Download realization of active filters using operational transconductance

110
V. Vasudeva Rao, R. Raghukulender and M.V. Krishna Murthy
5. MECHANICAL LOADING SYSTEM
The mechanical loading system consists of a C-frame that grips the test column with a
spindle above the loading pin and a base plate at the bottom of the test column. The applied
pressure is transmitted along the axial direction of the test column. This is ensured by the
two pairs of ball and conical seating arrangement, one at the top and other at the bottom of
the test column. The load on the test column is varied by adding the dead weights to the lever
attached to the C-frame. Proving ring that is attached to the spindle of the frame senses the
load that is transmitted to the test column and the load indicator displays the load in kN. The
C-frame described earlier takes care of the reactive forces developed during loading.
6. HEATING CIRCUIT
The heating circuit consists of a servo stabiliser of 3 k V A rating, single phase dimmerstats
of 0 to 250V range with 8Amps rating, analog Watt meters of 0 to 600W range. The heat
generation in the heaters is by simple ohmic heating. The source temperature can be varied
from room temperature to 250°C by changing the input voltage to the heater element by
regulating the dimmerstats. 3 kV A continuously varying servo stabiliser incorporated in the
heating circuit maintains the output voltage at 230V with a variation of less than I Volt. This
resulted in a constant heat generation and thereby a stable temperature of the heater-cooler
block. Analog Watt meter connected in series indicates the power dissipated in the heater.
7. COOLING CIRCUIT
A closed loop cooling circuit, consisting of a constant temperature bath, a centrifugal
pump, cooling coils embedded in the heater- cooler blocks is used. The temperature of the
heater-cooler block can be varied in the range of -195°C to 150°C and maintained with in
±1°C by choosing an appropriate coolant. When the temperatures are to be varied between
-30°C to 0°c ethylene glycol may be replaced for distilled water and Methyl cyclohexane may
be replaced to reach -85°C. Liquid Nitrogen may be used as a coolant to go down to -195°C.
LN2 can be circulated in the coils by pressurising the LN2 dewar with compressed Nitrogen
and can be let out to the atmosphere after it boils off.
8. INSTRUMENTATION
The basic measurements involved in contact conductance and thermal conductivity tests
are temperature measurements. The instrumentation in the present test facility consists of
temperature sensors (T-Type thermocouples), constant temperature reference bath, and a
precession multimeter. SWG 24 gauge thermocouple wire is drawn from a single spool and
cut to required lengths. The measuring junctions of the thermocouples are formed by fme
soldering. The beads so formed are shining bright and their diameter measured slightly over
1 mm. The wires are insulated by a layer of thin Teflon tape and are wrapped on the individual
wires. With all this the thermocouple junction measured 1.1 mm and tightly fits in to the drilled
holes of 1.2 mm diameter in the heat flux meters and test specimens. The total stretch of
thermocouple wires from measuring junction to the cold junction is continuous and
uninterrupted. Fourteen thermocouples are used to measure the temperature in the test column
and one thermocouple to measure the reference bath temperature. Thermocouple extension
cables are used to connect the cold junctions to the measuring instrument. The cold junction
of the thermocouple is maintained at O°C using a reference bath that contains melting ice in