TITLE

"TRANSFORMER CONTROL CIRCUIT"

FIELD OF THE INVENTION

The present invention relates to a transformer control circuit. In particular, although not exclusively, the invention relates to a transformer control circuit for use with outdoor lighting systems.

BACKGROUND OF THE INVENTION

Outdoor lighting systems are commonly used as a form of decoration as well as for highlighting elements of a garden, such as providing backlighting for plants, sculptures, and water features etc. Outdoor lighting systems are also used to illuminate paths and driveways to prevent accidents from occurring such as tripping over obstacles while providing a homeowner with a feeling of security when walking around their garden at night.

The use of outdoor lighting systems has grown in popularity in recent years due to the introduction of many do-it-yourself (DIY) outdoor lighting kits currently on the market. Generally, DIY outdoor lighting kits are provided as

12V or 24V systems, wherein a transformer is used to electrically isolate the

DIY outdoor lighting kit from the mains power supply and provide the DIY outdoor lighting kit with a relatively safe voltage level to reduce the risk of electrocution. Accordingly, DIY outdoor lighting kits are electrically safe, simple to connect and inexpensive to install since a qualified electrician is not required to connect the DIY outdoor lighting kit to mains power.

A common problem with DIY outdoor lighting kits is that they are frequently overloaded by unskilled users either by connecting more lights than

the kit was designed for or by replacing light bulbs with ones of a higher wattage. As a result the lifespan of the transformer may be reduced.

Another problem with DIY outdoor lighting kits is that the light bulbs tend to have relatively short life span. This is an inconvenient and expensive exercise for the user to frequently replace light bulbs.

A further problem is that the wire connecting the lights together is usually buried in the ground without a conduit and may suffer damage from garden implements or from weathering. Furthermore, the electrical plugs used to connect DIY outdoor lighting kits together are also prone to the ingress of water into their terminals and hence corrode. Any of the above situations may cause a short circuit to occur within the wiring. A short circuit will cause an excessive amount of electric current to flow through the electrical circuit and overheat the wiring connecting the lights together. This is particularly dangerous when lights are hung from trees or building structures and left on overnight, as in the case of Christmas and fairy lights, where an overheated wire may start an electrical fire.

Another problem is that DIY outdoor lighting kits commonly cause mains circuit breakers to trip or mains fuses to blow.

OBJECT OF THE INVENTION An object of the present invention is to overcome and/or alleviate one or more of the above disadvantages and/or provide the consumer with a useful commercial choice.

SUMMARY OF THE INVENTION

In one form, although it need not be the only, or indeed the broadest form, the invention resides in a transformer control circuit comprising:

a power supply module connected to a transformer; and a control module that regulates power supplied by the power supply module to the transformer; wherein the control module controls power supplied by the power supply module to the transformer to prevent protective devices such as mains fuses from blowing and mains circuit breakers from tripping or to protect the transformer from being damaged by an inrush current condition occurring.

Preferably, the control module gradually increases power supplied to the transformer upon energisation of the transformer control circuit. Ideally, the control module detects when either an overload or short circuit condition occurs between the transformer and an electrical load.

Preferably, the control module limits power supplied to the transformer when an overload condition is detected.

Preferably, the control module cuts power supplied to the transformer when a short circuit condition is detected.

Normally, the status detection module includes a visual warning to indicate when an overload or short circuit condition is detected between the transformer and load.

Additionally, the transformer control circuit includes an overload limit selection module that determines when the overload and short circuit condition is detected.

In another form, the invention resides in a method of preventing protective devices such as mains fuses from blowing and circuit breakers from tripping or protecting a transformer from an adverse electrical condition

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occurring between the transformer and an electrical load, the method including the step of:

(i) gradually increasing power to the transformer upon energisation to prevent an inrush condition occurring between the transformer and electrical supply.

Preferably, the method includes the additional steps of:

(ii) detecting an overload condition occurring between the transformer and electrical load; and (iii) limiting power to the transformer. Preferably, the method includes the additional steps of:

(iv) detecting a short circuit condition occurring between the transformer and electrical load; and (v) further limiting power to the transformer. Preferably, the method includes the additional step of: (vi) cutting power to the transformer if the short circuit condition is not removed within a preset time period.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be readily understood and put into practical effect, reference will now be made to the accompanying illustrations wherein:

FIG 1 shows a block diagram of the transformer control circuit according to one embodiment of the invention;

FIG 2 shows a circuit diagram of the transformer control circuit; and FIG 3 shows an exploded view of a transformer unit including the transformer control circuit of FIG 1 , a transformer, and a casing.

DETAILED DESCRIPTION OF THE DRAWINGS FIG 1 shows a block diagram of the transformer control circuit 10. The transformer control circuit 10 is connected between a transformer 20 and an AC mains supply 30, enabling it to sense the current load drawn by the transformer 20 while regulating the voltage being applied to the transformer 20. The transformer control circuit 10 includes a power supply module 40, a control module 50, a status detection module 60 and an overload limit selection module 70.

The power supply module 40 is used to supply power to both the transformer control circuit 10 and transformer 20. The power supply module

40 is designed to operate from a nominal 240Vac 50Hz mains power by connecting terminals X1 and X2 to the AC mains supply's 30 live and neutral lines.

The control module 50 is used to regulate power delivered from the power supply module 40 to the transformer 20 by connecting terminals X3 and

X4 of the power supply module 40 to the start and end of the primary coil of the transformer 20, respectively. By regulating power delivered to the transformer 20, the control module 50 prevents an inrush current condition occurring between the transformer 20 and an electrical supply. The control module 50 is also used to control the power supply module 40 when either an overload or short circuit condition occurs by limiting or cutting power to the transformer 20, respectively.

The status detection module 60 is used to determine when an overload or short circuit condition occurs at the transformer 20. The status detection module 60 is also used to indicate to a user that the transformer 20 is

operating normally or an adverse electrical condition is occurring between the transformer 20 and the electrical load, such an overload or short circuit condition.

The overload limit selection module 70 is used to set the power rating level that the transformer control circuit 10 uses to determine whether an overload or short circuit condition is occurring. The power rating level is set to within the maximum power rating of the transformer 20 as specified by the manufacturer to protect the transformer 20 from overheating and burning out.

FIG 2 shows a circuit diagram of the transformer control circuit 10. The functional blocks for each module is shown superimposed over circuit diagram of the transformer control circuit 10 to identify the component parts of the circuit that relate to each module.

The power supply module 40 comprises a first power circuit and a second power circuit. The first power circuit supplies power to the control module 50 and includes a voltage dropping resistor network R1-A to R1-H in series with a half wave rectifier D1 and capacitor C1. The second power circuit supplies power to the transformer 20 and includes triac Q1 in series with resistor network R8-A,R8-B,R8-C. The triac Q1 is controlled by the control module 50 to vary power supplied to the transformer 20, as will be discussed in detail later. A varistor R21 is included in the power supply module 40 to protect the transformer control circuit 10 from power spikes by shunting any transient currents away from the transformer control circuit 10.

The control module 50 comprises integrated circuit U1. Integrated circuit U1 is used to prevent an inrush current condition occurring when the transformer control circuit 10 is first energised by gradually increasing power

supplied to the transformer 20, as will be described in detail later. Integrated circuit U1 is also used to protect the transformer 20 when an overload or short circuit condition occurs by either decreasing or cutting power supplied to the transformer 20, as will be described in detail later. The status detection module 60 comprises integrated circuit U2 including two op amps U2A,U2B that are used in conjunction with resistors R6,R7,R8,R14,R17 to form a voltage comparator circuit. The voltage comparator circuit compares the voltage level of an output signal from the control module 50 at point TP8 with the voltage levels at points TP9.TP5 to determine if an overload or short circuit condition is occurring between the transformer 20 and load, as will be discussed in detail later. The voltage comparator circuit is also used to drive LED1 as a visual means of indicating when the transformer 20 is operating within its power rating or if an overload or short circuit condition is occurring between the transformer 20 and electrical load. LED1 is a bipolar, bicolour LED package comprising a red LED and green LED. The red and green LEDs of LED1 are driven either separately to emit a red or green light or simultaneously to emit an amber light from the package of LED1. Resistor R5 and capacitor C5 form an RC timer to cut power to the transformer 20 if a short circuit condition occurs over a prolonged period of time, as will be discussed in detail later.

The overload limit selection module 70 comprises capacitor C9, selectable resistor network R11 ,R13,R15,R16,R18,R19,R20 and solder jumper package JP1. The characteristics of capacitor C9 combined with one of the selectable resistors R11,R13,R15,R16,R18,R19,R20 will determine when the control module 50 will cause the power supply module 40 to limit or

cut power to the transformer 20 by setting a power rating level for the transformer 20. The output signal from integrated circuit U1 at point TP10 is a function of the current load drawn by the transformer 20 and provided to the control module 50 at point TP3 by resistor network R8-A,R8-B,R8-C through resistor R10.

It should be appreciated that the control module 50, status detection module 60 and overload limit selection module 70 may be replaced by a microprocessor or microcontroller. The microprocessor may monitor the power to the transformer 20 to determine whether the power is above a threshold set in software. The microprocessor may then signal the power supply module 40 to regulate the power delivered to the transformer 20.

FIG 3 shows an exploded view of a transformer unit 80 used to supply power to an electrical load. The transformer unit 80 comprises a transformer control circuit 10, a toroidal transformer 2OA, and a casing 90. The transformer control circuit 10 is mounted on a printed circuit board (PCB) 10A that is placed between the casing 90 and the toroidal transformer 2OA. The casing 90 includes a display window 91 that allows light from LED1 of the status detection module to be seen. The casing 90 is then filled with an epoxy resin or other similar substance which sets to ensure that all cavities inside the casing 90 are filled. This protects the transformer unit 80 from any hard knocks or bumps that may be delivered to the outside of the casing 90, while preventing any fluid from leaking into the casing 90 and damaging either the toroidal transformer 2OA or PCB 10A.

When the transformer control circuit 10 is first energised by connecting mains power across terminals X1 and X2 of the power supply module 40, the

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integrated circuit U1 applies a soft start signal to charge capacitor C3. The integrated circuit U1 also controls triac Q1 by applying an output pulse to its gate. As the voltage across capacitor C3 increases, the phase angle delay of the output pulse decreases accordingly. Decreasing the phase angle delay of the output pulse allows a greater amount of current to pass through triac Q1 which, in turn, causes the voltage potential between terminals X3 and X4 to gradually increase from OV to 240Vac over 1.5 to 3 seconds to 'soft start' the transformer.

The transformer control circuit 10 determines the power demand placed on the transformer by measuring the transformer's current load. Integrated circuit U1 monitors the current load at point TP3 through resistor R10 via resistor network R8-A,R8-B,R8-C. While the transformer is operating within the power rating level set by the overload limit selection module 70, the voltage level at point TP8 will be approximately OV. This will allow the voltage comparator circuit to drive the green LED on while keeping the red LED off to indicate that the transformer is operating within its power rating as determined by the capacitor C9 and one of the resistors of the selectable resistor network R11 ,R13,R15,R16,R18,R19,R20. While the voltage level at point TP8 remains at approximately OV, the power supply module 40 will allow 100% of the power rating to be delivered to the transformer.

An overload condition occurs when the current load causes the transformer to exceed its rated power load set by the overload limit selection module 70, causing integrated circuit U1 to increase the phase angle delay of the output pulse delivered to the triac Q1. Increasing the phase angle delay of the output pulse has the effect of reducing the amount of current allowed to

flow through the triac Q1 , thus limiting power supplied to the transformer. This also causes the voltage level at point TP8 to fall between -1V and -4V. The voltage comparator circuit detects that the voltage level at point TP8 has fallen below the voltage level set at point TP9, causing both the green LED and red LED to be driven simultaneously. Driving both LEDs simultaneously causes LED1 to emit an amber light, indicating an overload condition is occurring between the transformer and the electrical load.

A short circuit condition is detected when the current load of the transformer causes it to exceed its power rating by more than 200%. When this occurs, power is further limited to the transformer by the integrated circuit U1 reducing the amount of current allowed to flow through triac Q1. The voltage level at point TP8 is also further reduced to between -4V and -7V. The voltage comparator circuit detects that the voltage level at point TP8 has fallen below the voltage level set at point TP5, causing the green LED to be turned off, such that only the red LED is illuminated. Turning off the green LED indicates a short circuit condition is occurring between the transformer and the electrical load. The RC timer circuit of the status detection module 60 is selected such that if the short circuit condition is not removed within 4 to 5 seconds the voltage level at point TP6 to rise above -1.25V. This causes the integrated circuit U1 to block delivery of the output pulse such that current is prevented from flowing through the triac Q1 , cutting power to the transformer. Power to the transformer remains cut off until the short circuit condition is removed and the transformer control circuit 10 is reset.

FIG 4 shows a garden lighting system 200 including a transformer unit 80 connected to a series of garden lights 100A, 100B, 100n. An input to the

transformer unit 80 connected to an AC mains supply 30 and an output is connected to a plurality of garden lights 100A, 100B ... 100n. The transformer 20 in this case is rated to supply two garden lights 100A and 100B. When the mains supply 30 is first switched on, the garden lights 100A and 110B will gradually increase in brightness. As the transformer unit is operating within its rating the GREEN LED will be illuminated. When a third garden light 100C is connected, the current being drawn through the transformer will be between 100% and 200% of its rated current capacity and the RED and GREEN LEDs will be illuminated showing an amber light to a user. If a fourth garden light 100D is connected to the transformer unit 80, after approximately 5 seconds the RED LED only will be illuminated and power will be cut to the garden lights 100A, 100B ... 100n.

The transformer control circuit provides the advantage that gradually increasing power to the transformer allows the core of the transformer to balance any residual magnetism and bring the magnetic flux back to a balanced state. Soft starting the transformer also prevents an inrush current occurring between the transformer and the electrical supply. This is particularly useful in the case of using the transformer to power a DIY outdoor lighting system since it gives the filaments of each light bulb time to heat up, thus preventing the bulbs from blowing prematurely. Furthermore, soft starting the transformer reduces the probability of blowing a mains fuse or tripping a mains circuit breaker.

The transformer control circuit also prevents the electrical load placed on the transformer from overloading the transformer by limiting power supplied to the transformer when an overload condition is detected. Limiting

the power supplied to the transformer during an overload condition prevents the transformer from exceeding its maximum power rating as specified by the manufacturer and protects it from overheating and burning out.

Finally, the transformer circuit will cut power to the transformer if a short circuit condition is detected over a prolonged period of time to prevent the electrical load from overheating. This is particularly beneficial when using the transformer to supply power to Christmas or fairy lights, where a short circuit could cause an electrical fire.

While the transformer control circuit is shown to regulate power to a toroidal transformer, it is to be appreciated that the transformer control circuit may also be used to regulate power to other types of transformers, such as conventional E-I transformers. Furthermore, it is to be appreciated that while the transformer unit is described for supplying power in a garden setting to a DIY outdoor lighting kit, the transformer unit can be used for providing power to garden implements, such as whipper snippers, lawn mowers, hedge trimmers, etc. Furthermore, since the transformer unit is provided in a sealed casing, the unit may be submerged for supplying power to electric pumps or lights used in water features. The transformer unit may also be applied in other settings where electrical equipment is required to be isolated from mains power.

Although the transformer control circuit is designed to operate from a nominal 240Vac 50Hz mains power, a person skilled in the art will realise that the transformer control circuit may be modified to accept mains power supplied at different voltage and frequency ranges. A person skilled in the art will also realise that the transformer control circuit may be redesigned or

values of component parts may be changed without changing its purpose of regulating power supplied to a transformer to prevent inrush current, overloading, or short circuiting of the transformer and its electrical load.

Throughout the description and claims of this specification, the word "comprise" and variations of that word such as "comprises" and "comprising", are not intended to exclude other additives, components, integers or steps.

Throughout the description of this specification, the word "include" and variations of that word such as "includes" and "including", are not intended to exclude other additives, components, integers or steps. Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variations from the specific embodiments that will nonetheless fall within the scope of the invention.