Surge protection of full-wave rectifier by biased ionization tube

A television receiver includes a full-wave rectifier and filter capacitor coupled to the A.C. power line for generating an unregulated direct operating voltage for the receiver. In order to provide a start-up voltage for certain portions of the receiver when the receiver is initially turned on, a transformer has a primary winding coupled between the rectifier and the filter capacitor which is responsive to the initial surge current in the capacitor. In order to protect the diodes of the rectifier from voltage surges which may enter the receiver from the A.C. line, a glow or ionization lamp is coupled across the primary of the start-up transformer. The glow lamps have unpredictable start-up voltages when operated in the dark interior of the receiver housing. The lamps are ionized independent of the presence of surges by coupling to the horizontal deflection circuit. With independent ionization of the glow lamp, the voltage drop and speed of response to surges is improved.

BACKGROUND OF THE INVENTION 
This invention relates to arrangements for protecting reverse-biased diodes 
of a full-wave bridge rectifier against power-line voltage surges by use 
of a biased ionization tube. 
Electrically powered equipment is often energized with direct voltage 
obtained by rectification from the alternating-current power mains. 
Typically, such an equipment uses rectifiers to change alternating current 
into a pulsating unidirectional or direct current which is then applied to 
an integrating capacitor to obtain the desired direct or unidirectional 
voltage to power the equipment. As is known, transformers may be 
interposed between the power mains and the rectifiers in order to obtain 
galvanic or conductive isolation of the equipment from the mains, to 
provide a voltage transformation, or for other reasons. 
Surges having a duration of a few microseconds to twenty or more 
microseconds, and having a potential of up to 5,000 volts have been 
observed on the power mains. Voltages of this magnitude substantially 
exceed the maximum reverse voltage which power-supply rectifiers are 
commonly designed to withstand. Since the maximum voltage which such 
surges are capable of reaching is unknown, it is impractical to use 
power-supply rectifiers having maximum reverse voltages capable of 
withstanding the surges. In the ordinary silicon rectifier, application of 
excessive reverse voltage for more than a short period of time allows an 
avalanche current to build up which may destroy the junction. 
When the filter capacitor is coupled directly to the output of a bridge 
rectifier without the presence of an intervening impedance element, the 
maximum reverse voltage appearing across diodes of the rectifier cannot 
exceed the capacitor voltage. Since the capacitor voltage cannot change 
instantaneously, the diodes are protected against excessive reverse 
voltage surges. 
Copending application Ser. No. 750,632 filed Dec. 15, 1976 now U.S. Pat. 
No. 4,127,875; issued Nov. 28, 1978 in the name of R. E. Fernsler, et al. 
describes a start-up circuit for a television receiver including a 
transformer having its primary winding coupled between the power-supply 
rectifier and the filter capacitor. The transformer winding introduces an 
impedance in the charging path of the filter capacitor which allows the 
surge voltage to appear across the back-biased diodes of the bridge 
rectifier, thereby exacerbating the problem of diode destruction due to 
excessive reverse voltage. 
Copending application Ser. No. 901,232 filed Apr. 28, 1978 in the name of 
Robert Giger now abandoned describes a voltage controlled switch in the 
form of an ionization or glow lamp coupled across the transformer winding 
for limiting the reverse voltage across the diodes to the sum of the 
filter capacitor voltage and the offset voltage of the lamp. 
The glow lamps have been found to have variable turn-on speed and voltage 
drop characteristics when operated in a dark environment such as the 
interior of the cabinet of a television receiver. It is desirable to have 
consistent turn-on speed and voltage drop characteristics for reliable 
protection of the rectifier diodes. 
SUMMARY OF THE INVENTION 
A direct voltage supply for a television receiver adapted to be energized 
from a source of alternating current upon which voltage surges of 
uncontrolled magnitude may appear and which is intended to be enclosed 
within a housing includes a full-wave rectifier coupled to the source of 
alternating current for producing a pulsating direct current. An impedance 
couples the output of the full-wave rectifier to a filter capacitor for 
generating a direct voltage from the pulsating direct current. An 
ionization device is coupled across the impedance for coupling to the 
capacitor those surges exceeding the offset potential of the device. The 
ionization device is coupled to the horizontal deflection circuit for 
ionizing the device independent of the surges.

DESCRIPTION OF THE INVENTION 
In FIG. 1, a plug 10 carries two conductors adapted to be coupled to the 
power mains. One conductor of plug 10 is coupled by way of a 
radio-frequency interference (RFI) filter inductor 12b to a terminal 13 of 
a bridge rectifier designated generally as 16. The other conductor of plug 
10 is coupled through an RFI filter inductor 12a and through a fuse 14 to 
one end of an RFI suppression capacitor 17, the other end of which is 
coupled to terminal 13. The junction of fuse 14 and capacitor 17 is 
coupled by means of an on-off switch 18 to a low-value current-limiting 
resistor 20. A terminal 22 of rectifier 16 is coupled to resistor 20. 
Thus, terminals 13 and 22 are effectively coupled to the power mains. 
Pulsating direct current is generated at terminals 24 and 26 of rectifier 
16 and flows through the series path formed by a primary winding 50a of a 
transformer 50 and a filter capacitor 60. Capacitor 60 integrates the 
pulsating current to form an energizing voltage across its terminals of 
the polarity illustrated. A secondary winding 50b of transformer 50 is 
coupled to that portion of the television receiver requiring energization 
at turn-on. As illustrated, winding 50b is coupled to a horizontal 
oscillator 75. A horizontal deflection circuit illustrated as a block 70 
is coupled across capacitor 60 for being energized by the voltage 
thereacross. A glow lamp illustrated as a neon bulb or lamp 90 has its 
leads connected across winding 50a and is physically in proximity to 
horizontal deflection circuit 70. 
Neon lamp 90 has a voltage-current characteristic illustrated in FIG. 3. As 
illustrated, the curve is symmetrical about the current (I) axis denoting 
conduction symmetry. With 0 volts across the lamp, it is ordinarily not 
ionized and presents a high resistance to the flow of current. Curve 
portion 301 represents the low current which flows in the lamp in its 
nonionized state. When the voltage applied across the terminals reaches a 
value such as V1, the gas in the lamp becomes ionized by the electric 
field across its electrodes and the voltage drops to a value illustrated 
as V2. With the lamp in the ionized state, it has a relatively low dynamic 
resistance and small increments of applied voltage result in relatively 
large increases in the current therethrough. 
In normal operation of the arrangement of FIG. 1, a potential on the order 
of a few volts appears across winding 50a due to the flow of current from 
rectifier 16 to capacitor 60 to supply the requirements of horizontal 
deflection circuit 70. Horizontal deflection circuit 70 produces 
high-voltage radio-frequency (RF) pulses in the course of its operation. 
Such pulses may exceed 1,000 volts at a frequency of 15,750 Hz. The 
physical structure by which the deflection circuit generates the pulses is 
relatively large, and the electric fields in its vicinity are strong. With 
neon lamp 90 in physical proximity to deflection circuit 70, the neon gas 
within the envelope of the lamp is excited or ionized by the deflection 
current fields, as evidenced by a bright glow. This biases the lamp to 
region 302 of low dynamic impedance. No current flows through lamp 90, 
however, because the voltage applied across its terminals is less than 
offset voltage V2. 
With lamp 90 ionized by the field produced by deflection circuit 70, surges 
incoming to the television receiver from the power mains and appearing 
across winding 50a need only exceed voltage V2 of FIG. 3 to cause lamp 90 
to conduct and couple the surges to capacitor 60. If lamp 90 were not 
ionized by the deflection circuit field, the surge voltage would have to 
reach voltage V1 or possibly a greater voltage, depending upon the 
duration of the surge. Thus, the ionization of the lamps by the electric 
field of the deflection circuit reduces the magnitude of the surge 
appearing across the reverse-biased diodes of rectifier 16. 
In FIG. 2, details of horizontal deflection circuit 70 are illustrated. 
Elements of FIG. 2 corresponding to those of FIG. 1 are designated by the 
same number. In FIG. 2, deflection circuit 270 coupled across filter 
capacitor 60 includes an NPN transistor having its collector-emitter path 
serially coupled with a winding 274. A damper diode 276 is coupled across 
the collector-emitter conducting path of transistor 272. A retrace 
capacitor 278 and the serial connection of a deflection winding 280 and 
S-shaping capacitor 287 are coupled across damper diode 276. A conductor 
284 couples the output of horizontal oscillator 75 to the base of 
horizontal output transistor 272 for coupling horizontal drive pulses 
thereto. 
A neon lamp 290 has its leads coupled across primary winding 50a for 
coupling surges appearing between terminals 24 and 26 to capacitor 60. 
Lamp 290 may be located conveniently at a distance from the high-power 
portions of horizontal deflection circuit 272. A conductor 292 has one end 
wrapped about the glass envelope of lamp 290 and the other end connected 
of the collector of transistor 272. 
In normal operation, high-voltage retrace pulses appearing at the collector 
of transistor 272 are brought into close proximity of lamp 290 by lead 
292. The radio-frequency pulses create an electric field within the 
envelope of lamp 290 which ionizes the gas. It is believed that the 
capacitance between conductor 292 and the electrodes of lamp 290 provides 
a path for the flow of current by which the power required to maintain the 
lamp ionized is supplied. 
While the turns of conductor 292 around the envelope of lamp 290 provide 
reasonably sound mechanical coupling, it is only necessary for conductor 
292 to be in the vicinity of the lamp in order to achieve ionization. 
Those skilled in the art will recognize that the high-voltage radio 
frequency pulses may be obtained from other locations associated with the 
horizontal deflection apparatus.