GB1584370A - Battery charger and indicator circuit - Google Patents

Description

(54) BATTERY CHARGER AND INDICATOR CIRCUIT (71) We, SAFT-SOCIETE DES ACCUMU- LATEURS FIXES ET DE TRACTION, a Society Anonyme organised under the laws of France and having its registered office at 156 Avenue de Metz, 93230 Romainville, France, 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 battery chargers, and in particular it relates to a circuit for providing an indication that the power supply to the charger has failed.

Storage batteries and an associated charger are frequently used in situations where it is desired that an interruption of mains power supply should not interrupt some essential service. Such a service could be a fire detection and alarm system in a building. A danger with such storage battery systems is that the batteries may run down without the user being made aware that they were not being charged.

In the fire detection example mentioned above this situation could arise because of a fault in the power supply to the charger which could well pass unnoticed, especially if the fault did not affect any other equipment.

The present invention provides a battery charger including input terminals for connection to a power supply and output terminals for connection to a battery to be charged, wherein the charger further includes a lamp, a lamp flasher circuit so connected to the charger that it can draw power from the output terminals to cause the lamp to flash, and a flash inhibit circuit so connected to the charger that it is responsive to the application of power to the input terminals to cause the lamp to shine with a steady light, but that in the absence of power at the inputterminals it is not responsive to the application of power to the output terminals; whereby, in the event of a power supply failure during battery charging, the battery supplies power to cause the lamp to flash.

The advantages of a flashing lamp over a lamp which is simply extinguished are that it has a greater appearance of urgency and that it is impossible to dismiss it as a simple bulb failure. The use of a lamp which is normally shining is also advantageous; firstly because of the reduction in the number of lamps required to signal all the states that need signalling and secondly because the normally shining indication is a positive indication that the bulb has not failed.

The lamp is preferably a light emitting diode since these devices provide a relatively high efficiency and can therefore be run for longer at given brightness from the isolated battery than, say, a filament bulb.

They are also generally more reliable. If the battery charger is used in a fire detection and alarm circuit, the lamp should shine with a green light since a steady green light is a currently accepted convention used to signify that the mains or other external supply is present.

The flasher circuit is preferably a relaxation oscillator. In the preferred relaxation oscillator the timing is provided by a resistor-capacitor network with a timing capacitance being dischargeod through a unijunction transistor. The lamp should then be connected in series in the capacitor discharge circuit.

The resistor-capacitor network should be constituted by a series connection of a timing resistance and the timing capacitor connected across the output terminals of the battery charger. A discharge current limiting resistance may also be connected in series with the series connection. The firing potential of the unijuncton appears across the capacitor before discharge occurs and this potential should be kept low. This may be done by suitable biassing of the second base of the unijunction tran sistor, and it is preferably done by connecting it to an intermediate point in the timing resistance. In this way the firing potential may be kept to less than one third of the supply potential at the output terminals of the battery charger.

The flash inhibit circuit is then prefer ably in the form of a bias resistor connected from one of the input terminals to the battery charger to a point in the connection between the timing capacitance and the emitter of the unijunction transistor; a diode being provided between at least that one of the input terminals and the corresponding output terminal in order to isolate the bias resistor from the battery. The value of the bias resistor is chosen so that the unijunction transistor is maintained in its saturated positive resistance region by current flowing through the bias resistor.

This has the effect of keeping the capacitor charged to a potential which should be kept below the firing potential for flashing operation by suitable choice for the resistance of the bias resistor.

An embodiment of the invention is described by way of example with reference to the sole figure of the drawing accompanying the provisional specification which is a circuit diagram of a battery charger.

In the figure a battery trickle charger 10 has input terminals 12 for connection to a mains supply 14 and has output terminals 16 for connection to a battery 18. The essential components for charging the battery 18 comprise a transformer 20 which has a primary winding connected to the input terminals 12 via a fuse 22 and a secondary winding connecting the output terminals 16 via a diode bridge 24 and a charging current limiting resistor 26. Typical values for this part of the circuit are an unsmoothed DC output of about 40 volts, a battery potential of about 30 volts and a resistance of about 150 ohms for the resistor 26.

The charger has a lamp 28 in the form of a green light emitting diode (LED). The lamp 28 is connected in the discharge path of a lamp flasher circuit 31 in the form of a relaxation oscillator comprising a timing capacitor 32, a timing resistance 35 com- posed of a series connection of two resistors 34 and 36, a unijunction transistor 38 and a discharge current limiting resistor 39.

The timing capacitor 32 is connected in series between the timing resistance 35 and the current limiting resistor 39. This entire series connection is connected across the output terminals 16. The emitter of the unijunction transistor 38 is connected to the link between the timing resistance 35 and the timing capacitor 32 via the LED 28. The first base of the unijunction transistor 38 is connected to the same one of the output terminals 16 as the discharge current limiting resistor 39 and the second base is connected to the link between the two resistors 35 and 36 which constitute the timing resistance 35.

The remaining components form a flash inhibit circuit 41 which comprises a bias resistor 42 and an isolation diode 44. The bias resistor 42 is connected between the diode bridge 24 and the link between the timing capacitor 32, the LED 28 and the timing resistance 35; and the isolating diode 44 is connected in series between the diode bridge 24 and the resistor for limiting the battery charging current.

When there is a battery connected to the output terminals 16 but no mains power available at the input terminals 12 (i.e. a fault condition) the bias resistor 42 is isoc lated by the diode 44 and may be ignored in considering the operation of the lamp flasher circuit 31. Under these conditions the flasher circuit operates by periodic discharging of the capacitor 32 through the lamp 28, the unijunction transistor 38 and the current limiting resistor 39. After discharge the potential on the capacitor 32 builds up with current flowing into it via the timing resistance 35 until the firing potential of the unijunction transistor 38 is reached, whereupon the transistor conducts and discharges the capacitor again. Component values are selected to give a flash at about one second intervals and to keep the firing potential of the transistor at a low proportion of the supply potential.

Thus in the circumstances outlined above of a battery potential of about 30 volts, the component values or types are as follows: resistor 36 4.7 k ohms resistor 34 3.3 k ohms capacitor 32 100 jaF resistor 39 33 ohms transistor 38 2N2646.

This results in a firing potential of less than 10 volts (i.e. of less than 1!3rd of the supply) with the resulting advantages of being able to use a low voltage electrolytic capacitor for the capacitor 32 and of consuming as little power as possible while the battery 18 is not being charged. By way of indication, a fire detection and alarm circuit must retain enough charge in its batteries even after being off charge for three days (a long weekend) to be able to ring the fire alarm bells in spite of a constant drain during the three days from the flashing lamp and from the fire detection circuits.

During normal operation the bias resistor 42 is supplied with current from the bridge 24 and thus supplies current to the LED 28. The value of the bias resistor is such that it supplies enough current to keep the transistor 38 in a suitable state of conduction for the potential across the capacitor 32 not to fall below the firing potential. With the component values already given this requires the bias resistor 42 to be about 8 to 10 kilohms which will keep a potential of about 4 to 5 volts across the capacitor. This corresponds to the emitter of the transistor 38 being biassed to about 2.4 volts and a drop of about 2 volts across the LED.

Numerous modifications to this circuit are possible within the scope of the present invention. Firstly the component values will need to be changed appropriately for use in other applications where the battery potential is different from that described.

Sceondly the battery charger may be modified to differing requirements; it may include more sophisticated circuits than those described or it may include even fewer components, especially if a low voltage AC or even DC power supply happens to be available. Thirdly the elements of the circuit shown may be rearranged: for example it is possible to use a single current limiting resistor connected between the first base of the transistor and the capacitor for limiting both the charging current during normal operation and the flashing current during a failure of the charging supply; in other words if other circuit values are chosen so that the resistors 26 and 39 have the same resistance, then they may be replaced by a single resistor placed near the reference numeral 31 in the drawing; or the resistor connected to the second base could be independent of the timing resistance. Fourthly the flasher circuit could be replaced by one of different design, whether using a completely different configuration such as a multivibrator or an integrated circuit package. Finally the requirement that the lamp should shine with a steady light must be understood in relation to the persistence of the human eye; a lamp flashing at a high enough frequency looks steady and that is sufficient.

Thus the flash inhibit circuit could alternatively operate to increase the rate of flashing beyond the perceivable threshold, rather than operate to reduce the rate to zero. indeed, in the circuit described, there is probably a ripple in the brightness of the lamp due to the unsmoothed potential seen by the bias resistor 42.

WHAT WE CLAIM IS: - 1. A battery charger including input terminals for connection to a power supply and output terminals for connection to a battery to be charged, wherein the charger further includes a lamp, a lamp flasher circuit so connected to the charger that it can draw power from the output terminals to cause the lamp to flash, and a flash inhibit circuit so connected to the charger that it is responsive to the application of power to the input terminals to cause the lamp to shine with a steady light, but that in the absence of power at the input terminals it is not responsive to the application of power to the output terminals; whereby, in the event of a power supply failure during battery charging, the battery supplies power to cause the lamp to flash.

2. A battery charger according to claim 1 wherein the lamp is a light emitting diode.

3. A battery charger according to claim 1 or 2 for use in a fire detection and alarm circuit wherein the lamp shines with a green light.

4. A battery charger according to any previous claim wherein the flasher circuit is a relaxation oscillator.

5. A battery charger according to claim 4 wherein the relaxation oscillator includes a resistor-capacitor network with a timing capacitance connected to be discharged through a unijunction transistor.

6. A battery charger according to claim 5 wherein the lamp is connected in series in the capacitor discharge circuit.

7. A battery charger according to claim 5 or 6 and including a discharge-limiting resistance in the capacitor discharge circuit.

8. A battery charger according to claim 5, 6 or 7 wherein the resistor-capacitor network is constituted by a series connection of a timing resistance and the timing capacitance connected across the output terminals of the battery charger.

9. A battery charger according to claim 8 wherein an intermediate point of the timing resistance is connected to bias that one of the bases of the unijunction transistor which does not form part of the capacitor discharge circuit.

10. A battery charger according to claims 5, 6, 7, 8 or 9 wherein the flash inhibit circuit is in the form of a bias resistor connected from one of the input terminals to the battery charger to a point in the connection between the timing capacitance and the emitter of the unijunction transistor; a diode being provided between at least that one of the input terminals and the corresponding output terminal in order to isolate the bias resistor from the battery.

11. A battery charger substantially as herein described with reference to the drawing accompanying the provisional specification.

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. keep the transistor 38 in a suitable state of conduction for the potential across the capacitor 32 not to fall below the firing potential. With the component values already given this requires the bias resistor 42 to be about 8 to 10 kilohms which will keep a potential of about 4 to 5 volts across the capacitor. This corresponds to the emitter of the transistor 38 being biassed to about 2.4 volts and a drop of about 2 volts across the LED. Numerous modifications to this circuit are possible within the scope of the present invention. Firstly the component values will need to be changed appropriately for use in other applications where the battery potential is different from that described. Sceondly the battery charger may be modified to differing requirements; it may include more sophisticated circuits than those described or it may include even fewer components, especially if a low voltage AC or even DC power supply happens to be available. Thirdly the elements of the circuit shown may be rearranged: for example it is possible to use a single current limiting resistor connected between the first base of the transistor and the capacitor for limiting both the charging current during normal operation and the flashing current during a failure of the charging supply; in other words if other circuit values are chosen so that the resistors 26 and 39 have the same resistance, then they may be replaced by a single resistor placed near the reference numeral 31 in the drawing; or the resistor connected to the second base could be independent of the timing resistance. Fourthly the flasher circuit could be replaced by one of different design, whether using a completely different configuration such as a multivibrator or an integrated circuit package. Finally the requirement that the lamp should shine with a steady light must be understood in relation to the persistence of the human eye; a lamp flashing at a high enough frequency looks steady and that is sufficient. Thus the flash inhibit circuit could alternatively operate to increase the rate of flashing beyond the perceivable threshold, rather than operate to reduce the rate to zero. indeed, in the circuit described, there is probably a ripple in the brightness of the lamp due to the unsmoothed potential seen by the bias resistor 42. WHAT WE CLAIM IS: -

1. A battery charger including input terminals for connection to a power supply and output terminals for connection to a battery to be charged, wherein the charger further includes a lamp, a lamp flasher circuit so connected to the charger that it can draw power from the output terminals to cause the lamp to flash, and a flash inhibit circuit so connected to the charger that it is responsive to the application of power to the input terminals to cause the lamp to shine with a steady light, but that in the absence of power at the input terminals it is not responsive to the application of power to the output terminals; whereby, in the event of a power supply failure during battery charging, the battery supplies power to cause the lamp to flash.

2. A battery charger according to claim 1 wherein the lamp is a light emitting diode.

3. A battery charger according to claim 1 or 2 for use in a fire detection and alarm circuit wherein the lamp shines with a green light.

4. A battery charger according to any previous claim wherein the flasher circuit is a relaxation oscillator.

5. A battery charger according to claim 4 wherein the relaxation oscillator includes a resistor-capacitor network with a timing capacitance connected to be discharged through a unijunction transistor.

6. A battery charger according to claim 5 wherein the lamp is connected in series in the capacitor discharge circuit.

7. A battery charger according to claim 5 or 6 and including a discharge-limiting resistance in the capacitor discharge circuit.

8. A battery charger according to claim 5, 6 or 7 wherein the resistor-capacitor network is constituted by a series connection of a timing resistance and the timing capacitance connected across the output terminals of the battery charger.

9. A battery charger according to claim 8 wherein an intermediate point of the timing resistance is connected to bias that one of the bases of the unijunction transistor which does not form part of the capacitor discharge circuit.

10. A battery charger according to claims 5, 6, 7, 8 or 9 wherein the flash inhibit circuit is in the form of a bias resistor connected from one of the input terminals to the battery charger to a point in the connection between the timing capacitance and the emitter of the unijunction transistor; a diode being provided between at least that one of the input terminals and the corresponding output terminal in order to isolate the bias resistor from the battery.

11. A battery charger substantially as herein described with reference to the drawing accompanying the provisional specification.