Specifications

CIRCUIT IDEAS
S.C. DWIVEDI
S
CRs and Triacs are extensively
used in modern electronic power
controllers—in which power is con-
trolled by means of phase angle variation
of the conduction period. Controlling the
phase angle can be made simple and easy
if we set different firing times correspond-
ing to different firing angles. The design
given here is a synchronised program-
mable timer which achieves this objec-
tive.
The following equation for a sinewave
shows how firing time and the phase angle
are related to each other:
θ = 2πft or θ∝t
Here, θ is the angle described by a
sinewave in time t (seconds), while f is
the frequency of sinewave in Hz. Time
period T (in seconds) of a sinewave is
equal to the reciprocal of its frequency,
i.e. T = 1/f.
The above equation indicates that if
one divides the angle described during one
complete cycle of the sinewave (2π = 360
o
)
into equal parts, then time period T of
the wave will be divided into identical
equal parts. Thus, it becomes fairly easy
to set the different programmable tim-
ings synchronised with the AC mains
sinewave at zero crossing. The main ad-
vantage of such an arrangement, as al-
ready mentioned earlier, is that only the
firing time has to be programmed to set
different firing angles. It is to be noted
that the more precise the timer, the more
precise will be the power being controlled.
In this circuit, the time period of
mains waveform is divided into 20 equal
parts. So, there is a time interval of 1 ms
between two consecutive steps. The sam-
pling voltage is unfiltered full-wave and
is obtained from the diode bridge at the
output of the power transformer. The
timer is reset at every zero crossing of
full wave and set again instantly for the
next delay time. This arrangement helps
the timer to be set for every half of mains
wave—when the positive half of the mains
waveform starts building up, the timer is
set for that half and as it begins to cross
zero, it gets reset and set again
for negative half, when the
negative half begins to build up.
The process is repeated. Here,
instead of using two zero cross-
ing detectors—one for each half
of mains wave—a single detec-
tor is used to perform both the
functions. This is possible be-
cause the sampling wave for
negative half is inverted by the
rectifier diode bridge.
The 18V AC from power
transformer is fed to the four
diodes in bridge configuration,
followed by the filter capacitor
which is again followed by a
three-terminal voltage regula-
tor IC LM7812. The voltage so
obtained drives the circuit. The
unfiltered voltage is isolated
from the filter capacitor by a
diode and is fed to zener diode
D8, which acts as a clipper to
clip voltage above 6 volts.
This voltage is fed to the
base of transistor T1, which is wired
as zero crossing detector. When base
voltage reaches the threshold, it con-
ducts. It thus supplies a narrow posi-
tive pulse which resets the timer at
every zero crossing.
A 32.768kHz crystal is used to
get stable output of nearly
1 kHz (1,024Hz) frequency after five
stages of binary division by
an oscillator-cum-divider IC CD4060.
The 32.768kHz crystal is used be-
cause it can be found in unused
PRECISION DIGITAL AC POWER
CONTROLLER
PRATAP CHANDRA SAHU
85