04-10-2012, 12:11 PM
ELECTRONIC SYSTEM DESIGN LAB
ELECTRONIC SYSTEM.pdf (Size: 3.79 MB / Downloads: 397)
THEORY:
INSTUMENTATION AMPLIFIER:
Instrumentation amplifier is generally required in any measurement system
using electrical transducers to enhance signal levels often in low voltage less than mV. Also
it is required to provide impedance matching and isolation. When the desired input rides over
a common mode signal special amplifier are needed so that difference signals get amplified
to an acceptable level while the common mode signals get attenuated.
The physical quantities can be converted into electrical quantities by using
transducer. The output of the transducer needs to be amplified to get the meter readings. This
amplification is done by using instrumentation amplifier. The output of instrumentation
amplifier drives of indicator or display system. The important features of an instrumentation
amplifier are high gain accuracy, high CMRR, high gain stability with low temperature
co-efficient, low dc offset, low output impedance.
AC VOLTAGE REGULATOR:
If the SCR is connected to AC supply and load, the power flow can be
controlled by varying the RMS value of AC voltage applied to the load and this type
of power circuit is caused as AC voltage regulator. Applications of AC voltage
regulator are in heating on load transformers for changing light controls, speed
controls and polyphase controls, induction motors and AC magnet controls for power
transfer. Two types of power control are normally used.
(1) ON-OFF control
(2) Polyphase Angle control
AC regulators are those converter which converts fixed ac voltage directly to
variable ac voltage of the same frequency. The load voltage is regulated by
controlling the firing angle of SCRs. AC voltage controllers are thyristor based
devices.
The most common circuit is the inverse parallel SCR pair in which two
isolated gate signals are applied. Each of the two SCRs are triggered at alternate half
cycles of the supply and the load voltage is part of input sine wave. The SCR is an
unidirectional device like diode, it allows current flow in only one direction but unlike
diode, it has built-in feature to switch ON and OFF. The switching of SCR is
controlled by gate and biasing condition. This switching property of SCR allows to
control the ON periods thus controlling average power delivered to the load.
In this circuit SCR1 is forward biased during positive half cycle and SCR2 is
forward biased during negative half cycle. SCR1 is triggered at the firing angle ωt=α
and supply voltage is impressed on the load resistance(RL). It conducts from the
remaining positive half cycle, turning OFF when the anode voltage becomes zero at
ωt=π.
SCR2 is triggered at the firing angle ωt=α+π and conducts till ωt=2π. Hence
the load is alternating in polarity and is part of sine wave. The firing angle of both
SCRs is controlled by gate circuit. The conduction period of SCR is controlled by
varying gate signals within specified values of maximum and minimum gate currents.
For gate triggering, a signal is applied between the gate and cathode of the
device. AC sources are normally used as gate signals. This provides proper isolation
between power.