This is only a preview of the October 1994 issue of Silicon Chip. You can view 33 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Beginner's Dual Rail Variable Power Supply":
Items relevant to "Build A Talking Headlight Reminder":
Items relevant to "Electronic Ballast For Fluorescent Lights":
Items relevant to "Computer Bits":
Articles in this series:
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An electronic ballast
for fluorescent lamps
Do you hate fluorescent lights with their
inevitable flick, flick, flicker at switch-on, the
flicker while they are running & the buzz or hum
of the ballast? Now you can replace the internals
of your fluorescent light fittings with this elec
tronic ballast. It is highly efficient, gives instant
starting & has no flicker, buzz or hum.
By JOHN CLARKE
Fluorescent lights are good. They
are much more efficient than any
incandescent light, they are free of
glare and cast very little shadow. But
fluorescent lights can also be a pain,
especially when they are first turned
on. If the tube or the starter is a bit old
or the temperature is low, there will
be this inevitable flick, flick, flicker
and then maybe it will come on fully.
These and the other irritations associated with fluorescent lights can
be eliminated with this electronic
ballast. It fits directly into a stand-
WARNING!
This circuit operates at voltages which are potentially lethal. No part
of the circuit should be worked upon while it is connected to the
240VAC mains. If the project is to be used in a permanent domestic
installation, it should be connected to the 240VAC mains by a licensed
electrician.
42 Silicon Chip
ard fluorescent light batten and can
be built to suit 18W, 20W, 36W and
40W tubes.
The electronic ballast gives a virtually instant start and since the tube is
run at a very high frequency (around
100kHz), there is absolutely no sign
of flicker. Nor is there any audible,
buzz hum or whistle. As a bonus,
electromagnetic interference to radio
reception is low.
Power factor control
The electronic ballast design can
also be said to be “green” in that it
has less impact on the environment.
This comes about because of its use of
a power factor controller chip. Let’s
discuss this point.
Electricity supply authorities are
constantly after ways to reduce power
losses. This not only improves power
station efficiency (meaning that less
coal is burnt) but can keep costs
down for the consumer. One of the
major ways is to maintain the load
current directly in phase with the
supplied voltage. For loads such as
incandescent lights and bar radiators,
the current is in phase with the voltage but for inductive loads such as
Shown almost actual size, the
PC board is designed to fit into a
standard 18W or 36W fluorescent
batten fitting. It lights the tube almost
instantaneously & produces no
audible buzz or hum.
motors and conventional fluorescent
lights, the current lags the voltage
considerably.
Fig.1 shows roughly how the
current lags the voltage for a conventional fluorescent tube. Here the
current lags the voltage by 45° so
that the power factor is 0.7 (cosine
45°). Since the supplied power is the
RMS voltage x the RMS current x the
power factor, the supplied current
must therefore be some 41% greater
than if it was exactly in phase (ie,
power factor of 1 or unity) with the
applied voltage.
•
•
•
•
•
•
•
•
•
•
•
Fig.1: this diagram shows the phase relationship between the
voltage & current in a conventional fluorescent light fitting; the
current lags the voltage. Note that in reality, the fluorescent light
current is not sinusoidal but it is shown in this way for simplicity.
Features
Suitable for 18W/20W and
36W/40W tubes
Replaces existing ballast
and starter
High efficiency
Fast start without flicker
Noiseless operation
High frequency drive
Filament preheat
Constant lamp brightness
from 200V-280VAC input
Fuse protection for faulty
tubes
0.99 power factor
Low electromagnetic
radiation
Fig.2: in a conventional electronic ballast, pulses of current are
drawn at the crests of the 240VAC 50Hz waveform. This leads to
poor line utilisation & a less than desirable power factor.
October 1994 43
2.5V
REF
ZERO
CURRENT 5
DETECT
INPUT
CURRENT 4
SENSE
INPUT
8
UNDER VOLTAGE
DETECTOR
ZERO CURRENT
COMPARATOR
7 DRIVE
OUTPUT
LATCH,
PWM,
TIMER,
LOGIC
VREF
OVER VOLTAGE
COMPARATOR
1.08xVREF
1
MULTIPLIER 3
INPUT
Fig.3: block
diagram of the
MC34262 power
factor controller
IC. The heart
of the chip is
the two input
multiplier.
VCC
MULTIPLIER
ERROR
AMP
QUICKSTART
2
VOLTAGE
FEEDBACK
INPUT
VREF
6
A
+353V
0V
240VAC
C1
HIGH
FREQUENCY
BYPASS
N
Dx
Lx
3
1
Q1
7
IC1
MC34262 4
6
C2
STORAGE
Iavg
+400V
LOAD
R1
VIN
ILI
Iavg
Block diagram
ON
Q1
OFF
Fig.4: simplified boost circuit employing the MC34262 power factor
controller chip. Q1 is switched on & off for varying times during each AC
half-cycle so that the current drain is evenly spread out.
Fig.5: the fluorescent driver
circuit. This takes the 400V
DC from the boost circuit &
uses an oscillator running
at 100kHz to drive the
fluorescent tube. This gives
appreciably more light
output than the same current
at low frequency; eg, 50Hz.
44 Silicon Chip
This extra current requirement
when the power factor is less than
unity contributes to substantial power
losses in the mains distribution system
all the way back to the alternators at
the power stations. Since the power
losses follow a square law (ie, I2R),
increasing the current required by
41% will double the power losses! As
a consequence, most commercial and
industrial lighting installations are
required to include power factor correction in the light fittings, by adding
a capacitor across the supply. This is
not required in domestic light fittings
but perhaps it should be.
Most electronic ballasts (and indeed
all power supplies) have a similar
drawback as far as the energy authorities are concerned. This is because
they use a bridge rectifier and capacitor
filter. Typical electronic ballasts, as
used in compact fluorescent lamps,
use the circuit shown in Fig.2 to derive
a 353VDC supply. These circuits draw
a large pulse of current at the crest of
each mains half-cycle and while the
current is essentially in phase with the
voltage, the fact that it has such a short
duty cycle means that again, power
losses are higher than they otherwise
would be. The effective power factor
for this type of circuit is between 0.5
and 0.7.
By contrast, the SILICON CHIP electronic ballast incorporates a power factor controller chip which ensures that
the current drawn from the 240VAC
mains is spread more evenly over each
half-cycle, and thus reduces losses in
the distribution system.
Fig.3 shows the internal details of
the Motorola MC34262 power factor
controller IC while Fig.4 shows how
it is connected to boost the incoming
mains voltage. It drives a boost converter using Mosfet Q1, inductor Lx,
diode Dx and capacitor C2. The incoming 240VAC mains is fed to a bridge
rectifier to provide positive-going half
sinewaves. Capacitor C1 functions as
a high frequency filter. Mosfet Q1 is
rapidly switched on and off and each
time Q1 is switched off, the energy
stored in Lx is transferred to capacitor
C2 via diode Dx.
IC1 monitors the DC output voltage,
the current through Q1 (via resistor R1)
and the raw DC input waveform. As
a result, Q1 is switched on for longer
times at the beginning and end of each
A
47k
1W
750k
47k
1W
12-35VDC
470
35VW
750k
8
3
12k
22k
5
T1
S2
N2
.01
2
Q1
BUK547600B
10
G
7
S1
D6
BY229/
600
D
C2
6x1
400V
S
.018
1
150k
150k
150k
820k
680pF
3kV
12k
ZD1
12V
1W
330
D8
1N5062
+2.5V
27k
T2
N2
.018
27k
150k
R1
43k
6
F2
2x 330
1W
820k
D7
50822800
.0068
TP2
+400V
47
4
10
16VW
F1
N1
F2
IC1
MC34262P
T1 : EFD25/13/9 TRANSFORMER ASSY 3F3 CORE WITH 200um AIR GAP
T2 : RCC 12.5/7.5/5 3F3 RING CORE
L1, L2 : 26T 0.4mm DIA ENCU ON RCC/23/14/7 3F3 RING CORE
L3 : 60T 0.4mm DIA ENCU ON EFD/20/10/7 TRANSFORMER ASSY
3F3 CORE WITH 150um AIR GAP
D5
1N4936
DIAC1
ST2
0.1
63V
68k
2x 330
1W
0.12
A
D1-D4
4x1N5062
F1
.01
250VAC
L1
270k
240VAC
ZD2
12V
1W
330
N3
0.1
250VAC
270k
.01
250VAC
L2
TP1
CASE
+360V
0V
C3
.001
3kV
Q3
BUK457600B
D
G
S
D
A
C1
0.22
400V
N
FL1
FLUORESCENT
TUBE
C4
0.1
250VAC
L3
900uH
N1
22
100Hz NOTCH FILTER
Q2
BUK457600B
D
G
S
GD S
K A
E
CASE
T1
FL1
F1
F2
36W
5A
500mA
7T 0.25mm DIA ENCU
18W
5A
250mA
10T 0.25mm DIA ENCU
N2
T2
N1
N1
N2, N3
R1
84T 0.4mm DIA ENCU
14T
6T, 6T 0.4mm DIA ENCU
1.5
120T 0.4mm DIA ENCU
24T
3T, 3T 0.25mm DIA ENCU
3.3
ELECTRONIC BALLAST FOR FLUORESCENT TUBES
Fig.6: the circuit is more complicated than typical electronic ballasts because it
uses the MC34262 power factor controller (IC1). Note that the entire circuit is
powered directly from the 240VAC 50Hz mains supply.
half-cycle and for shorter times at the
crest of each half cycle, as depicted in
the waveforms associated with Fig.4.
So in effect, the current drain of the
circuit is spread more or less evenly
over each half-cycle and the power
factor is close to unity.
The MC34262 has a number of other
features which we will discuss later.
The 400VDC output from the power
factor controller circuit drives the fluorescent tube but it must be converted
into a high frequency AC voltage using
the scheme depicted in Fig.5. This uses
an oscillator to drive the tube via a
resonant circuit consisting of inductor
L3 and capacitor C3. A starter circuit
is also required to fire the tube after
which the oscillator is essentially free
running.
Main circuit
Fig.6 shows the complete circuit
of the electronic ballast. The 240VAC
mains is applied via fuse F1 and an
interference filter comprising L1 and
L2 and associated capacitors. L1 & L2
are wound onto a common toroid in
antiphase so that the inductor works to
eliminate common mode high frequency signals without saturation from the
line current. The .01µF capacitors act
to shunt high frequency signals to
ground while the 0.1µF capacitor in
conjunction with the inductance of
L1 and L2 forms a low pass filter to
block high frequency signals which
would otherwise be radiated by the
mains wiring.
October 1994 45
PARTS LIST
1 PC board, code 11309941,
362 x 45mm
1 18W or 36W fluorescent batten
with tube fitted
1 EFD25/13/9 3F3 core, former
and retaining clips (2 x Philips
4312 020 4116 1, 1 x 4322
021 3524 1, 2 x 4322 021
3516 1) - T1
1 RCC23/14/7 3F3 ring core
(Philips 4330 030 3499 1)
-L1,L2
1 RCC12.5/7.5/5 3F3 ring core
(Philips 4330 021 3515 1) - L3
4 M205 PC-mount fuse clips
1 5A M205 fuse (F1)
1 500mA M205 fuse (36W
version)
1 250mA M205 fuse (18W
version)
1 3-way mains terminal block
1 transistor insulating bush
6 9mm tapped standoffs
1 3mm Nylon screw & nut
2 small cable ties
12 3mm diameter screws 4mm
long
2 3mm diameter screws 12mm
long & two 3mm nuts
8 PC stakes
1 cord clamp
1 mains cord and plug
1 150mm length of 0.8mm tinned
copper wire
1 11.25-metre length of 0.4mm
enamelled copper wire
1 1-metre length of 0.25mm
enamelled copper wire
Semiconductors
1 MC34262P power factor
controller (IC1)
3 BUK457-600B Mosfets
(Q1-Q3)
The AC mains waveform is full
wave rectified using diodes D1D4 and partially filtered with the
0.22µF 400V capacitor. The resulting
raw DC waveform is fed to Q1 via
transformer T1 and to pins 3 & 8
of IC1 via series-connected pairs of
750kΩ & 47kΩ resistors. The 47kΩ
resistors provide the initial power
for the chip to pin 8 but once it is in
running mode, it derives its power
from the secondary winding of T1
via diode D5.
46 Silicon Chip
5 1N5062 800V 2A transient
protected diodes (D1-D4,D8)
1 1N4936 400V 1.5A fast
recovery diode (D5)
1 BY229-600 600V 7A fast
recovery diode (D6)
1 5082-2800 Schottky diode (D7)
1 ST2 Diac (DIAC1)
2 12V 1W zener diodes
(ZD1,ZD2)
Capacitors
1 470µF 35VW PC electrolytic
1 10µF 16VW PC electrolytic
6 1µF 400VDC metallised
polyester (Philips 2222 368
55105 or equivalent)
1 0.22µF 400VDC metallised
polyester (Philips 2222 368
55224)
1 0.12µF MKT polyester
2 0.1µF 250VAC metallised
polyester film & paper (Philips
2222 330 41104)
1 0.1µF MKT polyester
2 .018µF MKT polyester
2 .01µF 250VAC metallised
polyester film & paper (Philips
2222 330 1103)
1 .01µF MKT polyester
1 .0068µF MKT polyester
1 .001µF 3kV ceramic
1 680pF 3kV ceramic
Resistors (0.25W, 1%)
2 820kΩ
2 12kΩ
2 750kΩ
4 330Ω 1W 5%
2 270kΩ
2 330Ω
4 150kΩ
1 47Ω
1 68kΩ
1 22Ω
2 47kΩ 1W 5% 1 10Ω
1 43kΩ
1 3.3Ω 5%
2 27kΩ
1 1.5Ω 5%
1 22kΩ
The seriesed 750kΩ resistors and
a 12kΩ resistor divide the raw DC
waveform down to a level suitable
for the multiplier input at pin 3. The
multiplier has two inputs (which it
multiplies together): the input at pin
3 which provides phase and voltage
information on the incoming rectified
AC waveform, and the output of the
error amplifier at pin 2.
The error amplifier input at pin 1
monitors the +400V DC output from diode D6 via two 820kΩ resistors which
reduce the voltage to +2.5V before it is
fed via a 100Hz notch filter (to pin 1).
Thus, the internal multiplier has two
jobs to do as it controls the pulse width
modulation drive to the gate of Mosfet
Q1 via pin 7. First, it must regulate the
DC output to +400V and second, it
must ensure that Q1 is turned on and
off so that the current drawn from the
AC mains is evenly spread throughout
each AC half-cycle.
Note that while Q1 is draws current
from the raw DC input in the form of
very short pulses (typically about 20
microsec
onds) long, the pulses are
longer at the start and finish of each
AC half-cycle than they are at the crest.
This pulse current is filtered by the
input filter consisting of L1, L2, C1
and the associated 250VAC capacitors
so that the actual current drawn from
the AC mains is 50Hz with relatively
low harmonic content.
Q1 draws current through winding
N1 of transformer T1 (equivalent to
inductor Lx in Fig.4) and each time
Q1 turns off, diode D6 is forced to conduct and deliver charge to C2 which
consists of six 1µF 400V metallised
polyester capacitors. The secondary
winding of T1 drives diode D5 and a
470µF capacitor to provide power to
the chip itself.
Current limiting for Q1 is provided
by pin 4 which monitors the voltage
drop across R1. The current waveform
is filtered by the 47Ω resistor and a
.0068µF capacitor while Schott
ky
diode D7 is included to clip turn- off
voltage spikes due to the inductance
between ground and the source of Q1.
These spikes would otherwise cause
circuit instability.
OK, so we have a +400V DC supply
and this needs to be turned into high
frequency AC to drive the fluorescent
tube and a circuit is required to initially fire the tube. These functions are
performed by the fluorescent driver
circuit which is depicted schematic
ally in Fig.5. The circuit we have used
is very similar to that featured in the
fluorescent inverter circuit published
in the November 1993 issue of SILICON CHIP.
Fluorescent driver
The fluorescent tube driver comprises Mosfets Q2 and Q3, transformer
T2 and associated components. The
fluorescent tube is driven via inductor
L3 and the N1 winding of transformer
T2. The gates of Q2 and Q3 are driven
5
6 F2
S1 4
7
3
8
9
S2 2
10
F1 1
4
5
3
6
2
7
1
8
L3
T1
WINDING DETAILS
L1
N2
N1
N3
T2
WINDING DETAILS
L2
Fig.7: winding details for the toroid filters and ferrite
cored transformers. Note particularly that the two
windings of L1 & L2 are wound in different directions.
from the N2 and N3 windings which are connected in
antiphase.
When power is first applied, there is 400V DC between
the drain of Q2 and the source of Q3. The 0.1µF capacitor
adjacent to Diac1 begins to charge via two series 150kΩ
resistors. When the voltage reaches about 30V the Diac
breaks down and dumps the 0.1µF capacitor’s charge
into the gate of Q3. Zener diode ZD2 protects the gate
from overvoltage.
Mosfet Q3 now switches on and current can flow from
the +400V supply via the fluorescent tube top filament,
the .001µF 3kV capacitor, the second tube filament, the
0.1µF 250VAC capacitor, inductor L3 and the N1 winding
of T2. The current flow in N1 will apply gate drive to Q2
via N2 and switch off gate drive to Q3 via N3 (due to the
polarity of the windings of T2).
If oscillation does not occur, the Diac will again fire
Q3. Ultimately, when oscillation occurs, Mosfets Q2
& Q3 will switch on and off in alternate fashion. The
frequency of operation is set by the combined inductance of L3 and N1 which resonates with the .001µF
capacitor, C3.
The oscillator current heats the fluorescent tube’s fila
ments and after a short period (less than a second) the
tube ignites. Capacitor C3 is now effectively shunted by
the discharge within the tube and the oscillation frequency is set by the core saturation properties of T2. Current
through the tube is limited by the saturation of T2 and
the impedance of L3.
Once normal oscillation occurs, the start-up circuit
comprising Diac1 and the 0.1µF capacitor is disabled
by diode D8. This diode discharges the 0.1µF capacitor
every time Q3 switches on and hence prevents the Diac
from firing.
Gate drive to Q2 and Q3 is limited using two parallel
330Ω gate resistors and 12V zener diodes which clamp
the gate voltage to a safe value. The 330Ω resistor from
This shows the mains voltage waveform (the larger of the
two set to 10V/div) & the current waveform (set to 90mA/
div) when the electronic ballast is driving a 36W tube. Note
that the current is directly in phase with the voltage. The
flattening of the 240VAC waveform is not a circuit function
but was present at the time these photos were taken.
These are the starting pulses as seen at the drain of Q3
with no tube in circuit. Pulses from Diac1 drive the gate
of Q3 & switch it on. The voltage scale is 100V/div & the
frequency is about 1kHz.
This is the waveform at the drain of Q3 when driving a
36W tube. The vertical scale is 100V/div & the frequency is
about 100kHz.
October 1994 51
A
TP1
D5
.01
250VAC
750k
0.22 400V
IC1
1
.0068
0.18
TP0V
MC34262
470uF
43k
270k
N
L2
10uF
.01
12k
E
0.1
250VAC
270k
47k 1W
0.18
0.12
27k
27k
10
1
22k
Q1
820k
47k 1W
820k
47
R1
A
L1
K
D6
D1-D4
12k
68k
.01
250VAC
750k
F1
D7
T1
TO EARTH TERMINAL OF BATTEN
Fig.8 (above & facing page): the component overlay diagram of the PC board. Note that quite a few different
diodes & zener diodes are employed & they must not be mixed up.
This close-up view shows how transformer T2 is secured to the PC board with a
Nylon screw & nut & a transistor insulating bush.
gate to source provides a load for the
T2 windings to accurately set the core
saturation.
Q2 and Q3 switch on and off at
about 100kHz (150kHz for the 18/20W
version) but do not require heatsinks.
However, during the switch-over process, the Mosfet which is switched off,
is forced to commutate whereby its internal reverse diode briefly conducts.
This commutation can lead to high
dissipation in the Mosfets and must
be prevented otherwise they would
ultimately be destroyed. To reduce this
dissipation to a low value, a snubber
capacitor network comprising the
680pF 3kV capacitor and the series
22Ω resistor is connected from the
source of Q2 to the 0V line.
The two 150kΩ resistors connecting the snubber network to the 400V
supply provide a load for the circuit
if the fluorescent tube is not present
or is effectively open circuit.
Circuit variations
Depending on whether the circuit
is to be used with an 18W or 36W
fluorescent tube, there are a number
of variations to the winding details of
transformers T1 & T2, and the values
of fuse F1 and resistor R1. These are
shown on the table included on the
diagram of Fig.6. These changes are
also relevant to 20W and 40W tubes.
Transformer T2 has different wind
ings to set the frequency of operation
for each tube type. For the 18W tube
load, the frequency is set to around
150kHz, while for 36W loads the frequency is set to about 100kHz. This
difference in frequency allows us to
keep the same value of inductance
for L3.
The input filter, comprising L1 & L2 on a common toroid, is secured using two
plastic cable ties.
52 Silicon Chip
Construction
The PC board for the circuit is
K
Q2
TP2
150k
150k
330 1W
150k
D6
150k
A
T2
ZD1
1uF
400V
1uF
400V
1uF
400V
1uF
400V
D8
1uF
400V
ZD2
TP0V
coded 11309941 and measures 362
x 45mm. It is designed to fit inside
a standard 18W or 36W fluorescent
batten fitting.
Construction can begin by winding
the toroids and the transformers. Let’s
start with the larger of the two toroids
which has two windings, L1 and L2.
Fig.7 shows how they are wound,
using 26 turns of 0.4mm enamelled
copper wire (ENCU). Note that each
winding must be wound in the direction shown on the diagram; ie, L1 is
wound in a different direction to L2 so
that they end up in antiphase.
Transformer T2 is wound on the
smaller of the two toroids and again,
its windings must be wound as shown.
The wire gauge and number of turns
depend on whether you are building
the 18W or 36W version. Use the table on the circuit of Fig.6 to find the
number of turns for N1, N2 and N3.
22W
DIAC1
T1
N2
330
680pF
3kV
1
.001
3kV
TUBE
END
L3
N1
0.1
1uF
400V
F2
330 1W
330
TUBE
END
N3
330 1W
330 1W
0.1
250VAC
Q3
Both T1 and L3 are wound on ferrite
transformer bobbins. The larger of the
two is for T1. Both require the centre
leg of one of the core halves to be filed
down so that a precise air gap is formed
when the cores are clipped together.
You will need a small file and a set of
feeler gauges.
Initially, for each core set place the
two core halves together and observe
that there is no gap between the mating surfaces of the two. Now file the
centre leg of one core half, making
sure that you are filing squarely and
evenly across the face. The required
gap is 200µm (0.2mm) for the larger
core (T1) and 150µm (0.15mm) for L3,
the smaller core.
The whole process should not
take more that a few minutes since
the ferrite material is quite soft. Take
care when filing down the centre leg
of each core to ensure that you do not
exceed the gap required.
The secondary winding of T1 (N2) is
wound first using 0.25mm enamelled
copper wire – see Fig.6. Start the N2
winding on pin 2 and wind on the
required number of turns before terminating at pin 6. Now apply a layer
of insulating tape over the winding.
The primary (N1) of T1 can now
be wound using 0.4mm enamelled
copper wire. This must be wound in
the same direction as the secondary
winding. Start at pin 4 and wind on the
requisite number of turns neatly, side
by side, placing a layer of insulating
tape over each layer. The end of the
winding terminates on pin 1.
The transformer can now be assembled by fitting the core halves into the
bobbin and securing with the clips.
L3 is wound using 60 turns of
0.4mm ENCU wire, starting on pin 2
and finishing at pin 3. Again, insulate
RESISTOR COLOUR CODES
❏
No.
❏ 2
❏ 2
❏ 2
❏ 4
❏ 1
❏ 2
❏ 1
❏ 2
❏ 1
❏ 2
❏ 4
❏ 2
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
Value
820kΩ
750kΩ
270kΩ
150kΩ
68kΩ
47kΩ
43kΩ
27kΩ
22kΩ
12kΩ
330Ω
330Ω
47Ω
22Ω
10Ω
3.3Ω
1.5Ω
4-Band Code (1%)
grey red yellow brown
violet green yellow brown
red violet yellow brown
brown green yellow brown
blue grey orange brown
yellow violet orange brown
yellow orange orange brown
red violet orange brown
red red orange brown
brown red orange brown
orange orange brown brown
orange orange brown brown
yellow violet black brown
red red black brown
brown black black brown
orange orange gold brown
brown green gold brown
5-Band Code (1%)
grey red black orange brown
violet green black orange brown
red violet black orange brown
brown green black orange brown
blue grey black red brown
yellow violet black red brown
yellow orange black red brown
red violet black red brown
red red black red brown
brown red black red brown
orange orange black black brown
orange orange black black brown
yellow violet black gold brown
red red black gold brown
brown black black gold brown
orange orange black silver brown
brown green black silver brown
October 1994 53
between each layer with insulating
tape and apply a layer of tape over the
final windings. Assemble the ferrite
cores into the bobbin and secure with
the clips.
PC board assembly
Fig.8 shows the component layout
for the PC board. Before installing
the components, check the board for
shorts or breaks in the copper tracks
and make any repairs that may be necessary. Also check the holes for correct
sizing for each component. You will
need 3mm diameter holes for the six
PC board mounting holes, inductor
L3 and the cable tie holes for the large
toroid input filter (ie, L1 & L2).
Start the board assembly by inserting all the PC stakes plus the four M205
fuse clips. This done, insert the resistors, links and diodes, followed by IC1.
The diodes and IC must be oriented as
shown, while the ST2 (Diac1) can be
inserted either way around.
Take care with the diodes since there
are several types used on the board.
The 1N5062 diodes (D1-D4 and D8)
are axial lead types with spherical
bodies. The 1N4936 diode (D5) is an
Why is it called a ballast?
The circuit presented here is
called an “electronic ballast”
because it replaces the ballast
choke found in all conventional
fluorescent lamp fittings. Electronic ballasts are more efficient than
conventional ballast chokes and
the fact that they operate the
tube at very high frequencies also
improves the efficiency. Which
leads to the question “Why is
the choke in a fluorescent fitting
referred to as a ballast?”
A ballast or more correctly, a
ballast resistor, is used in a circuit
to limit the operating current to
a safe value. A fluorescent tube
requires a ballast because its
mercury vapour discharge has a
negative resistance characteristic, ie, if the current increases, the
voltage across the tube decreas54 Silicon Chip
es. If the ballast choke was not in
the circuit, the current through
the tube would not be limited
and it would be burnt out. Hence,
the ballast choke maintains the
current through the tube at a
more or less constant value.
And why are fluorescent light
fittings called battens?
Standard fluorescent light
fittings for use in domestic and
commercial installations are usually referred to as “battens”. This
is because they are screwed to
the timber battens which secure
the Gyprock or fibrous plaster
ceiling material to the rafters.
In the same way, incandescent
lamp holders which screw to a
wall or ceiling are usually sold as
“batten holders”.
axial lead type with a black and light
grey cylindrical body. D6 is a two-lead
TO220 encapsulation, while the two
zeners (ZD1, ZD2) are axial lead types
with an orange body. D7 is a small axial lead type with a clear transparent
cylindrical body.
When installing the capacitors,
take care with the orientation of the
electrolytic types which are polarised.
Note that the capacitors must be as
specified. In particular, don’t substitute 630V DC capacitors for those
specified at 250VAC.
Transformer T1 and inductor L3
must be installed with pin 1 oriented correctly. The input filter toroid
is mounted using two cable ties as
shown in the photos, while T2 is
secured using a transistor mounting
bush together with a Nylon screw
and nut.
Install the Mosfets (Q1-Q3) and
fit the fuses into the fuse clips. The
terminal block is mounted using two
3mm screws and nuts. Connections
from the PC board to the terminal block
are made with short lengths of tinned
copper wire.
Installation of the PC board
We recommend that the PC board
be installed into the fluorescent batten
before testing because the voltages on
the board are potentially lethal.
Before installation, the existing ballast, starter and starter socket should
be removed from the batten. The existing three-way insulated terminal block
should be left in place as it will still
be required to terminate the incoming
mains supply wiring.
Testing
Now it is ready for testing. Insert a
fluorescent tube into the fitting and
apply power. The tube should initially start with a blue glow at the tube
ends and then light up. After about
a second the power factor controller
will start up and the tube will reach
full brilliance.
If the circuit does not power the
tube, switch off immediately and dis-
connect it from the mains. Check that
the fuses are intact and if so check your
board for incorrectly located components. You should also check that the
inductors and transformers have been
wound correctly.
Voltage checks
Note that this circuit is potentially
lethal to work on and that all points
of the circuit float at mains voltage. If
you do use a multimeter to make voltage checks, make sure it has shrouded
probes and do not handle the meter
while you are actual
ly measuring
voltages.
Under no circumstance should an
oscilloscope be connected to the circuit unless it has differential inputs
or the circuit is powered via a line
isolating transformer.
You can check that the DC supply
section of the circuit is operating
by connecting a multimeter (set for
1000VDC) between TP0V and TP2. At
switch-on, the voltage will initially
be somewhat lower than 400V and
after a second or so it will settle at
400V DC.
The power supply for IC1 can
be measured between TP0V and
on the cathode of D5. This voltage
should gradually rise to about 12V,
whereupon the circuit will start and
the voltage should then sit at about
SC
20-25V.
Fig.9: this is the PC artwork reduced to 70.7%. To reproduce it full size, use a photocopier with an expansion ratio
of 1.41. Check the board carefully before mounting any parts.
Drill holes in the base of the batten
to accommodate the six PC board
standoffs. If the unit is to be used as
a free standing lamp, then any holes
in the metalwork of the batten should
be covered to prevent accidental
contact with the live PC board or its
components.
After installing the PC board into the
batten, the tube leads and mains wiring
should be connected to the PC board.
Use a 2-way insulated terminal block
to make the extension in the wires to
the far-end tombstone (tombstones are
the sockets used at each end of the
fluorescent tube).
It is important to earth the metal
case of the batten to the green/yellow
Earth wire in the mains lead. This
should be done using the earth contact provided on the batten via the
insulated terminal block mentioned
earlier. The centre terminal of this
contact is screwed onto an integral
lug in the batten.
The Active (brown) lead and the
Neutral (blue) lead should connect to
the A and N inputs on the PC board.
Clamp the cord so that it cannot be
pulled out of the terminal block.
The assembled PC board fits neatly
at one end of the batten fitting and is
secured with six screws. Remember
that the whole circuit is potential
lethal since it is powered directly
from the 240VAC mains supply.
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