WO2007119189A1

WO2007119189A1 - A circuit for detecting function status of a lamp and a ballast

Info

Publication number: WO2007119189A1
Application number: PCT/IB2007/051194
Authority: WO
Inventors: Etienne Eberson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-04-19
Filing date: 2007-04-04
Publication date: 2007-10-25
Also published as: CN101060737A

Abstract

The present invention proposes a circuit for detecting the function status of a lamp having at least one electrode, which circuit comprises: a voltage divider having a middle point, for outputting a voltage signal reflecting the function status of the lamp; a unilaterally conductive unit, coupled with said at least one electrode and coupled with said middle point, for blocking either a positive component or a negative component of a current signal flowing from the lamp towards the voltage divider. In addition, the invention also proposes a ballast using such a detection circuit. The present invention can decrease the unnecessary sensitivity of the known detection circuit and detect the function status of a lamp more accurately.

Description

A CIRCUIT FOR DETECTING FUNCTION STATUS OF A LAMP AND A BALLAST

FIELD OF THE INVENTION

The present invention relates to a detection circuit of a lamp and a ballast using such a detection circuit.

BACKGROUND OF THE INVENTION

Detection circuits are widely used in control apparatus of lamps. For example, a detection circuit for use in a ballast of a fluorescent lamp is used to detect safety problems that might arise during operation of the fluorescent lamp. When safety problems arise, such as false ignition, non-existence of a lamp tube and insufficiency of ignition voltage, they will be detected by the detection circuit, so that the lamp's security protection mechanism is triggered. Usually, the function status of a lamp is an important indicator of safety problems, and includes the following circumstances: where the lamp exists and works normally; where the lamp does not exist; and where the lamp exists but can not work normally due to damage.

However, practice shows that known detection circuits are sometimes oversensitive during the course of real operation. That is to say, even if the function status of a lamp is completely normal, the known detection circuit might indicate that the function status of the lamp is abnormal, and then the security protection mechanism is triggered.

OBJECT AND SUMMARY OF THE INVENTION

The present invention attempts to propose a new detection circuit so as to avoid the problem that the function status of a lamp cannot be accurately detected under some circumstances.

Based on this object, the present invention proposes a circuit for detecting the function status of a lamp having at least one electrode, which circuit comprises: a voltage divider having a middle point, for outputting a voltage signal reflecting the function status of the lamp; a unilaterally conductive unit, coupled with said at least one electrode and coupled with said middle point, for blocking either a positive component or a negative component of a current signal flowing from the lamp towards the voltage divider.

Preferably, said unilaterally conductive unit is a diode.

Preferably, said circuit further comprises: a conductive unit connected between a reference voltage and said at least one electrode.

Preferably, said circuit further comprises: a comparing unit for comparing the voltage signal outputted by said voltage divider with a reference signal to generate a value indicating the function status of the lamp.

In addition, the present invention proposes a ballast for driving a lamp, said ballast comprising a circuit proposed in the present invention.

These and other aspects of the present invention will be further described and become more apparent from the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, for the purpose of illustration only, the present invention will be explained with reference to the accompanying drawings, wherein:

Fig. 1 schematically shows an embodiment of a detection circuit according to the present invention; and Fig 2 schematically shows another embodiment of a detection circuit according to the present invention;

Like reference signs designate the same, similar or corresponding functions and features throughout the figures above.

DETAILED DESCRIPTION OF THE DESCRIPTION

Fig. 1 is a circuit diagram of a detection circuit according to the present invention. As shown in the figure, a lamp L has two electrodes El and E2 coupled with one end of a conductive unit CU. The other end of the conductive unit CU is connected to a reference voltage Vr at point Pl. In the present figure, it is connected to a zero voltage, Vr = 0, i.e., is grounded. At the same time, said two electrodes El and E2 are coupled with the current outflow end of a unilaterally conductive unit D, for example, the cathode of a diode; the current inflow end of the unilaterally conductive unit D, for example, the anode of a diode, is coupled via a resistor Rl with a middle point P2 of a voltage divider VT. Resistors R2 and R3 are on two sides of the middle point of the voltage divider VT respectively, the former being grounded at point P3 and the latter being connected to a DC voltage source Vs at point P4. The voltage value on point P2 may be outputted to a comparing unit CM which is used for comparing the voltage value on point P2 with a reference signal so as to generate a value indicating the function status of the lamp. We assume that the direction in which a current flows from the lamp L towards the voltage divider VT is the reference direction of the current. The above unilaterally conductive unit only allows a current to flow in the direction opposite to the reference direction, i.e., allows a negative current to flow, and a positive component of a current signal will be blocked.

If the function status of the lamp L is normal, then a closed loop is formed between point Pl and point P2. Since both Pl and P3 are grounded and the electric potentials thereof are both zero, the current will flow from the higher electric potential P2 to Rl, D, E2, El, CU, Pl on the one hand, and to R2, P3 on the other hand. At this point, the resistors Rl and R2 jointly assume the role of voltage division. A resistor formed by connecting Rl and R2 in parallel is designated by R1//R2, and then the voltage value on point P2 is

V_p2-_I = (R1//R2)/(R3+R1//R2) x Vs If the lamp L is in an abnormal function status, i.e., L does not exist or has been damaged, for example, the electrode El or E2 has been damaged, then an open loop is formed between point Pl and point P2. At this point, the voltage value on point P2 is the voltage division value calculated in accordance with the resistors R2 and R3, that is,

Obviously, V_P2-i<V_P2-2, that is, the voltage value on point P2 when the lamp is in a normal function status should be less than the voltage value on point P2 when the lamp is in an abnormal function status, i.e., does not exist or has been damaged.

Based on the above principle, the voltage value on point P2 may be outputted to a comparing unit CM. The comparing unit may compare the voltage value on point P2 with a pre-set reference signal. The reference signal may be pre-set according to the input voltage

Vs of the voltage divider VT and the resistance values of Rl, R2 and R3. For example, the reference signal may be a 0.6V reference voltage value. If the voltage on point P2 V_P2 < 0.6V, the lamp is deemed to be in a normal function status; if V_p2 > 0.6V, the lamp is deemed to be in an abnormal function status, and then the security protection mechanism is initiated.

The above analysis does not touch upon in particular the role of the unilaterally conductive unit D. As a matter of fact, the unilaterally conductive unit plays a highly significant role in the detection circuit of the present invention.

There is a circumstance in practice in which there might be a positive current signal flowing from the lamp L to the voltage divider VT in the above circuit when the lamp is in a normal function status. The existence of a positive current and the resistance in the real circuit will occupy a role in boosting the voltage on point P2. In the meantime, a negative current will occupy a role in falling the voltage on point P2. The waveform of ordinary industrial AC is symmetric with the positive component corresponding to the negative component, so the voltage on point P2 will not be affected. However, the waveform of current might be asymmetric under certain circumstances, that is, the positive component in the current does not correspond to the negative component therein with the current presents a property of being a positive DC current or a negative DC current as a whole (called "asymmetric current"). In addition, the conventional DC current also belongs to asymmetric current in the above sense.

Causes of asymmetric current might be various. For example, during the course of igniting a fluorescent lamp, it is possible that one electrode of the lamp has been in an emission status while the other electrode has not reached an emission status due to too low temperature, so that a current flowing through the electrodes might produce asymmetry. This circumstance might be because that the service life of the lamp tube has approached the end, or that the lamp tube has not gone through a proper warm-up time due to times of fast ignition, or that the qualities of different electrodes are not consistent with each other, and the like. This circumstance only lasts for a very short time during the course of igniting the lamp tube, and the lamp tube still can work normally. However, as described above, it is very likely that this circumstance results in false initiation of the security mechanism and therefore affects the normal using of the lamp. Another circumstance might be the effects of a disturbance current generated in the circuit because of the generation of other interference source, other power supply or other part of the circuit. In the circuit as shown in Fig. 1, when the positive component in the current is less than the negative component, i.e., there is a negative asymmetric current, the current will present a property of being a negative current as a whole, so that the voltage on point P2 is reduced. The reduction of the voltage on point P2 will not trigger any security mechanism, and will not affect the realization of the function of the detection circuit of the present invention. On the contrary, when the positive component in the current is more than the negative component, i.e., there is a positive asymmetric current, the current will present a property of being a positive current as a whole, so that the voltage on point P2 is increased. When the voltage value on point P2 is outputted to the comparing unit CM, and when it exceeds the pre-set reference voltage value, the function status of the lamp is deemed to be abnormal, and then the security protection mechanism is triggered. Thus, due to the disturbance of the positive asymmetric current, the detection circuit might make a wrong judgment on the function status of the lamp.

As a matter of fact, the positive current in the reference direction is not necessary to the realization of the function of the above detection circuit. As long as the current flowing from P2 to Pl can be realized, i.e., the current is allowed to flow in a direction opposite to the reference direction when the lamp is in a normal function status, it can be ensured that Rl performs the above function of joint voltage division, and as a result, the detection function of the circuit is guaranteed. Therefore, as shown in Fig. 1, a unilaterally conductive unit D, e.g., a diode is provided between the lamp L and the voltage divider VT, i.e., between the middle point P2 of the voltage divider VT and the connection point electrode E2 of the lamp L. The current inflow end of the unilaterally conductive unit D, for example, the anode of a diode is coupled with the direction towards the voltage divider VT or P2, which is coupled with P2 via the resistor Rl as shown in the figure. The current outflow end of the unilaterally conductive unit D, for example, the cathode of a diode is coupled with the direction towards the lamp or E2. With the existence of the unilaterally conductive unit, not only it is ensured that the current can flow in a direction opposite to the reference current direction, so that the normal operation of the detection circuit is guaranteed, but also the positive flow of the current in the reference current direction is restricted, so that the disturbance on detection results by the positive asymmetric current flowing in the reference direction is lessened or eliminated.

Fig. 2 is a circuit diagram of another embodiment of the detection circuit proposed by the present invention. In the present figure, each electric element and the way they are connected are substantially the same as those in Fig. 1. And the differences lie in the following two aspects: first, in the circuit as shown in Fig. 2, the reference voltage Vr, to which the conductive unit CU coupled with the lamp L is connected, equals Vs, that is, the conductive unit CU is connected with the same voltage source as the external DC voltage source Vs connected to the voltage divider VT; and second, included in the present embodiment is a unilaterally conductive unit, for example, a diode, which has a conductive direction opposite to the conductive direction in the embodiment as shown in Fig. 1.

When the function status of the lamp L is normal, a closed loop is formed between point Pl and point P2. Rl and R3 are in a parallel relationship and jointly assume the role of voltage division. A resistor formed by connecting Rl and R3 in parallel is designated by R1//R3, and then the voltage value on point P2 is

Vp₂-I = R2/(R2+R1//R3) x Vs

If the lamp L is in an abnormal function status, i.e., does not exist or has been damaged, for example, the electrode El or E2 has been damaged, then an open loop is formed between point Pl and point P2. At this point, the voltage value on point P2 is the voltage division value calculated in accordance with the resistors R2 and R3, that is,

Obviously, V_P2-i>V_P2-₂, that is, the voltage value on point P2 when the lamp is in a normal function status should be greater than the voltage value on point P2 when the lamp is in an abnormal function status, i.e., does not exist or has been damaged.

Based on the above principle, the voltage value on point P2 may be inputted to a comparing unit CM. The comparing unit may compare the voltage value on point P2 with a pre-set reference signal. The reference signal may be pre-set according to the input voltage Vs of the voltage divider VT and the resistance values of Rl, R2 and R3. For example, the reference signal may be a 0.6V reference voltage value. If the voltage on point P2 V_P2 > 0.6V, the lamp is deemed to be in a normal function status; if V_P2 < 0.6V, the lamp is deemed to be in an abnormal function status, and then the security protection mechanism is initiated Contrary to the embodiment as shown in Fig. 1, in the present embodiment, as long as the function status of the lamp is normal, the current can flow from Pl towards P2, i.e., is allowed to flow forwardly in the reference direction, and it can be ensured that Rl performs the above role of joint voltage division, so the realization of the detection function of the circuit is guaranteed. The negative current in the reference direction is not necessary to the realization of the function of the detection circuit of the present invention. Moreover, the negative asymmetric current might result in reduction of the voltage on point P2 and cause the voltage value on point P2 to be lower than the reference voltage value, and as a result, the system might form a wrong judgment that the lamp is in an abnormal status, thus the security protection mechanism is triggered and the normal use of the lamp is affected. Therefore, in the present embodiment, a unilaterally conductive unit D, for example, a diode is provided between the lamp L and the voltage divider VT, i.e., between the middle point P2 of the voltage divider VT and the connection point electrode

E2 of the lamp L. The current inflow end of the unilaterally conductive unit D, for example, the anode of a diode is coupled with the direction towards the lamp or E2. The current outflow end of the unilaterally conductive unit D, for example, the cathode of a diode is coupled with the direction towards the voltage divider VT or P2, which is coupled with P2 via the resistor Rl as shown in the figure. With the existence of the unilaterally conductive unit, not only it is ensured that the current can flow forwardly in the reference current direction, so that the normal operation of the detection circuit is guaranteed, but also the flow of the current in the direction contrary to the reference current direction is restricted, so that the disturbance in detection results by the negative asymmetric current flowing in the reference direction is lessened or eliminated.

In the above embodiments, the external reference voltage Vr, to which the conductive unit CU is connected, is grounded, i.e., Vr=O, or is the same externally connected voltage as the voltage divider VT, i.e., Vr=Vs, respectively. The difference of externally connected reference voltage will lead to the difference of current directions when the lamp is in a normal function status, thereby affecting the connection direction of the unilaterally conductive unit D included in the present invention. Those skilled in the art should appreciate that the external reference voltage Vr, to which the conductive unit CU is connected, might also be other values besides being grounded or the externally connected voltage Vs of the voltage divider; as a result, the connection direction of the unilaterally conductive unit D might be affected and corresponding adjustments can be made in accordance with needs.

In the schematic views of the above two embodiments, a capacitor C is shown, one end thereof connected to the middle point P2 of the voltage divider VT and the other end thereof grounded. The roles of the capacitor C are to delay the speed at which the voltage value on point P2 changes, and to decrease the sensitivity of the reflection of the detection circuit and to be used in conjunction with other elements of the detection circuit of the present invention. Obviously, the capacitor is not necessary but optional to the present invention.

The resistor Rl in the above two embodiments may be either an independent resistor or an actual resistance value of each element in the circuit. In the meantime, the implementation of the resistor Rl is not necessary to the implementation of the present invention, and the description of the resistor in the above embodiments is only to take into account the resistance effect existing in a real circuit and for the purpose of convenient description of a related voltage or current relationship.

As the present invention has been illustrated and described in detail in the accompanying drawings and the preceding description, it is to be understood that the illustration and description is descriptive and illustrative, but not limitative. The present invention is not limited to the disclosed embodiments.

Those skilled in the art may understand and carry out other alterations to the disclosed embodiments through studying the accompanying drawings, the disclosed contents and the appended claims. In the claims, the wording "comprising" does not exclude other elements and steps and the wording "a" or "an" does not exclude the plurality. A single processor or other unit may perform the function of multiple items referred in a claim. Any reference sign in the claims should not be construed to limit the scope.

Claims

CLAIMS:

1. A circuit for detecting the function status of a lamp (L) having at least one electrode (El, E2), comprising: a voltage divider (VT) having a middle point (P2), for outputting a voltage signal reflecting the function status of the lamp; a unilaterally conductive unit (D), coupled with said at least one electrode (El, E2) and coupled with said middle point (P2), for blocking either a positive component or a negative component of a current signal flowing from the lamp (L) towards the voltage divider (VT).

2. A circuit as claimed in claim 1, wherein the unilaterally conductive unit (D) is a diode.

3. A circuit as claimed in claim 1, further comprising: a conductive unit (CU) connected between a reference voltage (Vr) and said at least one electrode (El, E2).

4. A circuit as claimed in claim 1, 2, or 3, further comprising: a comparing unit (CM) for comparing the voltage signal outputted by the voltage divider (VT) with a reference signal so as to generate a value indicating the function status of the lamp .

5. A ballast for driving a lamp (L), said ballast comprising a circuit as claimed in claim 1, 2, 3 or 4.