The invention relates to a driving circuit system for driving at least two DC signal lamps.

In electronic interlockings signal lamps are normally controlled via a two-wire circuit. For safety reasons these wires have to be supervised whether there is a short circuit to earth or between two wires to avoid the unintended lightening of a signal lamp.

Within conventional AC based lamp circuits, supervision of interwire short circuits has to be done periodically by supervision staff. Checking by staff is very expensive.

Although DC based signal lamp circuits have not been commonly used in the past, there is an increasing interest for using LEDs as signal lamps within interlocking.

It is the object of the invention to further develop a system and a method for automatic and safe detection of earth faults and interwire short circuits within a driving circuit system for driving at least two DC signal lamps.

This object is achieved, in accordance with the invention, by a driving circuit system for driving at least two DC signal lamps, comprising:

• a first DC power supply having a first voltage for driving the signal lamps,
• a second DC power supply having a second voltage for driving only one of the signal lamps, the negative poles of the first and second power supplies being on different potentials,
• a central amperemeter provided between the two power supplies,
• and driving circuits for each signal lamp, each driving circuit comprising:
  • a first two-way change-over switch connected to the positive pole of either the first or second power supply,
  • a second two-way change-over switch connected to the negative pole of either the first or second power supply,
  • a third two-way change-over switch connecting one wire of the signal lamp to either the first or the second switch,
  • a fourth two-way change-over switch connecting the other wire of the signal lamp to either the first or second switch,
  • two local amperemeters for measuring the current in both wires of the signal lamp,
  • a local voltmeter for measuring the voltage applied to the signal lamp, and
  • a control unit for controlling the four switches and for detecting both earth faults within a driving circuit and interwire short circuits between two driving circuits, based on the measured currents and voltages.

In a second aspect the above object is achieved, in accordance with the invention, by a method for detecting earth faults and interwire short circuits in the driving circuit system described above, wherein in a normal operation mode of the driving circuits, in which a signal lamp is turned on by connecting it to the positive and negative poles of the first power supply via the switches, an earth fault of a driving circuit is detected due to a leckage current measured by the central amperemeter, an interwire short circuit between wires of two driving circuits is detected due to a difference of the currents measured by the locals amperemeters of one of the two driving circuits, and an interwire short circuit between wires of one driving circuit is detected due to a comparison between the measured current and the expected current computed by both the measured voltage and the resistance of the wires and of the signal lamp.

According to the invention, a driving circuit system is provided which supports automatic supervision and detection of earth faults and interwire short circuits within safety-critical systems using DC current for switching electrical devices.

Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as an exhaustive enumeration but rather have exemplary character for the description of the invention.

The invention is shown in the drawing, in which:

Fig. 1

shows schematically a driving circuit system for driving two DC signal lamps.

As shown in Fig. 1, the driving circuit system 1 for driving two DC signal lamps 2a, 2b comprises:

• a first DC power supply 3 having a first voltage U₁ for driving both signal lamps 2a, 2b, here formed as LEDs,
• a second DC power supply 4 having a second voltage U₂ for driving only one of the signal lamps 2a, 2b, the negative poles of the first and second power supplies 3, 4 being on different potentials, wherein the negative pole of the second power supply 4 is connected to earth 5 and the second voltage U₂ is less than the first voltage U₁,
• a central amperemeter 6 provided between the two power supplies 3, 4, for measuring a leckage current I_E between the two power supplies 3,4,
• a switch S_E for connecting the negative pole of the first power supply 3 to the negative pole 4 of the second power supply 4, i.e. to earth 5.
• driving circuits 10a, 10b for each signal lamp 2a, 2b, and
• a master control 7 controlling the driving circuits 10a, 10b.

First power supply 3 is powerful enough to supply all signal lamps 2a, 2b simultaneously. Second power supply 4 may to be less powerful because it has to supply one signal lamp only. Power supply U_E resulting from the different potentials can be very weak because it is used for earth fault detection only. The central amperemeter 6 is placed between the two power supplies 3,4 to check earth faults. Periodical testing of the central amperemeter 6 is done by closing the switch S_E. In other words, the driving circuit system 1 specifies three voltages: U₁ and U₂ for signal lamp supply and U_E for earth fault detection.

Each driving circuit 10a, 10b comprises:

• a first two-way change-over switch S₁ connected to either the positive pole of the first or second power supply 3, 4,
• a second two-way change-over switch S₂ connected to either the negative pole of the first or second power supply 3, 4,
• a third two-way change-over switch S₃ connecting one supply wire 11 of the signal lamp 2a, 2b to either the first or second switch S₁, S₂,
• a fourth two-way change-over switch S₄ connecting the other supply wire 11 of the signal lamp 2a, 2b to either the first or second switch S₁, S₂,
• a DC-DC converter or voltage regulator 12 connecting the first and third switches S₁,S₃, for generating a variable output voltage for the signal lamp 2a, 2b, - two local amperemeters 13 for measuring the current I_H, I_R in both wires 11 of the signal lamp 2a, 2b,
• a local voltmeter 14 for measuring the voltage U_A applied to the signal lamp 2a, 2b,
• further local voltmeters 15_1-15₄ for measuring the voltages V₁-V₄ between the connections of each of the four switches S₁-5₄,
• two diodes 16 connected in parallel to the signal lamp 2a, 2b, and
• a control unit 17 for controlling the four switches S₁-S₄ and for detecting both earth faults EF within a driving circuit 10a, 10b and interwire short circuits ISC between two driving circuits 10a, 10b, based on the measured currents and voltages.

Each signal lamp 2a, 2b is connected with the switches S₃, S₄ by a two- wire cable, wherein the resistance of the signal lamp 2a, 2b is represented by R_v and the resistance of one wire 11 is represented by R_L. The switches S₁-S₄ can be implemented by relays or any semiconductor switch. The correct working of the switches S₁-S₄ is checked by measuring the voltage V₁-V₄ between the connections of the switches S₁-S₄ and the occurrence of the expected current and is done by the control unit 17. All local amperemeters 13 and voltmeters 15₁-154 are provided with low-pass filters (not shown). In addition to the diodes 16, further protective components (not shown) can be provided to avoid distortion caused by inductive load switching.

Each control unit 17 of the driving circuit 10a, 10b is controlled by the master control 7 and can be realized for example by a microcontroller. Each control unit 17 supervises the voltages U_A ,V₁-V₄ and currents I_E, I_H, I_R, actuates the switches S₁-S₄ and defines the output voltage of the DC-DC converter 12. The master control 7 commands a control unit 17 to switch on or off the signal lamp 2a, 2b, determines an operation mode of a control unit 17 and organizes the process of measurement of earth faults EF and interwire short circuits ISC. Errors which have been detected by control units 17 are reported to the master control 7 immediately.

In safety critical systems the master control, the control unit, voltmeters, amperemeters have to be duplicated to accomplish the requirements of such a system (two channels supervising each other). In this case, switching on signal lamps 2a, 2b can only be executed if both channels give the command for switching on (logical multiplication).

The driving circuits 10a, 10b can be operated in a "normal operation mode" and a "test operation mode".

In the normal operation mode of the driving circuits 10a, 10b, a signal lamp 2a, 2b is turned on by connecting it to the positive and negative poles of the first power supply 3 via the switches S₁-S₄. If a signal lamp 2a, 2b has to be turned on switches S₁ and S₂ are in position 1, SH is in position 1 and S_R in position 2. If a signal lamp has to be turned off switches S_EH and S_ER are in position 1, switches S_H and SR in position 2.

If an earth fault EF occurs leckage current I_E is caused and can be detected by every control units 17. If signal lamp 2a, 2b is turned on an interwire short circuit ISC between wires 11 of different driving circuits 10a, 10b may be detected due to a difference of I_H and I_R. There is no guarantee of detection however because the potential difference between the two connection points of interwire short circuit ISC may be too small depending on the position within the wires 11. Therefore a special test is necessary, which is explained hereinbelow.

Interwire short circuits ISC between the two wires 11 of one driving circuits 10a, 10b can be deduced by measurement of U_A and knowledge of I_H and Rv and R_L. The expected value of I_H can be calculated. I_H = U_A/ (R_v + 2 R_L). A deviation is an evidence for the occurrence of a short circuit or a wire breaking. R_L of wire 11 may however alter depending on temperature influences R_L can be measured in the test operation mode which is explained hereinafter. RL has to be known for the detection.

At any time exactly one of the driving circuits 10a, 10b can be in that operation mode. The master control 7 regulates the permission of a driving circuit 10a, 10b to switch into the test operation mode. If a driving circuit 10a, 10b has finished the test master control 7 is informed accordingly.

If, for example, driving circuit 10a is in the test operation mode the switches S₁, S₂ move into position 2 and the driving circuit 10a is connected to second power supply 4. If its signal lamp 2a was turned on it still remains turned on except for the short moment of alternating the power supply.

Now the electrical potential of the driving circuit 10a is below that of driving circuit 10b, i.e. of all other driving circuits. Any possible interwire short circuits ISC between this unit 10a and the others will lead to a leckage current I_E. The potential difference at least amounts to voltage U_E. Additionally the leakage current I_E can be detected by calculating the difference of I_H and I_R if its signal lamp 2a is turned on. If signal lamp 2a is turned off, leakage current I_E leads to values of I_H and I_R unequal to zero.

Earth fault detection is not guaranteed at all in this operation mode. This operation mode takes a short time though. If signal lamp 2a is turned off a further test step happens: The output of DC-DC converter 12 is reduced and S₄ changes into position 1 so that the signal lamp 2a is provided with reduced inverted voltage. The diode 16 with a low voltage drop (for example: Shottky diode) is connected in parallel to the signal lamp 2a. Taking into account the voltage drop, R_L can be calculated by measurement of U_A and I_H. The knowledge of value of R_L has been known for the detection of an interwire short circuit within one driving circuit 10a (see normal operation mode).

At the end a further test step is carried out: Switch S_E is closed and I_E must arise, otherwise a fault of switch SE, of amperemeter IE or ground connection must be supposed.

At the end of the test operation mode of a driving circuit 10a all switches S₁-S₄ and the voltage of DC-DC converter 12 regain the original state as defined in the normal operation mode.