Inverter power supply for incandescent lamp

An inverter is powered by a magnitude-modulated DC supply voltage derived by rectification from an ordinary 120Volt/60Hz electric utility power line. The inverter powers a low voltage (12 Volt) incandescent lamp with a magnitude-modulated high frequency (30 kHz) voltage. The magnitude modulation on the high frequency voltage is proportional to the magnitude modulation on the DC supply voltage.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to controllable power-line-operated 
inverter-type power supplies for incandesent lamps. 
2. Description of Prior Art 
Power-line-operated inverter-type power supplies are presently being used 
in a variety of applications. For instance, such power supplies are 
frequently being used for powering low-voltage incandescent lamps. 
When using such inverter-type power supplies in connection with powering 
various loads, such as low-voltage incandescent lamps or microwave 
magnetrons, it is sometimes desirable to be able by way of electrically 
actuatable means to control the inverter output voltage, thereby providing 
for control of the power provided to the load. However, to provide 
cost-effectively for electrically actuatable means to effect control of 
the output of inverters is not as simple as it might initially appear. 
Of course, to achieve such control, one might use an electrically 
actuatable variable-ratio transformer (Variac) between the power line and 
the input of the power supply. However, the cost and complexities 
associated with such an approach would be unacceptably high in most 
applications. 
Or, one might consider the use of a Triac-type voltage control means 
mounted between the power line and the power supply. However, Triac-type 
voltage control means simply do not function properly with the kind of 
input characteristics normally associated with power-line-operated 
inverter-type power supplies. 
Then, there is the possibility of using an inverter-type power supply with 
a special input circuit that would permit the use of a Triac-type control 
means; which input circuit would then have to make the inverter 
power-input-characteristics appear substantially like a resistive load. 
Even so, however, there is the cost and the electrical inefficiency of the 
Triac-type control to consider. 
The present invention represents yet another solution; which other solution 
is novel, less costly and electrically more efficient than that of using a 
Triac-type control means between the power line and the inverter input. 
SUMMARY OF THE INVENTION 
Objects of Invention 
An object of the present invention is that of providing a 
power-line-operated inverter-type power supply having electrically 
actuatable means to permit output voltage control. 
This as well as other objects, features and advantages of the present 
invention will become apparent from the following description and claims. 
Brief Description 
In its preferred embodiment, subject invention is a power supply adapted to 
be powered from the regular 60 Hz power line voltage and to provide an 
output of relatively high-frequency (30 kHz) substantially squarewave 
voltage. This output voltage is provided by an inverter that is powered by 
way of the pulsed DC voltage derived from unfiltered full-wave 
rectification of the 60 Hz power line voltage. Thus, the high-frequency 
inverter output voltage is pulse-amplitude-modulated at a 120 Hz rate--in 
correspondence with the pulse-amplitude-modulations of the pulsed DC 
supply voltage. 
The inverter is of a type that has to be triggered into oscillation. 
However, once triggered, it will continue to oscillate, but only for as 
long as the instantaneous magnitude of its pulsed DC supply voltage 
exceeds a certain threshold level. 
Since the pulsed DC supply voltage falls to zero magnitude between each 
pulse, the inverter stops oscillating between each pulse. Thus, as long as 
output voltage is desired, the inverter has to be re-triggered after each 
pulse of the DC supply voltage. 
Inverter triggering is accomplished by a Diac in combination with an RC 
integrating circuit; which means that--upon each application of a pulse of 
DC supply voltage--the inverter is triggered into oscillation only after 
the DC supply voltage has been present for some period of time; the length 
of this period being determined by the nature of the RC integrating 
circuit--much in the same way as phase-control is accomplished in an 
ordinary Triac-type incandescent lamp dimmer. 
Connected with the RC integrating circuit is a control transistor, the 
effective impedance of which can be varied over a wide range by way of an 
electrical control voltage. With this control voltage having a relatively 
low magnitude, the inverter is triggered into oscillation quite early in 
the period of each pulse of the DC supply voltage; whereas, with this 
control voltage having a relatively large magnitude, no inverter 
triggering takes place at all. 
For in-between magnitudes of the control voltage, inverter triggering takes 
place at substantially corresponding in-between delays relative to the 
onset of each DC pulse; which means that the net effective RMS magnitude 
of the output voltage can be adjusted by adjusting the magnitude of the 
control voltage. 
Thus, by providing a control voltage to a pair of control terminals, the 
magnitude of the inverter output voltage can be adjusted over a wide 
range: from a maximum and all the way down to zero output--with a response 
time equal to half a cycle of the 60 Hz power line voltage. 
By sensing the average or RMS magnitude of the inverter output voltage and 
by providing a control voltage to the control transistor that is 
effectively proportional to that average or RMS magnitude, output 
magnitude control can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Description of the Drawings 
In FIG. 1, a source S of 120 Volt/60 Hz voltage is connected with 
full-bridge rectifier FBR. Positive output terminal OTa of rectifier FBR 
is connected directly with a B+ bus; and negative output terminal OTb of 
rectifier FBR is connected directly with a B- bus. 
Between the B+ bus and the B- bus is connected a series-combination of two 
capacitors C1 and C2, which two capacitors are connected together at a 
junction CJ. 
Between the B+ bus and the B- bus is also connected a series-combination of 
two transistors Q1 and Q2. 
The secondary winding CT1s of positive feedback current transformer CT1 is 
connected directly between the base and the emitter of transistor Q1; and 
the secondary winding CT2s of positive feedback current transformer CT2 is 
connected directly between the base and the emitter of transistor Q2. 
The collector of transistor Q1 is connected directly with the B+ bus; the 
emitter of transistor Q2 is connected directly with the B- bus; and the 
emitter of transistor Q1 is connected directly with the collector of 
transistor Q2, thereby forming junction QJ. 
The series-connected primary windings CT1p and CT2p are connected directly 
between junction QJ and a point X; while the primary winding Tp of 
transformer T is connected between point X and junction CJ. 
Transformer T has a secondary winding Ts, which is connected directly with 
an incandescent lamp IL. 
A resistor R1 is connected with its one terminal to the B+ bus and with its 
other terminal to point X. Another resistor R2 is connected between point 
X and one terminal of a variable resistor R3. The other terminal of R3 is 
connected to junction DJ, to which junction is also connected one of the 
terminals of a capacitor C3. The other terminal of C3 is connected to the 
B- bus. 
A Diac D is connected between junction DJ and the base of transistor Q2. 
A rectifier R is connected with its anode to junction DJ and with its 
cathode to junction QJ. 
A control transistor CQ is connected with its collector to the junction JR 
between resistors R2 and R3, and with its emitter to the B- bus. A 
resistor R4 is connected between the control transistor's base and 
emitter; and a resistor R5 is connected between a control terminal CT1 and 
the base of the control transistor. Another control terminal CT2 is 
connected directly with the B- bus. 
The overall inverter is identifed with the letter I. 
Actual values and descriptions of the components used in the preferred 
arrangement in FIG. 1 are listed as follows. 
______________________________________ 
Output of Source S: 
120 Volt/60 Hz; 
Full Bridge Rectifier 
Four 1N4004's; 
FBR: 
Capacitors Cl & C2: 
0.47 .mu.F/200 Volt; 
Transistors Q1 & Q2: 
Motorola MJE13002's; 
Transistor CQ: 
Motorola MXT3904; 
Resistor R1: 33 kOhm/0.25 Watt; 
Resistor R2: 100 kOhm/0.25 Watt; 
Adjustable Resistor 
1.5 MegOhm Potentiometer; 
R3: 
Resistor R4: 22 kOhm/0.25 Watt; 
Resistor R5: 47 kOhm/0.25 Watt; 
Capacitor C3: 
22 nF/50 Volt; 
Rectifier R: 1N4004; 
Diac D: General Electric ST-2; 
Transformers CT1 & 
Wound on Ferroxcube Toroids 
CT2: 213T050 of 3E2A Ferrite Material with 
two turns of #27 wire for the primary 
windings and ten turns of #31 wire for 
the secondary windings; 
Transformer T: 
Wound on a Ferroxcube 2616 Pot Core 
of 3C8 Ferrite Material with 95 turns 
of #27 wire for the primary winding and 
20 turns of five twisted strands of #27 
wire for the secondary winding; 
Incandescent Lamp 
12 Volt/25 Watt. 
IL: 
______________________________________ 
The frequency of inverter oscillation associated with the component values 
identified above is approximately 30 kHz. 
In FIG. 2a, the waveform identified as Wa represents the voltage Vi present 
between the B- bus and the B+ bus as plotted against time t. The magnitude 
of voltage Vi at the time t1 when the inverter is triggered into 
oscillation is indicated as Vi1. The magnitude of voltage Vi at the time 
t2 the inverter drops out of oscillation is indicated as Vi2. 
In FIG. 2b, the waveform identified as Wb represents the inverter output 
voltage Vo plotted against time t; which output voltage exists across the 
secondary winding Ts of transformer T in FIG. 1, and which is the voltage 
provided to incandescent lamp IL. 
FIG. 3 illustrates one particular use of the controllable inverter power 
supply of FIG. 1. In particular, the circuit arrangement of FIG. 3 is 
identical with that of FIG. 1 except for having added an automatic 
feedback control arrangement by way of having placed a light sensitive 
resistor LSR, such as a selenium semiconductor means, in the proximity of 
lamp IL and such as to be exposed to part of the light emitted from IL. 
The light sensitive resistor LSR is connected between the positive 
terminal of a DC source DCS and control terminal CT1. The negative 
terminal of DCS is connected directly with control terminal CT2. 
Description of Operation 
The operation of the circuit arrangement of FIG. 1 is described as follows. 
Source S represents an ordinary 120 Volt/60 Hz electric utility power line, 
the voltage from which is rectified in full-wave fashion by full-bridge 
rectifier means FBR. Thus, in the absence of filtering means, the voltage 
present across output terminals OTa and OTb is substantially as depicted 
in FIG. 2a; which voltage is applied directly to the inverter circuit I. 
This inverter circuit, which consists of the two series-connected switching 
transistors Q1 and Q2 in combination with the two positive feedback 
transformers CT1 and CT2, represents a self-oscillating half-bridge 
inverter and operates in a manner that is analogous with circuits 
previously described in published literature, as for instance in U.S. Pat. 
No. 4,184,128 entitled High Efficiency Push-Pull Inverters. 
Since the DC voltage-supply feeding the inverter has no filtering 
capacitors, it is necessary to provide within the inverter a low impedance 
return path for the inverter current. Such a low impedance return path is 
provided by way of the two series-connected capacitors C1 and C2. However, 
it is necessary that the capacitance values of these capacitors be kept 
small enough not to represent significant energy-storing capacity in 
comparison to the amount of energy being drawn by the inverter over a 
half-cycle of the power line voltage. In this case, with the power drawn 
being about 25 Watt (which is about 208 milli-Joule per half-cycle of the 
60 Hz power line voltage) the energy stored by the two series-connected 
0.47 uF capacitors is indeed small in comparison (being only 2.6 
milli-Joule at 150 Volt). 
In the inverter circuit of FIG. 1, the bases of the transistors are--in 
terms of DC--shorted to their emitters; which implies that the inverter 
can not start oscillating by itself. However, by providing but a single 
brief pulse to the base of transistor Q2, this transistor is caused to 
conduct momentarily; which momentary conduction puts this one transistor 
into an amplifying situation; which is enough to trigger the inverter into 
oscillation--provided, of course, that there is adequate voltage present 
between the B- bus and the B+ bus. 
Once triggered into oscillation, the inverter will continue to oscillate 
until the voltage between the B- bus and the B+ bus falls to such a low 
level as to be inadequate for sustaining regenerative feedback. At this 
point, which is identified as Vi2 in FIG. 2a, oscillations cease. 
Inverter triggering is accomplished by way of a Diac; which Diac itself is 
triggered by the voltage on capacitor C3. 
The output of the half-bridge inverter circuit is a substantially 
squarewave 30 kHz AC voltage, which output is provided between point X and 
junction CJ, and across which output is connected the primary winding of 
transformer T. The peak-to-peak amplitude of this 30 kHz squarewave 
voltage is substantially equal to the magnitude of the DC voltage present 
between the B- bus and the B+ bus; and therefore, as the magnitude of this 
DC voltage varies, so does the amplitude of the 30 kHz squarewave output 
voltage. 
The incandescent lamp IL is connected directly across the secondary winding 
Ts of transformer T; which means that the voltage presented to the 
incandescent lamp is directly proportional to the inverter circuit output 
voltage. 
Being supplied with a pulsed DC voltage similar to that depicted in FIG. 
2a, the inverter circuit--even if oscillating at some given moment--will 
cease oscillating when the DC supply voltage falls below a certain minimal 
level (Vi2 in FIG. 2a). Thus, if the inverter is triggered into 
oscillation at some time during each of the unidirectional 
sinusoidally-shaped voltage pulses constituting the DC supply voltage, it 
will cease to oscillate at or near the end of each of these pulses. 
In other words, the inverter circuit of FIG. 1 behaves much like a Triac or 
a thyristor: it can be triggered ON, and will remain ON until the end of 
the power-cycle--that is, until current flowing to the load falls below a 
certain threshold level. And, just like a thyristor, it can be triggered 
at substantially any point within the power-cycle; which means that it can 
be phase-controlled in a manner analogous to that of a thyristor. 
In yet other words, the RMS or average magnitude of the voltage provided to 
the incandescent lamp can be controlled over a wide range simply by 
controlling the timing of the inverter trigger point (t1 in FIG. 2). 
Triggering of the inverter circuit is accomplished essentially the same way 
as is the triggering of a Triac, and phase control is accomplished in the 
same manner. 
In FIG. 1, resistors R2 and R3 in combination constitute a resistance means 
through which capacitor C3 is charged. By adjusting the magnitude of the 
combined resistance, the time to charge capacitor C3 is similarly 
adjusted; which implies that the phase-point (i.e., t1 in FIG. 2a) at 
which the inverter is triggered into oscillation is correspondingly 
adjusted. 
The purpose of rectifier R is that of making sure that capacitor C3 gets 
fully discharged after the inverter is triggered into oscillation; which 
implies that this capacitor will start each new power cycle in a fully 
discharged condition, thereby assuring time-consistent triggering. 
The reason for having R2 as a resistor physically separate from R3 is that 
of preventing the voltage at point X from being applied directly to 
capacitor C3, which could provide for a situation of actually preventing 
triggering from taking place. 
The purpose of resistor R1, the resistance value of which is quite small in 
comparison with that of R2 and R3 combined, is that of making sure that 
there is enough voltage at junction CJ (relative to the B- bus) to permit 
the inverter circuit to be triggered into oscillation. 
The function of control transistor CQ is that of providing for an 
electrically actuatable means by which the triggering of Diac D can be 
controlled. When there is no control voltage provided between control 
terminals CT1 and CT2, transistor CQ is non-conducting, and the trigger 
circuit (which consists of resistors R2 and R3, capacitor C3 and Diac D) 
will operate as if CQ is non-present. However, as an increasing positive 
voltage is provided to control terminal CT1, CQ will eventually start to 
conduct and thereby to shunt charging current away from capacitor C3. The 
more positive current that is provided into the base of CQ, the more 
charging current is shunted away from C3. Eventually, with a relatively 
high positive voltage provided at control terminal CT1, CQ gets so much 
base current that its shunting effect entirely prevents C3 to charge to a 
voltage high enough to provide triggering pulses. 
Thus, by providing a unidirectional control voltage between control 
terminals CT1 and CT2--with the positive terminal of the control voltage 
being connected with CT1--electrically actuatable inverter trigger control 
results; which implies that the 30 kHz inverter output voltage can be 
electrically switched ON and/or OFF, as well as continuously controlled in 
terms of magnitude. 
The arrangement of FIG. 3 demonstrates one way in which the control 
capability of the circuit of FIG. 1 can be put to use. The light output of 
lamp IL affects inverter triggering in such a way that increased light 
output will cause reduction in the RMS magnitude of the 30 kHz voltage 
output; which implies that--since light output is proportional to the RMS 
magnitude of the lamp voltage--the RMS magnitude of the lamp voltage will 
tend to remain constant even if the RMS magnitude of the power line 
voltage might change. 
Another application in which the power supply of FIG. 1 can advantageously 
be used is as an electrically controllable source of power for the 
magnetron in a microwave oven--i.e., where the load would be a magnetron 
and not an incandescent lamp. In such an application, it would be 
desirable to have an electronic programming means be able to control the 
amount of power supplied to the microwave magnetron; which, of course, can 
be readily accomplished by way of having this programming means provide 
appropriate control voltages to control terminals CT1 and CT2. 
Otherwise, the following comments are offered. 
(a) The concept of feeding an inverter with a pulsed DC voltage and to have 
its oscillations phase controlled (in relationship to the phasing of the 
DC pulses) is not limited to be used with a half-bridge inverter circuit. 
Most any type of self-oscillating inverter circuit may be used, the chief 
criterion being that the inverter circuit must be of such a nature as to 
have to be triggered into oscillation. 
(b) To achieve a reasonably wide range of control of RMS output voltage, it 
is important that the inverter be capable of sustained self-oscillation 
even at relatively low levels of DC supply voltage. In the circuit of FIG. 
1, stable inverter self-oscillation is sustained down to a DC supply 
voltage of about 20 Volt; below which voltage oscillations abruptly cease. 
It is believed that the present invention and its several attendant 
advantages and features will be understood from the preceeding 
description. However, without departing from the spirit of the invention, 
changes may be made in its form and in the construction and 
interrelationships of its component parts, the form herein presented 
merely representing the preferred embodiment.