Ballast shut-down circuit responsive to an unbalanced load condition in a single lamp ballast or in either lamp of a two-lamp ballast

An abnormal or unbalanced load operating condition is detected in a single lamp ballast circuit or in either lamp of a two-lamp ballast circuit by feedback voltage signals that are proportional to the flow of current through the cathodes of each lamp. The analog feedback signals are combined algebraically by a summing circuit that produces a null (zero) value corresponding with normal lamp operation, and produces a non-zero value in response to abnormal cathode current flow. The non-zero value is compared with the reference value to generate a shut-down signal. In one embodiment, the cathode currents are sensed by primary windings of a toroid transformer, and the feedback signals are the magnetic flux components that are generated in response to cathode current flow through the current sensing windings. In another protective circuit embodiment, feedback signals derived from the positive and negative portions of an asymmetrical waveform appearing across the power input pins of a lamp are compared and summed together to produce an output voltage that triggers a shut-down signal if the energy content of the positive portion or of the negative portion of the asymmetrical waveform is greater than a predetermined threshold value.

FIELD OF THE INVENTION 
This invention relates generally to ballast power supplies for energizing 
gas discharge lamps, and in particular to high-frequency electronic 
ballasts having protective shut-down circuitry for preventing lamp 
overheat damage caused by an open cathode, a short-circuit cathode or a 
depleted cathode at end-of-life. 
1. Background of the Invention 
Low-pressure gas discharge lamps, such as compact fluorescent lamps, have a 
pair of tungsten cathodes which are coated with a metal oxide that emits 
electrons when heated. The electrons ionize pressurized gas within the 
lamp envelope, and when a high voltage is applied across the cathodes, 
electrical current is discharged between the cathodes in the form of a 
plasma arc that emits ultraviolet radiation. The interior of the lamp 
envelope is coated with a material that responds to the ultraviolet 
radiation by emitting visible light. The plasma arc within the lamp has a 
negative resistance characteristic that requires a series ballast 
impedance to maintain stable current flow. 
The electron-emissive material gradually depletes through normal use of the 
lamp; consequently, the cathode current required to sustain the plasma arc 
increases substantially over a period of time. At the end of the useful 
life of the lamp, the power consumed by the depleted cathode may increase 
by a factor of ten or more, thus causing excessive heating of the cathode. 
Cathode over-heating can cause the cathode to break and come into contact 
with the glass envelope, thus causing a local hot spot at extremely high 
temperatures approaching 2,000.degree. C. Such localized over-heating can 
be intense enough to cause the surrounding glass envelope to crack and 
melt the plastic body of the base. This condition can also occur at any 
point in the life of the lamp if the cathode should break, come into 
contact with the glass wall of the lamp and the ballast continues to 
supply enough energy to maintain the arc. 
Localized over-heating can occur in low wattage, compact fluorescent lamps, 
for example fluorescent lamp types T2 and T4 corresponding with lamp 
envelope diameters of 1/4" and 1/2", respectively. The cathodes of such 
lamps are located close to the glass envelope as compared with larger 
diameter lamps, for example type T8 (one inch diameter). Because of the 
limited radical spacing between the cathode and the glass wall, the small 
diameter fluorescent lamp can be subjected to excessive local overheating 
and extremely high temperatures in response to excessive current flow 
through a short-circuit cathode or a depleted cathode that has broken and 
comes into contact with the glass wall of the lamp. 
Moreover, the gas discharge lamp operates on alternating current with the 
current flowing through the lamp in both directions, between the two 
electrodes that act alternately as a cathode and as an anode, and vice 
versa. Under end-of-life operating conditions, one of the two electrodes 
will become depleted of the alkaline metal oxide coating, thus losing its 
ability to emit electrons relative to the other electrode. This will cause 
the lamp to conduct electric current more readily in one direction than 
the other, thus generating a substantial direct current (DC) component 
that can damage the ballast circuit. Also, the direct current component 
can saturate the magnetic core of an inductive ballast, thus causing it to 
lose control of the AC voltage and current applied to the lamp. 
2. Description of the Prior Art 
Circuits providing over-voltage and over-current protection for solid-state 
high frequency electronic ballasts generally include means for detecting 
an abnormal operating condition, such as excessive lamp voltage, and a 
shut-down circuit which disables the inverter. This allows the entire 
ballast circuit to be shut-down when a lamp cathode has failed or the lamp 
has been removed, to avoid unstable oscillation of the ballast circuit. 
Some conventional protective circuits include a fusible element or reactive 
device for disconnecting or reducing power applied to a lamp in response 
to excessive cathode current flow, for example as shown in U.S. Pat. No. 
5,138,235 and U.S. Pat. No. 4,501,992. 
According to another protective circuit as shown in U.S. Pat. No. 
5,262,699, the inverter is disabled in response to the detection of a 
relatively large increase in the inverter current flow resulting from 
operating a lamp having an internally shorted cathode or having a depleted 
cathode. Other protective circuits shown, for example as shown in U.S. 
Pat. No. 5,111,114 and U.S. Pat. No. 4,503,363, sense an over-voltage 
condition on the output of the inverter, corresponding with an open 
cathode conditions, removal of the lamp or failure of the lamp to ignite. 
Yet another protective technique discussed in U.S. Pat. No. 5,475,284 
relies on the detection of a significant increase in ballast input power 
when the inverter input voltage is boosted in response to a load demand 
feedback signal from the lamp. 
Still another protective circuit disclosed in U.S. Pat. No. 5,475,284 
measures the magnitude of the DC voltage component of the lamp voltage 
when the lamp is operating at end-of-life with a depleted cathode, and 
disables the inverter in response to a predetermined increase in the DC 
component of the lamp voltage. 
A limitation on the use of conventional ballast protection circuits is that 
a substantial change in some operating parameter must occur and continue 
for a predetermined period of time before the inverter shut-down circuit 
will operate. That is, such protective circuits are not responsive to 
slowly changing circuit conditions. Consequently, potentially damaging 
current and voltage conditions are ignored or suffered until the 
occurrence of a cathode failure event such as an open cathode, 
short-circuited cathode or a depleted cathode. During the time elapsed 
from the onset of excessive voltage or cathode current flow corresponding 
with an abnormal or unbalanced load condition leading to cathode failure, 
over-voltage can damage ballast components and possibly accelerate the 
failure event. Moreover, unless protective action is taken quickly, 
localized overheating caused by excessive cathode current flow can damage 
the lamp. 
SUMMARY OF THE INVENTION 
According to the present invention, a protective circuit is provided for 
quickly sensing an abnormal or unbalanced load operating condition in a 
single lamp ballast circuit or in either lamp of a two-lamp ballast 
circuit. In the single lamp embodiment, the cathodes are separately 
supplied with operating currents I.sub.K1 and I.sub.K2. Under normal, 
balanced lamp operating conditions, the total current into the lamp is 
equal to the total current out, 
EQU I.sub.L +I.sub.K1 =I.sub.L +I.sub.K2 
and the current flow I.sub.K1 through cathode K1 is equal to the current 
flow through the lamp cathode K2. 
Analog feedback signals are generated that are proportional to the flow of 
current through each cathode. The analog feedback signals are combined 
algebraically within a summing circuit that provides a predetermined 
output, for example a null (zero) value, corresponding with normal lamp 
operation. The summing circuit output will be a non-zero value for an 
unbalanced cathode current condition. The non-zero value is compared with 
a reference value to generate a shut-down signal. 
In the dual lamp embodiment, and assuming balanced load conditions, the 
lamps have substantially identical properties and are energized by equal 
driving voltages, so that the are current I.sub.L1 through lamp 1 of a 
commonly connected lamp pair lamp 1, lamp 2 is equal to the arc current 
I.sub.L2 through lamp 2. Consequently, the total current flow into the 
lamps is equal to the total current flow out of the lamps during balanced 
load operation. For the commonly connected lamp pair, this implies that 
the sum of the current I.sub.K1 flowing through the independently 
connected cathode K1 of lamp 1, plus the current I.sub.K flowing through 
the independently connected cathode K.sub.4 of lamp 2 is equal to the sum 
of the currents flowing through the commonly connected cathodes K2, K3. 
This is expressed algebraically as follows: 
EQU I.sub.K1 +I.sub.K4 -(I.sub.K2 +I.sub.K3)=0 
When the current flow through any cathode of either lamp increases above a 
predetermined acceptable level, for example corresponding with increased 
current flow through a depleted cathode operating at or near end-of-life 
conditions, or changes in response to an open cathode, a short-circuited 
cathode or lamp removal, the summing circuit produces a DETECT analog 
voltage signal that is compared to a reference voltage VREF in a shut-down 
comparator. If the magnitude of the DETECT analog voltage signal exceeds 
the reference value, the shut-down comparator transitions from a logic low 
value, corresponding with normal, balanced cathode current flow 
conditions, to a logic high value, corresponding with an abnormal cathode 
current flow condition. The logic high output of the shut-down comparator 
constitutes a trigger signal that is input to the SET input of a set-reset 
latch. The latch output Q changes state and sends a shut-down enable 
signal to a normally closed electronic switch. Opening of the electronic 
switch disables the inverter of the ballast circuit and thus removes all 
operating power from the lamp driver. 
In the preferred embodiment, the analog signals are magnetic flux signals 
that are generated within the core of a toroid transformer. The current 
sensing elements are formed by primary windings that are connected in 
series with each independent lamp cathode, and in series with the power 
supply to the commonly connected lamp electrodes. The toroid transformer 
includes a secondary winding which produces a null or zero output voltage 
when the flux components are in balance, corresponding to normal lamp 
operation. The sense of the primary windings connected in the supply 
circuit of the independently connected lamp cathodes is established 
relative to the sense of the primary winding for the commonly connected 
cathodes so that the magnetic flux components are summed algebraically to 
be in balance when the cathode currents are equal. 
The toroid transformer serves as a magnetic flux summing circuit, and its 
secondary winding produces an output voltage in response to a non-zero 
flux component in the toroid core. A non-zero flux component will develop 
immediately in response to differential current flow through any one of 
the lamp cathodes. The analog AC voltage signal produced by the secondary 
winding of the analog signal flux summing circuit is rectified and then 
input through a low pass filter to the non-inverting input of the 
shut-down comparator. The level of the reference voltage signal can be set 
arbitrarily close to the null voltage, so that the ballast can be safely 
shut-down immediately in response to the detection of an open cathode, a 
short-circuited cathode, lamp removal or a depleted cathode, and before 
the lamp is damaged by localized overheating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the description which follows, the block and schematic diagrams 
illustrate major components of a related conventional functional group, 
wherein the present invention may be more readily understood. 
Referring now to FIG. 1, a general block diagram of a solid-state, high 
frequency electronic ballast 10 provides operating power to a pair of four 
pin fluorescent lamps, lamp 1 and lamp 2. A single phase source 12 of AC 
line voltage VAC operating at 60 Hz is input to a full wave bridge 
rectifier 14, producing direct current voltage VDC. The rectified output 
voltage VDC is input to a boost regulator 16 which steps up the output 
voltage VDC to a desired high voltage level HVDC. The voltage output HVDC 
of the boost regulator 16 is coupled to an inverter 18 which chops the 
stepped-up DC voltage HVDC at a high frequency thereby producing a high 
voltage AC waveform HVAC. The output HAVC of the inverter 18 is preferably 
transformed by a reactance circuit Z in a lamp driver 20 to provide a high 
Q and a matching impedance for applying starting and operating current to 
lamp 1 and lamp 2. 
Referring now to FIG. 2, operating power for the independently connected 
cathode K1 of lamp 1 is supplied from the lamp driver 20 through a 
conductor 22, and the other cathode K2 of lamp 1 is connected in common 
with one of the cathodes K3 of lamp 2 through a conductor 24. Operating 
power is supplied by the lamp driver 20 to the commonly connected cathodes 
K2, K3 through a conductor 26. The independently connected cathode K4 of 
lamp 2 receives its operating power from the lamp driver 20 through a 
conductor 28. 
Referring again to FIG. 2, sensors 30, 32 and 34 are coupled to the power 
supply conductors 22, 26 and 28, respectively, for generating analog 
feedback signals 30F, 32F and 34F, respectively. Each analog feedback 
signal is proportional to the current flowing through the cathode that is 
being supplied by the power conductor to which it is coupled. According to 
the present invention, when the lamps are operating normally, the lamp arc 
currents I.sub.L1 and I.sub.L2 are equal. Consequently, the total current 
flow flowing through the commonly connected cathodes K2, K3 will equal the 
sum of the currents flowing through the independently connected cathodes 
K1, K4, and the algebraic sum of these currents may be expressed as 
follows: 
EQU I.sub.K1 +I.sub.K4 -(I.sub.K2 +I.sub.K3)=0 
The analog signals 30F, 32F, 34F that correspond to the cathode currents 
are combined algebraically within a summing circuit 36. The output of the 
summing circuit 34 is designated DETECT, which has a null value (zero) 
when the cathode currents are in balance according to the equation given 
above. However, if the current flow through any one of the cathodes should 
change, the equation given above will no longer sum to zero, and instead 
will sum to a non-zero offset value N, so that the following equation is 
satisfied: 
EQU I.sub.K1 +I.sub.K4 -(I.sub.K2 +I.sub.K3)=N 
The DETECT signal is input to the inverting input of a shut-down comparator 
38. When the magnitude N of the DETECT signal rises above a predetermined 
reference voltage level VREF, the output of the shut-down comparator 38 
transitions from a logic low value, corresponding with normal, balanced 
cathode current flow conditions (N=O), to a logic high value, 
corresponding with an abnormal cathode current flow condition in one or 
more of the cathodes. The logic high output of the shut-down comparator 38 
is input to a latch 40 which sends a shut-down signal 42 to a normally 
closed electronic switch 44. When actuated, the electronic switch 44 
disables the inverter 18 and thus removes operating power from the lamp 
driver 20. 
Referring now to FIG. 3, FIG. 4 and FIG. 5, in the preferred embodiment the 
lamp driver 20 is a ferrite core transformer T1 having a primary winding 
46 that is energized by the inverter 18. The transformer T1 has three 
secondary windings 48, 50 and 52 which supply operating voltage through DC 
isolation capacitors C1, C2, C3, C4, C5 and C6 to the lamp cathodes. The 
secondary winding 48 has a tapped winding section 48A that provides 
operating power for the independently connected cathode K1 of lamp 1 
through conductor 22. The secondary winding 50 provides operating power to 
the commonly connected lamp cathodes K2, K3 through the primary conductor 
26. Likewise, operating power is supplied to the independently connected 
lamp cathode K4 from the secondary winding 52 through the power conductor 
28. 
In this embodiment, the analog current sensors 30, 32 and 34 comprise the 
three primary windings 30, 32 and 34 wound on a ferrite core F of a toroid 
transformer T2. The primary windings 30, 32 and 34 are connected in series 
electrical relation with the power conductors 22, 26 and 28, respectively. 
Current flow through the sensor windings generates magnetic flux signals 
.PHI..sub.30, .PHI..sub.32 and .PHI..sub.34 in the ferrite core F. The 
toroid transformer T2 also includes a secondary winding 54 which produces 
a null or zero output voltage when the flux components .PHI..sub.30, 
.PHI..sub.32 and .PHI..sub.34 are in balance. 
The current sense of the primary sensing windings 30, 34 connected in the 
supply circuits of the independently connected lamp cathodes K1, K4 is 
established relative to the current sense of the primary sensing winding 
32 for the commonly connected cathodes K2, K3 so that the magnetic flux 
components .PHI..sub.30, .PHI..sub.32, .PHI..sub.34 sum to zero within the 
toroid core F when the cathode currents are equal. 
The toroid transformer T2 performs as a magnetic flux summing circuit 36, 
and its secondary winding 54 produces an output voltage E in response to a 
non-zero total flux component in the toroid core. A non-zero total flux 
component .PHI..sub.F will develop immediately in response to unbalanced 
current flow through any one of the lamp cathodes K1, K2, K3 or K4. The 
analog voltage signal E produced by the secondary winding 54 of the analog 
signal flux summing circuit 36 is rectified by a diode D1 and is input to 
a low pass filter 56. The DC output of the low pass filter 56 is then 
input as the signal DETECT to the shut-down comparator 38. 
Referring now to FIG. 6 and FIG. 7, another protective circuit is shown in 
combination with two lamps, lamp 1 and lamp 2, having commonly connected 
cathodes K2 and K3 and independently connected cathodes K1 and K4. In this 
embodiment, the independently connected cathodes K1, K4 of lamp 1 and lamp 
2 are separately energized through a resonant driver circuit 20 formed by 
inductor reactors L1 and L2 and series capacitors C8, C9, respectively. 
Return current flow is conducted through cathodes K2, K3 to ground 
reference. Node X provides a common connection for the series connected DC 
isolation capacitors C1, C2, the series connected Zener diodes D3, D4 and 
the series connected cathodes K2, K3. A feedback voltage signal is derived 
from the cathode voltage on node X as described below. 
During normal, balanced operation, assuming all cathodes are intact, and 
that each lamp is in its socket, the cathode current sensing windings 30, 
32 produce flux components .PHI..sub.A and .PHI..sub.B that are combined 
algebraically within the summing circuit 36. As a result of the change of 
flux through the secondary winding 54 of the toroid transformer T3, an AC 
voltage appears across the secondary winding 54. The AC waveform is 
rectified by diode D1, thus producing a first feedback voltage V.sub.F1. 
At the same time, the AC voltage appearing at node X is conducted through 
diode D2 and appears as a second feedback voltage V.sub.F2. The two 
feedback voltages appear at the output node Y. Because one of the diodes 
D1, D2 will be reverse biased, the feedback voltage having the greatest 
magnitude will be input to the low pass filter 56, where it appears as the 
DETECT voltage. 
For balanced, normal operation of lamp 1 and lamp 2, the voltage on node X 
is zero, and the feedback voltage signal V.sub.F2 is zero. At the same 
time, the feedback voltage V.sub.F1 is a positive, non-zero value 
corresponding with the flux component .PHI..sub.F in the core of 
transformer T3. The comparator reference voltage VREF is adjusted to be 
slightly higher than V.sub.F1 so that the output of the shut-down 
comparator 38 remains at the logic low condition during normal operation 
of the lamps. 
However, if one of the independently connected cathodes K1, K4 should 
become open, the voltage at node X rises to a non-zero AC value, and the 
DC feedback voltage V.sub.F2 rises to a positive value. At the same time, 
one of the flux components .PHI..sub.A or .PHI..sub.B will drop to zero, 
thus causing a one-half reduction in the voltage feedback signal V.sub.F1. 
The higher voltage on node Y, V.sub.F2, is input as the DETECT voltage 
signal, causing the shut-down comparator 38 to transition, thus triggering 
a shut-down signal. 
If one of the commonly connected cathodes K2, K3 should open, the voltage 
on node X will be the clamped Zener voltage provided by the back-to-back 
connected Zener diodes D3, D4. The clamped Zener voltage on node X is 
input as feedback voltage V.sub.F2 at node Y. The Zener diodes D3, D4 are 
selected to provide a clamped Zener voltage V.sub.F2 which is greater than 
the comparator reference voltage VREF, thus providing for ballast 
shut-down in the event of a failure of one of the commonly connected 
cathodes. 
The protective method and circuitry of the present invention is applicable 
to a single lamp ballast circuit as shown in FIG. 8. In that arrangement, 
the analog signal detection circuit provides an algebraic sum of the 
analog flux signals .PHI..sub.A, .PHI..sub.B corresponding with current 
flow through the electrodes K1, K2, respectively. During normal operation, 
a non-zero flux component .PHI..sub.F =.PHI..sub.A +.PHI..sub.B is input 
through the core F of a toroid transformer T3 having two primary current 
sensing windings 30, 32 and a secondary winding 54. The analog feedback 
flux component .PHI..sub.F produces an AC voltage that is rectified by the 
diode D1 and then is input through the low pass filter 56 to the 
non-inverting terminal of the shut-down comparator 38, thus providing a 
non-zero reference voltage VREF. 
Referring again to FIG. 8, a feedback signal 30F is derived by a direct 
connection 30 from the HVAC voltage applied to cathode K1 of the single 
lamp. A feedback signal 32F is derived by a direct connection 32 from the 
ground reference. The feedback signals 30F, 32F are current signals that 
are fed through a voltage divider formed by resistors R2, R3. A diode D5 
conducts positive half-cycle voltage from the voltage divider across a 
charging capacitor C10 at node El. The DC voltage across capacitor C10 
rises to a positive value which is input as the DETECT signal to the 
non-inverting input (+) of the comparator 38. 
During normal lamp operation, with the single lamp having non-depleted 
cathodes K1, K2, the current flow into the lamp, I.sub.L +I.sub.K1, is 
equal to the current flow out of the lamp, I.sub.L +I.sub.K2. 
Consequently, under normal operating conditions, the cathode current 
I.sub.K1 is equal to the cathode current I.sub.K2, and a non-zero flux 
component .PHI..sub.F is produced in the core of the sensing toroid 
transformer T3. The flux component .PHI..sub.F generates a voltage across 
the secondary winding 54 of toroid transformer T3. The secondary voltage 
is rectified by the diode D1 and is input to the low pass filter 56 and 
appears as a positive DC voltage level VREF on the inverting (-) input of 
the shut-down comparator 38. 
If either cathode K1, K2 should open, the flux component .PHI..sub.F will 
fall to zero, and the reference voltage VREF will fall to ground 
reference. The DETECT signal voltage El, on the other hand, remains at a 
positive, non-zero voltage level across capacitor C10, thus causing the 
comparator 38 to transition to logic high. The logic high output of the 
comparator 38 is input to the SET terminal of the SR latch 40, which then 
conducts a shut-down signal 42 to the electronic switch 44, thus 
disconnecting the HVAC output of the inverter 18. 
Another failure mode of the single lamp embodiment of FIG. 8 is a depleted 
cathode. If that should occur to either cathode, the lamp arc current 
I.sub.L would continue to flow, but the cathode currents I.sub.K1 and 
I.sub.K2 would be unbalanced and unequal. Increased current flow through 
either cathode causes an increase in the flux component .PHI..sub.F. This, 
in turn, causes VREF to rise to a higher DC level. At the same time, the 
DETECT voltage E1 output from the summing circuit 36 increases 
proportionally, since the total current flow I.sub.T =I.sub.K1 +I.sub.K2 
also increases. As a result, the DETECT voltage E1 is greater than VREF, 
and shut-down signal is generated. Also, if either cathode opens, the VREF 
voltage falls to the ground reference potential (zero) and the DETECT will 
increase, since the total current flow is then conducted through the 
summing circuit 36. 
Provided that R2=R3, the voltage at E1 will be zero during normal, 
balanced, cathode current flow conditions. If the lamp is removed from its 
socket, current flow through the cathodes K1, K2 is interrupted, and the 
reference voltage VREF falls to zero. The DETECT voltage E1 remains at a 
positive, non-zero value, thus causing the shut-down comparator to 
transition. The latch 40 is then set and conducts the shut-down signal 42 
to the electronic switch 44. 
Referring now to FIG. 9, an alternative protective circuit embodiment is 
shown for a single lamp that is supplied from a transformer T4. In this 
embodiment, a magnetic flux feedback signal .PHI..sub.F having two 
components .PHI..sub.A and .PHI..sub.B are summed within the core of a 
toroid transformer T3. The primary windings 30, 32 are connected so that 
the flux components .PHI..sub.A and .PHI..sub.B are oppositely directed 
with respect to each other. The primary sensing winding 30 is connected in 
series with the main power conductor from the power supply transformer T4 
which supplies lamp operating current and filament voltage to cathode K1. 
Likewise, the primary current sensing winding 32 is connected in series 
with the filament secondary winding of transformer T4 and cathode K2. 
During normal operation of the lamp, when the cathodes K1 and K2 are in 
good condition and the lamp is in its socket, the flux components 
.PHI..sub.A and .PHI..sub.B are equal and produce a net zero flux 
component .PHI..sub.F within the secondary winding 54 of toroid 
transformer T3. Consequently, a zero DETECT voltage level is input to the 
inverting input of the shut-down comparator 38, whose output remains at a 
logic low condition. However, if one of the cathodes should open, become 
depleted or short-circuited, the flux feedback component .PHI..sub.F 
becomes a non-zero value, which causes the DETECT voltage level on the 
inverting input of the comparator 38 to rise above the REF, thus producing 
a shut-down signal as previously described. 
Yet another protective circuit arrangement for a single lamp ballast is 
shown in FIG. 10. In this embodiment, the lamp is a two-pin lamp having a 
glow bottle starter. The glow bottle starter circuit is formed by a 
capacitor C.sub.0 connected in parallel with a thermal switch SW. The 
parallel connected capacitor and switch are connected in series with the 
cathodes K1, K2. When operating voltage is first applied across pin 1 and 
pin 2, the thermal switch SW is closed, thus permitting rapid heating of 
the cathodes K1, K2. After a short time interval, the thermal switch 
opens, and the cathodes are connected in series circuit relation by the 
capacitor C.sub.0. The capacitor C.sub.0 and the inductor L.sub.0 in the 
lamp driver 20 are selected to provide a resonant circuit after the lamp 
arc I.sub.L is established. 
The protective circuit shown in FIG. 10 is capable of detecting an open 
cathode condition in either cathode K1 or cathode K2. The cathode currents 
flowing through pin 1 and pin 2 are sensed on direct coupling nodes 30, 
32. Voltage divider resistors R4, R5 provide a reduced amplitude AC 
voltage E2 from which feedback signals 30F, 32F are derived. Diodes D6, D7 
are connected to the node E2 between R5 and R7 so that negative portions 
of the AC waveform at E2 are conducted as feedback signal 30F, and the 
positive portions of the AC waveform appearing at E2 are conducted as a 
feedback signal 32F. 
According to the present invention, the analog feedback signals 30F, 32F 
are input to the summing circuit 36 and are applied to the input terminals 
of a voltage divider circuit formed by resistors R6, R7. The resistors R6, 
R7 have equal value, so that under balanced, normal lamp operating 
conditions, the voltage at node E3 is zero. Electrolytic capacitors C12, 
C13 are connected between ground reference potential and the input signals 
30F, 32F, respectively. By this arrangement, the electrolytic capacitor 
C12 charges to a negative DC voltage level (-V.sub.C12), and the capacitor 
C6 charges to a positive voltage level (+V.sub.C13). 
If either cathodes should fail, for example by depletion or by opening, the 
voltage waveform across pin 1 and pin 2 becomes asymmetrical. Under 
asymmetrical power waveform conditions, the energy content of the positive 
waveform portions and the negative waveform portions are not equal. The 
asymmetrical condition is detected according to the circuit shown in FIG. 
10 by separately inputting the positive and negative waveform portions 
into the summing circuit 36. Because the positive and negative waveform 
portions are unequal, the charging capacitors C12, C13 will charge to 
different voltage levels, thus producing a non-zero voltage level on node 
E3. Consequently, the voltage appearing at node E3 will either be positive 
or negative, depending upon the asymmetry of the power voltage waveform 
applied across pin 1 and pin 2. 
The node voltage E3 is filtered by capacitor C14 and is applied through a 
current limiting resistor R8 as an input to the base of a pair of 
switching transistors Q1, Q2. Q1 is a PNP transistor, and Q2 is an NPN 
transistor. Initially, both transistors Q1, Q2 are off, assuming normal 
lamp operation and a zero node voltage at E3. However, if the voltage on 
node E3 rises to a positive or negative value greater than the 
base-to-emitter voltage of either Q1 or Q2, one of the transistors will 
turn on, thus pulling the base of DETECT transistor Q3 to ground reference 
potential. DETECT transistor Q3 is normally on, as a result of a bias 
voltage developed by a resistor R9 that connects+V.sub.0 bias voltage to 
the base of NPN transistor Q3. However, when either Q1 or Q2 conduct, the 
base of DETECT transistor Q3 is brought to ground reference potential, 
thus causing it to turn off. 
The inverting and non-inverting inputs of the comparator 38 are initially 
biased by bias resistors R10, R11 and R12, R13 to maintain the comparator 
output in the logic low condition when the lamp is operating normally and 
the differential voltage at node E3 is a zero or some value less than a 
predetermined threshold limit. The capacitor C15 and a capacitor C16 
filters transient voltages that could produce a false DETECT signal. 
Initially, the DETECT transistor Q3 is turned on, thus maintaining the 
non-inverting input of the comparator 38 at a value less than the 
reference voltage VREF. However, if either transistor switch Q1 or Q2 
turns on, the DETECT transistor Q3 is turned off, thus permitting the 
voltage on the non-inverting input of the comparator 38 to rise to a value 
that exceeds the reference voltage VREF. As the comparator 38 transitions 
from logic low to logic high, the latch 40 is set, thus producing a 
trigger signal 42 that opens the electronic switch 44. This disconnects 
the HVAC operating power from the lamp. 
Referring now to FIG. 11, a protective circuit that is responsive to an 
asymmetrical waveform condition is interconnected with a ballast circuit 
that supplies operating power to two lamps, lamp 1 and lamp 2, that have 
commonly connected cathodes K1, K2. The current flow into cathode K1 is 
sensed by a feedback resistor R14, and current flow through cathode K2 is 
sensed by a feedback resistor R15. The feedback resistors R14, R15 are 
connected to resistor R16 in a voltage divider arrangements, with the 
voltage E4 appearing at the voltage divider node E4 being input to the 
summing circuit 36. The positive half-cycle of the AC waveform on node E4 
is conducted as feedback signal 32F through the diode D7. Likewise, the 
negative portion of the waveform appearing at node E4 is input to the 
summing circuit through the diode D6 as feedback signal 30F. The feedback 
voltages 30F, 32F are combined algebraically and are compared to produce a 
DETECT signal in the same manner as discussed in connection with the 
operation of the summing circuit 36 in FIG. 10. The operation of the 
protective circuit of FIG. 11 for producing the turn-off signal 42 is the 
same as discussed in connection with the two pin lamp of FIG. 10.