GB2045020A

GB2045020A - Battery charging circuit

Info

Publication number: GB2045020A
Application number: GB8007323A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-03-16
Filing date: 1980-03-04
Publication date: 1980-10-22
Also published as: GB2045020B

Abstract

A battery charging circuit comprises a transformer 10 connected to a bridge rectifier 11 which charges a battery through a current sensitive circuit 3 and a thyristor 14. A battery voltage sensitive circuit 1 connected between one battery terminal 15 and a rail 19 controls the thyristor 14 so as to reduce the charging current as the battery voltage approaches a predetermined level. In order to charge batteries of differing voltages, the rail 19 may be connected to the other battery terminal 13 either directly through a switch 28 or through a voltage increment circuit 2, which may include an adjustable zener diode. <IMAGE>

Description

SPECIFICATION Battery charging circuit This invention relates to a battery charging circuit of the known kind including a series current control means connected in series with a d.c. source and the battery under charge and a battery voltage sensing circuit controlling said series current control means to reduce the current supplied to the battery as the battery voltage approaches a predetermined level during charging.

With such a system it is more difficult to provide a charger which will charge batteries of differeing voltages than it is with the simpler type of charger in which no voltage control is attempted. In the latter case a tapped transformer giving the required charging voltages can be used, but this will be ineffective if there is a series current control means controlled by a voltage sensing circuit.

The use of a switched sensitivity voltage sensing circuit is undesirable because of the increased complication of setting the circuit up, especially where two-stage control of charging is envisaged.

In a battery charging circuit in accordance with the invention the voltage sensing circuit which controls the series current control means is connected between one battery terminal and another terminal, means being provided for connecting said other terminal either directly to the other battery terminal or via a voltage increment circuit of said other battery terminal.

The voltage increment circuit may comprise a transistor, the base current of which is controlled by a zener diode.

Alternatively, the voltage increment circuit may comprise an adjustable zener diode and an adjustable resistive setting network therefor.

In the accompanying drawings: Figure 1 is a circuit diagram showing one example of a battery charging circuit in accordance with the invention; Figure 2 is fragmentary circuit diagram showing a modification of the example shown in Figure 1; Figure 3 is a fragmentary circuit diagram of another modification; Figure 4 is a fragmentary circuit diagram of a further modification, and Figure 5 is a fragmentary circuit diagram of yet another modification.

The battery charging circuit shown in Figure 1 includes a current source in the form of a mains transformer 10 and a bridge rectifier lithe positive terminal of the bridge rectifier 11 being connected by a fixed resistor 12 to the positive battery terminal 13 and the negative terminal of the bridge rectifier 11 being connected to the cathode of a thyristor 14, the anode of which is connected to the negative battery terminal 15, the thyristor 14 acting as a series current control means.

The thyristor is controlled by a voltage sensitive circuit 1 including a pnp output transistor 16, the collector of which is connected buy a resistor 17 to the gate of the thyristor 14. The emitter of the transistor 16 is connected by a resistor 18 to a rail 19. A pnp transistor 20 has its emitter connected to the rail 19 and its collector is connected by two resistors 21,22 in series to the negative battery terminal 15. A resistor 23 is connected between the rail 19 and the base of the transistor 20, which is also connected to the cathode of a zener diode 24, the anode of which is connected by a resistor 25 to the terminal 15 and by a variable resistor 26 and a resistor 27 in series to the rail 19.

The rail 19 is connected to a switch contact 28 which, when closed, connects the rail 19 to the positive battery terminal 13. The rail 19 is also connected to the rail 13 through a voltage increment circuit 2 comprising a trim resistor 6 and a resistor 7, connected in series between rail 13 and rail 19. A tapping point on the resistor 6 is connected to the anode of a zener diode 8, the cathode of which is connected to the base of a pnp transistor 9. The emitter of transistor 9 is connected to the rail 13 and the colector is connected to rail 19.

There is also provided a current sensitive circuit 3.

This includes a pnp transistor 33, the collector of which is connected to the junction of the resistors 21, 22 and the base of which is connected to the terminal 13 by a resistor 34. The emitter of transistor 33 is connected by a resistor 35 to the cathode of a diode 36, the anode of which is connected to the slider of a potentiometer 37 connected across the resistor 12, a capacitor 38 being connected between the cathode of the diode 36 and the terminal 13.

In use, the switch 28 closed, the circuit may be used to charge a nominally 12v battery. When the battery voltage is low there is insufficient voltage across the zener diode 24 for it to conduct, so that transistor 20 turns off and transistor 16 turns on providing ample bias current to the gate of the thyristor 14. As a result, thyristor 14turns on whenever it is forwardly biased and turns off again each time the output of the transformer approaches zero, reverse biasing the thyristor. As the battery becomes charged the point is reached where the zener diode 24 commences conducting weakly, so that transistor 20 turns on weakly and the bias current passed by transistor 16 is reduced, resulting in the conduction angle of the thyristor being reduced.At a predetermined battery voltage (about 14v for a nominally 12v battery) the zener diode 24 is fully on and there is no bias current provided. At this stage trickle charging is provided, that is to say a pulse of charging current only passes if there is a fall in battery voltage.

With the switch open, the circuit may be used to charge a norminally 24v battery. For this purpose the trim resistor 6 is adjusted so that a 14v drop appears between rails 13 and 19. With this arrangement, the switch over to trickle charging will occur at about 28v i.e. 4v above the nominal voltage which is the differential required. Thus, the sensitivity with the switch open as a proportion of the battery voltage is the same as with the switch closed.

It will be noted that the transistors 16 and 20 still operate as though in a nominally 12v circuit and higher voltage transistors are not needed.

Turning now to Figure 2, in this arrangement the rail 19 is connected to the rail 13 by an adjustable zener diode 29 and its bias resistor network 30, 31, 32 as shown In this arrangement, the adjustable zener diode is set up for a 14v drop.

The use of an adjustable zener diode makes it possible to adjust the voltage at which trickle charging occurs and-avoids the necessity of using a precision zener diode.

In both Figures 1 and 2, the transistor 33 acts to provide a current limit irrespective ofthe system voltage in use. If the currentthrough-resistor 12 exceeds a predetermined limit transistor 33 will turn on and cause transistor 16to be turned off. Capacitor 38 and diode 36 act as a peak store.

Turning now to Figure 3 it wilt be seen that two thyristors 14a, 14b may be used instead of a bridge rectifier, the transformer 1 0a having a centre tap.

Two diodes 40, 41 and two resistors 17a, 17b connect the gates of these thyristors 14a and 1 4b to the collector of transistor 1-6.

The modification shown in Figure shows a pnp output transistor 42 connected in circuit with the adjustable zener diode 29. The cathode of the adjustable zener diode 29 is connected to the base of transistor 42 instead of terminal 13. The emitter of the transistor 42 is connected to terminal 13 and its collector is connected to the anode of the adjustable zener diode 29.

By using the adjustable zener diode to control the base current of transistor 42, the adjustable zener diode itself is protected from high currents.

Figure 5 shows a further modification in which a switch 43 is included in the bias circuit of the programmable zener diode 29 so that either of two presettable bias circuits can be selected.

Claims

1. A battery charging circuit comprising a series current control means connected in series with a d.c.

source and the battery under charge, and a battery voltage sensing circuit controllng said series current control means to reduce the current supplied to the battery as the battery voltage approaches a predetermined level during charging, said voltage sensing circuit being connected between one battery terminal and another terminal; and means being provided for connecting said other terminal either directly to the other battery terminal or via a voltage increment circuit to said battery terminal.

2. A circuit according to Claim 1 in which the voltage increment circuit comprises a transistor, the base current of which is controlled by a zener diode.

3. Acircuit according to Claim 1 in which the voltage increment circuit comprises an adjustable zener diode and an adjustable resistive setting network therefor.

4. A circuit according to any one of the preceding claims in which the series current control means comprises a single thyristor.

5. A circuit according to any one of Claims 1 to 3 in which the series current control means comprises a pair of thyristors.

6. A battery charging circuit substantially as hereinbefore described with referenceto and as shown in Figure 1 or Figure 1 as modified by Figure 2 or Figure 3 or Figure 4 or Figure 5.