Specifications
CIRCUIT
IDEAS
CIRCUIT IDEAS
T
he SMPS described here is suit-
able for high-wattage stereos and
other similar equipment. The cir-
cuit employs two high-voltage power tran-
sistors (BU208D) which have built-in re-
verse-connected di-
odes across their
collectors and emit-
ters. It can supply
about 250-watt out-
put.
The circuit
uses a ferrite core
transformer of
14mm width,
20mm height, and
42mm length of E-
E cores. An air gap
of 0.5 mm is re-
quired between E-
E junction. Good
insulation using
plastic-insulating
sheets (Mylar) is to
be maintained be-
tween each layer of
winding.
The number of primary turns required
is 90 with 26 SWG wire. The secondary
winding employs 17 SWG wire (for 4A
load current). Each turn of the secondary
develops approximately 2 volts. The
reader can decide about the output volt-
age and the corresponding secondary
turns, which would work out to be half
the desired secondary voltage. The volt-
age rating of capacitors C7 and C8 should
be at least twice the secondary output of
each secondary section. BY396 rectifier
diodes shown on the secondary side can
be used for a maximum load current of 3
amperes.
Two feedback windings (L1 and L2)
using two turns each of 19 SWG wire are
wound on the same core. These windings
are connected to transistors T1 and T2
with a phase difference of 180
o
,
as shown
by the polarity dots in the figure. First
wind the primary winding (90 turns us-
ing 26 SWG wire) on the former. Then
wind the two feedback windings over the
DEEPU P.A.
S.C. DWIVEDI
TOILET INDICATOR
T
he circuit shown here displays
the message ‘toiLEt’ or ‘bUSY’,
using just six 7-segment common-
anode displays. Such a display can be fixed
on the toilet door and operated using a
reed switch and a ferrite magnet pair,
suitably fixed on toilet door and its frame,
such that the reed switch is closed when
the toilet is busy.
Those segments of the displays for
each letter that are common to both the
display words ‘toiLEt’ and ‘bUSY’ are con-
nected through resistors to ground (indi-
cated by bold lines in the figure). These
segments are permanently lit. ‘A’ rail is
connected to segments that make up the
word ‘toiLEt’ and ‘B’ rail for the word
S.C. DWIVEDI
K.S. SANKAR
secondary (output). Ensure that each
winding is separated by an insulation
layer.
Two separate heat sinks are to be pro-
vided for the two transistors (BU208D).
The filter capacitor for mains should be
of at least 47µF, 350V rating. It is better
to use a 100µF, 350V capacitor. If the
output is short-circuited by less than 8-
ohm load, the SMPS would automatically
turn off because of the absence of base
current.
The hfe
min
(current amplification fac-
tor) of BU208D is 2.5. Thus, sufficient
base current is required for fully satu-
rated operation, otherwise the transistors
get over-heated.
At times, due to use of very high value
of capacitors C7 and C8 (say 2200
µ
F or
so) on the secondary side or due to low
load, the oscillations may cease on the
primary side. This can be rectified by in-
creasing the value of capacitor C6 to
0.01
µ
F.
SIMPLE SWITCH MODE
POWER SUPPLY
169