GB2435724A

GB2435724A - TRIAC dimming of LED lighting units

Info

Publication number: GB2435724A
Application number: GB0604410A
Authority: GB
Inventors: David Thomas Summerland
Original assignee: LIGHT Ltd E; MOOD CONCEPTS Ltd
Current assignee: E-LIGHT LIMITED
Priority date: 2006-03-04
Filing date: 2006-03-04
Publication date: 2007-09-05
Also published as: GB0604410D0

Abstract

Conventional TRIAC dimmers are not effective with LED lighting units. A lighting circuit of the invention provides a dummy load 33 in parallel with the lighting units 34a,b. The dummy load 33 presents a sufficient impedance when the dimmer unit 31 is operating at low levels to ensure that the TRIAC continues to operate properly. At higher dim levels the impedance may vary with a positive relationship with the voltage until it is no longer required and therefore does not waste power unnecessarily.

Description

ELECTRONIC DIMMER LOAD

The present invention relates to lighting systems. and in particular to controlling lighting systems with dimmer switches.

Phase control dimmers are widely used to control the light output of modern incandescent lighting circuits. due to their compactness and high efficiency. Their operation relies ri timed switching of an AC voltage supply, for example by means of a circuit comprising a triac, or one or more thyristors, triggered by a variable RC timing circuit. During each half cycle of the supply, the triac is switched on for a proportion of the time. This proportion is dependent upon the set values of the timing circuit components. A variable resistor may he used as part of the timing circuit, together with a capacitor, to control the time after each zero point at which the triac is switched. The total electrical power over each half is cycle that is able to he transferred to a lighting unit is thereby controlled.

increasing the value of the resistor will increase the time constant of the RC circuit and thereby increase the time delay before the triac is switched. Varying the resistance from a low value to a high value will vary the proportion of the cycle being switched on from a minimum to a maximum, thereby controlling the total power over each cycle passed to the lighting unit through the triac 15.

Shown in figure 1 is a circuit diagram for a typical triac-switched dimmer 1 0. The dimmer 10 comprises a triac 15, a variable resistor 13 and a capacitor 14. The dimmer 10 is connected to an electrical lighting circuit via live terminal 11 and switched terminal 12. The triac 15 is controlled via the potential difference across the capacitor 14, which is in turn determined by the value of the resistor 13. As the capacitor 14 charges, the input to the triac 15 rises, and at a predetermined voltage level (dependent on the type of triac) the triac switches to a conducting mode. Current can then flow through the triac from live terminal 11 to switched terminal 12.

The effect of the circuit 1 0 is to enable an AC vohage supply to be switched over portions of each positive and negative cycle, dependent on the selected value of the variable resistor 13. As shown in figure 2. which illustrates voltage (y axis) as a function of time (x axis). the input AC voltage 22 is switched on during periods indicated at 21a. 21b, over the positive half and negative half of each cycle respectively. The resistor 13 and capacitor 1 4 network determine the time delay 23a, 23b between each zero of the voltage cycle 22 and the onset of each on period 21a. 21b.

Since current is only allowed to pass through the triac 15 during each on period 21a, 21b. the total amount of electrical power over each voltage cycle available to a load connected to the circuit 1 0 varies with the value of the resistor 1 3.

Typically, such phase control dimmers have minimum load requirements, so as to ensure proper operation over a substantial proportion of the operable range of the dimmer. Such a minimum load requirement, being that which will be drawn by a light at full supply voltage, may typically be of the order of 50W. This minimum rating is to ensure that the electrical load presented to the dimmer is of sufficiently low impedance to allow the triac to switch at low dimmed levels.

For incandescent lighting, such minimum load requirements are normally below the rated output of typical lighting units. Also, the impedance of incandescent filaments tend to be substantially lower when cold. Such a filament will therefore tend to draw a higher current initially at low voltages. However, when using more efficient forms of lighting such as LEDs (light emitting diodes) in conjunction with phase control dimmers, this minimum load requirement can become a problem. At low dimmed levels, an LED lighting unit may not present a sufficiently low impedance to enable current to be drawn through the triac.

LEDs operate at low DC voltages, typically in the region of 5V. Regulated transformers are therefore required to ensure that a correct supply is fed to the LEDs and to smooth out variations in the AC power supply. Such a transformer, when connected to an electronic dimmer, may not draw power through the whole mains cycle for the reasons above.

it is an object of the invention to provide an electronic load connectable to a dimmer circuit that enables control of LED lighting units at low dimming levels.

It is a further object of the invention to achieve this control in an energy efficient way.

According to a first aspect, the invention provides a lighting apparatus comprising: a lighting unit; and a variable impedance unit connected electrically in parallel with the lighting umt, wherein the variable impedance unit is adapted to provide a first impedance when an input voltage applied to an input of the lighting apparatus is below a first threshold voltage level and to provide a second higher impedance when the input voltage is above the first threshold voltage level.

According to a second aspect, the invention provides an electronic dimmer unit comprising: a switching circuit comprising a triac or a thyristor; and a variable impedance unit connected electrically with an output of the switching circuit, wherein the variable impedance unit is adapted to provide a first impedance when an output voltage of the switching circuit is below a first threshold voltage level and to provide a second higher impedance when the output voltage is above the first threshold voltage level.

According to a third aspect, the invention provides a variable electrical load, comprising: a rectifier having an input connectable to an adjustable mains voltage supply; and a voltage controllable current source electrically connected to an output of the rectifier, wherein the voltage controlled current source is adapted to provide a first impedance across the input when a voltage of the adjustable mains voltage supply is below a first threshold voltage level and to provide a second higher impedance when the voltage of the adjustable mains voltage supply is above the first threshold voltage level.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 illustrates a circuit diagram ola typical triac switching circuit Figure 2 illustrates an exemplary voltage output signa] from a triac switching circuit; Figure 3 illustrates schematically a dimmer controlled lighting circuit; Figure 4 illustrates a block diagram of an exemplary variable electrical load; Figure 5 illustrates an exemplary circuit diagram of a variable electrical load of one aspect of the invention; and Figure 6 illustrates an exemplary voltage and current variation plot for a variable electrical load of the invention.

Shown in figure 3 is a schematic diagram of a dimmer controlled lighting circuit.

A dimmer unit 31 is connected to the live line L, controlling the output of lighting units 32a, 32h via transformer units 34a, 34b connected to the dimmer output and the neutral line N. A dummy load 33 is connected in parallel with the transformer units 34a, 34b. The dummy load 33 is configured to present a low impedance when the dimmer unit 31 is operating at low levels at which the transformer units 34a, 34b do not by themselves provide a sufficiently low impedance to operate the triac 15 in the dimmer unit 31.

An exemplary dummy load 33 is shown in block diagram form in figure 4. The dimmer output and neutral N lines are connected to a bridge module 41, which operates to recti the AC supply from the dimmer unit 31. The rectified supply is fed to a voltage to current converter module 42. Current provided by the voltage to current converter module 42 is passed through an electrical load 44. The electrical load 44 is preferably substantially non-reactive, i.e. presents a substantially resistive load to the AC supply. Optionally, a time delay module 43 is included, which determines a time period after which the current through the load 44 may he shut off or substantially reduced. thereby maintaining a high impedance. For example, if a high power level is indicated by th dimmer unit 31, it may he preferable to shut off the current supplied to the load 44. since the transformer units 34a. 34h present a sufficiently low impedance to avoid the need for a low impedance to he presented by the variable load 33.

Shown in figure 5 is an exemplary electronic circuit fhr a dummy load 33, comprising a bridge module 51, a timing module 52 and a voltage to current converter module 53. The bridge module comprises diodes Dl, D2, D3 and D4, and is shown connected to an AC voltage VI, which is controlled by a phase controlled dimmer unit to vary up to +/-340V peak-to-peak (i.e. 240V RMS) at 50Hz. The current through the timing module 52 and the voltage to current converter module 53 is thereby always in the same direction, with the voltage across each module 52, 53 in the form of a full wave rectified signal, as in figure 2.

At low voltages, the base input to transistor Q 1 is essentially that provided by the resistor network consisting of R4 and Ri, which in this case maintains a low potential at the base of Qi. The base input of transistor Q2 is thereby held at a higher potential given by the values of resistors R6 and R5. Provided this voltage is sufficiently high to operate the transistor Q2, both transistors Q2 and Q3 are switched into conducting mode, since transistor Q3 is configured to ftllow the output of transistor Q2. A current I is therefore drawn through the collector and emitter of transistor Q3 and the resistor R7. The effect of this arrangement is to present a low impedance across the input terminals 54a,b of the dummy load 33, with the electrical power largely being drawn by the transistor Q3.

As the output voltage of the bridge module 51 rises, the base input of transistor QI rises, and the base input of transistor Q2 consequently falls. The output current of transistor Q3 therefore falls, and impedance presented across the terminals 54a,b rises.

As the dimmer unit provides a higher proportion of the mains voltage signal. the average voltage over each AC cycle increases. This raises the average voltage across the resistor R2, as the capacitor Cl is charged during each half cycle. once the average voltage increases above a threshold level, the voltage across R2 s becomes sufficiently high to drive the zener diode Zi into conducting mode. This further increases the voltage applied to the base input of transistor QI, thereby further increasing or maintaining the high impedance presented at the input terminals 54a.h. This is maintained for as long as the capacitor Cl remains charged above the level required to operate the zener diode Zi. However, the resistor R2 will tend to continually discharge the capacitor CI. If the average voltage over each AC cycle is above a certain threshold level, the charge across capacitor Cl will remain high enough to operate transistor QI and thereby keep the impedance across the terminals 54a.h high, despite the output voltage of the bridge module falling to zero after each half cycle.

The operation of the dummy load as detailed above thereby provides for a load that, at low dimmed levels, exhibits a low impedance, which will tend to enable the triac in the dimmer to operate. At higher dimmed levels, the impedance of the dummy load varies as a function of the input voltage as the transformer units 34a,b draw more current. Preferably, the impedance of the dummy load at higher dimmed levels varies in a positive relationship with the applied voltage, that is to say the value of dZ/dV remains positive as the voltage V rises, where Z is the magnitude of the impedance presented by the dummy load. At yet higher dimmed levels the impedance is further increased or maintained at a higher level, as the current being drawn by the transformer units 34a,b is high enough to not require the dummy load 33.

The effect of the operation of the dummy load 33 is illustrated in figure 6, which shows a voltage and current plot as a function of time. In this case, the input voltage 61 (right hand scale), i.e. that provided across the timing and voltage to current converter modules, is a full wave rectified 340V peak-to peak (240V RMS) 501-Iz AC signal. The current 62 (left hand scale) at the emitter output of transistor Q3 initially peaks to a maximum 63. and as the voltage 61 continues to rise, the current 62 falls, as the impedance of the dummy load increases.

The capacitor Cl and resistors R2. R8 may he chosen such that the impedance is maintained at a high level once the voltage 61 averages above a threshold level over a predetermined time period. Preferably. the components are chosen such that the predetermined time period, when the maximum voltage is applied, is less than that of a half cycle, which in this instance is I Oms. For example. a predetermined time period 64 may be chosen to be 8rns at maximum applied voltage. A high impedance is thereby maimained despite the voltage ihlling back to zero at the end of the half cycle, which would otherwise cause the impedance to fall and result in a further maximum current 65 to be drawn, followed by a repeat of the current characteristic. Provided that the voltage 62 averages over a threshold value over a plurality of cycles, the high impedance is maintained. If the supply provided by the dimmer is then reduced such that the average voltage falls below the threshold value, the dummy load returns to providing a low impedance at low voltage levels.

The dummy, or variable impedance, load may be provided as a separate unit connectable to a lighting circuit, for example as shown in figure 3. Alternatively, the dummy load may be provided as part of a lighting or transformer unit. The lighting and transformer units may he combined in an integrated lighting unit.

Further, the dummy load may be comprised within an electronic dimmer unit.

A particular advantage of the dummy load when used with high efficiency lighting circuits is that the load consumes very little additional power, due to the increase in impedance at higher applied voltages. For a typical application, the worst-case power loss in the dummy load at full applied voltage would he 1 Watt. This power loss would increase at dimmed levels to a maximum of around 3 Watts.

However, as compared to the power losses through heat in incandescent lighting this is a small contribution. and an overall large power saving in the lighting circuit can still be achieved.

Other embodiments are envisaged to he within the scope of the invention, as defined by the appended claims.

Claims

CLAIMS

1. A lighting apparatus comprising: a lighting unit and a variable impedance unit connected electrically in parallel with the lighting unit, wherein the variable impedance unit is adapted to provide a first impedance when an input voltage applied to an input of the lighting apparatus is below a first threshold voltage level and to provide a second higher impedance when the input voltage is above the first threshold voltage level.

2. The lighting apparatus of claim I wherein the first impedance and second impedance are both substantially non-reactive.

3. The lighting apparatus of either claim I or claim 2, wherein the second impedance varies as a function of the input voltage.

4. The lighting apparatus of any of the previous claims wherein the second impedance varies in a positive relationship with the input voltage.

5. The lighting apparatus of any of the previous claims further comprising a timing module adapted to maintain at least the second impedance after a predetermined time period when an average value of the input voltage over a plurality of cycles is above a second threshold voltage level.

6. The lighting apparatus of claim 5 wherein the timing module comprises a capacitor and a resistor network, wherein a charge on the capacitor determines when the second impedance is to be maintained or increased despite the input voltage falling below the first threshold voltage level, the resistor network determining a charging time constant and a discharging time constant for the timing module.

7. The lighting apparatus of any of claims I to 6 wherein the lighting apparatus includes one or more semiconductor lighting units.

8. The lighting apparatus of claim 7 wherein the one or more semiconductor lighting units are light emitting diodes.

9. An electronic dimmer unit comprising: a switching circuit comprising a triac or a thyristor and a variable impedance unit connected electrically with an output of the switching circuit, wherein the variable impedance unit is adapted to provide a first impedance when an output voltage of the switching circuit is below a first threshold voltage level and to provide a second higher impedance when the output voltage is above the first threshold voltage level.

10. The electronic dimmer unit of claim 9 wherein the first impedance and second impedance are both substantially non-reactive.

11. The electronic dimmer unit of either claim 9 or claim 10, wherein the second impedance varies as a function of the output voltage.

12. The electronic dimmer unit of any of claims 9 to 11 wherein the second impedance varies in a positive relationship with the output voltage.

13. The electronic dimmer unit of any of claims 9 to 12 further comprising a timing module adapted to maintain at least the second impedance after a predetermined time period when an average value of the output voltage over a plurality of cycles is above a second threshold voltage level.

14. The electronic dimmer unit of claim 13 wherein the timing module comprises a capacitor and a resistor network, wherein a charge on the capacitor determines when the second impedance is to be maintained or increased despite the input voltage Calling below the first threshold voltage level, the resistor network determining a charging time constant and a discharging time constant for the timing module.

A variable electrical load, comprising: a rectifier having an input connectable to an adjustable mains voltage supply: and a voltage controllable current source electrically connected to an output of the rectifier, wherein the voltage controllable current source is adapted to provide a first impedance across the input when a voltage of the adjustable mains voltage supply is below a first threshold voltage level and to provide a second higher impedance when the voltage of the adjustable mains voltage supply is above the first threshold voltage level.

16. The variable electrical load of claim 15 wherein the first impedance and second impedance are both substantially non-reactive.

17. The variable electrical load of either claim 15 or claim 1 6, wherein the second impedance varies as a function of the voltage of the adjustable mains voltage supply.

18. The variable electrical load of any of claims 15 to 17 wherein the second impedance varies in a positive relationship with the voltage of the adjustable mains voltage supply.

19. The variable electrical load of any of claims 1510 1 8 further comprising a timing module adapted to at least maintain the second impedance after a predetermined time period when an average value of the output voltage over a plurality of cycles is above a second threshold voltage level.

20. The variable electrical load of claim 19 wherein the timing module comprises a capacitor and a resistor network, wherein a charge on the capacitor determines when the second impedance is to he maintained or increased despite the input voltage falling below the first threshold voltage level, the resistor network determining a charging time constant and a discharging time constant for the timing module.