GB2249229A

GB2249229A - Mains overvoltage and undervoltage protection

Info

Publication number: GB2249229A
Application number: GB9021887A
Authority: GB
Inventors: Christopher James Cook
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-10-09
Filing date: 1990-10-09
Publication date: 1992-04-29
Also published as: GB9021887D0

Abstract

A mains connector apparatus prevents energisation of the load through RLY when the amplitude of the source is outwith a range, e.g. 200-275 volts, determined by Zener diode circuits around Z2 and Z3, that allows Q4 to conduct current from a supply provided by BR1, Z1, Q1-Q3, C1. The apparatus may be incorporated in a plug, and relay status/supply fault indicated by LED's L1 and L2 respectively. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO CONNECTORS The present invention relates to a method and apparatus for connecting electrical equipment to an electrical supply.

Electrical equipment is generally designed to operate with a supply at a particular voltage. In fact, the manufacturer normally specifies a working voltage range which can be applied to the equipment without detriment or damage that equipment. For example, in the United Kingdom, the normal mains supply is 240 volts, at 50 Hz, and most equipment rated for connection to the mains will work at 240 volts + 10%.

However, if the voltage level is outside of the stated range, or is degraded so that it is not sinewave form, this can cause damage to the equipment. Some electronic equipment is also provided with protective circuitry enabling it to shut down if it detects a fault.

However, there is the difficulty that the equipment has to be connected to the supply voltage.

If the incoming voltage is incorrect, it may still have been applied to the equipment before it is detected and before fuses have opened the circuit, or the protective circuitry has shut down the equipment. Thus, damage can be.caused to the electrical equipment before the protection systems provided are operative.

There is a particular problem with theatrical electrical supplies. In the theatre, lighting units are normally connected to the supply by way of a dimmer unit arranged to control the voltage applied between zero and the rating of the supply, 240 volts in the UnitedKingdom. To prevent equipment other than lighting units from being connected to the dimmer units, the lighting unit cables are connected to non-standard plugs arranged to be connected to the dimmer units.

Traditionally, other than lighting units, little electrical equipment is used on stage. Often there are very few standard mains sockets on or near the stage to which other electrical equipment can be connected.

Therefore, any equipment which would normally be connected into standard mains sockets, may have a non-standard connector attached to it, rather than an ordinary plug, so that it can be plugged into a proximate dimming unit.

Of course, if electrical equipment is plugged into a dimming unit, it is probable that it will be supplied with less that 240 volts and it is thus liable to damage. Not only is the equipment liable to be run under voltage, but the voltage waveform from the dimming unit is not a pure sinewave but is chopped.

This means that any in-built protective circuitry can fail to recognise fault conditions, thereby putting the equipment at risk.

The present invention seeks to reduce the problems identified above.

According to a first aspect of the present invention there is provided connector apparatus for connecting electrical equipment to an electrical supply, said connector apparatus comprising means defining one or more electrical supply paths, a normally open switch provided in at least one of said supply paths, and means for monitoring the electrical supply available, wherein said monitoring means its operable to close said normally open switch only when the monitored electrical supply available meets predetermined criteria.

In use, connector apparatus of the invention may be interposed between electrical equipment and the mains supply. For example, the connector apparatus may be housed within the housing of a plug or other connector connected to the electrical equipment.

Because at least one of the supply paths is interrupted by the normally open switch, the electrical equipment only receives power from the mains supply if that power meets the predetermined criteria. These criteria are preferably chosen to protect the equipment to be supplied by way of said connector apparatus.

Said monitoring means may be arranged, for example, to monitor the voltage of the supply available. The normally open switch may be closed to enable a supply to the electrical equipment only when the supply voltage is below a predetermined maximum, and/or only when the supply voltage is above a predetermined minimum.

In a preferred embodiment, the normally open switch is closed only when the supply voltage is within a predetermined range. For example, the range may be determined to be 200 to 275 volts. If the supply voltage is outside this range, the normally open switch remains open and the electrical equipment is isolated from the supply.

Where the electrical equipment is to be used in the USA, for example, where the usual mains supply is rated at 120 volts, the predetermined range may be set to 100 to 135 volts.

In an embodiment, said monitoring means comprises first means for establishing if the supply available is above a predetermined level. For example, said first means may comprise a comparator arranged to compare the available voltage with a pre-set reference level. The monitoring means further comprises means to close the normally open switch if the supply is below said reference level. The reference level may be, or may be determined by, a zener voltage of a zener diode.

In a preferred embodiment, said monitoring means comprises both first means for determining if the available supply is above said predetermined level, and second means for establishing if the available supply is below a predetermined level.

Said second means may comprise a comparator arranged to compare the voltage available with a pre-set reference level. Said monitoring means preferably comprises means to close the normally open switch if the voltage available is above the reference level. The pre-set reference may be, or may be determined by, the zener voltage of a zener diode.

Said connector apparatus may comprise live, neutral, and earth supply rails arranged for connection to an electrical supply and to electrical equipment.

Said normally open switch may be provided in said live or in said neutral rail.

Preferably, the connector apparatus comprises rectifier means connected to said supply paths for receiving power from the supply and applying rectified power to said first and/or said second means of the monitoring means. The rectifier may be a full wave rectifier, and in an embodiment is a bridge rectifier.

The rectifier means is preferably arranged to tap current from live and neutral supply rails and apply it to said monitoring means. Where the power supply available is suitable for the electrical equipment this has the advantage that the monitoring means does not distort the power applied to the electrical equipment by the supply rails.

In an embodiment, means providing a regulated power supply are provided and are arranged to receive rectified power from said rectifier means. Said regulated power supply means comprise a comparator arranged to compare the maximum voltage from said rectifier means with the zener voltage set by a zener diode. Said regulated power supply means is arranged to produce an output signal to which switch means, comprising said normally open switch, is arranged to be responsive to close said switch.

The response of said switch means to said output signal is controlled by said first means. Said first means comprises a comparator which is arranged to enable the application of the output signal from said power supply means to said switch means where the rectified voltage is below a reference level determined by the zener voltage of a zener diode.

In a preferred embodiment, said switch means comprises a relay having normally open relay contacts.

The application of an output signal to the relay is arranged to energise it whereby the relay contacts are closed.

Preferably, the connector apparatus is provided with indicator means signalling the passage of power to the electrical equipment. Fault indicator means may also be provided to indicate that power is applied to the first means of the monitoring means but the relay contacts have not been closed. Preferably, damping means are provided for the relay, to prevent local oscillations.

It has been suggested that connector apparatus of the invention may be housed within a conventional plug or other connector. Alternatively, the connector apparatus may be provided within the electrical equipment itself. In this instance, the predetermined levels of the apparatus can be pre-set to accord to the characteristics of the equipment. Additionally and/or alternatively, the apparatus may be provided with adjusting means enabling its charateristics to be adjusted.

The invention also extends to a method of connecting electrical equipment to an electrical supply, said method comprising the steps of interposing between the electrical equipment and the supply a connector apparatus including a normally open switch, and inhibiting closing of the normally open switch unless the electrical supply available meets predetermined criteria.

The invention also extends to a method of connecting electrical equipment to an electrical supply using connector apparatus as defined above.

An embodiment of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which the single Figure shows a circuit diagram of a connector apparatus of the invention.

The connector apparatus shown in the Figure is arranged to be incorporated in electrical equipment.

It has three power rails L, N and E extending therethrough. The input ends of these power rails L, N and E are arranged to be connected to the mains supply, for example by way of cable and an appropriate plug, whereas the output ends thereof are connected to the electrical equipment. As can be seen, the live rail L is interrupted by the normally open contact of a relay RLY so that when the connector apparatus is initially connected to the mains supply, no power is fed to the electrical equipment in which the apparatus is incorporated.

The mains supply available, and to which the apparatus is connected, is tapped into the connector apparatus by way of a full wave bridge rectifier BRl.

The output of the bridge rectifier BRl is applied to a voltage comparator comprising a resistor R1 and a zener diode Z1 connected via a resistor R2 to the base of a transistor Q1. The zener diode Z1 provides the reference voltage for the comparator. It will be appreciated that when the voltage output from the bridge rectifier BRl is below the zener voltage, the zener diode Zl will not conduct. In this condition, the transistor Q1 will be rendered non-conductive. A second transistor Q2 has a base connected to the collector of the first transistor Ol and an emitter connected to the base of a third transistor Q3.When the zener diode Zl is not conducting the first transistor Ql is switched off, and the power output from the bridge rectifier BRl is applied to, and will turn on, the transistors Q2 and Q3. The emitter of the transistor Q3 is connected to one pole of a capacitor C1 which is charged thereby.

If the output of the bridge rectifier BRl does not exceed the zener voltage, the capacitor Cl will be charged and then the circuit will attain a steady state. However, if the rectified output voltage does exceed the zener voltage, the zener diode Zl will conduct turning on the transistor Ql which thereby grounds the base of transistor Q2 and turns it off.

The transistor Q3 is turned off in its turn so that the capacitor C1 may then discharge through the relay RLY as discussed below.

Thus, the voltage comparator Zl, Rl, R2 and Ql acts to provide a regulated power supply for the circuit. The transistor Ql switches off the transistors Q2 and Q3 if the voltage exceeds the reference voltage determined by the zener diode Zl.

This prevents excess voltage being applied across transistors Q2 and Q3 and thereby prevents excessive heat being generated therein. There is therefore no need to provide heat sinks. If the voltage periodically exceeds the reference voltage determined by the zener diode Z1, the capacitor Cl provides a source of pulses for energising the relay RLY.

To supply a current to the electrical equipment it is necessary to energise the relay RLY to close its contacts. It will be appreciated that the capacitor C1 will discharge through the relay if a transistor Q4, whose collector-emitter path is connected in series between the relay RLY and a common ground, is switched on. In this respect, the rectified output of the bridge circuit BRl is also applied to a voltage divider formed by series resistors R7 and R8 and the voltage at their node is applied to a second zener diode Z2. If that voltage exceeds the break down voltage of the zener diode Z2, the diode will conduct and thereby switch on the transistor Q4. Thus, the pulsed output from the capacitor C1 will be fed by way of the relay RLY through the transistor Q4 thereby energising the relay coil and closing its contacts.In this way, the mains supply is applied to the electrical equipment by way of the rails L, N and E. A series combination of a resistor R5 and the light emitting diode Ll is connected across the relay coil RLY. The current flowing in the light emitting diode Ll provides a visual indication that power is being fed to the electrical equipment. A capacitor C2 connected across the relay RLY prevents local oscillations, whilst a diode Dl connected thereacross prevents the generation of back emf when the relay contacts open.

The zener diode Z2 is arranged such that it will conduct whenever the voltage across the voltage divider R7,R8 is within the required range. In this respect, a capacitor C3 in parallel with the resistor R8 acts as a smoothing capacitor for the voltage applied to the zener diode Z2.

The connector apparatus is provided with further means for preventing the application of power to the electrical equipment if the voltage from the mains is too large. In this respect, a further voltage divider R9, R10 applies a proportion of the rectified input voltage to a third zener diode Z3. This zener diode Z3 is connected to the base of a further transistor Q5 and it will be appreciated that the transistor Q5 will be turned on by the zener diode Z3 whenever the voltage applied thereto exceeds the zener voltage of the diode Z3. The collector of the transistor Q5 is connected to the base of the transistor Q4 and hence, when the transistor Q5 conducts it will ground the base of the transistor Q4 turning the transistor Q4 off and thereby preventing the flow of sufficient power through the relay to energise it. In this instance, the relay contacts will remain open.

It will be therefore appreciated that the first means comprised of the voltage divider R9, R10, zener diode Z3 and transistor Q5 requires that the rectified voltage be below a predetermined level if the relay RLY is to be energised. In addition, the transistor Q4 is only turned on if the voltage determined by the voltage divider R7 and R8 is at the correct level determined by the zener diode Z2. In this respect, the zener diode Z3 has a smoothing capacitor C4 connected thereto. It is most important that the relative values of the capacitances C3 and C4 are correct and that the capacitance C4 is smaller than that of capacitor C3 so that the transistor Q5 is turned on by any excess voltage to thereby ground the base of transistor Q4.If the transistor Q5 is not operated in advance of the transistor Q4, the apparatus will momentarily turn on thereby applying power to the electrical equipment which could cause damage.

It will be seen that a series combination of a resistor R6 and a light emitting diode L2 is connected between the relay RLY and ground. Illumination of the diode L2 by the passage of current therethrough shows an error state as it indicates that power is being supplied to the relay RLY when the contacts thereof should be open as shown by the fact that the transistor Q4 is off.

Clearly, by adjustment of the size of the various resistors and appropriate choice of the characteristics of the zener diodes, it is possible to select different values for the voltage range within which power is supplied to the electrical equipment. For example, for the United Kingdom and Europe, where the mains voltage is 220 or 240 volts, the transistor Q4 can be arranged to be turned on by a voltage of at least 200 volts, whereas the transistor Q5 can be arranged to conduct when a voltage of 275 volts or more is applied thereto.

The unit will therefore only enable the application of power to the electrical equipment when the voltage of the mains is within the range 200-275 volts.

In the United States, mains voltage is generally rated at 120 volts. In this case, the voltage range for the unit could be, for example, 100-135 volts.

The circuit illustrated can be made to be small and compact and is therefore easily housed within the electrical equipment to be protected. Alternatively, the apparatus may be provided in a rated plug for the electrical equipment.

It will be appreciated, that the connector apparatus taps power from the mains supply to establish if it is within the correct range for application to the electrical equipment. In this manner, the operation of the connector apparatus does not adversely affect the power being applied to the equipment.

It will be appreciated that variations and modifications to the apparatus described above can be made within the scope of the present invention.

Claims

1. Connector apparatus for connecting electrical equipment to an electrical supply, said connector apparatus comprising means defining one or more electrical supply paths, a normally open switch provided in at least one of said supply paths, and means for monitoring the electrical supply available, wherein said monitoring means its operable to close said normally open switch only when the monitored electrical supply available meets predetermined criteria.

2. Connector apparatus as claimed in Claim 1, wherein said monitoring means is arranged to monitor the voltage of the supply available and is operable to close said normally open switch only when the supply voltage is below a predetermined maximum, and/or only when the supply voltage is above a predetermined minimum.

3. Connector apparatus as claimed in Claim 2, wherein said monitoring means is operable to close said normally open switch only when the supply voltage is within a predetermined range.

4. Connector apparatus as claimed in any preceding claim, wherein said monitoring means comprises first means for establishing if the supply available is above a predetermined level.

5. Connector apparatus as claimed in Claim 4, wherein said first means comprises a comparator arranged to compare the available voltage with a pre-set reference level, and wherein said monitoring means further comprises means to close the normally open switch if the supply is below said reference level.

6. Connector apparatus as claimed in Claim 5, further comprising a zener diode arranged such that its zener voltage is, or determines, said reference level.

7. Connector apparatus as claimed in any of Claims 4 to 6, wherein said monitoring means further comprises second means for establishing if the available supply is below a predetermined level.

8. Connector apparatus as claimed in Claim 7, wherein said second means comprises a comparator arranged to compare the voltage available with a second pre-set reference level, and wherein said monitoring means further comprises means to close the normally open switch if the voltage available is above said second reference level.

9. Connector apparatus as claimed in Claim 8, further comprising a second zener diode arranged such that its zener voltage is, or determines, said second reference level.

10. Connector apparatus as claimed in any preceding claim, said apparatus comprising live, neutral, and earth supply rails arranged for connection to an electrical supply and to electrical equipment, and wherein said normally open switch is provided in said live or in said neutral rail.

11. Connector apparatus as claimed in any preceding claim, further comprising rectifier means connected to said supply paths for receiving power from the supply and applying rectified power to said monitoring means.

12. Connector apparatus as claimed in Claim 11, further comprising means providing a regulated power supply, said regulated power supply means being arranged to receive rectified power from said rectifier means, and said regulated power supply means comprising a comparator arranged to compare the maximum voltage from said rectifier means with the zener voltage set by a zener diode, and said regulated power supply means being arranged to produce an output signal to which switch means, comprising said normally open switch, is arranged to be responsive to close said switch under the control of said monitoring means.

13. Connector apparatus as claimed in Claim 12, wherein said switch means comprises a relay having normally open relay contacts.

14. Connector apparatus as claimed in any preceding claim, further comprising indicator means signalling the passage of power to the electrical equipment.

15. Connector apparatus as claimed in any preceding claim, further comprising fault indicator means for indicating that power has been applied to said monitoring means but that the normally open switch has not been closed.

16. A plug or connector for electrical equipment incorporating connector apparatus as claimed in any preceding claim.

17. Electrical equipment incorporating connector apparatus as claimed in any of Claims 1 to 15.

18. A method of connecting electrical equipment to an electrical supply, said method comprising the steps of interposing between the electrical equipment and the supply a connector apparatus including a normally open switch, and inhibiting closing of the normally open switch unless the electrical supply available meets predetermined criteria.

19. A method of connecting electrical equipment to an electrical supply using connector apparatus as claimed in any of Claims 1 to 17.

20. Connector apparatus substantially as hereinbefore described with reference to the accompanying drawing.