Specifications

CONSTRUCTION
of the LOT winding, in place of the mains
rectified 200V DC, as shown in Fig. 2.
For testing, one is not required to use
230V directly at all. The same in Fig. 2.
For testing, one is not required to use
230V directly at all. The same 12V, or
the unregulated 12V prior to the 7812
regulator, can be connected. In Fig. 2,
the BA159 anode is shown connected to
the mains rectified supply at the posi-
tive terminal of the 100uF, 400V electro-
lytic capacitor. But, for the present, con-
nect the unregulated 12V (may be 16V
or slightly more) to the circuit at the
anode of BA159.
4. After switching 'on' the supply,
observe the voltage on the EHT wind-
ing, which comes from the LOT, on a DC
multimeter kept at its maximum range
(say, 500 or 1,000V DC). The meter
should show a deflection of above 500V.
5. The presets in the circuit can be
adjusted to tune the ferrite transformer,
for this voltage to be a maximum. Then,
adjust the 1-kilo-ohm potentiometer VR3
so that it shows the possibility of vary-
ing the voltage over a limited range,
above a threshold value.
6. Now, the 12V transformer supply
can be disconnected. The 12V low-voltage
generation part has to be separately
tested. For this, remove the connection to
the LOT from the MOSFET. Also remove
the CMOS ICs from sockets. Then, on the
PCB, one can easily check for zener volt-
age of 12V. If this voltage is less than 12
volts, adjust the value of 82-ohm series
resistor to a lesser value, say, 68-ohm.
Lab. note. At EFY, this part of the
circuit has been modified, and it is pos-
sible to get correct 12V output at AC
input voltage of 120 volts. Only the modi-
fied circuit is included in Fig. 2.
The circuit will ordinarily work even
at 200-volt mains, but not below that.
The mains input can go up to 240V, but
not more.
Lab note. During testing it was ob-
served that fusible resistors R13 and
R14, rated 10W, got red-hot if voltage
was increased beyond 160 V AC.
Construction of the discharge tube
(Fig. 7). A simple method for construct-
ing the discharge tube is presented here,
which is suitable for any hobbyist. An
aluminium tube of 20cm length and about
1cm diameter is taken. Antenna scrap
tube can be used, provided the same does
not have kinks, bends, or burrs. Fig. 7
shows the construction of the discharge
tube. This aluminium tube is blocked on
the inside with a small amount of M-seal
compound, so that no air can pass di-
rectly through its middle hole.
M-seal comes in a pack of two parts.
The sealing compound is prepared as
and when required, bu taking equal quan-
tities of the two and mixing them to-
gether throughly. One of the compounds
is black and the other is of cream colour.
The two are taken, each about 1 cc, and
then mixed will. This mixture is inserted
into the tube with a pencil and spread to
attach to the inside wall of the aluminium
tube, blocking any air path. Then, two
side holes of 2mm diameter are made on
the tube at the two ends, about 33 cm
from each end. These holes can be on the
opposite faces of the aluminium tube.
To provide the discharge gap, a
thinwalled glass tube, commonly used as
chemistry test tube, is required. It should
have an inner diameter about 1.2-1.5 mm
greater than thet of the aluminium tube,
i.e., if a 10mm outer dia aluminium tube
is taken, a glass tube of 11.5 mm inner
diameter should be used. This will en-
sure the best performance with an air
gap of 0.75 mm all around. If the gap is 0.6
mm, it is still better, but then the metal
tube should be extremely perfect.
The glass tube is cut such that it
covers the length of the aluminium tube,
except for about 1.5 cm at each end.
Thus if a 20cm long aluminium tube is
taken, the glass tube will be 17 cm long.
The metal tube should be able to go
freely in it Now, the glass tube may be
rotated over a gas burner to soften the
ends of the tube. It should now be made
chamfered on to the metal tube, such
that, at the edges, the glass tube fits the
metal tube with no gap. Still, the glass
tube should be able to slide over the
aluminium tube.
After the glass tube is so positioned
over the metal tube, the ends of the
glass tube are taped using Teflon tape.
The tube assembly, with the glass enve-
lope taped, is held at its edge, leaving
about 15 cm free at either end, and
clamped to the wall of the plastic box, as
shown in Fig. 8. Clamps meant for TV
antennae, which are made of plastic
mouldings, can be used for this purpose.
Preparation of the hot electrode.
The hot electrode, to which a voltage
greater than 6,000 volts is applied at
high frequency, is made by closely wrap-
ping plain aluminium foil around the
outer side of the glass tube. The foil is
wrapped leaving 1 cm uncovered area on
either ends of the tube. The foil is to be
taped for tightness on the outer glass,
using cellulose tape, and a piece of Teflon-
insulated wire connected to the alu-
minium foil brought out. This wire is
connected to the EHT lead from the LOT
on the circuit board.
Assembling the unit. The unit is
easy to assemble. First, the circuit board
is fixed on the bottom of the box with
plastic bushes and screws. If screws are
not needed externally, the bushes can be
pasted on to the box. Then, clamps are
fixed for discharge tube. Polythene tubes
(transparent plastic) are fitted to the
glass tube ends.
At the bottom of box, the air pump,
with its outer plastic casing removed, is
fixed to the bottom with a screw. The
inside of the diaphragm pump is shown
in Fig. 8. The casing of the pump is not
needed for two reasons: to save space
needed for fixing it within the bread box,
and the vibrator part is now accessible.
A small plastic sheet is fixed by applying
glue (Araldite) to
the vibrating ar-
mature, so that it
serves as a
simple fan for the
inside. The mains
supply is con-
nected to the
PCB in parallel
with the supply
to the air-pump.
Fig. 7: Construction details of discharge tube
92