GB2051505A - A.C./D.C. input circuit for inverter - Google Patents

Abstract

An input circuit for an inverter comprises a full-wave rectifier 12, the output of which is connected to a swinging choke comprising a choke 17 and capacitor 19. A.C. or D.C. can be applied to the input terminals 10, 11, as might happen when running either from the mains or, during an emergency, from batteries, and the circuit supplies, from terminals 15, 18, unaltered D.C. when using a D.C. battery supply, or, when using mains supply, a slightly smoothed full wave signal having a maximum value equal to the RMS value of the input voltage. Capacitor 20 acts as a radio frequency filter. The A.C. output of the inverter operates fluorescent lamps. The A.C. supplied to 10, 11 can also supply incandescent lamps. During battery operation the incandescent lamps will also receive D.C. <IMAGE>

Description

SPECIFICATION Improvements relating to maintained lighting systems The invention relates to maintained lighting systems, namely lighting system which normally operate on mains power, but which can be switched over to battery power, without interruption of the lighting, in the event of an emergency such as mains failure. Such system are commonly used in hospitals for example.

Maintained lighting systems frequently utilize both incandescent lamps and fluorescent lamps. The fluorescent lamps require A.C. to operate but the A.C. is supplied by inverters which require D.C. Incandescent lamps will operate from A.C. or D.C. so known systems utilize a D.C. supply. When the batteries are being used in emergencies the D.C. from the batteries is supplied to the inverters of the fluorescent lamps and the incandescent lamps as necessary. When mains power is being used, a stabilised power pack is required to convert the mains A.C. to D.C. and supply it to the inverters of the fluorescent lamps and to the incandescent lamps. However stabilised A.C. to D.C. power packs are expensive, and furthermore running incandescent lamps permanently on D.C. considerably shortens their life.

We have appreciated that the use of a stabilised A.C. to D.C. power pack could be eliminated, and incandescent lamp life could be increased, if the inverters could be arranged to accept A.C. or D.C. During mains operation, which occurs for most of the time, the incandescent lamps could then receive A.C., thereby increasing their life, and the inverters would also accept A.C. During the occasional emergency situations which may arise, when the batteries would be used, the incandescent lamps could run off D.C. and the inverters could also accept D.C.

The logical way to adapt an inverter to accept A.C. is to use an input circuit comprising a full-wave rectifier and an electrolytic capacitor to smooth the output of the rectifier.

However we realised that the rectified and smoothed ouput voltage which would be fed to the inverters would have a voltage in the region of the peak voltage of the A.C. input.

This voltage would be significantly higher than the D.C. voltage which would be received by the inverter during battery operation and existing inverters cannot cope with a wide range of input voltages. If the A.C. input voltage were reduced, so that the peak value of the rectified and smoothed voltage was comparable with the battery D.C. supply voltage, then the incandescent lamps would not function satisfactorily during mains operation, since they would be receiving a reduced A.C.

supply.

The invention provides an input circuit for an inverter to enable the inverter to operate from an A.C. or a D.C. supply, the input circuit comprising a full-wave rectifier the input of which will, in use, receive A.C. or D.C., the output of the rectifier being connected to a swinging choke which, when the input circuit receives A.C. limits the output voltage of the input circuit to the RMS value of the input voltage to the input circuit.

The swinging choke preferably comprises an iron dust core choke connected to a capacitor.

A second capacitor may be arranged to act as a radio frequency filter to prevent radio frequency signals from being fed back to the power supply from an inverter coupled to the input circuit.

By way of example, a specific embodiment of input circuit for an inverter according to the invention will now be described, with reference to the accompanying drawing, which is a circuit diagram illustrating the input circuit.

The input circuit forming the subject of this embodiment has a pair of input terminals 10 and 11. In use these terminals will receive A.C. during mains operation, and D.C. during emergency battery operation. Terminal 10 is connected to one input of a full-wave bridge rectifier 12, via a fuse 1 3. Terminal 11 is connected to the other input terminal of the full-wave bridge rectifier 1 2.

One output of the rectifier 1 2 is connected directly to a first output supply line 14 which leads to a first output terminal 1 5. The other output terminal of the bridge 1 2 is connected to a second output supply line 1 6 via an iron dust core choke 17. The supply line 16 leads to a second output terminal 1 8.

A point on the output line 1 6 lying between the choke 1 7 and the terminal 1 8 is connected to the output line via a capacitor 19.

The choke 1 7 and capacitor 1 9 together act as a swinging choke.

A second capacitor 20 is connected between the output line 14 and the junction of the bridge rectifier 1 2 ahd the choke 1 7.

In use the terminals 1 5 and 1 8 are connected to an inverter which in turn is connected to one or more fluorescent lamps.

During mains operation, the terminals 10 and 11 are supplied with A.C., for example at 24 volts. This A.C. is full-wave rectified by the rectifier 1 2 and the effect of the swinging choke comprising choke 1 7 and capacitor 1 9 is that the terminals 1 5 and 1 8 receive a slightly smoothed full wave signal having a maximum value equal to the RMS value of the input voltage. The capacitor 20 acts as a radio frequency filter and prevents any radio frequency signals transmitted by the inverter from being passed back to the electricity supply. It will be seen that the inverter receives slightly smoothed D.C. which it then converts to A.C. for the operation of the fluorescent lamps.

The circuit shown, connected to inverters and fluorescent lamps, can be included in a lighting system which also has incandescent lamps and when running on mains power, the A.C. which is supplied to the terminals 10 and 11 can also be supplied to the incandescent lamps and will operate these in the most effective manner.

During occasional emergency conditions when D.C. is supplied from batteries, the rectifier 1 2 and the swinging choke will have no effect and the battery D.C. also at 24 volts for example, is simply transmitted via the terminals 1 5 and 1 8 to the inverter and the inverter again converts the D.C. into A.C. for use in operating the fluorescent lamps. The capacitors 1 9 and 20 together with the choke 1 7 operate as a radio frequency filter.

During battery operation the incandescent lamps will also receive D.C., and while running incandescent lamps on D.C. shortens their life, the circuit forming the subject of this embodiment enables a lighting system to be provided in which the incandescent lamps are only run on D.C. during accasional emergency periods when the batteries are used. At other times the incandescent lamps receive A.C.

If the swinging choke were not used, and an electrolytic capacitor was used to smooth the full-wave rectified current, the voltage of the rectified current would be approaching the peak value of the voltage of the A.C. supplied to the terminals 10 and 11. This is approximately 1.4 times greater than the RMS value and this in turn would have the effect that the inverter would have to receive a higher voltage during A.C. mains operation than during battery operation, or alternatively the voltage of the A.C. supply would have to be made less than the voltage of the D.C. supply. If the inverter was fed with a significantly higher voltage during A.C. supply than during D.C.

operation, the inverter would quickly burn out; since known inverters cannot cope with a large range of input voltages. If on the other hand the voltage of the input A.C. was reduced so that the inverter received a similar voltage under all conditions of operation, then the incandescent lamps in the lighting system would receive a lower voltage under A.C.

operation and the incandescent lamps would therefore operate unsatisfactorily.

The invention is not restricted to the details of the foregoing embodiment. For example the ratings of the components of the circuit may be such that the circuit can be used with supply voltages other than 24 volts, for example 50 volts or 110 volts.

Claims (5)

1. An input circuit for an inverter to enable the inverter to operate from an A.C. or a D.C. supply, the input circuit comprising a full-wave rectifier the input of which will, in use, receive A.C. or D.C., the output of the rectifier being connected to a swinging choke which, when the input receives A.C., limits the output voltage of the input circuit to the RMS value of the input voltage to the input circuit.

2. An input circuit as claimed in Claim 1, in which the swinging choke comprises an iron dust core choke connected to a capacitor.

3. An input circuit as claimed in Claim 2, including a second capacitor arranged to act as a radio frequency filter to prevent radio frequency signals from being fed back to the power supply from an inverter coupled to the input circuit.

4. An input circuit constructed and arranged substantially as herein described with reference to the accompanying drawings.

5. An input circuit as claimed in any one of the preceding claims, connected to an inverter.