistors.

10. A circuit as claimed in any one of Claims 1-8, wherein the switching circuit includes switching devices comprising bipolar transducers.

11. A circuit as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

therefore proposed that the demand input bits could be fed to the logic circuitry in a serial fashion over a two-wire data link. By the use of sufficient bits, the steps couid become so small that the brightness control appears to be as smooth as an analogue control, whilst still retaining the advantages of the digital system.

Such advantages are:- a) Repeatable and digitally-defined levels of brightness, which are exactly reproduced in all units driven in "parallel" and in all production units without involved setting up procedures.

b) Digital signals are used between the system controller and all electronic ballasts.

c) A custom l.C. or U.L.A. is easily produced in digital circuitry-preferably in low-power logic (e.g. CMOS) so that production costs are minimal and circuit confidentially ensured. Low-power logic enables lower power drain from the mains and hence efficiency at low cost.

d) Digital circuitry is more immune than analogue circuitry to the electrical noise generated by the switching devices and/or outside sources.

The high-frequency operation of the circuit gives more light output from fluorescent tubes than is obtainable at mains frequency. It also gives silent operation.

In the above embodiment, the interconnection of the drive for the transistor 1 2 with the circuit of the transistor 1 3 gives an economical circuit.

However, an alternative, but more expensive, technique would be to drive the transistor 12 independently of the transistor 13 by use of a transformer or an opto-coupler.

Although the circuit operation has been described with reference to a single lamp, two lamps 1 and 2 and associated capacitors and inductors may be connected as shown, or any other suitable number of lamps and associated components may be similarly connected.

Claims (filed on 25 Feb. 83) 1. A lamp control circuit for controlling the operation of a discharge lamp, comprising a switching circuit for cyclically switching on and off an electrical supply to the lamp; and a control circuit for digitally controlling the ratio of on-time to off-time of the switching circuit.
2. A circuit as claimed in Claim 1, wherein the control circuit includes means for generating a digital command representing a required lamp brightness; a clock pulse generator; a counter for counting the clock pulses; comparison means for detecting correspondence between the clock pulse count and the digital command; and means to cause the switching circuit to switch on at the beginning of the count and to switch off when said correspondence is detected.
3. A circuit as claimed in Claim 2, wherein the means for generating a digital command includes manually-operable switch means which produces binary code elements for controlling the lamp brightness.
4. A circuit as claimed in Claim 3, wherein the means for generating a digital command includes means for over-riding the setting of the manuallyoperable switch means during a lamp starting sequence.
5. A circuit as claimed in Claim 4, wherein during a running phase of the lamp the switching of the electrical supply by the switching circuit is controlled to occur at a frequency substantially equal to the resonance frequency of a drive circuit in which the lamp is connected, and during a "pre-heat" phase of the starting sequence the switching of the electrical supply is controlled to occur at a frequency which does not cause resonance of the drive circuit, the ratio of on-time to off-time then being substantially at a maximum value.
6. A circuit as claimed in Claim 5, wherein during a first part of a "soft start" phase of the starting sequence the switching of the electrical supply is controlled to occur substantially at said resonance frequency but with a ratio of on-time to off-time less than that required to cause firing of the lamp.
7. A circuit as claimed in Claim 6, wherein during a second part of said soft start phase the ratio is increased so that the lamp fires.
8. A circuit as claimed in any one of Claims 47, wherein the overriding means includes a second clock pulse generator; a register which is stepped by the second clock pulses; and means responsive to the stepping of the register to cause the control circuit to step through the pre-heat, soft start and running phases in turn.
9. A circuit as claimed in any preceding claim wherein the switching circuit includes switching devices comprising field effect transistors.
10. A circuit as claimed in any one of Claims 1-8, wherein the switching circuit includes switching devices comprising bipolar transducers.
11. A circuit as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.