GB1595921A

GB1595921A - Electrical supply system

Info

Publication number: GB1595921A
Application number: GB4249977A
Authority: GB
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1978-05-03
Filing date: 1978-05-03
Publication date: 1981-08-19

Description

(54) ELECTRICAL SUPPLY SYSTEM (71) We, CHLORIDE GROUP LIMITED, a Company registered under the Laws of England, of 52 Grosvenor Gardens, London SWIW OAU. 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 statement::- This invention relates to the supply of electrical loads from a supply whose voltage may vary, and though it is applicable particularly to maintained emergency lighting equipment in which a lamp is normally supplied from the mains but is automatically switched over to battery operation if there is a mains failure, it is also applicable to sustained equipment in which a second lamp switches on when a mains lamp fails, and to non-maintained equipment according to which the lamp is illuminated only when the mains supply has failed, and in general to equipment for supplying other loads.

A problem with many battery systems is that the battery voltage falls during the discharge period so that the lamp output also falls and the fall may be beyond acceptable levels, particularly when the cells of the battery start off hot as is usual with maintained equipment.

According to the present invention, an electrical supply system includes a rechargeable battery connected to a load, through an inverter including a transformer having a primary winding connected to the battery circuit. a secondary winding connected to the load, and a feedback winding connected to a drive circuit. and a compensator arranged in response to a fall in battery voltage to increase the current in the drive circuit, and hence maintain the power into the inverter and provide at least partial regulation of power to the load In one form of the invention, the primary winding is connected in the collector circuit of a transistor, whose base drive circuit includes the feedback winding and has a bleed path for current, the current in the bleed path being arranged to decrease automatically with decreasing battery voltage.

Thus, as the battery voltage falls, the current bled through the bleed path will also fall and a larger current will be available for driving the inverter to keep the output power constant.

In a preferred form of the invention, emergency lighting equipment comprises a supply for providing d.c. from the mains to the inverter and means responsive to mains failure to make the battery voltage available to the inverter.

Preferably the battery means are arranged normally to be trickle charged from the mains so that a full charge will always be available when a mains failure occurs.

The battery means can be arranged to be switched to the inverter automatically in response to mains failure by an electronic switch having a bias voltage derived from the mains and holding the switch open as long as the main voltage is high.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the drawing accompanying the Provisional Specification of which the single figure is a circuit diagram of a maintained single point unit emergency lighting equipment.

The equipment is used for lighting an 8 watt fluorescent tube 11 from an inverter shown generally at 12 supplied with dc. at a nominal 5 volts.

The supply is normally derived from the mains at 13 by way of a transformer T2 having a secondary winding with a centre tap forming the negative side of the d.c. supply, and its two ends connected through respective semi-conductor rectifiers D 1 and D2 to provide the positive side of the d.c. supply.

The rectified a.c. is smoothed by an electro Iytic capacitor C2.

The inverter for providing the necessary alternating supply to the fluorescent tube 11 is a single ended saturating transistor type of inverter employing a transformer T3 with the tube 11 energised from its secondary wind ing, and having a primary winding in series with the collector/emitter path of the inverter transistor TR3, and having a feed-back winding connected in the base/emitter circuit of the transistor TR3 and in series with a rectifier D10 across a capacitor C3 which is connected across the d.c. supply in series with a rectifier D9, a resistor R5 and a variable resistor VRl which can be adjusted to set the discharge current into the inverter.

A pack of secondary cells 14 is provided for replacing the d.c. supply from the transformer T2 if there should be a mains failure, and the cells 14 are normally trickle charged from the mains through a full wave rectifier 15 and a resistor R2. A light emitting diode LED I is connected in series with a resistor R I across the resistor R2 to be illuminated as long as there is voltage across the resistor R2 showing that the mains are healthy.

While the mains are healthy a capacitor Cl is charged through a rectifier D7 and its positive plate is connected through a Zener diode ZDI and a rectifier D8 to the base of a transistor TR2 having a resistor R3 connected between the base and the negative side of the d.c. supply.With the mains healthy the voltage across the capacitor Cl holds the base of the transistor TR2 high so that that transistor is held off, and as its emitter is connected to the base of a transistor TRI connected between the positive side of the battery pack 14 and the inverter 12, the transistor TRI is normally held off so that the inverter is not supplied from the battery pack Now if the mains fail. or the full wave rectified voltage drops below a certain value set by the Zener diode ZDl. the transistor TR2 will start conducting and that will turn on the transistor TRI so that the inverter is supplied from the battery pack 14.

Now the output level of the tube 11 will depend upon the inverter's characteristic and its supply voltage. and as it is a characteristic of the nickel/cadmium cells which are typicalls used in this type of application, at the temperatures above normal ambient temperature which are often experienced, that the battery voltage falls significantly as the cells discharge. there is a danger that the light level of the tube 11 will fall below the required level in an unacceptably short time.

For example in one standard set for this type of emergency lighting equipment by the Greater London Council. it is a requirement that the light output at the end of a three hour discharge period should be not less than X5' of the mean value of the light output over the first 30cho of that three hour period.

Accordingly the base drive circuit to the transistor TR3 in the inverter 12 is provided with a bleed circuit including a transistor TR4 and a resistor R8. The base of the transistor TR4 is connected to the junction of resistors R6 and R7 connected in a chain with a Zener diode ZD2 across the output of the battery pack 14 so as to set the amount of current bled from the base drive circuit to the transistor TR3 in accordance with the battery pack voltage. The resistor R6 limits the current through the Zener diode ZD2 and the resistor R7 ensures a minimum Zener current.

As the battery pack voltage falls during the discharge period, the transistor TR4 is turned off linearly as its base bias falls, so that the current bled from the base drive to the transistor TR3 steadily reduces and more current is available for driving the inverter.

This provides compensation for the effect of the falling drive voltage and can enable the light output to remain within the set requirements.

The resistor R8 reduces the dissipation in the transistor TR4 and provides feedback to reduce the effect of gain variation due to the particular transistor selected for TR4 in a particular circuit.

WHAT WE CLAIM IS: 1. An electrical supply system including a rechargeable battery connected to a load, through an inverter including a transformer having a primary winding connected to the battery circuit, a secondary winding connected to the load, and a feedback winding connected to a drive circuit, and a compensator arranged in response to a fall in battery voltage to increase the current in the drive circuit and hance maintain power into the inverter to provide at least partial regulation of power to the load.

2. A system as claimed in Claim 1 in which the primary winding is connected in the collector circuit of a transistor, whose base drive circuit includes the feedback winding and has a bleed path for current, the current in the bleed path being arranged to decrease automatically with decreasing battery voltage.

3. A system as claimed in either of the preceding claims comprising a supply for providing d.c. from the mains to the load and means responsive to mains failure to make the battery voltage available to the inverter.

4. A system as claimed in Claim 3 in which the battery is arranged normally to be trickle charged from the mains.

5. A system as claimed in Claim 3 or Claim 4 in which the battery is arranged to be switched to the inverter automatically in response to mains failure by an electronic switch having a bias voltage derived from the

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Iytic capacitor C2. The inverter for providing the necessary alternating supply to the fluorescent tube 11 is a single ended saturating transistor type of inverter employing a transformer T3 with the tube 11 energised from its secondary wind ing, and having a primary winding in series with the collector/emitter path of the inverter transistor TR3, and having a feed-back winding connected in the base/emitter circuit of the transistor TR3 and in series with a rectifier D10 across a capacitor C3 which is connected across the d.c. supply in series with a rectifier D9, a resistor R5 and a variable resistor VRl which can be adjusted to set the discharge current into the inverter. A pack of secondary cells 14 is provided for replacing the d.c. supply from the transformer T2 if there should be a mains failure, and the cells 14 are normally trickle charged from the mains through a full wave rectifier 15 and a resistor R2. A light emitting diode LED I is connected in series with a resistor R I across the resistor R2 to be illuminated as long as there is voltage across the resistor R2 showing that the mains are healthy. While the mains are healthy a capacitor Cl is charged through a rectifier D7 and its positive plate is connected through a Zener diode ZDI and a rectifier D8 to the base of a transistor TR2 having a resistor R3 connected between the base and the negative side of the d.c. supply.With the mains healthy the voltage across the capacitor Cl holds the base of the transistor TR2 high so that that transistor is held off, and as its emitter is connected to the base of a transistor TRI connected between the positive side of the battery pack 14 and the inverter 12, the transistor TRI is normally held off so that the inverter is not supplied from the battery pack Now if the mains fail. or the full wave rectified voltage drops below a certain value set by the Zener diode ZDl. the transistor TR2 will start conducting and that will turn on the transistor TRI so that the inverter is supplied from the battery pack 14. Now the output level of the tube 11 will depend upon the inverter's characteristic and its supply voltage. and as it is a characteristic of the nickel/cadmium cells which are typicalls used in this type of application, at the temperatures above normal ambient temperature which are often experienced, that the battery voltage falls significantly as the cells discharge. there is a danger that the light level of the tube 11 will fall below the required level in an unacceptably short time. For example in one standard set for this type of emergency lighting equipment by the Greater London Council. it is a requirement that the light output at the end of a three hour discharge period should be not less than X5' of the mean value of the light output over the first 30cho of that three hour period. Accordingly the base drive circuit to the transistor TR3 in the inverter 12 is provided with a bleed circuit including a transistor TR4 and a resistor R8. The base of the transistor TR4 is connected to the junction of resistors R6 and R7 connected in a chain with a Zener diode ZD2 across the output of the battery pack 14 so as to set the amount of current bled from the base drive circuit to the transistor TR3 in accordance with the battery pack voltage. The resistor R6 limits the current through the Zener diode ZD2 and the resistor R7 ensures a minimum Zener current. As the battery pack voltage falls during the discharge period, the transistor TR4 is turned off linearly as its base bias falls, so that the current bled from the base drive to the transistor TR3 steadily reduces and more current is available for driving the inverter. This provides compensation for the effect of the falling drive voltage and can enable the light output to remain within the set requirements. The resistor R8 reduces the dissipation in the transistor TR4 and provides feedback to reduce the effect of gain variation due to the particular transistor selected for TR4 in a particular circuit. WHAT WE CLAIM IS:

1. An electrical supply system including a rechargeable battery connected to a load, through an inverter including a transformer having a primary winding connected to the battery circuit, a secondary winding connected to the load, and a feedback winding connected to a drive circuit, and a compensator arranged in response to a fall in battery voltage to increase the current in the drive circuit and hance maintain power into the inverter to provide at least partial regulation of power to the load.

mains, and holding the switch open as long as the main voltage is high.

6. A system as claimed in any of the preceding claims including a fluorescent tube or other lamp connected as a load to be supplied with bad current from the system.

7. An electrical supply system arranged substantially as herein specifically described with reference to the drawings accompanying the Provisional Specification.