GB2066595A

GB2066595A - Battery charger with transient suppression

Info

Publication number: GB2066595A
Application number: GB8038262A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-12-28
Filing date: 1980-11-28
Publication date: 1981-07-08
Also published as: DE3043760A1; NO803455L; NO151264C; NO151264B; FI68743B; DK548780A; SE417040B; FI68743C; GB2066595B; FI803839L

Abstract

Charging device principally for the charging of stationary accumulators and comprising a mains transformer (L1, K1, L2) connected via a rectifying device (D10-D13) with the accumulator (B) and designed to permit the use of capacitive current limitation in the primary circuit of the mains transformer without the risk of series resonance and consequent current surges. The charging device has, according to the invention, in series with the primary winding (I1) of the mains transformer both a current limitation condenser (C1) and a safety circuit (SK) which for the purpose of preventing current surges is transferable between a high ohmic and a low ohmic circuit condition, in which connection the safety circuit for control between the two conditions is connected with a control circuit (ST) connected in series with the secondary winding L2 of the mains transformer. The control circuit is arranged to transfer the safety circuit to the low ohmic condition as soon as a predetermined value of the charging current has been reached. <IMAGE>

Description

SPECIFICATION Charging device The invention under consideration concerns a charging device which is intended principally, but not exclusively, for the charging of stationary accumulators, for use for example in emergency lighting plant or the like and which is equipped with a capacitive current limiter in series with the primary winding of the transformer through which the accumulator obtains its charging current via the secondary winding of the transformer and a rectifying circuit.

In connection with the charging of accumulators of different types, a device is required to regulate the charging current during the process of charging, since the terminal voltage of the accumulator varies greatly during this operation and thus without regulation the charging current could easily attain far too high a value at the beginning of the charging process. In many situations such properties of the charging device as physical size, power loss, generation of heat, length of life or weight are considered by and large to be of no importance. This can be considered to be the case, for example, with such charging devices that are solely intended for the occasional charging of car batteries.Where however charging devices of the type referred to in the introduction are concerned, it nearly always happens that they must be built in to confined spaces which furthermore can be difficult to keep cool, and that they are permanently connected to the mains. This means that the physical dimensions must be as small as possible, that the power loss and consequently also the heat generation must be, broadly speaking, zero and that the length of life must be comfortably large.

Where the design of the regulator for the charging device - in practice a current limiter for the limiting of the magnitude of the charging current at the beginning of the charging process is concerned, it has previously been necessary to resort to resistive or inductive current limitation in the primary circuit of the mains transformer, or to resistive, capacitive or inductive current limitation in the secondary circuit of the transformer. Up to now it has not been possible to apply capacitive current limitation in the primary circuit of the transformer since under certain working conditions, mainly when idling, series resonance has occurred due to the inductance of the primary winding.

In connection with the limitation of current in the secondary circuit of the transformer, the transformer must be designed for the sum of the VA figure that is governing for the charging function itself and the VA figure that is governing for the regulating function, which as a rule is double the VA figure that referred to the charging function. Naturally in such a design the transformer will be larger and the heat generation more troublesome than if the current limiter were placed in the primary circuit. Furthermore, if the current limiter is of capacitive type, electrolytic condensers must be used for practical reasons, and these electrolytic condensers have a high internal consumption, modify capacitance with time, and cannot withstand high ambient temperatures.

Placing the current limiter in the primary circuit is in itself advantageous but the problem is that both the resistive and the inductive current limitation have greater losses than what a capacitive would have had if it cbuld be used without complications.

The purpose of the present invention is therefore to produce a charging device of the kind referred to above which is designed in such a way that capacitive current limitation can be used in the primary circuit of the mains transformer without the risk of series resonance and consequent injurious current surges.

This object is achieved, according to the invention, if a charging device of the type referred to in the introduction is characterized by the connection of a condenser for the limitation of the charging current in series with the primary winding of the mains transformer, and that also a safety circuit is arranged in series with this which for the prevention of injurious current transients is transferable between a low ohmic and a high ohmic circuit condition, and that a control circuit is connected in series with the secondary winding of the mains transformer which is so arranged that when a predetermined charging current through the secondary winding is exceeded the safety circuit is transferred to the low ohmic circuit condition.

In a favourable design, according to the invention, the safety circuit comprises a transistor which in saturated condition via collector-emitter effectively short-circuits the safety circuit in its low ohmic circuit condition and which via collector emitter and base emitter if non-saturated condition directs current through the safety circuit in its high ohmic condition, wherein the control circuit is connected with the base and emitter of the transistor.

It is furthermore advisable, according to the invention, that the control circuit includes a further transformer, the primary winding of which is connected in series with the secondary winding of the mains transformer and the rectifying device, while its secondary winding is connected via a rectifying circuit to the transistor's base and emitter.

The invention will now be described in more detail by means of a practical example illustrated in the attached wiring diagram for the charging device.

The charging device comprises in the normal manner a mains transformer with primary winding L1, core K1 and secondary winding L2. The primary winding is connected via a condenser C1 to the mains R, in connection with which the task of the condenser is to limit the current during the initial stage of the charging process so that the accumulator or the components included in the charging device are not damaged by excessive currents. The other end of the primary winding of the mains transformer is connected via a safety circuit SK to the other mains terminal N.The secondary winding L2 of the mains transformer is connected with the accumulator B that is to be charged, via a rectifying circuit consisting principally of the four diodes D10, D1 1, D12 and D13. The rectifying circuit also comprises other components whose purpose will be stated below.

On account of the condenser Cl and the inductance in the primary winding L1 of the mains transformer the resonance phenomenon could occur, especially when idling, that could give rise to very high currents that could damage the charging device. In order to prevent such surges of current the safety circuit SK, whose principal components are the transistor T1 and the diodes D1, D2, D3, D4 and D5, together with the resistances R1 and R2, is arranged in series with the primary winding of the mains transformer.

For the control of the safety circuit, a control circuit ST is connected in series with the secondary winding L2 of the mains transformer and the rectifier circuit D10--D13. The control circuit comprises a transformer with primary winding L3, core K2 and secondary winding L4, and the current which passes from the second winding L2 of the mains transformer to the rectifier circuit passes through winding L3. The winding L4 is connected via a rectifier bridge with the diodes D6, D7, D8 and D9 with its plus side to the base of the transistor T1 and with its minus side to the emitter.

When an alternating voltage is applied to the mains terminals R and N, an initial current flows from R through C1, L1, D1, R1 and D5, after which the current branches via the base-emitter to T1 and R2 and then reunites via D4. Alternatively (during the other half period) the initial current flows from N through D3, R1 and D5 before it branches via T1 and R2, to reunite via D2 and flow through L1 and C1 to R. The initial current via the base-emitter in T1 is so adjusted that the circuit condition via the collector-emitter is high ohmic and does not permit of any damaging current surge.The maximum current in this condition (idling, for axample with the secondary circuit of the mains transformer open) through the primary circuit of the mains transformer will therefore be the sum of the transistor base-emitter current and its collector-emitter current, which later attains the value of the current amplification factor multiplied by the base-emitter current. The initial current, limited to a maximum value byT1 (idling current) through L1 induces via the core K1 a voltage in L2 whose instantaneous value exceeds the terminal voltage of the accumulator B by an appropriate amount.

When charging is to take place, in other words, when the secondary circuit of the mains transformer is closed, the voltage induced in the secondary winding L2 gives rise to a current which also passes through the winding L3 in the control circuits transformer, which conveys to the transistor T1 via the rectifier D6-D9 a control current to the base-emitter. This control current is so adjusted that it is sufficiently large to saturate the transistor T1 in the collector-emitter transision, which broadly speaking means that the safety circuit is short-circuited by the transistor and therefore the charging current is only limited by the instantaneous value of the mains voltage and the condenser C1.

In order to prevent overcharging of the accumulator and to permit trickle charging, a voltage relay Sv is connected across the accumulator B which gives an optical signal when the accumulator is fully charged. The voltage relay is optically connected to a photo-resistance Fm which together with a transistor T3 and a resistance R3 are included in the safety circuit, as can be seen from the wiring diagram.

When the accumulator B is fully charged and therefore the charging current must be greatly reduced, the voltage relay Sv gives an optical signal to the photo-resistance Fm, which in the illuminated state conveys current via the baseemitter in the transistor T3. The initial current (at the beginning of the half period) through the safety circuit no longer goes through the baseemitter in the transistorT1 but is instead diverted via the collector-emitter in the transistor T3. This means that the transistor T1 does not receive that control current that is required for the current in the secondary circuit of the mains transformer via the control circuit ST to generate the control current that is required to saturate the transistor T1, by which means the charging current is thus prevented from reaching its full value.The current now goes from R via C1, L1, D1, after which it branches both via R1, collector-emitter in T3 and via Fm, base-emitter in T3 and parallel with this via R3, after which the branch currents are reunited via D4 to N. Alternatively (during the second half period) the current flows from N to D3 where in branches both via Fm, base-emitter in T3 and R3 and via R1 and the collector-emitter in T3, after which the branch currents are reunited via D2 and flow through L1 and C1 to R.

By a suitable choice of the resistance R1 the current through the mains transformer is given such a value at fully charged accumulator (in principle with T1 disconnected) that the current through its secondary circuit, in other words the charging current, will be suitable for the trickle charging of the accumulator B. This current may however not be made so large that the voltage that is induced in the secondary winding L4 of the control circuits transformer exceeds the threshold value for the diodes D6-D9, since in such a case control current would be given to T1, which thereby was saturated and the charging current again attained full strength.

If the accumulator is discharged, for example via an emergency lighting plant during power failure, its terminal voltage will fall. This will also occur with time if it only receives trickle charging.

When this has happened to such an extent that it falls below a certain predetermined value of the voltage relay Sv, this gives an optical signal to the photo-resistance Fm which thereby changes to a non-conducting condition, whereby the blocking effect of the transistor T3 is broken so that the transistor T1 can again receive control current and thereby the charging current can again attain full strength until the accumulator is once more fully charged and the process is repeated.

In order to protect the charging device against damage caused by large voltage transients that can arise if the accumulator should (by mistake) be disconnected under full charging current, a condenser C2, a transistor T2 and a resistance R4 are arranged in the rectifier circuit as shown in the wiring diagram. The voltage impulse occurring when the accumulator is disconnected charges the condenser C2 via the base-emitter in the transistor T2, which thereby becomes more or less saturated in the collector-emitter transition so that the dangerous voltage transient is prevented.

The purpose of the resistances R2, R3 and R4 is to provide the bases in the corresponding transistors T1, T3 and T2 respectively with potential adequate for the attainment of the desired base-emitter current.

The invention can be modified within the scope of the following patent rights claim. Thus for example it would be possible to replace the transistor T1 with a relay, possibly connected with a resistance. It is also possible to arrange an optical or acoustical transmission of information about the magnitude of the charging current (through L2) as an alternative to the transformer L3, L4, K2. For example the winding L3 can be replaced by an incandescent lamp whose luminous efficiency reflects the current in the charging circuit, and then such a light signal can be sensed in the control circuit ST which will control this and thereby the state of the safety circuit.

Claims

1. A charging device for charging an electrical accumulator and comprising a principal transformer (11, L1, L2) whose secondary winding (L2) is connected via rectifying device (D10--D13) with the accumulator (B) characterised by the connection of a condenser (C1) for limitation of charging current in series with the primary winding (Li) of the principal transformer, when a safety circuit (SK) is also arranged in series with this which for the purpose of preventing current transients is transferable between a low ohmic and a high ohmic circuit condition, and by the connection of a control circuit (ST) in series with the secondary winding (L2) of the principal transformer which is so arranged that when a predetermined charging current through the secondary winding (L2) is exceeded it transfers the safety circuit to the low ohmic circuit condition.

2. A charging device as claimed in claim 1 characterised in that the safety circuit (SK) comprising a transistor (Ti) which in saturated condition via collector-emitter effectively shortcircuits the safety circuit in its low ohmic circuit condition and which via collector-emitter and base-emitter in non-saturated condition leads current through the safety circuit in its high ohmic circuit condition, when the control circuit is connected with the base and emitter of the transistor.

3. A charging device as claimed in claim 1 or 2 characterised in that the control circuit (ST) comprising a second transformer (L3, K2, L4) whose primary winding (L3) is connected in series with the secondary winding (L2) of the first transformer (L1, K1, L2) and the rectifier device (D10--D13), while its secondary winding (L4) is connected via a rectifier circuit (D6, (D6, D7, D8, D9) to the base and emitter of the transistor (to).

4. A charging device for charging an electrical accumulator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.