Detection of a hazard condition of a load

Abstract

An electrical circuit is described for detection of an electrical hazard condition of a load 20, in particular of an OLED lighting element comprising driving terminals A, C. An electrical hazard condition, such as an overvoltage or short circuit is to be detected between terminals 22a, 22b of the circuit. A disabling element 24 is connected to one of the terminals 22a, 22b to disable the load. A monitoring circuit is connected to monitor a voltage V or current magnitude at at least one of the terminals 22a, 22b. The monitoring circuit comprises a maximum or minimum value detector 26 to deliver a maximum or minimum value V.sub.max of the voltage or current magnitude over time. The monitoring circuit is disposed to monitor the maximum or minimum value V.sub.max to detect the electrical hazard condition. A monitoring circuit is connected to activate the disabling element 24 if an electrical hazard condition is detected. The electrical circuit, a lighting arrangement including an LED or OLED lighting element and the electrical circuit, and a detection method allows to operate a load with different types of power supply, in particular also by PWM.

Claims

1. An electrical circuit for detection of a short circuit or overvoltage or both condition(s) of a load, comprising: a power source delivering a gapped electrical power supply which is a regulated current that is time-modulated such that the electrical power is supplied intermittently in a first time interval of constant current amplitude whereas no electrical current is supplied in a second time interval following the first interval, terminals for said load between which the short circuit or overvoltage or both condition(s) are to be detected, a disabling element connected to at least one of said terminals to disable said load, a monitoring circuit connected to monitor a voltage or current magnitude at least one of said terminals to detect the short circuit or overvoltage or both condition(s), said monitoring circuit comprising a maximum or minimum value detector to deliver a maximum or minimum value of said voltage or current magnitude over a time at least longer than the second time interval in the power supply, such that said monitoring circuit is disposed to monitor said maximum or minimum value over the time to detect the short circuit or overvoltage or both condition(s), and wherein said monitoring circuit is connected to activate said disabling element when the short circuit or overvoltage or both condition(s) are detected.

2. The electrical circuit according to claim 1, where said disabling element is a controllable switching element connected between said terminals, and said monitoring circuit is connected to close said switching element if the short circuit or overvoltage or both condition(s) are detected.

3. The electrical circuit according to claim 1, where said monitoring circuit further comprises a delay timer circuit for providing an activation signal when a detection signal is applied to said delay timer circuit for a delay time interval, and said delay timer circuit is connected to supply said activation signal to said disabling element.

4. The electrical circuit according to claim 1, where at least one electrical energy storage element is connected to at least one of said terminals to receive and store electrical energy, and where at least said monitoring circuit is connected to be supplied with electrical energy from said electrical energy storage element when no electrical energy is supplied at said terminals.

5. The electrical circuit according to claim 1, where said monitoring circuit comprises at least a comparator circuit to compare said maximum or minimum value of said voltage or current magnitude over time at least longer than the second time interval to a threshold value.

6. The electrical circuit according to claim 5, where said monitoring circuit comprises at least a first comparator circuit to compare said maximum or minimum value of said voltage or current magnitude over time at least longer than the second time interval to a lower threshold value and a second comparator circuit to compare said maximum or minimum value of said voltage or current magnitude over time at least longer than the second time interval to an upper threshold value.

7. A lighting arrangement including at least one LED or OLED lighting element comprising driving terminals and an electrical circuit according to claim 1 connected in parallel to said driving terminals.

8. The electrical circuit of claim 1 where the time modulated current of the gapped electrical power supply from the power source is modulated by pulse width modulation (PWM).

9. The electrical circuit of claim 1 where the load is an OLED device.

10. A method of detection of a short circuit or overvoltage or both condition(s) between terminals of a load in an electrical circuit, comprising the steps of: providing a power source delivering a gapped electrical power supply which is a regulated current that is time-modulated such that the electrical power is supplied intermittently in a first time interval of constant current amplitude whereas no electrical current is supplied in a second time interval following the first intervals, monitoring a voltage or current magnitude at least one of said terminals and controlling a disabling element connected to at least one of said terminals to disable said load in case of the short circuit or overvoltage or both condition(s), wherein said monitoring step includes detecting a maximum or minimum value of said voltage or current magnitude over a time at least longer than the second interval in the power supply, and monitoring said maximum or minimum value to detect the short circuit or overvoltage or both condition(s) over the time, and activating said disabling element when the short circuit or overvoltage or both condition(s) are detected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter.

(2) In the Figures

(3) FIG. 1 show a schematic diagram of a lighting arrangement,

(4) FIG. 2 shows a schematic diagram of an electrical hazard protection device within the lighting arrangement of FIG. 1,

(5) FIG. 3 shows an embodiment of circuit diagram for the protection device of FIG. 2.

(6) FIG. 4 is a current vs time graph illustrating a gapped electrical power supply which is a regulated current (34) that is time-modulated such that the electrical power is supplied intermittently in a first time interval (36) of constant current amplitude whereas no electrical current is supplied in a second time interval (38) following the first interval.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) FIG. 1 shows in a schematical representation a lighting arrangement 10 comprised of a power supply 12 and, connected thereto, three OLED lighting assemblies 14.

(8) The power supply 12 comprises a mains connector 16 for connection to mains electrical power and a current modulator 13 and delivers at power supply terminals 18 electrical power with voltage and current values adapted to operate the OLED assemblies. Since in the present example the OLED assemblies 14 are intended for lighting purposes, a certain electrical power is necessary to obtain sufficient luminous flux. Generally, it is preferred to use OLED elements 20 with a nominal electrical power of at least 1 W, further preferred at least 3 W. For example, one type of OLED assembly may be operated at nominal values of 14V and 500 mA, thus has a nominal power of 7 W.

(9) The electrical power supply 12 may be any type of power supply delivering a regulated voltage or current at the terminals 18 via the current modulator 13. Preferably, power supply 12 is one out of a plurality of known power supply circuits to deliver a regulated constant-current supply to the OLED assemblies 14. As will become apparent, electrical power and therefore luminous flux may be controlled by modulating the current I supplied to the OLED assemblies 14 over time, in particular by AM (amplitude modulation) or PWM (pulse width modulation) control.

(10) The OLED assemblies 14 each comprise an OLED lighting element 20, i.e. a flat surface with an OLED covering electrically connected to form (between an anode connection A and cathode connection C) an organic light emitting diode surface. Since OLED technology is generally known to the skilled person, and the invention is not limited to a specific type thereof, further details will not be explained here

(11) The OLED lighting assemblies are, as shown in FIG. 1, connected in series to the terminals 18 of power supply 12. In order to operate each OLED assembly 14, the power supply 12 supplies a regulated current I, which may e.g. have a value of 500 mA or 300 mA and will be supplied to all three OLED assemblies 14 due to the series connection.

(12) In order to control operation of the OLED assemblies 14, the regulated current I may be delivered time-modulated, and in particular with pulse-width modulation (PWM), i.e. such that electrical power is supplied intermittently in first intervals of constant current amplitude (pulses), preferably at the nominal current of the OLED element, whereas no electrical current is supplied in second time intervals following the first intervals. In operation, control of the luminous flux by each OLED assembly 14 may be effected by appropriately choosing the duty cycle, i.e. the duration of the first intervals (“on”) relative to the duration of the second intervals (“off”).

(13) Within the surfaces of the OLED element 20 of each OLED assembly 14, it is possible that certain defects may lead to electrical hazard conditions. For example, local defects within the surface area may cause a local drop of resistance, i.e. a short circuit condition. In consequence, during operation an increased amount of heat is generated at the position of the defect, which may lead to strong overheating and could potentially become a fire hazard. Likewise, if the surface area deteriorates over the lifetime of the OLED element 20 or due to mechanical effects, such that the resistance increases, operation with a regulated current I may lead to a strongly increased driving voltage applied, causing overvoltage as a further potential electrical hazard situation.

(14) In order to supervise the operation of the OLED element 20 and to prevent hazards such as e.g. overvoltage or overheating in case of short circuit conditions occurring, each OLED assembly 14 comprises an electrical hazard protection circuit SCP connected in parallel to the OLED element 20.

(15) FIG. 2 shows in a schematic representation elements of a short circuit protection circuit SCP. Each SCP comprises terminals 22a, 22b by which the SCP is connected in parallel to an OLED element 20 (FIG. 1), and between which terminals 22a, 22b an electrical hazard condition should be detected.

(16) Within each SCP, a disabling switch 24 is provided as a disabling element, which is connected between the terminals 22a, 22b such that if the disabling switch 24 is closed the terminals 22 are directly connected to each other. In this case, the connected OLED element 20 is bridged, such that a current delivered at the terminals 18 of the power supply 12 will flow mainly through the terminals 22a, 22b and the disabling switch 24, bypassing the OLED element 20.

(17) In normal operation the disabling switch 24 is opened such that the current I delivered passes through the OLED element 20 and operates it to emit light.

(18) Within each SCP, the first terminal 22a is connected to a maximum voltage value detector, or peak detector 26. This element accepts as input the time-varying value of the voltage V at the first terminal 22a and delivers as output a maximum value V.sub.max thereof. The maximum voltage value detector 26 may thereby operate with a certain time constant, i.e. the delivered maximum voltage value V.sub.max may then correspond to the maximum value delivered within a chosen time interval of e.g. 300 ms.

(19) The maximum voltage value V.sub.max is processed by a threshold comparator 28, comparing it to a pre-stored voltage threshold value V.sub.th. The threshold value V.sub.th may be an an upper threshold to detect an overvoltage condition. In a case where the delivered maximum voltage value V.sub.max is found to be above the upper threshold V.sub.th, a detection signal D is delivered to a delay circuit 30 to signal an electrical hazard condition. Alternatively, the threshold value V.sub.th may be a lower threshold, such that a detection signal D signaling a short circuit condition is delivered in case V.sub.max drops below V.sub.th.

(20) During normal operation, if no electrical hazard condition is detected, the detection signal D is inactive.

(21) Delay circuit 30 operates with a certain pre-defined time constant of e.g. 0.5 to 3 s and delivers a control signal S if the detection signal D is supplied for this pre-chosen length of time. As long as the detection signal is inactive or has only been active for less than the time constant, no control signal S is applied. This serves to avoid false alarms, in particular during a run-up phase of current control.

(22) The control signal S operates the disabling switch 24, i.e. if the control signal S is present because the detection signal D has been applied to delay circuit 30 for the specified length of time, the disabling switch 24 is closed.

(23) In operation of each of the OLED assemblies 14, the SCP thus operates as follows: In normal operation, the disabling switch 24 is opened. The current I flows through the OLED elements 20. Consequently, the voltage at the first terminal 22a of each SCP will be at an expected nominal value. Thus, comparator circuit 28 will deliver a value V.sub.max within a normal voltage window, i.e. below an upper threshold V.sub.th1 and above a lower threshold V.sub.th2 and no detection signal D will be applied, such that disabling switch 24 will remain open.

(24) In case of a short circuit condition occurring at an OLED element 20, the voltage at the first terminal 22a will drop, such that (after the time interval associated with the maximum voltage value detector 26) correspondingly also the maximum voltage value V.sub.max delivered by the maximum voltage detector circuit 26 will decrease. Eventually comparator circuit 28 will find V.sub.max<V.sub.th2 and deliver the detection signal D. After a period of time pre-set in delay circuit 30, this will lead to activation of the control signal S, thereby closing the disabling element 24 and thus bypassing the defective OLED element 20. Likewise, in case of an increase of resistance in the OLED element 20, the voltage V and thus the maximum voltage value V.sub.max will increase, until at V.sub.max>V.sub.th1 the detection signal D for an overvoltage condition will be activated, leading—after the delay time—to activation of the disabling element 24.

(25) This behavior of each SCP is independent of the mode in which electrical power is supplied, i.e. operation is the same, regardless whether the electrical current delivered may be constant, amplitude modulated or pulse-width modulated. In each case, maximum voltage detection circuit 26 will deliver the maximum voltage value V.sub.max. For example, in a case of regular operation at constant current (and consequently voltage), the maximum voltage detector 26 will also deliver a constant value V.sub.max. Likewise, in operation in PWM mode, maximum voltage detection circuit 26 will continue to deliver the constant value V.sub.max even during the off-periods, as long as the time constant of the peak detector circuit 26 is longer than the “off”-period of the PWM voltage.

(26) In cases of a short circuit condition, the voltage value at the first terminal 22a will strongly decrease, whereas it will increase in an overvoltage condition, regardless of the mode of supply of electrical power. Thus, each OLED assembly 14 equipped with an SCP as described is secured against electrical hazard conditions occurring within the OLED element 20, regardless of how electrical power is supplied. The OLED assemblies 14 may thus be safely operated by different kinds of power supplies 12.

(27) FIG. 3 shows a circuit diagram of an embodiment of an SCP circuit, including terminals 22a and 22b. An energy storage circuit 32 is connected to the first terminal 22a, which includes a capacitor bank 34. In PWM operation, capacitor bank 34 is charged during each pulse and delivers an operating voltage V.sub.O for the active components of the SCP circuit.

(28) Further, the SCP circuit comprises a maximum voltage detector 26 (peak voltage detector circuit) connected to the first terminal 22a. The voltage V delivered at the first terminal 22a is stabilized at a first capacitor C1 and delivered to a first operational amplifier OP1, which is coupled back over a Zener diode D1. The output corresponds to a maximum voltage V.sub.max, stabilized over a second capacitor C2. As the skilled person will notice, within the peak voltage detector circuit 26 the voltage V is not directly applied to the operational amplifier OP1, but only a certain proportion thereof given by a voltage divider, because the operating input window of OP1 is limited by the supply voltage. Further, the skilled person understands that the peak detector circuit will operate within a time window defined by the values of C.sub.1 and C.sub.2, i.e. if the input voltage V drops below a previous value V.sub.max for longer than the time window, the delivered peak value V.sub.max will also decrease.

(29) The delivered maximum voltage over time V.sub.max is delivered to a comparator circuit 28 comprised of a first comparator circuit 28a for an upper threshold and second comparator circuit 28b for a second, lower threshold. Each of the comparator circuits 28a, 28b are built substantially identically, except for connection of the respective comparator. In the first comparator circuit 28a, the maximum voltage V.sub.max is delivered to the positive input of an operational amplifier OP2 used as a comparator, with the operating voltage V.sub.O delivered over a voltage divider R1/R2 to the negative input of the comparator OP2 as upper threshold V.sub.th1. Likewise, in the second comparator circuit 28b, the maximum voltage V.sub.max is delivered to the negative input of an operational amplifier OP3, and the operating voltage V0 is delivered over a voltage divider R3/R4 as a lower threshold value V.sub.th2 to the negative input of the comparator OP3. By the value of R1/R2, the first, upper threshold V.sub.th1 may be set in the upper threshold comparator circuit 28a, whereas by choosing the corresponding values of resistors R3/R4 in the second, lower threshold comparator circuit 28b the lower threshold value V.sub.th2 may be chosen.

(30) In each comparator circuit 28a, 28b, the output of the comparators OP2, OP3 is delivered as a detection signal D if V.sub.max is outside of the normal operation window, i.e. above V.sub.th1 or below V.sub.th2.

(31) The detection signal D does not instantaneously lead to activation of a disabling element, but is first processed by a delay circuit 30 comprised of individual delay circuits 30a, 30b. Each delay circuit 30a, 30b is an R/C circuit comprised of an adjustable resistor R5 (R6) and capacitor C3 (C4).

(32) Thus, the delay/driver circuit 30 accepts as input the detection signals D from both comparator circuits 28a, 28b, indicating that the delivered maximum voltage value V.sub.max is outside of the normal operation window defined by the upper and lower threshold voltages V.sub.th1, V.sub.th2. If the detection signal D is present for the amount of time specified by the R/C-circuits 30a, 30b, which may be chosen by the resistance values R5, R6 and the capacitance values C3, C4, a control signal S is generated and supplied to a driver circuit 32.

(33) In case of a presence of a control signal S, a further operational amplifier OP4 of a driver circuit 32 drives over an RC circuit comprised of a capacitor C5 and a resistor R7 a thyristor 24 working as the disabling switch, and being connected between the first and second terminals 22a, 22b.

(34) Thus, if a peak V.sub.max of the voltage V, detected by the peak voltage detector circuit 26 is found to be above the upper threshold V.sub.th1, or below the lower threshold V.sub.th2 for the amount of time given by the time constants of the delay circuit 30, the disabling switch 24 is activated and bridges the load connected between terminals 22a, 22b.

(35) The invention has been illustrated and described in detail in the drawings and foregoing description. Such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

(36) For example, instead of monitoring the voltage at the first terminal 22a, it is also possible to monitor another electrical value, such as a voltage at the second terminal 22b or a current value.

(37) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention, from the study of the drawings, the description and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single unit may fulfill the function of several items recited in the claims. The mere fact that certain measures are recited in mutually different depended claims does not indicate that a combination of these measures cannot be used to advantage. Any references signs in the claims should not be construed as limiting the scope.