GB2073964A

GB2073964A - Switching circuit for battery charger

Info

Publication number: GB2073964A
Application number: GB8011057A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1980-04-02
Filing date: 1980-04-02
Publication date: 1981-10-21

Abstract

A switching circuit for selectively connecting a battery charging circuit 26 to a 110V or a 240V mains supply. An input terminal 12 is connected to one end of the primary winding of a transformer 21 and a terminal 10 is normally connected by the contactor of a relay 18 to the other end of the primary winding. When the coil 18e of the relay 18 is energised the terminal 10 is connected to an intermediate tapping of the primary winding. The coil 18e is included in a control circuit which is arranged to energise the coil 18e when the terminals 10, 12 are connected to a 110V supply. When the terminals 10, 12 are connected to a 240V supply, the coil 18e is not energised a delay circuit R4, C4 preventing energization of the coil as the mains supply rises through 110V. The secondary winding is connected to the switching circuit 26. <IMAGE>

Description

SPECIFICATION Switching circuit This invention relates a switching circuit for an electrical apparatus slectively operable at a higher or lower supply voltage value.

A known circuit of this kind includes switching means for adapting the apparatus to the supply voltage having first and second states associated respectively with the higher and lower supply voi tage values and normally assuming the second state, and control means responsive to the voltage across the input terminals for driving the switching means into the first state upon detecting the higher supply voltage. Such apparatus suffers from the disadvantage that the switching means may remain momentarily in the second state when the apparatus is connected to a voltage supply at the higher value thereby causing an overload and possible damage to the appratus.

According to one aspect of this invention there is provided a switching circuit for an electrical appratus selectively operable at a higher or lower supply voltage value, said switching circuit including a pair of input terminals for connection to a power supply, switching means for adapting the appratus to the supply voltage having first and second states associ ated respectively with the higherand lower supply voltage values and normally assuming the first state, and control means responsive to the voltage across the input terminals for driving the switching means into the second state upon detecting the lower supply voltage.

Preferably, the control means includes delay means for preventing the switching means from being driven into the second state as the voltage of a power supply having said higher voltage value varies through the lower voltage value.

The switching means may comprise relay contacts operated by a relay coil forming part of the control means.

The switching circuit may include a transformer having a primary winding associated with the input terminals, a secondary winding associated with a pair of transformer output terminals, and an inter mediate tapping on one of said windings, the switching means selectively connecting one end of said one winding or said tapping to the associated terminal. In a prefered arrangement, the intermedi ate tapping is on the primary winding.

According to another aspect of this invention there is provided the combination of a switching circuit according to the first aspect of this invention and an electrical apparatus selectively operable at the high er or lower supply voltage value.

The apparatus may comprise a battery charging circuit.

This invention will now be described in more detail, by way of example, with reference to the accompanying drawing which is a circuit diagram of a combined switching circuit and battery charging circuit.

Referring now to the drawing, there is shown a switching circuit having a pair of input terminals 10, 12 connected respectively to a pair of rails 14, 16.

The rail 14 is connected through switching means in the form of a set of contacts 18a, 18b of a double throw relay 18 and through a fuse 20 to one end of the primary winding of a transformer 21, the other end of the primary winding being connected to the rail 16. The rail 14 is also connected through the other set of contacts 1 8c, 1 8d of relay 18 and a fuse 23 to an intermediate tapping on the primary winding. When the relay 18 is in its de-energised state, it connects the rail 14 to the end of the primary winding and when it is in its energised state it connects rail 14 to the intermediate tapping.

The secondary winding of transformer 21 is connected to a pair of transformer output terminals 24a, 24b, which are connected to a battery charging circuit 26. The battery charging circuit 26 may be a conventional battery charging circuit or may be a battery charging circuit as shown in our co-pending application no. 7909374 or 7925803.

The rails 14 and 16 are also connected to a control circuitforthe relay 18 and this circuit includes a diode D1 the anode of which is connected to the rail 14 and the cathode of which is connected to a rail 28.

The rail 28 is connected to the rail 16 through a potential divider comprising a resistor R1 and a resistor R2 connected in series. The rail 28 is also connected through the winding 1 8e of relay 18 and a capacitor C1 connected in parallel, to the anode of a thyristor SCRl,the cathode of which is connected to the junction of resistors R1 and R2.The rail 28 is also connected through a resistor R3 to the cathode of a zener diode ZD1 the anode of which is connected to the rail 16. The junction of resistor R3 and zener diode ZD1 is connected to the anode of a diode D2, the cathode of which is connected to the gate of thyristor SCR1.The cathode of diode D2 is also connected through a delay circuit in the form of a resistor R4 and capacitor C4, connected in parallel, to the cathode of thyristor SCR1.

In operation, the terminals 10 and 12 may be connected either to a 110 volt AC mains supply or a 240 volt AC mains supply. When the terminals are connected to the 240 volt supply, a half wave rectified voltage will appear between rails 28 and 16.

The resistance values of resistors R1 and R2 and the breakdown voltage of zener diode ZD1 are chosen so that the voltage applied to the cathode of thyristor SCR1 exceeds the voltage at the junction of resistor R3 and zener diode ZD1 and so relay 18 is not energised. Consequently, rail 14 is connected through the contacts 18a, to one end of the primary winding. The windings of transformer 21 are arranged so that in this state the voltage appearing across terminals 24a, 24b will be the correct supply voltage for the charging circuit 26.

When the 110 volt supply is connected to terminals 10, 12, the half wave rectified voltage will again appear between rails 28 and 16. The value of resistors R1 and R2 and zener diode ZD1 are chosen so that in this condition the voltage at the cathode of thyristor SCR1 is less than the voltage at the junction of resistor R3 and zener diode ZD1. Consequently, during each half cycle of the main supply for which the diode D1 conducts, gate current is supplied to thyristor SCRi -thereby energising the relay winding 18e, the capacitor C1 ensuring that the winding 1 8e remains energised during the half cycles for which diode D1 does not conduct.Energisation of winding 18 drives the relay so that rail 14 is now connected to the intermediate tapping of transformer 21. The tapping of transformer 21 is selected so that the correct voltage is now supplied to the charging circuit 26.

When the terminals 10, 12 are connected to the 240 volt supply, the delay circuit comprising resistor R4 and capacitor C4 prevent the thyristorSCRl from conducting as the supply voltage rises through the 110 volt level.

As the relay 18 is normally in the state for the 240 volt supply, there is no danger of high voltages being mementarily supplied to the charging circuit 26 when the terminals 10, 12 are connected to the 240 volt supply. Also, when connected to the 110 volt supply if a voltage surge occurs the relay 18 will assume the state for the 240 volt supply until the surge subsides.

As may be appreciated, the relay 18 should be insulated as it operates at the mains voltage and by providing the relay on the primary side of transformer 21 this may easily be achieved.

Although the switching circuit described above is combined with a battery charging circuit, it may be used with any electrical apparatus which is required to operate from a higher or lower voltage supply.

Claims

1. Aswitching circuitforan electrical apparatus selectively operable at a higher or lower supply voltage value, said switching circuit including a pair of input terminals for connection to a power supply, switching means for adapting the apparatus to the supply voltage and having first and second states associated respectively with the higher and lower supply voltage values and normally assuming the first state, and control means responsive to the voltage across the input terminals for driving the switching means into the second state upon detecting the lower supply voltage.

2. Aswitching circuit as claimed in Claim 1,in which the control means include delay means for preventing the switching means from being driven into the second state as the voltage of a power supply having said higher voltage value varies through the lower voltage value.

3. A switching circuit as claimed in Claim 1, or Claim 2 in which the switching means comprise relay contacts operated by a relay coil forming part ofthe control means.

4. A switching circuit as claimed in any one of the preceding Claims, including a transformer having a primary winding associated with the input terminal a secondary winding associated with a pair of transformer output terminals, and an intermediate tapping on one of said windings, the switching means selectively connecting one end of said one winding or said tapping to the associated terminal.

5. A switching circuit as claimed in Claim 4, in which the intermediate tapping is on the primary winding.

6. The combination of a switching circuit as claimed in any one of the preceding Claims, and an electrical apparatus selectively operable at the higher or lower supply voltage value.

7. .The combination as claimed in Claim 6, in which the apparatus comprises a battery charging circuit.

8. A switching circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

9. A combined switching circuit and battery charging circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.