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Precision
Digital AC Power Controller
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| SCRs
and Triacs are extensively used in modern electronic power
controllers—in which power is controlled 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 corresponding to different firing
angles. The design given here is a synchronised programmable
timer which achieves this objective.
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The
following equation for a sinewave shows how firing time
and the phase angle are related to each other:
q
= 2pft or qµt
Here,
q 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.
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| The
above equation indicates that if one divides the angle
described during one complete cycle of the sinewave (2p
= 360o) 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 timings
synchronised with the AC mains sinewave at zero crossing.
The main advantage of such an arrangement, as already
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 sampling 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 crossing
detectors—one for each half of mains wave—a single detector
is used to perform both the functions. This is possible
because 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 regulator
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 conducts. It thus supplies a narrow
positive 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 because it can be found in unused
quartz clocks and is readily available in the market.
But use of a 1kHz crystal using a quad-NAND IC CD4093
as clock generator, as shown in Fig. 2, is better as it
provides the exact time interval required. In that case,
CD4060 oscillator/divider is not required. |
The
CD4017B counter-cum-decoder IC then divides this 1kHz
signal into ten equal intervals, which are programmed
via the single-pole, 10-way rotary switch. Once the delayed
output reaches the desired time interval, the corresponding
output of CD4017 inhibits the counter CD4017 (via pole
of rotary switch and diode D6) and fires the Triac. Transistor
T2 here acts as a driver transistor. The reset pin of
4017 is connected to zero crossing detector output to
reset it at every zero crossing. (The load-current waveforms
for a few positions of the rotary switch, as observed
at EFY Lab, are shown in Fig. 3.)
The
Circuit can be used as power controller in lighting equipment,
hot air oven, universal singal-phase AC motor, heater
etc. |
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Click on the Image for its larger version
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