Safety circuit for electro-luminescent lamp ballast

A safety circuit is provided for a ballast of an electro-luminescent (EL) lamp. The EL lamp had an electrical ground shield on at least a portion thereof and has a separate ground lead. The ballast is an isolated ballast with a line input being isolated from an output. The ballast is constructed and arranged to be electrically connected with the separate ground lead. The circuit includes a current sensing structure constructed and arranged to connect between the separate ground lead and a potential ground of the ballast. In the event a user contacts the EL lamp to replace the EL lamp or contacts a defective EL lamp, the ballast is shut down based on a value of current sensed by the current sensing structure.

FIELD OF THE INVENTION

The invention relates to Electro-luminescent (EL) lamps and, more particularly, to a safety circuit, for an EL lamp system, herein called Smart Faults Detection and Protection (SFD&P).

BACKGROUND OF THE INVENTION

Typical EL lamps are applied in frame systems. Thus, the size of the EL lamp can be 2,500 square inches or more and the applied voltage can be between 100-350 VAC or even higher. If people touch the electrode of EL lamp, current may flow through the human body to ground, thus possibly causing physical injury.

It has long been known to apply a Ground Fault Interrupt (GFI) circuit to the ballast for fluorescent lamps. A conventional GFI circuit uses a current sensor to measure unbalanced current between input live and neutral. Most of the ballasts are non-isolated circuits. These GFI circuits can be applied to the non-isolated ballast for an EL lamp. Thus, when a person touches the electrode of EL lamp, the conventional GFI circuit will shut down the ballast. However, when EL lamp is put into a grounded lighting frame or fixture, there is a large parasitic capacitance (a few nF) between the fixture and EL lamp, thus resulting in high leakage current from the EL lamp to ground. This stray leakage current will trip the GFI circuit and shut down the ballast. Hence, the conventional GFI circuits cannot accurately discriminate between stray leakage current and the leakage current that occurs due to a true fault condition.

In an isolated ballast, line input is isolated from an output high voltage terminal. Thus, an isolated ballast could prevent stray leakage current flow to a grounded fixture since there is no return current path back to a secondary of the ballast. An isolated ballast with a GFI circuit could detect leakage current when a person touches the electrode of an EL lamp. However, when a person replaces an EL lamp, if the lighting fixture of the EL lamp is floating, there is still a leakage current coupled to the lighting fixture which can flow to the human body and back to ballast secondary output. The magnitude of the coupled leakage current depends on parasitic capacitance between the El lamp and the fixture, and at times can be up to 10 mA. Thus, even with an isolated ballast, there is still a risk of getting shocked when replacing such an EL lamp.

An EL lamp with a ground shield and an isolated ballast could solve the above-mentioned problem, since there is no parasitic capacitance from the EL lamp to the lighting fixture. However, if an isolated ballast is connected with a ground shield of the EL lamp and if the EL lamp is destroyed by a metal tip, if a person touches the defective part, leakage current will flow through the human body to the ground shield and couple back to the EL lamp. This shock current could be as high as 60 mA and depends on the parasitic capacitance between the ground shield and the rear electrode of EL lamp.

Thus, there is a need to provide a new safety circuit for an EL lamp to protect a user when replacing an EL lamp and when touching a defective EL lamp.

SUMMARY OF THE INVENTION

An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is obtained by providing a safety circuit for a ballast of an electro-luminescent (EL) lamp. The EL lamp had an electrical ground shield on at least a portion thereof and has a separate ground lead. The ballast is an isolated ballast with a line input being isolated from an output. The ballast is constructed and arranged to be electrically connected with the separate ground lead. The circuit includes a current sensing structure constructed and arranged to connect between the separate ground lead and a potential ground of the ballast. In the event a user contacts the EL lamp to replace the EL lamp or contacts a defective EL lamp, the ballast is shut down based on a value of current sensed by the current sensing structure.

In accordance with another aspect of the invention, a method is provided for controlling a ballast for powering an electro-luminescent (EL) lamp. The ballast has a line input isolated from an output. The method provides an EL lamp having an electrical ground shield on at least a portion thereof and having a separate ground lead. The ballast is electrically connected with the separate ground lead. Current between the separate ground lead and a potential ground of the ballast is sensed. The ballast is shut down based on a value of current sensed by the current sensing structure.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

With reference toFIG. 1, a safety circuit, generally indicated at10, for an EL lamp ballast, generally indicated at11, is shown in accordance with the principles of the present invention. The ballast11is an isolated ballast with a line input being isolated from an output. The ballast11powers an EL lamp12having a ground shield. The safety circuit includes a Smart Faults Detection and Protection (SFD&P) circuit, generally indicated at13. The ballast11, with the SFD&P circuit13, together with the EL lamp12defines an EL lamp system19.

In the embodiment, the isolated ballast11includes a three-wire input, generally indicated at14, an isolated AC-DC converter, generally indicated at15, and a DC-AC inverter, generally indicated at16, providing AC current to the EL lamp12.

Preferably, an aluminum foil17is laminated on at least a portion of a rear side of the EL lamp12and a separate ground lead18is electrically connected to the SFD&P circuit13. More particularly, in the embodiment, the SFD&P circuit13includes a current sensing structure, preferably in the form of a resistor20(e.g., a few ohms), placed between the separate ground lead18and potential ground (PG) of the ballast11.

When a person or user touches the electrode of the EL lamp12or fixture of the EL lamp, the leakage current will flow through human body to the ground lead18to the current sensing resistor20. Since the voltage drop is low, an active rectifier22and an amplifier24are connected to the sensing resistor20providing an amplified signal21. The voltage drop (signal21) of the sensing resistor20is preferably measured by an analog to digital converter23and a controller25, such as a microprocessor. The converter23can be part of the controller25. The controller25will shut down the ballast11when current sensed by the sensing resistor20exceeds a trip or threshold value of current. Instead of using the microprocessor25, a trip latch circuit can be connected to the amplifier to shut down the ballast11when a leakage current flow to a person is measured.

Thus, it can be appreciated that the safety circuit10protects a user when replacing an EL lamp and when touching a defective EL lamp.