GB2208192A

GB2208192A - Back-up power supply

Info

Publication number: GB2208192A
Application number: GB8816146A
Authority: GB
Inventors: Robert Thomas Cowan
Original assignee: LOKER ANDREW STEVEN
Current assignee: LOKER ANDREW STEVEN
Priority date: 1987-07-08
Filing date: 1988-07-07
Publication date: 1989-03-08
Also published as: GB8816146D0

Abstract

Electronic equipment 12, particularly a computer is energised by a battery 40 via a diode 42 in the event of failure of a mains-derived supply 22, 24, and means 106 and 130 are provided respectively to indicate the mains supply status and a low energy level in battery 40. A transformer 44 and rectifier 52 provide a supply to energise relays, 60, 62, 64 and to charge an auxiliary battery 74 operable to energise the monitoring and indicating circuits. Relays 60, 62, 64 respectively have normally open contacts 60-1, 60-2, normally closed contacts 62-1, 62-2, and changeover contacts 64-1, whereby LED devices 106 and 130 both normally give a steady green light respectively indicating pressure of the mains supply and closure of a standby switch 50. If the mains supply fails, device 106 gives a flashing red light, a sounder 116 is energised, and contacts 62-2 close to connect the backup battery 40 to a comparator which cause device 130 to give a steady green light if the battery voltage is above a reference. If the voltage of a battery 40 is below the reference, device 130 gives a flashing red light and a sounder 140 is energised. A trickle charging supply may be provided for battery 40 if it is rechargeable. The relays may be replaced by solid state devices. Equipment 12 may have a switched mode supply maintaining a substantially constant power drain with in variation in the supply voltage. <IMAGE>

Description

BACK-UP POWER SUPPLIES DESCRIPTION The invention concerns back-up power supplies, in particular back-up power supplies for electronic equipment such aS #computers.

With increasing use of computers there is ever growing danger of losing information held therein in the event of failure of the mains power supply thereto. It has been suggested that the power supply to a computer may be maintained using emergency generators or large scale battery back up systems including inverters and the like; however, all such systems known to us are expensive and difficult to install and maintain.

Objects of the invention include the provision of apparatus enabling the safe backing up of data in electronic equipment which is less costly than has until now been available, which provides a warning that the back up power supply is in danger of failing, and which desireably provides a indication both of the status of the mains supply and that the back~up power supply is connected to the equipment and on standby.

In one aspect the invention provides a back-up power supply for interposition between electronic equipment and the mains supply thereto, which power supply comprises a source ot electrical power normally disabled but enabled, when the mains supply fails, to supply electrical power to the electronic equipment, and means for checking and indicating to a user the level of electrical energy remaining in the source of electrical power.

The back-up power supply may desireably include means fot indicating the condition of the mains supply.

The electrical power source preferably comprises a battery for connection to the electronic equipment via a diode normally reverse biased by an operating voltage supplied to the electronic equipment, but forward biased by the said battery whenever the operating voltage supplied to the electronic equipment fails.

The checking means may with advantage comprise circuitry which when the mains supply is uninterrupted indicates to a user that the back-up power supply is on stand-by and which when the mains supply fails compares the output of said battery with a reference voltage to provide said indication of the level of electrical energy therein.

The reference voltage may be provided by a second battery in the back-up power supply.

The checking means preferably comprises a comparator enabled when the mains supply fails to receive as one input a predetermined voltage derived from current flow from said second battery and as a second input a voltage indicative of the state of charge of said first mentioned battery, the output of said comparator energising an indicator visible to a user of the relative values of the voltages applied to its inputs and for energising an audible warning device if the first voltage input thereto exceeds the second.

The visible indicator of the checking means advantageously comprises a tri-state LED device driven to a first state by the output of the comparator to emit light of one colour when the mains fails and the first voltage input to the comparator is less than the second voltage input thereto and which is driven by the output of the comparator to a second state to emit light of another colour when the mains supply fails the first voltage input to the comparator is greater than the second voltage input thereto.

The back-up power supply may include circuitry adapted to drive said tri-state LED device to said first state whilst the mains supply is uninterrupted to indicate the back-up power supply is on standby.

The means for indicating the condition of the mains supply includes an indicator visible to a user of the state of the mains supply and means for providing an audible warning in the case of mains failure. The audible warning of mains supply failure may be disabled by a user for a predetermined period.

Desireably the visible indicator of mains supply status comprises a further tri-state LED device driven to emit light of one colour when the mains supply is acceptable and to emit light of another colour when the mains supply fails.

In a second aspect the invention provides a backup power supply including a battery and control circuit for interposition between switch mode power supplied electronic equipment and the rectified mains voltage supply thereto, wherein the battery and control circuit includes a battery and a diode coupled in series across the DC power input terminals of the equipment such that when the mains supply is uninterrupted the diode is reverse biased preventing battery passing power to the equipment and which diode, in the event of a mains power failure, becomes forward biased allowing said battery to discharge and maintain the voltage supplied to the DC power input terminals of the equipment.

The back-up power supply preferably further includes mains supply indicator and battery check circuits powered by the battery and control circuit which includes a step-down transformer the primary of which is coupled to receive the alternating mains voltage supply and the secondary of which is rectified to drive relays operational to control the said a mains supply indicator and battery check circuits, and to provide power to a second battery included in the battery and control circuit.

Back-up power supplies embodying the invention are particularly useful in securing power supplies of that class of computer presently available having an internal switch mode power supply in which the alternating mains supply is rectified without being first transformed to a lower alternating voltage and which draw substantially constant power from the supply. Such computers are capable of operating with a DC supply voltage varying by up to 50 per cent of the nominal DC supply voltage - the current drawn by them varying to maintain a substantially constant power drain on the supply.

Qther aspects features and advantages of the invention will become apparent from the following description of an embodiment thereof now made with reference to the accompanying drawing in which: Figure 1 schematically illustrates the connection of the back-up power supply to a computer, and Figure 2 schematically illustrates in more detail the circuitry of the back-up power supply.

As can be seen from Figure 1 the normal power supply 10 to the computer 12 includes a double pole switch 14 operable to couple lines 16 and 18 to a source of 240v AC. After passing through a mains filter and surge protection circuit 20 the alternating current supply is rectified in bridge rectifier 22, smoothed by capacitor 24 and the resultant DC voltage (approximately 300v DC) passed via lines 26 and 28 to terminals 30 and 32 to the computer 12.

The back-up power supply of the present invention is provided as additional components comprising a battery and control circuit 34 coupled to lines 16 and 18 and lines 26 and 28, a mains supply indicator circuit 36 and a battery check circuit 38. The battery and control circuit 34 includes aX battery 40 (of approximately 200v DC) and a diode 42 coupled as shown across lines 26 and 28. During normal operation - with no break in the continuity of the mains supply diode 42 is reversed biased preventing battery 40 discharging and passing power to the computer 12 .In the event of a mains power failure, however, the voltage applied to lines 26 and 28 from bridge rectifier 22 will rapidly fall until (when the voltage thereacross is approximately 200 v DC) diode 42 becomes forward biased and permits battery 40 to discharge maintaining the voltage on lines 26 and 28 at about 200v DC, and thereby maintaining power to computer 12.

The mains supply indicator circuit 36 is operational to provide an indication of the state of the mains supply having an LED display which glows green whilst the mains supply is uninterruped and an LED display which flashes red whenever the mains supply fails. The battery check circuit 38 is enabled only when the mains supply fails and is operational to provide an indication of the state of charge of battery 40 an LED display which normally glows green whilst (the mains supply is interruped) and battery 40 has sufficient charge to power computer 12 and an LED display which flashes red when the charge of battery 40 has fallen to a level at which the safe operation of computer 12 cannot be assured from the output of battery 40.

Additionally mains supply indicator circuit 36 and battery check circuit 38 include tone sounders which sound an audible alarm if the mains supply fails or battery 40 has discharged to a predetermined voltage level.

To describe the circuitry in more detail; mains filter and. surge protection circuit 20 includes a surge suppressor and mains filter of known configuration, and its output is fed both to bridge rectifier 22 and to the primary winding of a step-down transformer 44 in battery and control circuit 34. Battery and control circuit 34 also includes a diode 46 coupled across lines 26 and 28 in parallel with one pole of a standby switch 50 serially connected to battery 40 and diode 42.

The low voltage secondary winding of transformer 44 is coupled to a bridge rectifier 52 which provides a 9 V DC output - smoothed by a capacitor 54 - across lines 56 and 58 in battery and control circuit 34. In parallel with capacitor 54 are three relays 60, 62 and 64 and, on the other side of a diode 66 in line 56, lines 56 and 58 are bridged byra a diode 68, a resistor 72 and a second battery 74. The 'battery 74 is a 9 v DC rechargable battery and is used to power the elements of the mains supply indicator circuit 36 and the battery check circuit 38 as will be described below.

The junction of resistor 72 and battery 74 is tapped as shown to a second pole of stand-by switch 50 which, when closed, connects battery 74 via a diode 76 to line 56 and, via resistor 78, to the base of transistor 80.

The collector of transistor 80 is connected to line 56 as shown and its emitter connected to contact 62.1 of relay 62 and contact 60.1 of relay 60 in the battery check circuit 38. Line 56 is connected to the circuit 36.

The junction of battery 40 and diode 42 is tapped and connected via contact 62.2 of relay 62 and resistor 90 to one end of a variable resistor 94 the other side of which is connected to ground. The tap of variable resistor 94 provides a further input to battery check circuit 38.

Within mains supply indicator circuit 36 the feed from line 56 is taken to a resistor 86 coupled by contact 60.2 of relay 60 to ground. The junction of resistor 86 and contact 60.2 is connected by a resistor 96 to the the base of a transistor 98 and by resistor 100 to the base of transistor 102. The emitter of transistor 98 and the collector of transistor 102 are connected as shown to receive the voltage on line 56. The voltage on the collector of transistor 98 forms one input 104 of a tristate LED device 106 (via a resistor 108) and the the emitter of transistor 102 is connected via a resistor 110 to another input 112 of tri-state LED device 106. Device 106 is further connected to ground as shown.

The emitter of transistor 102 also connected to tone sounder 114 in the emitter-collecter circuit of transistor 116 as shown. The base of transistor 116 connected via resistor 118 to the output of a timer/driver circuit 120 including a counter 121.

The emitter of transistor 80 is connected via contact 62.1 of relay 62 to the emitter of a transistor 122, to the collector of a transistor 124 and to form one input power supply to an IC comparator 126. The emitter of transistor 80 is also connected by contact 60.1 of relay 60 to form an input 128 of a second tri-state LED device 130 via resistor 132. A diode 134 connects the collector of transistor 122 to the junction of contact 60.1 and resistor 132.

The output of comparator 126 is connected by resistor 136 to the the base of transistor 122 and by resistor 137 to the the base of transistor 124. The emitter of transistor 124 is connected to ground by a diode 138 and a second tone sounder 140, and by a resistor 142 to a second input 144 of tri-state LED device 130 as shown.

Device 130 is further connected to ground as shown.

Comparator 126 has two inputs, the first a source of reference voltage derived from the emitter circuit of transistor 80 and including a resistor 146 connected by zener diode 148 to ground, and the second taken from the tap of variable resistor 94 as shown.

In normal operation - when the AC power supply 10 is uninterrupted - the direct current voltage across lines 56 and 58 drives relay 60 such that its contacts are CLOSED, drives relay 62 such its contacts are OPEN, and drives relay 64 such that its contact 64.1 connects timer/driver circuit 120 to ground.

Whilst this situation is maintained bridge rectifier 22 puts approximately 300v DC across lines 26 and 28, reverse biasing diode 42 and thereby preventing battery 40 supplying current to terminals 30 and 32.

The base of transistor 98 is held at Ov DC (via contact 60.2 and resistor 96) and transistor 98 is therefore ON connecting input 104 of tri-state LED device 106 to the nominal 9v DC supply from battery 74. In this state the LED device glows a steady green indicating the mains supply is present.

The Ov DC at the the base of transistor 102 (via contact 60.2 and resistor 100) holds transistor 102 OFF; and the output of timer/driver circuit 120 is held at approximately 9v DC by its internal circuitry thereby biasing transistor 116 OFF. As a result tone sounder 114 does not sound.

The 9v DC on the base of transistor 80 (from line 56) holds transistor 80 ON and in turn it supplies approximately 6v DC to input 128 of tri-state LED device 130 (via contact 60.1 and resistor 132) and in this state LED device 130 glows a steady green indicating the stand-by switch 50 is closed.

As the contacts of relay 62 are OPEN the battery check circuit 38 is disabled and battery 40 is isolated from resistor 90 and variable resistor 94, thereby preventing battery 40 discharging.

Diode 68 acts to trickle charge battery 74 maintaining it in a fully charged condition.

In the event of a failure of the mains power supply 10 the voltage on lines 16 and 18 immediately falls to 0V and the output of bridge rectifier 22 rapidly falls toward zero. When voltage across lines 26 and 28 is approximately 200 V DC diode 42 becomes forward biased allowing battery 40 to pass power to terminals 30 and 32 and to computer 12.

As the the voltage across lines 26 and 28 falls the voltage across lines 56 and 58 also falls allowing relays 60, 62 and 64 to relax. Thus the contacts of relay 60 OPEN, whilst those of relay 62 and relay 64 CLOSE. The diodes 66 tand 68 act to prevent battery 74 feeding back into lines 56 and 58 keeping these relays in their driven states.

As contact 60.2 of relay 60 is now OPEN the voltage at the base of transistor 98 rises to approximately 6v DC (via resistors 86 and 96 from line 56) and transistor 98 is driven OFF. At the same time the voltage at the base of transistor 102 rises (via resistors 86 and 100) and transistor 102 is driven ON. Thus the voltage at input 104 of tri-state LED device 106 falls to Ov DC and the voltage at input 112 of the device rises to approximately 6v DC.

The optical output of tri-state LED device 106 therefore changes to a flashing red indicating that the mains power supply has failed.

As contact 64.1 of relay 64 swjtchpr# to cet Ct timer/driver circuit 120 to line 56 the output of circuit 120 (applied to the base of transistor 116) falls to Ov DC, and transistor 116 turns ON energising the tone sounder 114.

Tone sounder 114 may be silenced by the user closing silence switch 152. When this is done the timer/driver circuit 12.0 holds the kase of transistor 116 at approximately 6v DC for a period of approximately 120 seconds (determined by resistors 154 and 156 and capacitors 158 and 160) thereby driving transistor 116 OFF for this period. At the end of this period transistor 116 will again be driven ON if the mains power has not returned (and switches 14 and 50 are still CLOSED).

The opening of contact 60.1 of relay 60 would cause the the 6v DC applied to input 128 of tri-state LED device 130 to fall, however, the simultaneous closing of the contacts of relay 62 enables the rest of battery check circuit 38 and - depending upon the condition of'battery 4O - device 130 continues to emit a steady green light.

With contact 62.2 CLOSED battery 40 is connected to variable resistor 94. The voltage at tap of variable resistor 94 is compared in comparator 126 with a stable 3.6v DC provided by resistor 146 and zener diode 148. Variable resistor 94 is preset to provide at its tap a voltage of 3.6v DC when the output voltage of battery 40 is at 150v DC.

All the time the voltage output of battery 40 is at or above 150v DC the the voltage at the tap of variable resistor 94 applied to one input 162 of comparator 126 is greater than the 3.6v DC applied to its other input 164 from resistor 146 and zener diode 148. In this condition the output comparator 126 is held at 0v DC and this voltage is applied to the base of transistor 122 (via resistor 137) driving it ON, and the base of transistor 124 (via resistor 136) driving it OFF. Whilst transistor 122 is held ON and its collector current maintains (via diode 134) the optical output of tri-state LED device 130 green which now indicates the state of charge of battery 40 is acceptable.

As the voltage of battery 40 falls the voltage at the tap of variable resistor 94 also falls until the voltage on the two inputs of comparator 126 are equal. At this time the output of the comparator rises to approximately 6v DC driving transistor 122 OFF and transistor 124 ON. Thus the voltage at input 128 of LED device 130 falls and that at input 144 rises with the result the optical output of the tri-state LED device 130 changes to a flashing red.

Whilst transistor 124 is ON tone sounder 140 is driven to sound.

If the mains power is restored the contacts of relay 60 are driven to CLOSE, contacts of relay 62 are driven to OPEN and the contact 64.1 of relay 64 is driven to couple th! input of timer/driver circuit 120 to ground and reset the timer circuit therein.

A capacitor 164 is provided to smooth the input to comparator 126 thereby preventing alarms being triggered by noise in the circuit and zener diode 166 is provided to protect comparator 126 by limiting the voltage applied to the input 162 of comparator 126 to a practical a upper limit of approximately 6v DC.

The nominal level of charge required for the battery 40 will vary depending upon the equipment with which the back-up power supply is used. With one well known computer sold at present a battery 40 having a nominal charge of 200v DC (provided by a sealed pack of 15 12v DC dry cell batteries) has been found to continue safely powering the computer for up to one hour following a mains failure. That is to say the voltage of the battery pack reduced to approximately 150v DC in the sixty minutes following mains interruption - below which voltage safe operation of the computer could not be assumed. This time period is, of course, very much longer than would be required to safely download data in the machine onto disc or other storage medium or to finish a printing operation.

Assuming it is possible to download data held in the machine and turn the machine of in an average of two minutes it will be seen that such a battery will have a useful life of up to thirty mains failures and it is likely it would not discharge to a level making it unusable in the two to three year periods we recommend between having the battery pack reconditioned.

The state of charge of battery 40 may be checked by a user simply by closing switch 14 and stand-by switch 50 to connect the computer 12 (with the back-up power supply in circuit) to the mains supply and then open switch 14. This will enable battery check circuit 38 and cause tri-state LED device 130 to emit a steady green light if the state of charge of battery 40 is acceptable but to flash red if that is not the case. It is thought that in normal use (i.e.

with no mains failures) the state of the battery 40 should be be checked in this way at approximately, monthly intervals.

Jt will be appreciated that many variations may be made to the disclosed arrangements without departing from the scope of the present invention - the relays may be replaced with solid state switching devices for example and, if desired the battery 40 may be provided as a rechargable unit which is constantly trickle charged whilst the back-up power supply is connected to the mains (in the manner proposed for the battery 74 described above).

Claims

1. A back-up power supply for interposition between electronic equipment and the mains supply thereto, which power supply comprises a source of electrical power normally disabled but enabled, when the mains supply fails, to supply electrical power to the electronic equipment, and means for checking and indicating to a user the level of electrical energy remaining in the source of electrical power.

2. A back-up power supply as claimed in claim 1 further including means for indicating the condition of the mains supply.

3. A back-up power supply as claimed in claim 1 or claim 2, wherein the electrical power source comprises a battery for connection to the electronic equipment via a diode normally reverse biased by an operating voltage supplied to the electronic equipment, but forward biased by the said battery whenever the operating voltage supplied to the electronic equipment fails.

4. A back-up power supply as claimed in claim 3, wherein the said checking means comprises circuitry which when the mains supply is uninterrupted indicates to a user that the back-up power supply is on stand-by and which when the mains supply fails compares the output of said battery with a reference voltage to provide said indication of the level of electrical energy therein.

5. A back-up power supply as claimed in claim 4, wherein the reference voltage is provided by 'a second battery in the back-up power supply.

6. A back-up power supply as claimed in claim 4 or claim 5, wherein said checking means comprises a comparator enabled when the mains supply fails to receive as one input a predetermined voltage derived from current flow from said second battery and as a second input a voltage indicative of the state of charge of said first mentioned battery, the output of said comparator energising an indicator visible to a user of the relative values of the voltages applied to its inputs and for energising an audible warning device if the first voltage input thereto exceeds the second.

7. A back-up power supply as claimed in claim 6, wherein said visible indicator comprises a tri-state LED device driven to a first state by the output of the comparator to emit light of one colour when the mains fails and the first voltage input to the comparator is less than the second voltage input thereto and which is driven by the output of the comparator to a second state to emit light of another colour when the mains supply fails the first voltage input to the comparator is greater than the second voltage input thereto.

8. A back-up power supply as claimed in claim 7, including #circuitry adapted to drive said tri-state LED device to said first state whilst the mains supply is uninterrupted to indicate the back-up power supply is on standby.

9. A back-up power supply as claimed in claim 2 and any one of claims 3 to 8, wherein the means for indicating the condition of the mains supply includes an indicator visible to a user of the state of the mains supply and means for providing an audible warning in the case of mains failure.

10. A back-up power supply as claimed in claim 9, wherein the audible warning of mains supply failure may be disabled by a user for a predetermined period.

11. A back-up power supply as claimed in claim 9 or claim 10 wherein said visible indicator comprises a further tri-state LED device driven to emit light of one colour when the mains supply is acceptable and to emit light of another colour when the mains supply fails.

12. A back-up power supply including a battery and control circuit for interposition between switch mode power supplied electronic equipment and the mains supply thereto, wherein the battery and control circuit includes a battery and a diode coupled in series across the DC power input terminals of the equipment such that when the mains supply is uninterrupted the diode is reverse biased preventing battery passing power to the equipment and which diode, in the event of a mains power failure, becomes forward biased allowing said battery to discharge and maintain the voltage supplied to the DC power input terminals of the equipment.

13. A back-up power supply as claimed in claim 12, further including a mains supply indicator circuit and a battery check circuit, and wherein the battery and control circuit includes a step-down transformer the primary of which is coupled to receive the alternating mains voltage supply and the secondary of which is rectified to drive relays operational to control the said a mains supply indicator and battery check circuits, and to provide power to a second battery included in the battery and control circuit.

14. A back-up power supply as claimed in claim 1 or claim 12 and substantially as herein described with reference to the accompanying drawings.