Ground fault detector for gas discharge tubing

A ground fault circuit comprising: a power supply, and a transformer having a primary winding driven by the power supply and a high voltage secondary winding, a shutdown control circuit having a controllable switch and a control input coupled to the controllable switch for causing operation of the controllable switch when a trigger voltage applied to the control input is exceeded, the shutdown control circuit being coupled to the power supply for controlling shut-down of the power supply when the switch is in operation, a circuit connected to the high voltage secondary winding for detecting leakage current from the transformer to ground, for short circuiting an A.C. component of the leakage current passing through the detector to ground, and for deriving a D.C. voltage from D.C. leakage current from the transformer to ground, and a circuit for applying the derived D.C. voltage to the control input of the shutdown control circuit, whereby the power supply may be shut down in the presence of leakage current in excess of the trigger voltage which is derived exclusively from D.C. current leakage from the transformer to ground.

FIELD OF THE INVENTION 
This invention relates to the field of ground fault detectors, and 
particularly ground fault detectors used in high voltage circuits. 
BACKGROUND TO THE INVENTION 
Ground fault detectors are used in high voltage circuits such as in 
ballasts which drive gas discharge tubes such as neon display tubes. A 
ground fault detector is used to safeguard current carrying circuits, 
detecting leakage current to ground and shutting off the power supply of 
the ballast in the event the leakage current exceeds a predetermined 
value. Excessive leakage current can cause arcing, which can cause fire or 
can be lethal, and therefore maximum leakage is regulated by standards. 
One of the conventional ways of controlling the shut-off of the high 
voltage is to monitor the alternating current which is returned to ground 
carried by the center tap of the high voltage secondary winding of a high 
voltage transformer. In the event the high voltage leads of the secondary 
winding are conducting to ground, causing current to flow from the center 
tap to ground, a current transformer reflects this back to a shutdown 
circuit. Examples of circuits which use this principle are described in 
U.S. Pat. No. 4,663,571 to Nilssen and U.S. Pat. Nos. 4,613,934 and 
5,089,572 to Pacholok. 
Another conventional way of controlling shut-down of a high voltage circuit 
is to detect the inherent phase shift between current and voltage when the 
high voltage is radiated capacitively to ground. However, the realized 
circuit requires a phase discriminator circuit and a high parts count, 
which is costly. 
A typical ground fault detector is comprised of a solid state switch which 
accepts a trigger voltage and conducts to operate a relay, etc. when the 
trigger voltage is exceeded. The relay operates switch contacts in the 
power supply, shutting down the power supply. The trigger voltage is 
derived by detecting the leakage current and converting this current to a 
voltage which is rectified and is applied to the solid state switch, which 
will operate if the voltage, and thus the current, is large enough. Ground 
fault detector circuits of this type are described, for example, in U.S. 
Pat. No. 4,114,089 to Ahmed, U.S. Pat. No. 3,899,717 to Legatti et al and 
U.S. Pat. No. 4,138,707 to Gross. 
The leakage current detected in the aforenoted structures constitutes 
radiated or reactive alternating current, similar to current emitted from 
a radio frequency transmitter. The return energy is purely capacitive to 
ground. If the energy emitted by both high voltage leads (e.g. the 
antennae) of the secondary winding of the high voltage transformer of the 
ballast is not balanced capacitively, a current will flow through the 
center tap of the secondary winding to ground, creating an A.C. voltage 
which is detected as leakage current, and causing a false shutdown of the 
power supply. 
It has been determined that hazardous arcing to ground can be detected 
solely from the D.C. current flowing from a D.C. biased winding to ground, 
rather than from the A.C. reactive current to ground. The systems 
described above shut down in the presence of A.C. reactive current, even 
without the presence of additional resistive current, which causes the 
false shutdown. The prior art systems are thus not reliable detectors of 
the hazardous currents. 
SUMMARY OF THE INVENTION 
The present invention is a ground fault detector circuit which ignores the 
A.C. leakage current caused by radiation, unbalanced radiation current, 
etc., and provides a trigger voltage which is caused by true direct 
current leakage during dielectric breakdown. It generates the trigger 
voltage by short circuiting A.C. leakage current, and detects only D.C. 
(resistive path) leakage current, applying a voltage derived from the D.C. 
leakage current to the trigger input of a solid state switching device 
such as a programmable shunt regulator. 
The present invention thus provides reliable detection and high voltage 
shut down for D.C. leakage current which would otherwise be hazardous, 
ignoring A.C. radiated current. 
In accordance with an embodiment of the invention, a method of shutting 
down a power supply which drives a transformer having a center-tapped high 
voltage secondary winding, comprises short circuiting A.C. leakage current 
that may flow between the secondary winding and ground, detecting D.C. 
voltage caused by D.C. leakage current which may be conducted between the 
D.C. biased secondary winding and ground, applying the D.C. voltage to the 
control input of a switch, and controlling shut-down of the power supply 
by means of the switch. 
In accordance with another embodiment, a ground fault circuit comprises (a) 
a power supply, and a transformer having a primary winding driven by the 
power supply and a high voltage secondary winding, (b) a shutdown control 
circuit having a controllable switch and a control input coupled to the 
controllable switch for causing operation of the controllable switch when 
a trigger voltage applied to the control input is exceeded, the shutdown 
control circuit being coupled to the power supply for controlling 
shut-down of the power supply when the switch is in operation, (c) a 
circuit connected to the high voltage secondary winding for detecting 
leakage current from the D.C. biased transformer to ground, for short 
circuiting an A.C. component of the leakage current passing through the 
detector to ground, and for deriving a D.C. voltage from D.C. leakage 
current from the transformer to ground, and (d) a circuit for applying the 
derived D.C. voltage to the control input of the shutdown control circuit, 
whereby the power supply may be shut down in the presence of leakage 
current in excess of the trigger voltage which is derived exclusively from 
D.C. current leakage from the transformer to ground.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
A power supply 1, for example one which drives a neon gas discharge tube, 
has a shutdown control input which, when driven by a voltage VCC, shuts 
down the power supply in a conventional manner. A loosely coupled 
transformer is comprised of a primary winding 3 which is driven by the 
power supply, and a high voltage secondary winding 5. Ends V1 and V2 of 
the secondary winding are to be connected to a gas discharge tube. 
The secondary winding has a center tap CT, and a balanced pair of taps A 
and B on either side of the center tap for obtaining a small 
representative portion of the high voltage produced by the secondary 
winding. A pair of diodes D1 and D2 having their cathodes connected to the 
respective taps, have their anodes connected together and to the negative 
terminal of a capacitor C1. The positive terminal of capacitor C1 is 
connected via resistor R1 to the center tap of secondary winding 5. 
The diodes rectify the small portion of the high voltage, resulting in a 
D.C. voltage across capacitor C1 which has been charged through resistor 
R1. This D.C. voltage creates a D.C. bias of the center tap with respect 
to the potential at the negative terminal of capacitor C1. In a successful 
embodiment, the D.C. voltage across capacitor C1 was 25 volts. 
Capacitor C2 and resistor R2 are connected in parallel between the negative 
terminal of capacitor C1 and ground. This provides an A.C. shunt path to 
ground, to maintain a low A.C. voltage from the center tap CT of the 
secondary winding 5 to ground, via the path R1, C1, C2 and R2. 
If there is A.C. radiated leakage current between the secondary winding 5 
and ground, this current is short circuited by capacitor C2. 
If there is no D.C. leakage current from the D.C. biased transformer 
secondary winding to ground (e.g. caused by dielectric breakdown), there 
will be no net D.C. voltage across capacitor C2. Thus the D.C. potential 
at the negative terminal of capacitor C1 will be the same as that at the 
positive terminal of capacitor C2, i.e. ground potential. 
However, if there is D.C. leakage to ground, capacitor C2 will begin to 
charge. The voltage will appear across resistor R2, which is applied to 
the control input of a shutdown control circuit 7. The value of resistor 
R2 should be chosen to trigger control circuit 7 if a predetermined D.C. 
leakage current level is reached. 
The shutdown control circuit 7 is preferably comprised of a programmable 
shunt regulator U1, which has a switch control input 9. The shunt 
regulator is functionally similar to an NPN bipolar transistor in 
operation, except that the threshold voltage to turn it on (into a 
conducting state from anode to cathode) is about 2.5 volts. 
The cathode of shunt regulator U1 is connected to one terminal of the 
photodiode (e.g. light emitting diode) of an optocoupler U2, the other 
terminal of which is connected via a current limiting resistor R4 to the 
positive terminal of capacitor C1. The anode of shunt regulator U1 is 
connected to the junction of the negative terminals of capacitors C1 and 
C2 and the anodes of diodes D1 and D2. 
Resistor R3 is connected between the control terminal of shunt regulator U1 
and ground. Thus the voltage across resistor R2 is applied between the 
control input of shunt regulator U1 through resistor R3 and the anode of 
shunt regulator U1. 
In operation, when the D.C. voltage caused by D.C. leakage to ground from 
the D.C. biased secondary winding 5 of transformer T1 is equal to or is in 
excess of the turn-on voltage of shunt regulator U1, e.g. 2.5 volts or 
higher, shunt regulator U1 is triggered, and it becomes conductive between 
its cathode and anode. Current resulting from the rectified voltage which 
appears across capacitor C1 from tansformer T1, passes through the 
photodiode of optocoupler U2 and shunt regulator U1, causing the 
phototransistor in the optocoupler which is connected between the VCC and 
the shutdown terminals of the power supply 1 to conduct, causing the power 
supply to shut down. The inverter in the ballast is thereby triggered to 
shut down. 
In addition, it is preferred to connect resistor R5 across the light 
emitting diode of optocoupler U2, to prevent the optocoupler from turning 
on as a result of anode to cathode leakage current of the shunt regulator. 
A diode D5 is also preferred to be connected across the photodiode of the 
optocoupler U2 in oppositely poled direction to the light emitting diode, 
to prevent damage to the photodiode from reverse voltage spikes. A diode 
D4 is connected between the control input and the anode of the shunt 
regulator U1, with its anode connected to the anode of the shunt 
regulator, to protect the shunt regulator from reverse voltage spikes. 
A diode D3 is preferably connected across the shunt regulator U1 in the 
same polarity direction, as a limiting shunt to protect the shunt 
regulator from damage in case the secondary winding voltage goes to high, 
and to protect the shunt regulator from reverse voltage spikes. If diode 
D3 is caused to conduct from one of these two events, it will cause the 
optocoupler to operate, and will thereby trigger the power supply and 
therefore the inverter to shut down as if the shunt regulator had turned 
on from D.C. ground fault current. Thus the inverter is provided with 
monitoring of excessive voltage in the high voltage secondary winding 5. 
A person skilled in the art understanding the above description may now 
consider other embodiments using the principles described above. All such 
embodiments which are within the spirit and scope of the claims appended 
hereto are considered to be part of the present invention.