SWITCHEDDUMMYLOAD FORMONITORED AC LOADS

Field of the Invention

The present invention relates generally to the monitoring of electrical loads and, in particular, for the provision of a switched dummy load to facilitate reliable monitoring for a load at a remote location.

Background

Until relatively recently, most traffic light controllers relied on relays to operate lamps or lanterns associated with traffic light systems, hi such instances, there was typically no monitoring of lamp operation, but to give a reasonable level of safety, a wiring method called "interlocking" was used. As a simple example, the green lamps (and often the yellow) for, say, the east-west direction at an intersection were powered from the supply for the north-south red lamps. Thus, it was not possible for the east-west green lamps to be driven unless the north-south red lamps were powered. This arrangement protected against faults within the controller that may accidentally cause the wrong lamps to be driven. However, such did not protect against wiring faults or short circuits at the intersection. Also, the response to a fault of inhibiting the output of the green lamps could leave motorists to assume that there were no traffic lights at that intersection which of itself may be dangerous. Further, the "interlocking" wiring to the controller in any but the simplest of intersections was quite complex, the procedure talcing some hours for installation and being prone to much error.

More recently, traffic light controllers have used an output switched on or off using a software controlled semiconductor device, such as a TRIAC. The voltage across each output was monitored. A combination of TRIACs monitored as "on" was then checked by two independent circuits and, if either reported a conflicting combination, the intersection would be switched to flashing yellow or off, thus indicating to motorists the presence of a fault in the traffic light controller. Such a method is presently used in traffic light controllers around the world and has the advantage that no intersection specific

"interlocking" wiring is required, and that specific conflict information is stored in a memory module and able to be assessed by software within the controller.

For safety reasons, the voltage applied to a traffic lamp output is monitored. If the voltage is too high when the lamp is meant to be off, and thereby sufficient to cause the lamp to illuminate, even dimly, then such may result in a dangerous situation. If such a false indication is present on a green or even a yellow lamp, motorists may interpret such to mean that it is safe to proceed, when in fact vehicles may be crossing their path. This is normally called a "conflict".

The normal procedure within modern controllers when such a "conflict" occurs is to either turn all lamps off, or to fall back to flashing yellow lamps only.

Besides the basic problem of incorrect lamp displays occurring, the lamp voltage monitoring circuit may also report an output as being driven when in fact it is not. This is also dangerous since such may result in a controller shutting down or going to flashing yellow when in fact such is not necessary. In the applications described above, traffic lamps represent an electrical load for which monitoring of operation is desired. Importantly, since the load is often remotely located from its controller, and interconnecting wiring being of the lengths of perhaps 100 metres or more, the load can be considered "remote" and thus being susceptible to voltages being induced upon unpowered cables from adjacently located powered cables. Further, with traffic lamps, the nature of the load can often vary, examples of which include traditional incandescent lamps, low voltage transformer driven halogen lamps, and even light emitting diode (LED) arrays.

Summary of the Invention

It is an object of the present invention to substantially overcome, or at least ameliorate one or more deficiencies in existing arrangements to enable reliable monitoring of remote loads, particularly traffic lamps.

In accordance with one aspect of the present invention there is disclosed electrical control apparatus comprising:

a first switch configured to couple an electrical supply to a primary electrical load according to a value of a control signal; a second switch configured to couple the primary load to a dummy electrical load according to an inverse value of the control signal; and means for applying the control signal to the first switch and an inverse thereof simultaneously to the second switch.

Specific embodiments provide a switched dummy load for an electrical system presenting a load voltage. The load is formed of a shunt network, arranged to shunt the load voltage, and a biasing circuit. The biasing circuit is arranged to be operative on an inverted version of the switching drive, used to supply the load voltage, to bias the shunt network to a low impedance when the load voltage is intended to be disabled. The biasing circuit also biases the shunt network to a high impedance when the magnitude of the load voltage is intended to be above the predetermined value.

Brief Description of the Drawings A number of aspects of the prior art and embodiments of the present invention will now be described with reference to the drawings in which:

Fig. 1 schematically illustrates the prior art operation of a remote load; Fig. 2 is a schematic representation of a prior art traffic controller arrangement; Fig. 3 is a schematic illustration of a prior art dummy load configuration used in traffic controllers;

Fig. 4 is a schematic illustration of another prior art dummy load arrangement used in traffic controllers;

Fig. 5 A is a schematic block diagram representation of a first mode of operation of an arrangement according to the present disclosure; Fig. 5B is a schematic block diagram representation of a second mode of operation of the arrangement of Fig. 5 A;

Fig. 6 is a schematic circuit diagram of a traffic light control circuit including a switched dummy load; and

Fig. 7 is a schematic circuit diagram of a variation of the dummy load circuit of Fig. 6.

Detailed Description including Best Mode

Fig. 1 shows a traditional arrangement 100 for supplying a remote load 112 via a controller 106. A mains supply 102 is provided to the controller 106 and a switch 108 having a control terminal 110 is used to selectively connect the mains supply 102 to cables 114 that supply the load 112. The cables return to an earth connection 104 representing part of the mains supply 102. Such a configuration applies generally to traffic lamp systems but also to other control arrangements such as security systems including lighting, surveillance cameras and industrial control arrangements including valve and motor operation, to name but a few. The switch 108 can be a relay but, more recently semiconductor devices such as SCR's and TRIACs have found favour with designers in view of the ease of driving such devices from computer control equipment.

Fig. 2 shows a traditional traffic light control system 200 in which a mains supply 202, having an earth connection 204, is used to operate a number of traffic lamps via a controller 206. In Fig. 2, only those components relating to the operation and monitoring of a red lamp 228 are numerically identified for the sake of clarity, the circuits for yellow and green being otherwise identical. As seen, the mains supply 202 couples to a switch 210 via a current sensing resistor 208. The switch 210 is electrically controllable, via a control terminal 212, by a logical computer module (not illustrated) generally formed within the controller 206. Where the switch 210 is formed from a TRIAC, a snubber circuit 216 formed from series network, including a capacitor 218 and resistor 220, is arranged in parallel with the switch 210 and operates to protect the electronic switch 210 from potentially harmful transient voltages. A fuse 222 is provided at an output of the switch 210 to provide for over current (short circuit) protection for the cabling 224, 226 which supplies the lamp(s) 228. The resistor 208 allows the controller 206 to monitor the load current 214 as drawn by lamps. The sensed load current may be compared against a predetermined set of values to assess under-current or over-current

conditions indicating mis-operation of one or more of the lamps. Further, the controller 206 includes a voltage sensing arrangement by which the voltage applied to the lines 224, 226 supplying the lamp 228 is monitored via a resistive divider formed of resistors 234 and 236. The voltage sense signal 238 allows the controller to specifically monitor the voltage being applied to the lamp 228 to thereby provide for monitoring the operation of the switch 210. Whilst the voltage sensing arrangement of Fig. 2 is shown referenced to the earth connection 204, a similar arrangement referenced against the supply 202 may be used.

As illustrated, it is highly desirable for all control and monitoring circuits in the system 200 to be found or located within the controller 206 thereby leaving only the remote loads (the lamps 228) and lengthy interconnecting cabling 224, 226 external to the controller 206.

Where the lamp 228 is an incandescent lamp, when a particular output is not being driven, the voltage at that output would be close to zero due to the relatively low impedance of the lamp 228. However, if the load presented by the lamp 228 is absent, due normally to a failed (open circuit) lamp, a high voltage would appear at the output, even when not driven, due to the capacitance of the snubber circuit 216 and also virtual capacitances 230, 232 arising from proximity with other cables within the traffic light system 200. Such a situation can cause a false "conflict" to occur, thereby shutting down the controller 206 when in fact no conflict was present.

The simple solution to this false conflict problem was to add a "dummy" load directly across each output. Such a load must be sufficient to keep the coupled voltage below the safety threshold under worst expected conditions.

The simplest form of dummy load is a resistive dummy load, an example of which is shown in Fig. 3. In the arrangement 300 of Fig. 3, a lamp 304 is supplied by cables 302, representing the cables 224, 226 of Fig. 2. A resistor 306 is arranged in parallel with the lamp 304 to add additional load to the circuit 300. The resister 306 is depicted arranged within the associated controller.

To keep the voltage across the lamp below a critical level, where the mains supply is 240 volts AC, a resistor value of the order of 10,000 ohms or less is required. The disadvantage arising from such an arrangement is that typically a power value of approximately 5 watts will be dissipated in the resistor when the lamp 304 is turned on. This results in unacceptable heat being generated in the controller, which can be significant at an intersection where a large number of lamps are used.

An alternate approach is to use a capacitive dummy load 406, an example of which is shown in Fig. 4, where the components illustrated operate in a complementary fashion to those shown in Fig. 3. In Fig. 4 however, the power consumed is reactive and therefore negligible heat is generated. Such an arrangement works well when most traffic lamps are formed from normal incandescent light bulbs.

More recently however, incandescent lamps have been replaced by extra low voltage halogen bulbs usually operated via a step-down transformer. In such a situation, when the bulb fails, the inductance of the transformer can resonate with the capacitive dummy load 406 and the cable capacitance (eg. 230, 232). When such a circuit is not driven, voltage from other cables can thereby induce a voltage on the non-driven resonant circuit that can well exceed the safety limit. Such is not presently a serious problem since the step down transformers used are generally "lossy", being made of low grade steel and hence the voltage induced is often within design limits. However, traffic lamp assemblies are now being introduced that use LED arrays in place of incandescent lamps. Such lamp arrangements use very little energy and the transformers used with such LED arrays have been designed to be low loss in order to capitalise upon the energy saving capabilities of such lamps. Further, when such LED lamps are un-powered, they present a high impedance to the control circuit, contrasting halogen and incandescent lamps which present a low impedance. This can result in a resonant circuit being formed due to the capacitance of the snubber 216, and/or 406 and the inductive windings of the transformer. Resonance of such a circuit can easily reach traditional safety limits. Since the power drawn by some LED lamps is very low, the

induced voltage may not be sufficiently dampened by the traditional dummy load, even when there is no failed lamp.

The solution proposed by the present inventor and forming the basis of the present disclosure is to provide a switched low value load that will dampen any induced voltage to a low level but, through its switched operation, will not dissipate large amounts of power if the associated load output is turned on.

Fig. 5A is a schematic representation of a circuit in accordance with the present disclosure presented in a fashion similar to that of the prior art arrangement of Fig. 1. Fig. 5A shows a control circuit 500 in which a mains supply 502 is input to a controller 506 for the supply of a load 512 via cable connections 514. The controller 506 incorporates a controllable switch 508 which, according to the value of a control signal 510, is arranged to selectively connect the mains supply 502 to the load 512. Coupled between an output of the switch 508 and an earth return 504, and therefore in parallel with the connections 514 and load 512, is a series connection of a further controllable switch 554 and a dummy load 556. The switch 554 can therefore sample the voltage output from the switch 510 and on the connections 514. An inverter 550 is arranged between the control signal 510, representing a control input of the switch 508, and the control input 552 of the switch 554. The inverter 550 operates to invert the logic of the control signal 510 which is simultaneously supplied to the switch 554. In this fashion, the controllable switches 508 and 552 are thereby configured to operate at the same time but in anti-phase such that when one switch is open, the other switch is closed, and vice versa. As seen in Fig. 5A, the switch 508 is open and the switch 554 is closed. In this configuration, the dummy load, in this example represented by a resistor 556, is effectively connected in parallel with the load 512, thereby operating as a shunt network to reduce the overall impedance presented at or by the connecting lines 514.

As seen in Fig. 5B, when the control signal 510 is such that the switch 508 is closed, the switch 554 is open through operation of the inverter 550. This operates to

remove the dummy load 556 from the circuit and to apply the main supply 502 to the load 512.

The circuit arrangement of Figs. 5A and 5B has a specific advantage where the voltage being supplied to the load 512 is monitored and for times that when no voltage is intentionally supplied to the load, that no voltage is seen upon the load 512. This is achieved by using the dummy load 556 and the controllable switch 554 to effectively shunt the load 512 at the supply side of the connecting lines 514 thereby ensuring that any voltage that may be induced upon the connecting lines 514 is induced upon a low impedance and thus does not contribute any significant voltage upon the lines 514. Fig. 6 shows a control circuit 600 developed along the lines of the arrangements of Figs. 5A and 5B and specifically configured for the supply of a traffic lamp assembly 612 and which may be utilised to supplement the prior art arrangement shown in Fig. 2 of this patent specification. Fig. 6 in this regard only shows control circuitry for a single lamp assembly and omits the monitoring circuitry shown in Fig. 2. Where the traffic lamp assembly 612 operates using an LED array, such may include a step-down transformer, rectifier and filter, and constant current circuitry, or a switched mode power supply, for driving the LED array.

In the example of Fig. 6, the traffic lamp assembly 612 is connected via supply lines 614 to a traffic controller 606 which supplied from a mains AC supply 602 having a mains neutral return 604. A logic supply 660 having an associated logic earth 678 is typically used to generate and supply logic control signals for the specific traffic light configuration being controlled. In some applications, and as illustrated, the logic earth 678 may be isolated from the mains neutral 604.

In the arrangement of Fig. 6, a logic control signal 610, separately generated elsewhere in the controller 606, is used to control the operation of an electronic switch formed by a TRIAC 608 which selectively couples the mains AC supply 602 via the lines 614 to the traffic lamp load 612. As illustrated, arranged in parallel with the

TRIAC 608 is a snubber circuit formed by a series connection of a capacitor 618 and a resistor 620. The TRIAC 608 supplies the lines 614 via a fuse 622.

In Fig. 6, representing an alternative to the configuration of Figs. 5 A and 5B, the logic control 610 is fed to the TRIAC 608 via a bipolar junction transistor (BJT) 650 operating as an inverter, and also via a buffering BJT transistor 672. The transistors 650 and 672, together with the illustrated combination of resistors 664, 662, 668, 670, 676 and 674, achieve an appropriate logic control bias required for switching the TRIAC 608 into and out of conduction.

The logic control signal 610 also couples to a dummy load circuit 654 via a resistor 666. The resistor 666 couples the control signal 610 to a diode formed as part of an optocoupler 684, connected to provide electrical isolation between the logic supply side (660, 678) of the controller 606 and the dummy load circuit 654. These circuits operate at different potentials due to the presence of a full-wave bridge rectifier 682 in the circuit 654 which samples the voltage on the lines 614 forming a pair of inputs to the dummy load circuit 654.

The dummy load circuit 654 of Fig. 6 is connected with the output connections 614 and the traffic lamp load 612 to shunt voltages thereon. A high voltage varistor 680 is configured at the input of the dummy load circuit 654 to limit the applied voltage to a safe value approximating that of the nominal maximum value of the mains AC supply 602. The AC signal sampled from the lines 614 is input to the full wave bridge rectifier 682 which creates (+) and (-) line outputs as shown. The dummy load circuit 654 also incorporates a field effect transistor (FET) 692, having a conduction channel the drain of which connects to the (+) line from the rectifier 682, and the source connecting to a load resistor 656, which in turn connects to the (-) line. The FET 692 is self-biased by operation of a resistor 688 interconnecting gate of the FET 692 to the (+) line. A zener diode 686 is arranged between the gate of the FET 692 and the (-) line to limit the voltage applied between the gate of the FET 692 and the (-) line at the load resistor 656. An optically coupled transistor of the optocoupler 684 is connected to the

gate of the FET 692 and is operative to shunt biasing current away from the gate of the FET 692 thereby disabling the self-biasing operation of the resistor 688.

In operation, when the logic control signal 610 is at a relatively high logic level, the diode within the optocoupler 684 will be biased "off thereby ensuring the optically coupled transistor therein is also off. This results in the voltage at the gate of the FET 692 being limited to a maximum value established by the zener diode 686, whilst permitting the FET 692 to be self-biased "on" via the resistor 688 and the magnitude of the (+) line. Accordingly, the dummy load will then become active should a voltage be output from the rectifier 682 that is sufficient to bias the FET 692 "on" via the resistor 688. When this occurs, the FET 692 conducts, effectively placing the resistor 656 and the FET 692 in parallel with the connecting lines 614 via the rectifier 682 thereby shunting the voltage on the lines 614. Where the voltage on the (+) and (-) lines exceeds that of the zener diode 686, the voltage at the gate of the FET 692 will be limited to that of the zener diode 686, also limiting the voltage across the resistor 656, and thereby forming a constant current load across the connecting lines 614. As a consequence, where the voltage on the (+) and (-) lines continues to increase, the constant current operation of the dummy load circuit 654 will provide for only a linear increase in power (ie. P=VI), thereby managing the overall power consumption (dissipation) whilst effectively dampening induced voltages on the lines 614. In contrast, if the FET 692 were configured to operate as a simple switch, as illustrated in the modified dummy load circuit 654a of Fig. 7, the repositioned load resistor 656a would be effectively placed in parallel with the applied voltage and thus the power dissipated therein would increase with the square of the voltage (P=V ² R ^"1 ). In such an implementation, the zener diode 686 may be omitted. Returning to Fig. 6, when the logic control signal 610 is high, the inverter transistor 650 will also be turned "off, as will the driver transistor 672 and also the TRIAC switch 608. Thus, the mains AC supply 602 will be prevented from passage to the connecting lines 614 and thus the lamp assembly 612 is intended to be off.

Accordingly, when the logic control 610 is "high", the dummy load 654 is effectively "in circuit" and operative to prevent the build up of high induced voltages that may cause a monitoring circuit (not illustrated in Fig. 6) to misinterpret or misrepresent a relatively high impedance lamp assembly 612 as being "on". When the logic control signal 610 is relatively "low", the inverter transistor 650 will turn on, thus biasing the driver transistor 672, on which in turn will bias the TRIAC 608 into conduction. This will thereby connect the mains AC supply 602 to the lines 614 to turn on the traffic lamp assembly 612. The logic low on the control signal 610 will also bias or switch the diode within the optocoupler 604 to be "on", which in turn will bias or switch the optically coupled transistor "on". This will ensure that the gate of FET 692 is pulled down to the (-) line from the rectifier 682 thereby ensuring that the FET 692 is switched "off and non-conductive. As such the only load thereby exposed to the lines 614 by the dummy load circuit 654 will be that presented by the resistor 688 and the transistor of the optocoupler 604. Typically the magnitude of current that flows through that series connection is very small, thereby dissipating minimal power. hi Australia, where the mains AC supply 602 is nominally 240V AC, an appropriate selection of the resistors 688 and 656, will be sufficient to ensure that the dummy load circuit 654 can impart a relatively low impedance load upon the lines 614 during those periods when the TRIAC 608 is turned off. For example, the resistor 688 may have a value in the range 100 kohm to 680 kohm, and the resistor 656 being in the order of 1 kohm to 10 kohm. As a consequence, voltage sensing circuits such as those shown in Fig. 2 will accurately read that the lamp voltages are intended to be off and not misread lamp voltages due to the presence of high impedances on the lines 614 or due to the presence of high impedance loads such as LED lamp assemblies.

The foregoing describes only a number of embodiments of the present invention and modifications can be made thereto without departing from the scope of the present invention.

Additionally, whilst the described arrangements find particular application in AC supply systems, because of virtual coupling capacitances, such may also be used in DC supply systems to aid monitoring DC loads. In DC applications, the rectifier 682 may be omitted and the (-) line may be connected to the (mains neutral) earth 604. Whilst the described arrangement finds particular application to traffic light controllers, such may be used for monitoring the operation of other types of loads, such as remote machinery, remote sensors, appliances and the like.

Further, whilst the embodiments of Figs. 5A to Fig. 6 make use of a dedicated inverter, the two (inverse) control signals need not be generated through operation of a discrete inverter device. For example, in logic circuits, flip-flops and some other devices have dual outputs representing the inverse of each other. As such where the logic control signal is developed in a flip-flop, the outputs thereof may be directly connected to each of the controllable switches 508 and 554.

The types of lamps to which the present arrangements are particularly suited also include other high-impedance lamps such a neon lamp circuits and back-lit liquid crystal displays.