CA1110317A - Two level inverter circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An emergency lighting system includes a two-level switching transistor inventor circuit for starting and operating a fluorescent lamp from a battery upon failure of line voltage. One of the switching transistors of the two transistor inverter is arranged for connection first to a tap, then to one of the feed-in terminals of an output transformer coupled with the lamp. A timing controller effects this switching thereby to provide, first, a high power level for starting the lamp, and then, a second lower power level for operating the lamp after starting.

Description

~03~

The present invention relates to a two level inverter circult for operating a gaseous dis-charge lamp, and more particularly, to an emergency lighting circuit for operating a gaseous discharge -~
lamp from a DC energy source upon failure of the primary AC source, the circuit being provided with two power level operation to conserve energy in the DC source.
Electric power Failures due to inclement weather conditions and equipment breakdowns have been a plague for many years. A power failure, no matter what may be the cause, may very well jeopardiæe human life due to lighting system failure. There are, therefore, many installations which require some type of emergency lighting system which will automa-tically come into operation upon the occurrence of a power -failure. The high efFiciency of a Fluorescent lamp ~-makes it especially valuable for use in an emergency lighting system.
Presently available emergency lighting ` systems are generally of the type using transistor - switching inverter circuitry. In a typical arrange- ;
ment, a single lamp (or group of lamps3 is used ~or both normal AC operation of the lighting system and for the emergency system, a rechargeable battery being the power source ~ r energizing the transistor inverter when the AC line voltage fails. A principle limitation of such emergency lighting systems resides in battery capacity, that is, the amount oF lamp operating time a given battery will provide. To be taken into consideration is the fact that a higher power level is required to star-t such a lamp than is .
,.

~V3~7 58-BD-628]
t required to maintain it in operation once started.
This especially is the case in an arrangement wherein the lamp is eight feet in length, and over. Operating the lamp after starting at the same power level as is required to effect starting results in the operating li~e of the battery, and therefore of the system, being less than recommended.
It is desirable therefore to provlde an emergency lighting system capable of maximizing the operating time on a given battery charge.
Accordingly, it is an object of the present invention to provide an emergency lighting system having an inverter capable of operating on two power levels: a high power level for starting the lamp and a bwer power level for running the lamp after starting.
In accordance with the present invention, there is provided an improved invlQrter circuit for operating at least one gaseous discharge lamp from a DC electrical energy source. Incluaed are means for operating the circuit on at least two power levels:
a first power level for starting the lamp and a second lower power level for operating the lamp after starting.
In the preferred embodiment, there is provided an improved emergency lighting circuit for operating at least one gaseous discharge lamp including means for connection to an AC electrical energy source for energizing the lamp during normal conditions when the AC source voltage is above a predetermined value and an inverter and an auxiliary electrical energy source for operating the lamp during emergency conditions when the AC source voltage is below a predetermined ~ 31~ 58-BD-6281 value. Included are means for operating the inverter `~
on at least two power leveIs for better matching the inverter output capabilities to the lamp starting requirements, and after starting/ to lamp run ~
requirements. -~-EIGURE 1 is a detailed schematic representation of the preferred embodiment of the circuit of the present invention; ~`
FIGURE 2 is a detailed schematic representation of another ~orm of the preferred embodiment; and FIGURE 3 is a graphical representation showing operating characteristics of the inverter circuit of FIGURE 1 and of a typical fluorescent lamp.
; Referring to FIG. l, there is shown an ; emergency lighting system circuit which will automatically become operative upon the failure of the primary electric source. Such a system is disclosed and claimed in U.S. Patent 3,921,00S - Watrous, dated November 18, 1975, assigned to the same assignee as the present invention. A tuned inverter lO includes a pair of power transistors QA and QB capable of operation in a low 105s switching mode to energize a load ;~ such as gaseous discharge lamps 12. ~n auxiliary electrical energy source in the form of a rechargeable battery 14 provides the power necessary to operate the inverter lO. A buffer inductance Ll serves to - enable transistors QA and QB to operate in a low loss switching mode. A first transformer Tl couples the inverter 10 with the lamp 12 and is resonated with capacitors C101 and C102 to set the operating frequency .

~3~7 58-BD-628l I
of the inverter and to establish a sinusoidal output voltage. Inductor Ll is electrically connected with center tap 15 of primary winding Pl forming a part ;~
o transormer Tl.
A control circuit 20 (in this preferred embodiment, an integrated circuit) is provided for supplying base drive for switching transistors QA
and QB at zero collector voltage: that is, when the instantaneous voltage across transformer Tl varies at fundamental frequency, the voltage at point 22 and hence across inductor Ll varies at twice the fun-damental frequency. The current through Ll is DC
with a second harmonic component. This same current r,:
is alternately carried by the two transistors QA and QB~ While the transistors are required to switch collector current, they do so at essentially zero collector voltage with a resultant low power dissipation.
An auxiliary winding S2 magnetically coupled with the primary winding Pl of first transformer T1 provides ~-timing in~ormation to the control clrcuit 20 for ef-! ~ fecting switching o the respective transistors QA

~i~ and QB in step with the natural resonant frequency of the inverter. Thus, the control circuit 20 tracks the resonant requency of first transformer Tl and insures that transistor switching occurs when the voltage across capacitor Cl01 is zero. Winding TlS2 ; is not used as a source of energy for driving the ;~
~' transistors QA and QB because of its sinusoidal waveform.
,~., A feedback transformer T2 is provided to supply a feedback current to the control circuit 20 for effecting transistor base drive proportional to )3~
ss-sr)-62sl transistor collector current theraby to effect hiqher ef~iciency in the inverter lO. FeedbacX transformer -T2 has a feedback winding D magnetically coupled to the respective collectors of the transistors QA and Q~ through a pair of windings A and B respectively.
Thus, the power consumed by the control circuit 20 can be limited to that required to start and control the oscillation of the inverter 10. Lamps 12 are coupled to an AC source through a ballast 16 Eor operation during normal conditions when the AC source voltage is above a predetermined value. An impedance limited transformer T3 provides, among other things, means for charging battery 14. Battery 14 is connected to a secondar~ winding S2 o~ transformer T3 through ~`
a diode D]01, ~ The emergency lighting systam circuit shown ,~ in FIG. 1 operates generally as ~ollows. Control circuit 20 includes a first sensor (not shown) ~or sensing the voltage of the batter~. Further details o~ the operation of the control circuit 20 ma~v be had by referring to the above mentioned Watrous patent.
Con-trol circuit 20 also includes loqic means (not shown) combining the outputs of the first sensor and the second sensor ~o enable inverter 10 when the battery voltage is above a predetermined level and the AC
voltage is below a predetermined level and to disable the inverter when the battery voltage is below a predetermined level or the AC voltac~e is above a ~ -predetermined level.
Assuminq now that the inverter 10 is enable to run, control circuit 20 supplies a smal] base drive signal to one of the transistors Q~ or QB~

. , -~ ~, , : ,, )3:~7 58-B~-6281 Assuming that the hase drive is applied to QA~ tran-sistor Q~ turns on and current starts to flow rom battery 14 through inducto.r ~,1, cente.r tap 15 of the primary Pl of t~sformer Tl, thence through Pl and through the A winding of f~ec~ack transforme.r T2~
through transistox QA thence back to the battery.
The base drive originally supplied to transistor QA
is augmented by a current flowi.ng from winding ~
oE feedback -transformer T2 to the control ci.rcuit 20 to exit from pin 1 thereof thence to flow through the ~ase of txansistor Q. This base drive then is proportional to the collector current of transi.stor QA and is designed to be adequate to keep the t.ransistor in saturation.
At some volt-seconcl product, feedback ~i :
transformer T2 saturates sharply, suddenly reducin~ ;
the output current of winding D thereof, thereb~ ~-reduciny the base drive to transistor QA. ~ sudden rise in collector-emitter voltage on transistor QA
20 sharply reduces the rate of current rise in this DC
circuit. This change in collector current with respect to time reverses the polarity o the S2 ~ ~
winding of transformer Tl and hence the polarity of ~:
~ the voltage on pins 3 and 4 of the control ci.rcuit .
; 20. This reversal of polarity signals the control circuit to change the base drive from transistor Q~
to transistor Q3.
; Control circuit 20 now supplies a small ~:~
amount of base drive through pin 9 to the base of transistor QB and simultaneously connects the base of Q~ to the emitter thereof to hasten the turn-off process of transistor QA. Transi.stor QB starts to . ;. . . . .
. .. .:: -: . .; ,.

3~l~
58-B~-6281 concluct as a resul-t of -the sma].l base drive s.ignal from the control circuit and currént flows through winding B of feedback transformer T2 to induce a curxent in winding D thereof. Thi.s current is supplied to control circuit 20, Control circuit 20 ~ ~
now suppl:;es this current as base drivP out ~r of pin 9 to the base of Q~; thus the base dri~e of QB is proportional to the collector current thereo such that the transistor is kept i.n saturation~
Transformer Tl has an air gap and operates as a nearly linear inductor. When the voltage across winding Pl of transformer Tl, and thereby the voltage on winding S2 of that trans:Eormer, reaches zero, this event is signaled to the control circuit 20 throuc~h pins 3 and 4 thereo:E. The control circuit again switches the base drive circuitry to transistor QA and connects the base of QB to tha emittex thereo.f ~ -to hasten the switching off o:E the transistor Q~. The circuit is then ready to go through the next half cycle with A conducting.
If switching could be accomplished in absolute ~.ero time, the above described circuit operation would be entirely correct. However, the switching is :
accomplished normally in periods of less than one microsecond and the current 10w from the battery -~
14 is essantially at a constant level with a small - .
ripple content~ This ripple content is determined by the inductance of Ll which adds or subtracts from the battery voltage as applied to the center tap 15 on the primary winding o:E transormer Tl, It is this inductor I.l which adjusts the voltage at point 22 in such a wa~v thatthe transistors may be switched at 3~7 zero collector voltage. As long as this inductance Ll has a value exceeding a critical value, the circuit will function as described. In the event that both transistors QA and QB are in the ofE state, the rate of current change in Ll forces the voltage thereacross to a value where zener diodes D104 start conducting to limit the voltage applied to the circuit. This clipping action rapidly reduces circuit efficiency and hence is an operational mode -to be avoided. Such 10 clipping action can occur momentarily during the ~ , starting process or when the inverter is turned of and under these conditions represents an acceptable design operating condition~ ~
The load for the inverter 10 which includes ~;
lamp 12 is connected to winding S1 of transEormer Tl.
For Eluorescent emergency lighting purposes! the bal-lasting is done by capacitors C102 w~lich determine ;~
the load current through the lamp 12. These capacitances in conjunction with C101 and the inductance o~ TlP1 determine the operational frequency of this system (The inductance of the P1 winding and the capacitance --of C101 determine the oscillating fre~uency when S1 is unloaded.) A double capacitive ballast system is used to reduce the voltage across a single unit and - thus enhance the reliability of the complete s~stem.
The voltage output of the inverter circuit is high enough to instant start eight Eeet long instant start ; ~
lamps under fairly adverse conditions. -As stated above, ~attery charging is acc-omplished through winding S2 of 60 Hz transformer T3, Half wave charging current is coupled to a non-linear load, the battery 14, throuyh diode D101 and is '' _~~ ;

.

3~ 7 58-BD~6281 limlted in magnitucle by the impedance oE the trans-former. ~ecause of the transEormer impedance~ the sinusoidal voltage at the terminals of winding S2 is clamped at the battery voltage when diode Dl01 conducts. On the alternate half c~cle, diode D103 conducts half wave current through indicator lamp P~
and the dual prong battery plug. Thus, the battery must be plugged in and 120 volt AC power available to energize lamp PL indicating that the battery i9 charging. Using the alternate half cycle reduces the volt-amp rating of the transformer T3. For monitoring the AC source voltage, means are provided for coupling secondary winding Sl of the transformer T3 with a linear load during an alternate half cycle. To this end, during the half cycle alternate from that in which the battery is charged, capacitor C104 is charged through diode D102~ This DC monitoring voltage is connected to the irst sensor means through pin 7 of control circuit 20 and through a linear load, 20 resistor div;der R10~ and R105. The DC voltage at pin 7 is proportional to the average value of the 60 Hz supply voltage and is not influenced by the afore-said clamping action oE t~e battery. Thus, transformer T3 serves a dual purpose.
As hereinbefore stated, it is desirable that.
upon failure of line voltage, a battery serves to operate a fluorescent lamp in an emergency lighting system for at least one and a half hours. In acc-ordance with the present invention, there is provlded in an inverter circuit for operating at least one gaseous discharge lamp from a DC electrical energy source, means for operating the circuit on at least ._9_ ,:

3:~
5~BD-6281 two power levels includi.ng a first power level :Eor starting the l.amp and a second lower power level for operating the lamp aftex starting thereby to conserve the DC energy source. In the preferred embodiment, as shown in FIG. 1, inverter circuit 10 includes output transformer T1 including primary winding P1 for coupling the inverter 10 with the lamp load.
Primary winding Pl of transformer Tl includes a pair of power feed-in terminals 100 and 101 and a tap 105.
10 A snap action relay switch 110 is provided having its ~.
common terminal 111 connected through winding A of Eeedback transformer T2 to transistor QA ~ Switch ~
has a contact 112 moveable between :Eeed-in connector 100 and tap 105. Means are provided for automatically ;~
switching switch 110 between feed-in terminal 100 and tap 105. This takes the form oE control means 120, the operation of which will now be discussed.
When 60 Hz is applied to the input terminals of the pri.mary winding P of transformer T3, capacitor C125 is quickly charged through zener diode D133 and diode D126~ This voltage causes current to flow through ~ .
resistor R124 to the base of the Darlington pair Q130 to energize relay coil 122 (part of snap action relay switch 110) over either diode D127 or D128. The in~
verter 10 is not operating because an inhibit signal appears on pin 7 of the controlling integrated circuit 20~ When AC power disappears from the input of trans- :
former T3, inverter 10 starts and runs in -the high power mode since switch contact 11.2 is in engagement with tap 105. As capacitor C125 di~charges through resistor R124, the voltage drop across the Darlington Q130 gradually increases until the relay contac-t 112 .. ~ . .. .. .

)3~l7 switch Erom the high power start mode in contact with -tap 105, to the normal run mode in contact with feed-in terminal 100~ Thj.s is so because the turns ratio of transformer Tl is lower thus providing a lower output vo:Ltage on winding Sl thereo:E than previously when transistor QA was in circuit with tap 105 through relay switch contact 112. Diode 1~9 ' suppresses transients caused by the relay coil 1~2. ~ , Capacitor C131 suppxesses transient voltage which may be picked up by the base of the Darlington Q130.
Resistor R132 serves as a linear discharge resistor for capacitor C125 to,insure a more uniEorm rate of discharge giving sharper relay drop out. Zener diode D133 prevents low AC line voltages from feeding back and keeping capacitor C125 charged. Any breakdown' device with a suitable range may be substituted ~or the æener diodeO It is noted that this circuit is o:E the quick reset type, that is r iE the line voltage temporarily returns, the timing cycle will be repeated, ReEerring now to FIG. 2, there is shown another form of the preferred embodiment of the present invention. With the arrangement as shown ' in FIG. 2, inverter 10 in addition to having transis-; tors Q~ and QB~ includes a -third switching transistor QC connected through a secondary winding C o Eeedback transformer T2 to a tap 205 on the primary Pl of ; trans:Eormer Tl. While a power transistor may be switched between taps as with a relay, another ~unction is to perform the switching with solid state circuitry ; 30 by utilizing the two main power transistors QB and QC
and swi.tching the drive Erom one to the other (QA
remains always an opera-tlng inverter transistor).

. .. . .

~ 3~7 58-BD-628l ~

In the preferred embodiment as shown in F'IG. 2, such switching is accomplished on a timed basis using ~ -a contro3.1er 200 connected in ci.rcuit with the emer-gency lighting system. While timer controller 200 is shown in block diagram form, any device is suitable which is capable of providiny, on line 202, a positive signal and after a given period of time~ allowing the positive voltage to disappear thence to allow the control lead 202 to be connected to the circuit ground.
When control lead 202 is plus to resistor R213, transistor Q215 is turned on allowing the switching signal from pin 9 of the controller 20 to be connected to the base of transistor QC~ This allows QC to turn on and conduct current through secondary winding C of feedback transformer T2~ The~ ~
higher than normal voltage induced on transistor QB;;
is blocked by diode D206. When the timer 200 times out and the control lead 202 is grouned~ transistor 20 Q215 is turned of and transistor Q214 is tu.rned on.
allowing the base drive signal ~oming from pin 9 of controller 20 to be transferred to ~he base of the main power transistor QB~ Operating all the time --are transistors Q216 and Q217 which serve to conduct the stored charges which may exist on transistor Q~
or QC to ground and thus hasten the switching of the main power transistor. Transistors Q216 and Q217 - ;
duplicate some of the functions existing in the integrated circuit controller 20 and their bases 30 are driven by the current from pin 1 o~ contxoller 20. ;~
To prevent QA~ Q216 and Q217 from fighting over the drive current available, a resistor R211 is established ;

in the base of transistor QA to limit the drive current therein.
FIG. 3 illustratas how such a two level:~
inverter can be applied to operate an ei.ght foot long instant start fluorescent lamp of the t~pe designated as F96T12/IS. Referring to curve A, there is shown that the lamp starts conducting in the vicinity of eight milliamperes when approximately 250 volts are applied across its electrodes. At this time, the electrode phenonema appar~ntly are most important in the lamp since, when current through the lamp is caused to : increase, substantially more vol.tage is re~uired across the electrodes in order to allow the flow of this increased current~ By the time the lam~ terminal voltage reaches approximately 500 volts, about 35 milliamperes are flowing in the lamp. At this point, enough eneryy is liberated at the elactrodes such that one, and then subsequently two, of the el.ectrocles form electron emitting hot spots which reduce the voltage drop at the electrodes to a relatively low value. All that remains is the voltage drop across~ ;
the ei~ht feet o~ conducting plasma in the lamp 4 This ~ *
plasma voltage drop i~ almost invarient with the current going through it as shown by the horizont.al line labeled "Lamp Hot Spotted Operation". This curve was taken at approximately 4 to 5 kHz operation and this effect may not be necessarily so at greatly reduced or greatly increased frequency spread. It should be pointed out that in the lamp glow mode where the operation is limited by the voltage drops at the electrodes, such voltage drops consume considerable power yet do nothing to produce light output. Thus, ''' : ' .

~ 317 58-BD-6281 to secure efficient light production from the fluore-scent lamp, it is necessary to secure most of the power usage in the lamp in the gas plasma to accomplish the desired electron orbital transitions.
Superimposed on the same graph of FIG. 3 ; as is the lamp characteristic curve A are the output load lines, both high and low power designated Curve B and Curve C respectively, of the inverter constructed as shown in either of the FIGS. 1 or 2. Thus, if the inverter were operated in the low power mode as shown in Curve C, the inverter would be incapable of driving the lamp from the glow mode into the hot spotted mode because load line C does not intersect the lamp load line A at a point beyond the power input to the lamp which is required for a hot spot transistion. The inverter would operate stably at the intercept point marked "I" on curve C. Such an inverter could possibly cause the lamp to occasionally go into the hot spot mode because of extraneous radiation on the lamp.
Also the possibility of transients occurring because of switching of power supplies could temporarily force the inverter load line into the unstable mode of the lamp thereby cause the lamp to transition. If the lamp by some method could be transitioned into the ho~ spot mode, i would operate in equilibrium at point II on -~;
the diagram. The advantage of being able to transition ~-to point II can be seen by noting that at point I
slightly over 15 watts are being dissipated in the lamp. At point II, only approximately II watts are ~-30 being dissipated yet point II is generating more light ~;
output than is point I (light output is related app-roximately to lamp current). To reliably secure inver-ter , . ;: .

~ 317 58-BD-6281 lamp operation at point II, the inverter is first operated in the high power mode according to the load line B. The only place that load line B intercepts the lamp load line A is at point III. This forces the lamp to operate in the hot spotted mode because sufficient energy is supplied to the lamp to generate the required electron emitting spots on the electrodes.
After a given period of time, or possibly after some other criterion such as lamp voltage has been satis-fied, the inverter then may transition to the lowpower mode C and hence operate at the intercept point II on the curve.
Once the lamp is operating in the hot spotted mode, as long as this mode of operation is - not interrupted, it will continue to do so until the level of lamp operation load line comes close to intercepting the lamp glow mode curve. For emergency lighting purposes, this is advantageous because it means the lamp will stay in this highly efficient light generating condition until the light level produced by the lamp has degenerated considerably.
The emergency lighting circuit including ~ the inverter circuit of FIG. 1 has been built and ;~ has operated satisfactorily with components having the following values:
transistors QA' QB GE D42C10 transformer T1 primary winding Dl -; 72 turns, tapped at 36 turns load winding Sl - 1788 turns feedback winding S2 - 12 turns transformer T2 collector windings A & B -8 turns 3~7 output winding D - 160 turns transformer T3 prLmary windiny P - 1058 turns secondary winding Sl - 260 turns secondary winding S2 - 224 turns inductor Ll 120 turns .0359`' -~
lamps 12, 12' F96T12/IS
battery 14 16.8 VDC
resistor R101 15 K ohms resistor ~102 56 K ohms resistor R103 56 K ohms resistor R104 18 K ohms resistor R105 270 K ohms capacitor C101 0.68 uF.
capacitor C102, C102' 5000 pF
capacitor C105 .01 UF
capacitor C104 .33 uF
diodes D101, D102, D103, I~ 4004 ~ " Dlll, D112, D115, D116 DA 1701 zener diodes D104 21 V ~ 5%, 1/2 W (each) ' 20 Relay 110 Form C reed Relay 24 V dc coil Resistor R124 6.8 Megohm Resistor R132 10.0 Megohm Capacitor C125 2.2 ufd ;~
Capacitor C131 .01 ufd :
~iodes D126, D127, D128 IN 4004 : :.
: Diode D129 DA 1701 . Zener Diode D133 21 V Zener Darlington Q130 2 N 5308 The emergency lighting circuit of FIG. 2 has been built ~ 30 and has operated satisfactorily with additional components ; having the following values:
transistor QC GE D42C10 3~7~

transistor Q214 2 N 5811 transistor Q215 2 ~ 5810 transistor Q216 2 ~ 5810 transistor Q217 2 ~ 5810 Resistor R211 4.7 ohms Resistor R213 2700 ohms Resistor R221 180 ohms Resistor R222 1 K ohms `-~
R~sistors R223, R224 100 ohms ~ -10 Diode D206 I~ 4004 Transformer T2 output winding C-8 turns - ~`
The specific invarter circuit and/or emer-gency lighting circuits herein described are intended as exemplary and not limitative of ths invention.
For example, an inverter need not be coupled to a load by a transformer; direct coupling has been `~
contemplated, however, with appropr:iate circuitry changes. In such applications, other power level switching arrangements will occur to those skilled in the art. Other inverter configurations certainly have been contemplated and include the bridge switching ~-type and others. The appended claims are intended to include such modifications, and others which may occur to those sXilled in the art, as coming within the true spirit and scope of the invention.

. ,

Claims (11)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In an inverter circuit for operating at least one gaseous discharge lamp from a DC electrical energy source, the improvement comprising:
switching means in the circuit for automatically operating the circuit on at least two power levels including a first power level for starting the lamp and a second lower power level for operating the lamp after starting thereby to conserve the DC energy source.

2. The invention of claim 1 wherein the inverter circuit further comprises an inverter having an output, means for connecting the inverter with the DC energy source and an output transformer for coupling the inverter with the lamp, said output transformer having a primary winding including a pair of power feed-in terminals and at least on tap for connection to the inverter output.

3. The invention of claim 2 wherein the switching means includes means for automatically switching one side of the inverter output from the tap on the primary winding of the output transformer to one of the power feed-in terminals thereof.

4. In an emergency lighting circuit for operating at least one gaseous discharge lamp including means for connection to an AC electrical energy source for energizing the lamp during normal conditions when the AC source voltage is above a predeter-mined value and an inverter having an output and an auxiliary electrical energy source for operating the lamp during emergency conditions when the AC source voltage is below a predetermined value, the improvement comprising:
switching means in the circuit for automatically operating the inverter on at least two power levels for better matching the inverter output capabilities to the lamp starting requirements and after starting, to lamp run requirements thereby to conserve energy in the auxiliary electrical energy source.

5. The invention of claim 4 wherein the circuit includes an output transformer having a primary winding including a pair of power feed-in terminals connected to the inverter output for coupling the inverter with the lamp.

6. The invention of claim 5 wherein the inverter is of the transistor switching type having two transistors capable of operation in a push-pull arrangement, a first of the transistors being connected to a first of the power feed-in terminals of the primary winding.

7. The invention of claim 6 wherein the primary winding of the output transformer includes at least one tap, the switching means including means for connecting a second of the switching transistors upon inverter turn-on to the tap and after a pre-determined time, to a second of the power feed-in terminals of the primary winding.

8. The invention of claim 5 wherein the switching means includes means for automatically switching one side of the inverter output from a tap on the primary winding of the output transformer to one of the power feed-in terminals thereon.

9. The invention of claim 8 wherein the means for automatically switching includes a switch operable in response to the operation of a timer.

10. The invention of claim 6 wherein:
a second of the transistors is connected to a second of the power feed-in terminals of the primary winding; and a third transistor is included connected to a tap on the primary winding.
11. The invention of claim 10 further including a controller having timing means for controlling the operation

Claim 11 continued:
of the third transistor and the first transistor whereby, upon energization of the inverter, the third transistor and first transistors are operated in the switching inverter mode, and after a predetermined time, the third transistor is switched to the off state and the first and second transistors then operate in the switching inverter mode.