GB2075297A - Circuit for an electronic switch for high load currents, particularly for the light circuit of motor vehicles - Google Patents

Abstract

T1 and T2 form a bistable latch which can be set to a first state with T1 + T2 conducting when S is opened, and a second state with T1 and 12 non-conducting when S is closed, capacitive coupling via C1 turning T1 on and off, which turns T2 on and off. Protection against short-circuit of the load LST is provided via R2, as, if the voltage across LST is too low, T1 is held OFF turning T2 off. R6 provides current limiting on switch-on. The arrangement may be used for switching filament lamps in motor vehicles. <IMAGE>

Description

SPECIFICATION Circuit for an electronic switch for high load currents, particularly for the light circuit of motor vehicles The invention relates to a circuit for an electronic switch for high load currents, particularly for the light circuit of motor vehicles, comprising a switching transistor which is in series with the load to be switched and which is driven into the off-state or on-state by a driver stage after an on-off switch is actuated.

Known circuits of this type, for example for motor-actuated and lifting magnet control systems in motor vehicles and in industry, have recently been manufactured and marketed in the form of an integrated circuit in the "bilitic circuit arrangement". In this case components having an integrated "Darlington" stage are used for operations requiring over 10 A and up to 30 A. A driver stage is provided for driving the Darlington stage. In addition, components are provided to protect the integrated circuits against too high peak currents and short circuit loading. A disadvantage in this connection is the relatively complicated arrangement of circuits of this type, thereby rendering the integrated circuits relatively costly.

The object underlying the invention is to improve the known circuit to the extent that it can be manufactured more simply and more cheaply.

The invention achieves this object with the aid of the features of Claim 1.

The invention provides a circuit which can be constructed relatively simply, which withstands robust operation of motor vehicles and which is also protected against high peak currents and short-circuit loading.

Further embodiments of the invention are obtained from the subsidiary Claims.

The invention is described in more detail below with the aid of an exemplary embodiment shown in the drawing. In the drawing: Fig. 1 shows the circuit diagram according to the invention; Fig. 2 shows a part circuit diagram of a modified embodiment.

The circuit according to Fig. 1 shows a switch for high load currents, such as the switch which is necessary, for example, for controlling lights or lamps on motor vehicles. A Darlington transistor T, is used as an economical circuit element. By selcting a 15 A version, it is possible to achieve low saturation voltages (power loss) in the rated current range and adequate pulse strength in relation to the 'cold' current peaks which occur.

The Darlington transistor T2 of the p-n-p type is connected in series with the load L, for example one of a plurality of filament lamps of a motor vehicle.

Thisfilament lamp L is connected to the collector connection of the Darlington transistor and, on the other hand, to the negative supply line -Ubat of the vehicle battery. On the other hand, the emitter of the Darlington transistor T2 is connected to the positive supply line +Uba, of the vehicle battery via a resistor R.

The base electrode of the Darlington transistor T2 is connected to the positive supply line +Ubat of the vehicle battery via a resistor R5. R5 serves to divert the residual or cutoff current of the collector and base and to achieve short retention times. A Zener diode D2 is connected in parallel with the resistor Rs.

The base electrode of the Darlington transistorT2 is also connected to the collector of the driving transistor T1 of the n-p-n type via a feedback resistor R4.

The emitter of this driving transistor T1 is connected to the negative supply line Ubat of the vehicle battery. The base electrode of this driving transistorT, is connected to the negative supply line -Ubat of the vehicle battery via a resistor Ra and, on the other hand, is connected to the positive supply line +Ubat of the vehicle battery via a Zener diode D1 and a time function element, which is formed ofthe capacitor C, and the resistor Rut . R3 serves merely to divert the residual or cutoff current of the collector and base. A diode D3 can be connected in parallel with the capacitor C1.

A further feedback resistor R2 connects the collector of the Darlington transistor Ta to the connecting leads between the capacitor C1 and Zener diode D,.

A mechanical on-off switch S is connected in parallel with the series connection of C1, D1 and R3.

During normal operation the driving transistor T1 is driven into the conducting state when the switch S is opened via R1, C1 and D1. By means of the positive voltage which then drops at R4, the Darlington trans istor T2 is also driven into the conducting state so that a voltage drop occurs at the load Las a result of the current flowing through said load. This voltage drop keeps the driving transistorT1, via R2, in a conducting state until the switch S is closed again at a later time. When the switch S is closed, an off-state potential is transmitted via C1 or D3 to the base of T1 so that T, blocks whereby T2 is also driven into the off-state. This off-state is also kept stable by way of the feedback resistor R2 until a repeated 'on' pulse is transmitted by actuating the switch S.

As can be easily seen, the transistor arrangement T1, T2 combines with the feedback resistors R4, R2 to form a flip-flop circuit (either both transistors T" T2 are conducting or both transistors are blocked).

During normal operation this flip-flop circuit is controlled in its respective states by being dynamically driven by the switch S via C1.

The function of the diode Da in this case is to effect reliably the cutoff operation even when the Darlington transistor T2 has a long retention time and the capacitor C1 has a short time constant. Thus it is possible for C1 to be relatively small corresponding to a reliable turn-on characteristic, which has a positive effect in the case of the short-circuit characteristic yet to be described.

However, D3 can in principle be omitted. Then, corresponding to a reliable turn-off characteristic, C1 must be relatively large when T, has a long retention time, which can lead to prolonged short circuiting during short-circuit operation.

Because of the rough conditions when used on a motor vehicle, a circuit of this type must be short circuit proof. Moreover, it should be noted that the proposed use of the circuit for lamp load and the 'cold' current values obtained as a result constitutes an operation using dynamic partial short circuits.

The circuit meets the requriements embodied in the functions of load current limitation and short circuit protection.

The resistance of the spiral-wound filament of a lamp representing the load L in the cold state, when compared with that in the operating state, is approximately 1 :12 (for example, in the case of a 24 volt/21 watt lamp, Rcold is 2 Ohm; Rhot is 24 Ohm). When there is a surge of operating or circuit voltage applied to the lamp with a cold filament, which represents the load L, a buiid-up or rise of current from approximately 6 times the 'cold' value to the 'hot' value having a time constant of approximately 20 ms can be observed. If the Darlington transistor T2 were designed for the full high peak current, very costly and large components would be required.If it were designed only for the resistive 'hot' current, inadmissibly high peak currents and current densities would result in the switching transistor when the cold lamps representing the load were switched on, which would lead to destruction.

Forthis reason it is economically and technically ideal to use a switching transistor T2 which tolerates approximately 3 times the 'hot' current value as the peak current. In the case of resistive 'hot' current, the switching transistor T2 then shows good saturation characteristics (low power loss). The current is limited to three times the 'hot' value by current limitation or regulation.

In the illustrated embodiment R6 and D2 serve to limit the current. In a normal case the 'hot' current at the low-impedance resistor R6 generates only a slight voltage drop so that UR6+UBE T2 remains below the steep-drop voltage of D2. R6 and D2 are selected in such a way that, when the peak current value is to be limited, the voltage drop at RG and thus UD2=UR6 UBE T2 increase to such an extent that De becomes conductive. D2 takes over the base current, which is supplied by R4, forT2 and thus limits its base potential and its collectorlemitter peak current.

The Zener diode D, serves as a short circuit protect tion for this circuit. Where the load L in itself or the output (collector of T2) to earth are short-circuited, the collector potential is applied to earth so that the threshold voltage of the Zener diode D, prevents a connection via R2. The stable state, which is normally possible without short circuiting and in which both transistors T, and T2 are driven into the conducting state, cannot remain stable. If short circuiting takes place after turn-on, T, is immediately blocked and thus T2 is also blocked. The latter are consequently switched into the stable currentless state. If a turn-on test is performed (opening of the switch) while there is a short circuit, T, is kept in a conductive state by R1, C, for the trigger time which is thus achieved.Consequently, T2 is also conductive and performs the current limiting operation predetermined by D2 and R6. As a result of the short circuit, the collector potential cannot, despite the currentflowing through D2, be raised to such an extent that the stable "on-state" is achieved by R2, D,. Afterthe short turn-on pulse is damped via R1, , (when the switch S is open), T, is blocked again. Consequently, T2 also blocks again.

The circuit is switched into the stable "off-state". The turn-on test via R1, C1, when a short circuit occurs, therefore leads, as a resultofthe current limitation, only to a short-term operation with the permitted peak current (large, but permitted power loss). A destructive increase in current beyond all limits, which occurs with short circuiting, is thus reliably avoided.

Obviously, it is possible when meeting require ments to form from the same basic circuit diagram complementary circuit (load on one side connected to positive, electronic circuitry to earth) by changing the polarities and the like.

It is also easily possible to use as a control switch S a make contact to positive. Such an embodiment is shown in Fig. 2. The switch S is in this case connected directly to the positive supply line +Ubat of the vehicle battery. C, and R1 which form the time function element are connected to the switch S. The Zener diode D1, which is connected to the base of the driving transistor T1 (no longer shown), is again connected in series with the time function element.

In addition, the switch S is connected to the negative supply line -Ubat Of the vehicle battery via a resistor R7.

In place of the previously described mechanical on-off switch S acting as a control switch, it is also possible to use for corresponding functions a com ponentemploying an optical effect or the Hall effect or some other component.

Also, the dynamic on-off control system of the switch does not necessarily have to comprise the RC-network F1, C,. In an integrated arrangement, a preferred dynamic driving or triggering operation could be performed by means of transit-time elements.

In the case of an embodiment tested in practice, the individual components of the circuit had the following values: T, type bc 547 T2 type bdw 84 a L lamp 6 x 21W124V R, 7.5 kohm R2 5.6 kohm R3 10 kohm R4 560 ohm F5 2 kohm R6 70 milliohm F7 3kohm C1 2.2 nanofarad D1 bzy 87/1 V 8 D2 bzy8712V8 D3 1 n 4148 Ub,ltt 24 volts

Claims (11)

1. CLAIMS 1. Circuit for an electronic sw; h for high load currents, particularlyforthe light circuit of motor vehicles, comprising a switching transistor which is in series with the load to be switched and which is driven into the off-state or on-state by a driver stage after an on-off switch is actuated, characterised in that the switching transistor (T2) and the driving transistor (T,), which have feedback resistors (R2, R4) connecting each collector to each base, are arranged to form a flip-flop circuit, that a capacitor (C1) of a time function element is connected to the drive circuit or trigger circuit of the base of the driving transistor, and that the said capacitor can be connected via the on-off switch (S) to a potential which drives the driving transistor and thus also the switching transistor into the conducting or on-state or into the blocking or off-state.
2. 2. Circuit according to Claim 1, characterised in that a Darlington transistor is used as the switching transistor (T2).
3. 3. Circuit according to Claim 1 and/or 2, characterised in that approximately 3 times the value of the 'hot' current through the load (L) is selected as the peak current value for the Darlington transistor (T2), that a current limiting dropping resistor (R,) is inserted between the emitter of the Darlington transistor and the positive supply voltage (+), and that the base of the Darlington transistor is connected to the positive supply voltage via a voltage stabilizer diode (D2).
4. 4. Circuit according to Claim 1, characterised in that the base of the driving transistor (T,) is connected to the positive supply voltage via a voltage stabilizer diode (D, ) and the time function element (F1, C1) which is in series connection with the said diode.
5. 5. Circuit according to Claim 4, characterised in that a resistance (R3) is applied to the negative supply voltage by the base of the driving transistor (T1).
6. 6. Circuit according to Claim 1, characterised in that the capacitor (C1) of the time function element is connected, on the one hand, to the base of the driving transistor (T,) via the voltage stabilizer diode (D,) and, on the other hand, to the negative supply voltage via the mechanical on-off switch (S).
7. 7. Circuit according to any of the preceding claims, characterised in that the capacitor (C1), is connected to the base of the driving transistor (T,) via a voltage stabilizer diode (D,), and is connectd to the positive supply voltage via a series connection with a resistor (R1) and via the mechanical on-off switch (S) and is connected to the negative supply voltage via a resistor.
8. 8. Circuit according to Claim 7, characterised in that a diode (D3) is connected in parallel with the capacitor (C1) in the drive circuit of the driving transistor (T,).
9. 9. Circuit according to Claim 1, characterised in that the mechanical on-off switch (S) is replaced by a component employing an optical effect or the Hall effect or by some other component.
10. 10. Circuit according to Claim 4, characterised in that the RC-network (F1, C, ) is replaced by transittime elements.
11. 11. A switching circuit for the motor vehicle lights substantially as described herein with reference to Figure 1 or Figure 2 of the accompanying drawings.