GB1578081A

GB1578081A - Load failure detector

Info

Publication number: GB1578081A
Application number: GB4761/78A
Authority: GB
Original assignee: Welwyn Electric Ltd
Current assignee: Welwyn Electric Ltd
Priority date: 1978-02-07
Filing date: 1978-02-07
Publication date: 1980-10-29

Description

(54) LOAD FAILURE DETECTOR (71) We, WELWYN ELECTRIC LIMITED of Bedlington, Northumberland NE22 7AA, a British Company do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which It is to be performed, to be particularly described in and by the following statemcnt:-- This invention relates to apparatus for detecting failure, due to assuming an opencircuit condition, of a load energised from an alternating current supply through a transformer.It is particularly applicable to traffic signal lights, such as are employed at road junctions, and of the type utilising highintensity, low voltage bulbs operated from an alternating current mains power supply through a transformer, but although the fol- lowing description makes particular reference to such lights, it is not intended that the invention be limited thereto.

The provision of apparatus to detect failure of bulbs in traffic light systems is ccmpllcated by the use in such systems of a transformer to reduce the mains voltage to a level required for low voltage high intensity bulbs. The off- load magnetising current d such transfonners when supplied with a high voltage during daylight hours and with a failed, opencircuited bulb, is similar in magnitude to the total load current, including the transformer magnetising current component, when the transformer is supplied with a lower voltage after dark, for bulb-dimming purposes, and with a bulb in good operating condition.

Furthermore, the resistance of the bulb, and hence the operating current, varies with the age of the bulb and this complication tends to rule out a method of monitoring the two levels od current for daytime and night-time operation.

It would be possible to monitor the current flowing through the circuit containing the bulb and the secondary winding of the transformer and thereby detect an open-circuit condition resulting from failure of a bulb, but this would be highly disadvantageous and impractical in that it would entail providing additional wiring for feeding back in formation to the control area which will usually be very remote from the location of the traffic lights themselves.

It is an object of the present invention to minimise or overcome the above problems by utilising a technique whereby the magnetising current flowing in a pritnary winding of a transformer used to supply a load is compared with the magnetising current through a primary winding of another transformer having an unemployed secondary winding, or through an inductor, having substantially simi- lar magnetisation characteristics to the transformer supplying the load.

The present invention provides apparatus for detecting failure, due to assuming an open circuit condition, of an electrical load energised through a first transformer, a first winding of said first transformer being connected to an alternating current energising source and a second winding being connected to said load, said apparatus comprising a second transformer, or an inductor, having sub stay tially similar magnetisation characteristics to said first transformer and having a winding thereof connected to said alternating current source, means for developing first and second signals in dependence upon electric currents flowing through said first winding of said first transformer and said winding of said second transformer or of said inductor, res- pectively, and means for comparing said signals and for actuating a warning device or appropriate circuitry when a change in relative magnitudes of said signals occurs as a result Of failure of said load.

If required, a means may be provided for ensuring that when failure of said load occurs, said first and second signals assume substantially similar magnitudes.

The said means for developing first and second signals may comprise first and second electrical resistors, said first resistor being electrically connected in series with said first winding Of said first transformer and said second resistor being connected in series with said winding of said second transformer or said inductor, said first and second signals comprising first and second voltage potential differences across said first and second resistors respectively.

Alternatively the said means for developing first and second signals may comprise first and second current transformers arranged to detect the currents flowing through said first winding of said first transformer and said winding of said transformer of of said inductor, respectively, said first and second current transformers being connected to respective electrical resistors, said first and second signals comprising first and second voltage potential differences across said resistors.

In either of the above alernative embodiments, at least one of the two resistors may be a variable resistor which can be initially adjusted with said load disconnected from.

said second winding to arrange for said first and second potential differences to assume substantially similar values. In this way, with the load connected and operating normally, the values of the first and second potential differences will be different from one another and when open-circuit failure of the load occurs, the assumption of substantially similar values is utilised to actuate a warning device Of actuate appropriate circuitry.

The means for comparing said signals suitably comprises a voltage comparator whose output is utilised to actuate the said warning device or appropriate circuitry.

In a preferred embodiment, said second transformer is employed having a first winding thereof connected to the alternating current source and having a second winding connected to provide a power supply for the voltage comparator. In this embodiment circuits are preferably arranged whereby current is supplied to the comparator during a different half cycle of the alternating current power supply from that during which said signals are being compared, thereby ensuring that while the signal comparison is being effected there is minimal influence on the current flowing through the first winding of the second transformer, by the current required to supply the comparator.This is achieved by suitably phasing the windings on the second transformer and by incorporating a rectifier to provide half wave rectification in that circuit supplying the comparator and a further rectifier to provide half wave detection in that circuit involved in the signal comparison.

In this preferred embodiment, the comparator is preferably arranged in such a way that it does not draw current when the load is in a failed condition; current drawn in such a condition could affect the magnitude of the current flowing through the first winding of the second transformer and result in errors in making the signal comparison.The comparator is suitably arranged such that a first input thereof is cormected to the base of a first transistor, a second input thereof is connected to the emitter of said first transistor, the collector d said first transistor is connected through a resistor whose resistance value is selected to give a required output current from the comparator, to the base of a second transistor which is complementary to said first transistor, the collector of said second transistor being connected to form an output for the comparator and the emitter of said second transistor being connected to the power supply for the comparator.

As a warning device and indicator lamp, bell, buzzer or other alarm may be provided.

In the event of failure of said load a relay may also be operated to actuate isolated circuits to operate alarms or take remedial action. Such isolated circuits may, if desired, be logic based.

It is possible to monitor several loads whilst using only a single said second transformer or inductor and providing a further transformer, connected to the alternating cur rent supply, for supplying power to the comparators.

It is to be noted that in practice, the majority of standard available transformers have substantially similar magnetisation characteristics so that no special selection process will normally be necessary in order to provide the required second transformer or inductor.

In a particular application, the said load comprises a low voltage bulb used in traffic signal lights and the said first transformer provides a step-down from an alternating current mains power supply voltage to the voltage (e.g. 12 volts) required for the bulb.

The monitoring apparatus of the invention is unaffected in operation by the fact that the signal bulb being monitored is usually sup plied with a lower voltage at night-time than during the day in order to avoid excessive glare at night, since the magnetisation currents in the first transformer and second transformer or inductor are reduced in substantially similar manner.

The invention is now described by way od example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram illustrating a basic embodiment of the invention, Figure 2 is a circuit diagram illustrating an alternative and preferred embodiment Of the invention, Figure 3 is a circuit diagram illustrating a preferred form of comparator for use in the embodiment of Figure 2.

Referring to Figure 1, an alternating current mains power supply 1 is connected to a first winding 2 of a first transformer 3 and a second winding 4 of the transformer is connected to an electrical load 5 which is required to be monitored in order to detect when failure occurs due to it assuming an open-circuit condition. The lad 5 may, for example, be a high intensity low voltage lamp bulb such as is used in traffic light signalling systems and designed for operation at a voltage of the order od 12 volts.

Load failure detecting apparatus is provided as follows. A second transformer, or an inductor 6, having subtsantially similar magnetisation characteristics to the first trans former 3 has a winding thereof connected to the power supply 1. A first electrical resistor 7 is connected in series with winding 2 of transformer 3 and a second electrical resistor 8, whose electrical resistance value is capable of being varied, is connected in series with the winding of the second transformer 6.

A voltage comparator 9 has one input connected to the junction between transformer 6 and resistor 8 and another input connected to the junction between winding 2 of transformer 3 and resistor 7. An output terminal 10 of comparator 9 is connected to a warning device (not shown) such as an indicator lamp.

In operation, electric current flowing through the winding 2 od transformer 3 also flows through resistor 7 and current flowing through the winding of transformer or induc tor 6 also flows through variable resistor 8.

Signals in the form of voltage potential differences are developed across resistors 7 and 8 and are compared by comparator 9.

These potential differences are dependent upon the respective currents flowing through winding 2 of transformer 3 and the winding of transformer or inductor 6. With the load 5 disconnected from the winding 4, variable resistor 8 is adjusted until the potential difference across resistor 7 is substantially the same as that across resistor 8. When this occurs, there is no output from terminal 10 of the comparator 9 and the warning device (not shown) is caused to be actuated. When the load 5 is connected to the winding 4 and the load is in good operating condition, the current flowing through winding 2 is different from that which flowed when the load was disconnected. The voltage potential difference across resistor 7 is then different from that across resistor 8.As long as this condition holds, there is an output from terminal 10 of comparator 9 and the warning device (not shown) is not actuated. If the load 5 fails and assumes an open circuit condition, this produces the same effect as with the disconnected load. Hence the potential difference across resistor 7 again assumes the same value as that across resistor 8 and there is zero output from the comparator 9. The warning device is therefore actuated, indicating that the load has failed.

A preferred embodiment of the invention is illustrated in Figures 2 and 3. In these Figures, parts fulfilling the same or similar functions as those in Figure 1 are given the same reference numerals as in Figure 1.

Referring to Figure 2, an alternating current mains power supply 1 is connected to a first winding 2 of a first transformer 3 and a second winding 4 of the transformer is con nected to an electrical load 5 (e.g. a low voltage lamp bulb such as is used in traffic light signalling systems) which is required to be monitored to detect when failure occurs due to it assuming an open circuit condition.

Load failure detecting apparatus is provided as follows. A second transformer 6, having substantially similar magnetisation characteristics to the first transformer 3 has a winding 11 thereof connected to the power supply 1. A first electrical resistor 7 is connected in series with winding 2 of transformer 3 and a second electrical resistor 8, whose electrical resistance value is capable of being varied, is connected in series with the winding 11 of the transformer 6. A voltage comparator 9 has one input 18 connected to the junction between winding 11 of transformer 6 and resistor 8 through a diode rectifier 14 and another input 19 connected to the junction between winding 2 of transformer 3 and resistor 7. An output terminal 10 from comparator 9 is connected to a relay 17 which actuates a warning device or appropriate circuitry (not shown).

A second winding 12 on transformer 6 is connected in circuit with comparator 9 and provides a power supply to terminal 20 of the comparator. A diode 13 in this circuit provides half wave rectification of the supply.

As described for the embodiment of Figure 1, as a result ob electric current flowing through the winding 2 and resistor 7 and current flowing through the winding 11 and resistor 8, signals in the form of voltage potential differences which are dependent upon these respective currents are developed across resistors 7 and 8 and are compared by comparator 9. By appropriate phasing of the windings of transformer 6 and the incorporation of diodes 13 and 14, current is supplied to the comparator 9 during a different half cycle of the alternating current power supply 1 from that during which the voltage potential difference signals are being compared by the comparator 9.This ensures that while the signal comparison is being effected there is minimal influence on the current flowing through the winding 11 of the transformer 6 by the current required to supply the comparator 9.

Capacitors 15 and 16 are provided to maintain the voltages in the circuit supplying the comparator and the circuit employed in the comparison of the voltages developed across resistors 7 and 8, respectively, during the half cycles of the alternating current supply when they are not receiving power from the supply.

With the load 5 disconnected from the winding 4, variable resistor 8 is adjusted until the potential difference across resistor 7 is substantially the same as that across resistor 8.

When this occurs, there is no output from terminal 10 of the comparator 9 and a warning device or appropriate circuit (not shown) is caused to be actuated by the relay 17.

When the load 5 is connected to the winding 4 and the load is in good operating condition, the current flowing through winding 2 is different from that which flowed when the load was disconnected. The voltage potential difference across resistor 7 is then different from that across resistor 8. As long as this condition holds there is an output from terminal 10 of comparator 9 and relay 17 is held so that the warning device or appropriate circuit (not shown) is not actuated. If the load 5 fails and assumes on open-circuit condition, this produces the same effect as with the disconnected load. Hence the potential difference across resistor 7 again assumes the sarne value as that across resistor 8 and there is zero output from the comparator 9. The warning device or circuit is therefore actuated by the relay 17, indicating that the load has failed.

It is preferred that the comparator 9 does not draw current when the load is in a failed condition since any current flowing through winding 12 of transformer 6 in this condition could affect the magnitude of the current flowing through winding 11 of transformer 6 and introduce errors in the apparatus since the voltage potential differences developed across resistors 7 and 8 would not then be absolutely the same. To avoid such errors, comparator 9 is preferably constructed as shown in Figure 3.Input 18 of the comparator is connected to the base of a transistor T1 and input 19 of the comparator is connected to the emitter of transistor Tl. The collector of transistor T, is connected through a resistor 21 to the base of a further transistor T2, which is complementary to transistor Tl. The collector of transistor T2 is connected to form the output 10 of the comparator and the emitter of transistor T2 is connected to terminal 20 to which the power supply for the comparator is connected. The resistance value of resistor 21 is selected to give the required output current from the comparator.

It is possible to monitor several loads 5 whilst using only a single transformer 6 but it is then necessary to provide a further transformer, connected to the alternating current supply, for supplying power to the comparators applied to the various loads.

WHAT WE CLAIM IS: 1. Apparatus for detecting failure, due to assuming an open circuit condition, of an electrical load energised through a first transformer, a first winding of said first transformer being connected to an alternating current energising source and a second winding being connected to said load, said apparatus comprising a second transformer, or an inductor, having substantially similar magnetisation characteristics to said first transformer and having a winding thereof connected to said alternating current source, means for develop ing first and second signals in dependence upon electric currents flowing through said first winding of said first transformer and said winding of said second transformer or of said inductor, respectively, and means for comparing said signals and for actuating a warning device or appropriate circuitry when a change in relative magnitudes of said signals occur as a result of failure of said load.

2. Apparatus according to Claim 1 in which a means is provided for ensuring that when failure of said load occurs, said first and second signals assume substantially sirnilar magnitudes.

3. Apparatus according to Claim 1 or 2 in which said means for developing first and second signals comprises first and second electrical resistors, said first resistors being connected in series with said first winding of said first transformer and said second resistor being connected in series with said winding of said second transformer or said inductor, said first and second signals comprising first and second voltage potential differences across said first and second resistors respectively.

4. Apparatus according to Claim 1 or 2 in which said means for developing first and second signals comprises first and second current transformers arranged to detect the currents flowing through said first winding of said first transformer and said winding of said second transformer or of said inductor, respectively, said first and second current trans formers being connected to respective electrical resistors, said first and second signa!s comprising first and second voltage potential differences across said resistors.

5. Apparatus according to Claim 3 or 4 in which at least one of the two resistors is a variable resistor which can be initially adjusted with said load disconnected from said second winding to arrange for said first and second potential differences to assume substantially similar values.

6. Apparatus according to Claim 3, 4 or 5, in which the means for comparing said signals comprises a voltage comparator whose output is utilised to actuate the said warning device or appropriate circuitry.

7. Apparatus according to Claim 6 in which said second transformer is employed having a first winding thereof connected to the alternating current source and having a second winding connected to provide a power supply for the voltage comparator.

8. Apparatus according to Claim 7 in which circuits are arranged whereby current is supplied to the comparator during a different half cycle of the alternating current power supply from that during which said signals are being compared, thereby ensuring that

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. terminal 10 of the comparator 9 and a warning device or appropriate circuit (not shown) is caused to be actuated by the relay 17. When the load 5 is connected to the winding 4 and the load is in good operating condition, the current flowing through winding 2 is different from that which flowed when the load was disconnected. The voltage potential difference across resistor 7 is then different from that across resistor 8. As long as this condition holds there is an output from terminal 10 of comparator 9 and relay 17 is held so that the warning device or appropriate circuit (not shown) is not actuated. If the load 5 fails and assumes on open-circuit condition, this produces the same effect as with the disconnected load. Hence the potential difference across resistor 7 again assumes the sarne value as that across resistor 8 and there is zero output from the comparator 9. The warning device or circuit is therefore actuated by the relay 17, indicating that the load has failed. It is preferred that the comparator 9 does not draw current when the load is in a failed condition since any current flowing through winding 12 of transformer 6 in this condition could affect the magnitude of the current flowing through winding 11 of transformer 6 and introduce errors in the apparatus since the voltage potential differences developed across resistors 7 and 8 would not then be absolutely the same. To avoid such errors, comparator 9 is preferably constructed as shown in Figure 3.Input 18 of the comparator is connected to the base of a transistor T1 and input 19 of the comparator is connected to the emitter of transistor Tl. The collector of transistor T, is connected through a resistor 21 to the base of a further transistor T2, which is complementary to transistor Tl. The collector of transistor T2 is connected to form the output 10 of the comparator and the emitter of transistor T2 is connected to terminal 20 to which the power supply for the comparator is connected. The resistance value of resistor 21 is selected to give the required output current from the comparator. It is possible to monitor several loads 5 whilst using only a single transformer 6 but it is then necessary to provide a further transformer, connected to the alternating current supply, for supplying power to the comparators applied to the various loads. WHAT WE CLAIM IS:

1. Apparatus for detecting failure, due to assuming an open circuit condition, of an electrical load energised through a first transformer, a first winding of said first transformer being connected to an alternating current energising source and a second winding being connected to said load, said apparatus comprising a second transformer, or an inductor, having substantially similar magnetisation characteristics to said first transformer and having a winding thereof connected to said alternating current source, means for develop ing first and second signals in dependence upon electric currents flowing through said first winding of said first transformer and said winding of said second transformer or of said inductor, respectively, and means for comparing said signals and for actuating a warning device or appropriate circuitry when a change in relative magnitudes of said signals occur as a result of failure of said load.

while the signal comparison is being effected, there is minimal influence on the current flowing through the first winding of the second transformer by the current required to supply the comparator.

9. Apparatus according to Claim 8 in which the arrangement whereby current is supplied to the comparator during a different half cycle of the alternating current supply from that during which said signals are being compared is achieved by suitably phasing the windings on the second transformer and by incoxporat- ing a rectifier to provide half wave rectification in that circuit supplying the comparator and a further rectifier to provide half wave detection in that circuit involved in the signal comparisons.

10. Apparatus according to Claim 9 in which said comparator is arranged whereby it does not draw current when said load is in a failed condition.

11. Apparatus according to Claim 10 in which said comparator is arranged such that a first input thereof is connected to the base of a first transistor, a second input thereof is connected to the emitter of said first transistor, the collector of said first transistor is connected through a resistor whose resistance value is selected to give a required output current from the comparator, to the base of a second transistor which is complementary to said first transistor, the collector of said second transistor being connected to form an output for the comparator and the emitter od said second transistor being connected to the power supply for the comparator.

12. Apparatus according to any one of the preceding Claims in which an indicator lamp, bell, buzzer, or other alarm is provided as a warning device.

13. Apparatus according to any one of the preceding Claims in which in the event of failure of said load a relay is operated to actuated isolated circuits to operate alarms or take remedial action.

14. Apparatus according to Claim 13 in which said isolated circuits are logic based.

15. Apparatus according to any one of the preceding Claims in which several loads are monitored whilst using only a single said second transformer or inductor and providing a further transformer, connected to the alter- nating current supply, for supplying power to the comparators.

16. Apparatus for detecting failure in a load substantially as herein described with reference to Figures 1, 2 and 3 of the accom- panying drawings.

17. A traffic signal light including a load in the form of a low voltage bulb supplied from a power supplv through a transformer, and provided with apparatus for detecting failure in said load as claimed in any one of the preceding Claims.