AU2008331420A1

AU2008331420A1 - An LED driver method for traffic lanterns

Info

Publication number: AU2008331420A1
Application number: AU2008331420A
Authority: AU
Inventors: Malcolm Young
Original assignee: ALDRIDGE TRAFFIC SYSTEMS
Current assignee: ALDRIDGE TRAFFIC SYSTEMS Pty Ltd
Priority date: 2007-12-03
Filing date: 2008-12-03
Publication date: 2009-06-11
Anticipated expiration: 2028-12-03
Also published as: WO2009070828A1; WO2009070828A9; CN101896951A; ZA201004263B; AU2008331420B2; NZ586130A; EP2229667A1

Description

WO 2009/070828 PCT/AU2008/001778 AN LED DRIVER METHOD FOR TRAFFIC LANTERNS TECHNICAL FIELD The present invention relates to a light emitting diode (LED) driver 5 method and apparatus for traffic lanterns. BACKGROUND ART In the use of light emitting diodes (LEDs) in traffic lanterns, it is useful to know what proportion of the LEDs are faulty and to signal to the traffic control equipment when more than a chosen proportion of the LEDS have failed. 10 DISCLOSURE OF INVENTION This invention addresses the need to control the current in the LEDs and to discover the failed proportion. According to the invention, there is provided an LED driver for a traffic lantern comprising a current mirror adapted to control the current through an 15 LED of the lantern, a fault detector arrangement connected to an output of the current mirror and the LED and adapted to indicate a fault condition if the voltage measured is outside a predetermined range, and a voltage sensor arrangement adapted to provide an output voltage proportional to the average voltage of other LED drivers for the lantern which are not connected to faulty 20 LEDs. Preferably, the fault detector monitors the output voltage of the current mirror and produces first and second outputs, the first output being fed to a fault circuit in which the first output is combined with the first outputs from a plurality of other LED drivers and then compared, and the second output being 25 fed to the voltage sensor arrangement, and wherein the voltage sensor arrangement is also fed an output voltage of the LED. It is preferred that the output voltage of the voltage sensor arrangement is equal to the LED output voltage when a fault is absent and is equal to an open circuit when a fault is present.

WO 2009/070828 PCT/AU2008/001778 2 In a preferred form, the output voltage of the voltage sensor arrangement is fed to a voltage controller comprising a combining circuit and thence an LED power supply. BRIEF DESCRIPTION OF THE DRAWING 5 In order that the invention may be more readily understood and put into practical effect, reference will now be made to the following block schematic diagram (Fig. 1) of an LED driver according to a preferred embodiment of the invention. MODES FOR CARRYING OUT THE INVENTION 10 In general terms, the LED driver consists of a current mirror to control the current through the LED, a fault detector arrangement and a driver voltage sensor arrangement. The current mirror can usefully have a current ratio of about 20, meaning that a current into the control terminal of say ImA will result in a current in the 15 output of 20mA. The fault detector arrangement is connected to the output of the current mirror and the driven LED or LEDs and indicates a fault condition if the voltage measured is too low (indicating an open circuit LED) or too high (indicating a short circuit LED). 20 The voltage sensor arrangement provides an output voltage (or current) proportional to the average voltage of those LED drivers which are not connected to faulty LEDs. In excluding the LEDs with faults, the output of the fault detectors may be used. In operation with other subsystems, the present invention may be 25 applied as follows. The driver voltage sensor arrangement is used to adjust or control the voltage applied to the LEDs so that enough voltage is present to operate the LEDs and the other systems but no excess voltage is used as this would cause excessive power loss in the current mirrors. The fault detector WO 2009/070828 PCT/AU2008/001778 3 arrangement is used to either report the proportion of faulty LEDs via some communications devices or to disconnect the traffic lantern, thus causing a measurable change in consumed power. The LED driver apparatus and method are now described in greater 5 detail by reference to Fig.1. The LED current control input at 12 is fed to current mirror 14, the output 16 of which feeds power to LED 18. The fault detector 20 monitors the output voltage as described above and produces two outputs. The output at 22 is fed to a fault circuit comprising a combining circuit and thence a fault level comparator (not shown). The combining circuit 10 combines the outputs from a plurality of similar LED drivers. It is to be understood that Fig. 1 shows only one of many similar LED drivers that are used together. The fault detector output at 24 is fed to voltage sensor (or monitor) 26. The voltage sensor 26 also monitors the LED voltage via connection 28. The voltage sensor output at 30 is a simple function of the LED 15 voltage while the fault detector 20 indicates correct operation of the LED and is some other useful value at other times. Typically, the output of the voltage sensor 26 could be equal to the LED voltage when a fault is absent and an open circuit when a fault is present. This allows the output of the voltage sensor 26 to be indicative of only the correctly functioning LEDs. The output 30 20 is fed to a voltage controller comprising a combining circuit and thence the LED power supply (not shown). The LED power supply uses the combined voltage sensors to set its own voltage to work efficiently. Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention.

Claims

1. An LED driver for a traffic lantern comprising a current mirror adapted to control the current through an LED of the lantern, a fault detector arrangement connected to an output of the current mirror and the LED and adapted to indicate a fault condition if the voltage measured is outside a predetermined range, and a voltage sensor arrangement adapted to provide an output voltage proportional to the average voltage of other LED drivers for the lantern which are not connected to faulty LEDs.

2. The LED driver of claim 1 wherein the fault detector monitors the output voltage of the current mirror and produces first and second outputs, the first output being fed to a fault circuit in which the first output is combined with the first outputs from a plurality of other LED drivers and then compared, and the second output being fed to the voltage sensor arrangement, and wherein the voltage sensor arrangement is also fed an output voltage of the LED.

3. The LED driver of claim 2 wherein the output voltage of the voltage sensor arrangement is equal to the LED output voltage when a fault is absent and is equal to an open circuit when a fault is present.

4. The LED driver of claim 3 wherein the output voltage of the voltage sensor arrangement is fed to a voltage controller comprising a combining circuit and thence an LED power supply.

5. The LED driver of claim 1 wherein the current mirror has a current ratio of about 20.