Specifications

CONSTRUCTION
is more efficient and silent in its dis-
charge. One can easily assemble this por-
table ozone generator in a plastic
breadbox (used for storing one full bread),
which is all insulated(with no exposed
metal parts) The cost of making a simple
unit is much less than Rs 1,000.
The air pump used in this project is an
aquarium pump which costs less than Rs
100. This pump works on mains and has a
50Hz vibrator attached to a rubber bel-
lows that provides a pulsating airflows
that provides a pulsating airflow. This air
flows through the cylindrical space of dis-
charge tube as shown in Fig.1. The dis-
charge tube outlet gives ozonated air.
The circuit as shown in Fig.2 gener-
ates a controlled high-frequency AC volt-
age of above 5 kV. The circuit has been
designed such that all components used
in the circuit are economical and freely
available from TV spares shops.
The single-sided, actual-size PCB lay-
out for the complete circuit and its
component layout are shown in Figs 3
and 4 respectively. The entire assembly
of the unit-including the air pump, dis-
charge tube, circuit board, and the fuse-
can be comfortably fitted within th
breadbox, as shown in Fig.8. Because of
this compact packaging, no mains trans-
former (which is generally heavy) is em-
ployed. The unit should not be touched
after its assembly in the breadbox, nor
should its lid be opened after plugging
into the mains.
Lab note: During practical testing
of the circuit at EFY Lab, an auto-trans-
former for stepping down the mains volt-
age to about 120V AC had to be used to
avoid build-up of excessively high volt-
age-greater than 30 kV peak. At AC in-
put voltage greater than 160V (RMS),
overheating of resistors (parallel combi-
nation of resistors R13 and R14) in se-
ries with the primary of EHT winding
was also noticed.
The circuit
Pulse generator. A simple pulse gen-
erator is realised using two CMOS inte-
grated circuits. The CD4069 is a hex
buffer, while CD4011 is a quad NAND
gate. Two of the 4069 gates are used to
generate 15-20 kHz pulses. The fre-
quency of this oscillator can be varied by
10-Kilo-ohm preset VR1 on the board.
The width of the pulse can also be ad-
justed using preset VR2, but it is left at
33 per cent duty cycle. The circuit uses
an RC (resistance-capacitance) feedback
for generation of the square-wave oscil-
lations. The 330pF capacitor C1 used
here charges during one-half cycle
through the 110-ohm resistor R3 and
the 1-kilo-ohm width-setting variable
resistor VR2. During the other half, ca-
pacitor C1 discharges through 10k resis-
tor R2 and the adjustable-frequency pre-
set VR1. Diodes D1 and D2 differentiate
between the two half cycles.
Note: In Fig. 2, the line joining resis-
tor R1 to D1, D2, and the 3,300pF ca-
pacitor C1 represents a joint only and is
not ground.
The second oscillator, shown in Fig.
2, also uses the gates from same IC
CD4069. It is, however, wired using 0.1uF
capacitor C2, instead of the 330pF used
in the former oscillator. The 1k potmeter
VR3 in the circuit is for ozone output
control. This control is brought out, as
shown in Fig. 8, for slightly varying the
ozone output. This control is brought
out, as shown in Fig. 8, for slightly vary-
ing the ozone output. This second oscil-
lator works at around 2 kHz. Therefore
when the outputs of the two oscillators
are combined using the NAND gate N6
of CD4011 (IC2) and inverted by gate
N7, one gets a modulated output of high
and low frequencies. Such an excitation
of the discharge tube has been found to
be very efficient and less heat-producing
as compared to a plain high-frequency
or a plain low-frequency excitation.
The output pulse train from pin 11 of
the CD4011 gate N7, which is of the
same polarity after the second inversion
(first inversion takes place in gate N6),
is sent through the pair of complemen-
tary transistors T1 and T2 (2N2222 and
2N2907, respectively). These two buffer
the signal for giving adequate charging
current to drive the gate capacitance of
MOSFETs during the leading edges of
the square wave.
The 15-ohm resistor R11 and switch-
ing diode D9 (1N914) are needed to pre-
vent any negative signal input ot the
power MOSFET IRF840 gate. The power
MOSFET is a boon to switch-mode cir-
cuit operation. It looks like te 5V regula-
tor 7805 in TO-220 package, with which
everyone is familiar. It needs a small
aluminium heat-sink
The drain of the MOSFET is connected
in series with a 33-ohm, 10-watt
wirewound resistor (replaced by EFY Lab
with 2 x 47-ohm, 10W resistors R13 and
R14, in parallel). It is then connected to
EHT primary winding of the ferrite core
line output transformer (LOT)-also re-
ferred to as EHT transformer. Switching
diode BA159 is also placed in series with
the primary, as shown in Fig. 2. The sup-
ply is the rectified DC voltage, which is
derived form the mans voltage directly.
(During testing at EFY Lab, the mains
voltage was stepped down to 120V AC as
mentioned earlier.) An RC series network
comprising 33kpF (3000V rating) capaci-
tor C8 and 100-ohm (10W) resistor R12 is
placed across drain-source terminal of the
MOSFET. The source terminal of the
MOSFET is directly grounded.
Low-voltage supply. The ICs 4069
and 4011, and transistors T1 and T2,
require a low voltage of around 12V. A
separate 12V, 250mA transformer could
also be used with a rectifier bridge and
filter capacitor to derive the necessary
voltage. But, in this compact design, the
same is derived from mains using a ca-
pacitor and diode pair. The mains supply,
through the series limiting resistor of 82-
ohm, 1W (replaced at EFY Lab, with a 10-
kilo-ohm, 10W resistor R8) sends a cur-
rent via 0.47uF, 400V polyester capacitor
(replaced at EFY Lab with two such ca-
pacitors C4 and C5 in parallel) and diode
D8 to change 100uF capacitor C6 during
positive half cycle. Zener diode D7, with
a breakdown voltage of 12V, limits the
voltage across capacitor C6 to 12V. Diode
D6 provides an easy path during nega-
tive half cycle of the AC input. The stable
12V DC supply developed across capaci-
tor C6 is used for the ICs and transistors
2N2222 and 2N2907.
Ferrite-core transformer. The fer-
rite core transformer used here is the
commonly available B&W television
transformer, known as LOT (line output
transformer). AT2070 type used int he
Metering circuit
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