The invention relates to a drive circuit for driving a load connected to a power supply by means of said drive circuit, whereby both said power supply and said load comprise a first and a second terminal and whereby the first terminals of said power supply and said load are interconnected, which drive circuit comprises an electronic first switch comprising a power input terminal, which is connected to the second terminal of said power supply, a control input terminal and a drive output terminal connected to the second terminal of said load, whereby a second switch is connected in a series circuit with said load, and which furthermore comprises a processing circuit provided with an input terminal and with an output terminal connected to the control input terminal of said first switch for generating a control signal on its output terminal in dependence on said second switch being operated, which is detected by said processing circuit on its input terminal.

A drive circuit of this type may be considered to be known from US Patent No. 5,030,890.

The known drive circuit is connected to the electricity grid, and the load may be a heating, lighting or motor load. The first switch is a triac, and said triac is controlled by means of phase shift control. The second switch is an on/off switch, which is present at some distance from the load, a gas-discharge lamp, for example. The processing circuit senses the voltage across the triac and delivers a triggering signal for the triac on the basis thereof. More in particular, the processing circuit detects a power interruption caused by said second switch being operated. In response to a temporary interruption of the alternating current being applied a change in the power intensity or in the delivery of power to the load is effected for a predetermined length of time. Initially the second switch is closed and a power interruption is effected by quickly opening and closing the second switch. Various ways of driving the load may be selected by opening and closing the second switch briefly a number of times in succession. More in particular the power interruption is used to achieve full intensity lighting or more or less dimmed lighting by means of the gas-discharge lamp.

The object of the invention is to provide a drive circuit for driving direct-current loads, in particular for use in cars,

whereby a single conductor may be used both for driving the load and for signalling to the processing circuit the manner in which the load is to be driven.

In order to accomplish that objective the invention provides a drive circuit of the kind referred to in the introduction, which is characterized in that said power supply is a direct-current power supply, that said second switch is an automatically resetting normally- closed switch, which is connected between the drive output terminal of said first switch and the second terminal of the load, and that said first switch is provided with a current sensing input terminal connected to the drive output terminal thereof and with a detection output terminal connected to the input terminal of said processing circuit for generating a detection signal which indicates, both in the case of a driven load and in the case of a non-driven load, whether or not current is flowing through the series circuit of said second switch and said load.

The first switch is an electronic semiconductor device, such as a MOSFET, or a combination of devices of this kind.

The second switch may be an interrupter push-button, whereby the term interrupter push-button used herein is understood to mean a switch which makes a (conducting) connection between the two switch terminals during the idle state. When the interrupter push-button switch is depressed by hand, the connection is broken. The (conducting) connection is (automatically) restored again (by means of a spring, for example) when the interrupter push-button is released. When the interrupter push-button is operated, the electronic switch will detect a circuit interruption and not a power interruption, as is the case with the aforesaid US patent. This interruption is recognized as a switching action by the processing circuit of the drive circuit, as a result of which the load can be switched on or off, depending on the previous state. Said second switch may also be or comprise a touch-sensitive switch.

Furthermore it is possible for the drive circuit to effect a different switching action, dependent on the duration of the interruption. For example, in the case of a short interruption of, let us say, less than 1 second, the load may be switched on briefly, and in the case of a long interruption the load may be switched on for a longer period of time, or vice versa.

The processing circuit of the drive circuit may furthermore be set such that in the case of a brief interruption the load is switched off slowly and that another brief interruption will cause the load to switch on slowly, whilst the load is switched on or off directly in the case of a long interruption.

The advantage of the drive circuit according to the invention is that only one electric conductor (between the first switch and the second switch) is required both for driving and for detecting, which results in savings of cost as well as weight, which is of great importance, for example in the case of cars. In addition this leads to an enhanced reliability. Furthermore the load is fully driven by the electronics of the drive circuit according to the invention. Finally an automatic diagnosis of a load forming an open circuit is obtained. The fact is that when the interruption lasts longer than 25 seconds, for example, it may be concluded that the load forms an open circuit.

The power supply may be a car battery.

The load may be a car accessory, whereby the term car accessory is to be given a broad interpretation and may be of the lamp, heater, resistor, coil and motor type. When a motor is concerned, operation of the interrupter push-button might cause the processing device to reverse the direction of rotation of the motor.

Another aspect of the present invention is the fact that when several series circuits of first switch, second switch and load are used, there is a possibility of the processing circuit driving the load in another series circuit based on the operation of a second switch in a series circuit.

The invention will now be explained in more detail with reference to the drawings, in which:

Figure 1 is a combined block and circuit diagram illustrating a possible embodiment of the invention; and

Figure 2 shows a simplified extension possibility of the embodiment of Figure 1.

More in particular, Figure 1 shows an embodiment of a drive circuit for driving a load 2, in this case a lamp, for example, which is connected to a direct-current power supply 1 by means of said drive circuit, whereby power supply 1 and load 2 each have a first terminal

11 and 21 respectively and a second terminal 12 and 22 respectively, and

whereby said first terminals 11, 21 of said power supply 1 and said load 2 are interconnected. The drive circuit shown in Figure 1 comprises an electronic first switch 3 having a power input terminal 31 connected to the second terminal 12 of power supply 1, a control input terminal 32, a drive output terminal 33, a current sensing input terminal 34 connected to said drive output terminal 33, and a detection output terminal 35. Furthermore a second switch 4 is provided in the shape of an automatically resetting normally-closed switch connected between the drive output terminal 33 of first switch 3 and the second terminal 22 of load 2. Finally a processing circuit 5 is provided, which comprises an input terminal 51 connected to the detection output terminal 35 of first switch 3 for generating a detection signal which indicates, both in the case of a driven load 2 and in the case of a non-driven load 2, whether or not current is flowing through the series circuit of second switch 4 and load 2, and which also comprises an output terminal 52 connected to the control input terminal 32 of first switch 3 for generating a control signal in dependence on the detection on input terminal 51 of operation of second switch 4.

Opening switch 4 by hand causes an interruption of the circuit between the two supply terminals 11 and 12. This results in a voltage change on detection output terminal 35 (status check or current output) of electronic switch 3. The electronic switch 3 is for example provided with a status output terminal, in the case of a so-called smart power MOSFET, or with a current-sensing output terminal, in the case of a so-called current-sensing MOSFET. Also other types of transistors or suitable electronic components may be used for that matter. The MOSFET may for example be selected from the Siemens ProFET (registered trademark) or TempFET (registered trademark) series, such as type BTS412. The voltage change is detected as a switching action by the drive circuit, that is, a change from a disconnected load 2 to a connected load 2, or vice versa. Furthermore it is possible for the drive circuit to effect a different switching action, depending on the duration of the interruption. For example, in the case of a brief interruption of less than 1 second, for example, load 2 may be switched on briefly, and in the case of a longer interruption said load may be switched on for a long time, or vice versa. The drive circuit may also be set such that a brief interruption causes load 2 to be switched off slowly and another brief interruption causes said load to be switched on slowly, whilst a longer

interruption causes load 2 to be switched on or off directly.

Furthermore it is possible with the drive circuit according to the invention to have the processing circuit to ^" overrule" a manual switching ^" request ^" . Furthermore it is possible to detect by means of said voltage or current change that the load 2 in question forms an open circuit, which is concluded from the fact that the interruption lasts longer than a predetermined period of time of for example 25 seconds. This makes it possible to detect the open circuit condition both when load 2 is on and when load 2 should be on. This may be indicated on the signal output terminal 53 of the processing circuit, whereby a signal on signal output terminal 53 may be used for visible or audible signalling.

The processing circuit 5 may also be arranged in such a manner that: The load is switched on or off, depending on the previous condition, when second switch 4 is operated;

It drives load 2 in dependence on the length of time during which the second switch is operated, for example when second switch 4 is operated briefly, it switches load 2 on briefly, and when switch 4 is operated for a long time, it switches load 2 on for a long time, or vice versa, or it switches load 2 on or off slowly when second switch 4 is operated briefly, and it switches load 2 on or off directly when said switch is operated for a long time; and When it detects that no current is flowing through the series circuit of second switch 4 and load 2 for a period of time longer than a predetermined period, it signals an open circuit condition of load 2 on signal output terminal 53.

In the case of a lamp, for example, an open circuit condition indicates that the lamp is defective. The lamp, being load 2, and second switch 4 may be jointly housed in a lamp holder.

Figure 2 shows an extension of Figure 1, whereby power supply 1 has been left out of Figure 1 for the sake of convenience.

Figure 2 shows two sub-circuits A and B, but a larger plurality of sub-circuits may be selected at will. Each sub-circuit A and

B comprises a series circuit of an electronic first switch 3, a second switch 4 and a load 2. The control input terminal 32 and the detection

output terminal 35 of first switch 3 of each sub-circuit A, B are the input terminal and the output terminal respectively of sub-circuit A, B. Processing circuit 5 is provided with a number of input terminals 51 and output terminals 52 connected to the output terminals 35 and the input terminals 32 respectively of a respective sub-circuit A, B. The processing circuit 5 is arranged in such a manner that processing circuit 5 will generate a control signal on its output terminal 52 connected to another sub-circuit A or B in dependence on the detection on input terminal 51 of processing circuit 5 of the operation of the second switch 4 of the sub-circuit B or A connected to said input terminal . This makes it possible to drive loads 2 independently of the switch 4 to which said loads are respectively connected in series. In the example of Figure 2 load 2 of sub-circuit A may be driven by second switch 4 of sub-circuit B, and load 2 of sub-circuit B may be driven by second switch 4 of sub-circuit A. It will be apparent that various minor modifications to a drive circuit according to the invention are possible without departing from the essence of the invention.

The essential aspect of the drive circuit according to the invention is the fact that processing circuit 5 can determine at all times whether the load 2 in question may/must be switched on or off.