U.S. patent number 3,787,751 [Application Number 05/279,445] was granted by the patent office on 1974-01-22 for ballast circuits for discharge lamps.
This patent grant is currently assigned to Thorn Electrical Industries Limited. Invention is credited to Victor Francis Farrow.
United States Patent |
3,787,751 |
Farrow |
January 22, 1974 |
BALLAST CIRCUITS FOR DISCHARGE LAMPS
Abstract
A ballast circuit for a discharge lamp is arranged to supply a
uni-directional voltage to the lamp which is made up of at least
two current components which differ in phase. Flicker at the supply
frequency f and at 2f is thus much reduced. A preferred ballast for
use with a three-phase supply employs ballast capacitors in each of
the three supply lines connected to a three-phase bridge rectifier,
the output of which is applied to the lamp. A preferred ballast for
use with a single phase supply includes a ballast capacitor and a
ballast inductor connected through respective bridge rectifiers
which have some components in common.
Inventors: |
Farrow; Victor Francis (London,
EN) |
Assignee: |
Thorn Electrical Industries
Limited (London, EN)
|
Family
ID: |
23069005 |
Appl.
No.: |
05/279,445 |
Filed: |
August 10, 1972 |
Current U.S.
Class: |
315/137; 315/205;
315/207; 315/244; 315/DIG.5; 315/227R; 315/283 |
Current CPC
Class: |
H05B
41/232 (20130101); Y10S 315/05 (20130101) |
Current International
Class: |
H05B
41/20 (20060101); H05B 41/232 (20060101); H05b
041/16 () |
Field of
Search: |
;315/137,139,141,205,207,244,227R,247,283,DIG.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Mullins; James B.
Attorney, Agent or Firm: O'Connell; Robert F. Dike,
Bronstein, Roberts & Cushman
Claims
1. A ballast circuit for a discharge lamp, comprising:
first and second input terminals for receiving an alternating
voltage;
first and second output terminals for applying a uni-directional
voltage to a discharge lamp;
two oppositely-poled diodes connected between said first input
terminal and said first and second output terminals
respectively;
a capacitor and an inductor both connected to said second input
terminal;
two oppositely-poled diodes connected between said capacitor and
said first and second output terminals respectively; and
two oppositely-poled diodes connected between said inductor and
said first
2. In combination with a discharge lamp, a ballast circuit
comprising:
three input terminals for connection to a three-phase alternating
current supply;
two output terminals for applying a uni-directional voltage to said
discharge lamp;
three ballast capacitors each connected to a respective input
terminal; and
a three-phase bridge rectifier connected between said capacitors
and said output terminals, whereby said voltage applied to said
discharge lamp has a ripple frequency which is six times the
frequency of the alternating current received by said input
terminals.
Description
BACKGROUND OF THE INVENTION
This invention relates to ballast circuits for discharge lamps.
Gas discharge lamps are conventionally operated from alternating
current sources by using a ballast circuit. A basic type of ballast
circuit includes an inductor in series with the lamp to absorb the
difference between the supply voltage and the lamp voltage; because
the inductor is reactive it consumes a relatively small, though
significant, amount of power. A capacitor may be connected across
the series combination of the inductor and lamp to correct the
power factor presented to the supply.
Modifications of this basic circuit include the use of
auto-transformers and auto-transformers with built-in leakage
reactance. However, capacitors have not been used in place of the
inductor because they give rise to undesirable current waveforms,
the rapid charging and discharging being sufficient to damage the
lamp and also produce marked flicker. Thus, where a leading power
factor is required inductors and capacitors are used in series,
with the capacitive reactance being greater than the inductive
reactance. The inductive reactance has the effect of improving the
lamp current waveform while the overall ballast effect is
capacitive.
All these ballasts produce flicker in the light output at the mains
supply frequency f and at 2f. The psychological implications of
this flicker are not fully understood, but it is known that the
flicker can cause some considerable distress.
We have appreciated that one way of eliminating flicker is to
operate the lamp on a direct current supply. We have therefore been
concerned to provide circuits which will enable a lamp to be run on
an effectively direct current from an alternating current source,
while still providing a ballast effect and not being unnecessarily
complex, and in such a way that unacceptable flicker is at least
substantially reduced.
SUMMARY OF THE INVENTION
According to this invention there is provided a ballast circuit for
a discharge lamp, comprising input terminals for receiving an
alternating voltage, output terminals for applying a
uni-directional voltage to a discharge lamp, and means connected
between the input and output terminals for applying to the output
terminals at least two current components which differ in
phase.
Another disadvantage of known ballasts is that they require a bulky
iron-cored inductor, which is expensive and which results in an
overall efficiency of only about 90 percent, measured as a
percentage of the power input from the source which actually
reaches the lamp. An added advantage of one ballast circuit
embodying this invention is that it does not require the use of an
inductor.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example and with
reference to the accompanying drawings, in which:
FIG. 1 is a circuit diagram of a ballast circuit for operating a
discharge lamp on direct current from a three-phase alternating
current source, the ballast circuit not including any
inductors;
FIG. 2 is a waveform diagram showing the line current and phase
voltage for one phase of the supply in FIG. 1;
FIG. 3 is a waveform diagram showing the output current from the
ballast circuit of FIG. 1;
FIG. 4 is a circuit diagram of another ballast circuit for
operating a discharge lamp on direct current from a three-phase
alternating current source;
FIG. 5 is a circuit diagram of a ballast circuit for operating a
discharge lamp on direct current from a single-phase alternating
current source;
FIG. 6 is a waveform diagram showing the input current to the
ballast circuit of FIG. 5;
FIG. 7 is a waveform diagram showing the output current from the
ballast circuit of FIG. 5; and
FIG. 8 is a waveform diagram showing certain voltages in the
circuit of FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows three input terminals 10 for connection to a
three-phase alternating current supply. The terminals 10 are
connected through fuses 12 to respective capacitors 14, and the
capacitors 14 are connected to three input terminals of a
three-phase bridge rectifier 16. The rectifier 16 comprises six
diode elements 18 and has two output terminals 20. In this case two
lamps 22 are connected in series, together with a surge-limiting
resistor 24, across the terminals 20.
The alternating currents through each of the capacitors 14 are
identical in form but phase displaced relative to each other by
120.degree.. The line current relative to phase voltage for one
capacitor is shown in FIG. 2. The output current from the bridge
rectifier 16 is shown in FIG. 3 and is seen to consist of a
uni-directional current with a ripple the frequency of which is six
times the supply frequency. The ripple amplitude is relatively
small, the ratio A:B being typically about 1:3.
At this ripple frequency the afterglow of the lamp phosphors is
significant, and this gives a further reduction in light output
ripple. Thus this ballast circuit provides a very substantial
reduction in ripple without the use of a complex ballast circuit.
Surprisingly, it is found to operate well while using capacitors
alone without inductors, and the efficiency is thereby increased to
upwards of 95 percent. The ballast circuit provides a leading power
factor which can be used to help correct a lagging load in factory
installations.
If the supply voltage is 415 volts, the prestrike voltage
generated, namely 415 .times..sqroot.2 volts, is sufficient to
strike two normal 400 watt high pressure mercury vapour lamps in
series.
FIG. 4 shows a similar ballast circuit in which the three
capacitors 14 are replaced by a three-phase inductor 30. In this
case the reactance presented to the supply is inductive, and the
circuit will provide a lagging power factor which can be used in
conjunction with the capacitor ballast circuit of FIG. 1 to at
least partially correct the power factor. The three-phase inductor
30 could be replaced by three individual inductors connected in the
three lines. In other respects the ballast circuit of FIG. 4 is
similar to that of FIG. 1, and includes a bridge rectifier the
output of which is connected to a discharge lamp.
Another ballast circuit is shown in FIG. 5. Two input terminals 40
are connected to a discharge lamp 42 through a double bridge
circuit 44. The circuit 44 may be considered as two superimposed
bridge circuits, with half of each bridge being common. The common
elements are diodes 46 and 48 which are connected in opposite
senses between one of the terminals 40 and respective output
terminals 41 which are connected to the lamp 42. To the other
terminal 40 are connected both a capacitor 49 and an inductor 50.
Four further diodes 52, 54, 56 and 58 are then connected as
follows: diode 52 between one end of the lamp 42 and the capacitor
49, diode 54 between the same end of the lamp and the inductor 50,
diode 56 between the other end of the lamp and the capacitor, and
diode 58 between the said other end of the lamp and the inductor.
The cathode of diodes 46, 52 and 54 are all connected together and
to one end of the lamp, and the anodes of diodes 48, 56 and 58 are
all connected together and to the other end of the lamp.
Thus the lamp is supplied through both the capacitor 49 and the
inductor 50 with current which leads and lags the voltage
respectively, and the lamp 42 thus receives two current components
of differing phase. The effect is to produce an input line current
as shown in FIG. 6, and an output current as shown in FIG. 7, in
which alternate peaks correspond to component currents through the
capacitor 49 and inductor 50 respectively. The fundamental of the
line current is in phase with the supply voltage. It will be seen
that the output current includes a large d.c. component and a
ripple of predominantly four times the supply frequency.
To reduce the twice-line-frequency ripple component to a minimum,
the capacitor reactance (1/2]fC) should be approximately equal to
the inductor reactance (2 .pi.fL). This results in a larger
capacitor than is normally used for power factor correction.
The open-circuit prestrike voltage V.sub.42 across the lamp is seen
to consist of the sum of the open-circuit voltages V.sub.46 and
V.sub.48 across the diodes 46 and 48 respectively. The forms of the
voltages V.sub.46, V.sub.48 and V.sub.42 are shown in FIG. 8. The
peak open-circuit prestrike voltage (V.sub.42) produced by the
circuit of FIG. 5 with a 240 volts r.m.s. input is 240.sup..
.sqroot.2 volts, which is sufficient to strike a high pressure
mercury vapour discharge lamp.
* * * * *