Specifications

CIRCUIT IDEAS
The reference terminal of regulator
(IC3) is kept at 3.9V using LED2, LED3,
and diode D6 in the common lead of IC3
to obtain the required regulated output
(15.9V), in excess of its rated output,
which is needed for proper operation of
the circuit. This output voltage is fed to
the base of transistor T3 (BC548), which
along with transistor T4 (2N3055) forms
a Darlington pair. You get 14.5V output
at the emitter of transistor T4, but be-
cause of a drop in diode D7 you effec-
tively get 13.8V at the positive terminal
of the battery. When Schmitt trigger
switches ‘on’ relay RL1, charging is at
high current rate (boost mode). The fast
charging path, starting from transformer
X2, comprises diode D5, N/O contacts of
relay RL1, and diode D7.
The circuit built around IC4 and IC5
is the voltage monitoring section that
provides visual display of battery volt-
age level in bar graph like fashion.
Regulator 7805 is used for generating
reference voltage. Preset VR1 (20 kilo-
ohm) can be used to adjust voltage lev-
els as indicated in the circuit. Here also
a potmeter arrangement using resistors
R7, R8, and R9 is used as ‘divide by 3’
circuit to sample the battery voltage.
When voltage is below 10V, the buzzer
sounds to indicate that the safe dis-
charge limit has been exceeded.
I
f wires of two dissimilar metals are
joined at both the ends and the jun-
ction formed at one of the ends is
heated more than the other junction, a
current flows in the circuit due to See-
beck thermal emf. This effect is used in
thermocouple (TC) temperature sensors.
The Peltier effect is converse of the
Seebeck effect, which means that if a
current is forced through junctions of
dissimilar metals, one junction starts
getting hot while the other starts get-
ting cold, depending on direction of the
applied emf. This effect is used to make
small portable refrigerators.
It is known that one of the junc-
tions is the sensing or hot junction (T
mes
)
and the other junction is the terminat-
ing or cold junction (T
ref
). The voltage
between terminals ‘a’ and ‘b’ is propor-
tional to T
mes
- T
ref
(as given in the Table
I). The formula being V
ab
= a(T
mes
-T
ref
),
where a is the Seebeck coefficient of
the thermocouple.
In the circuit, use only metal film
resistors (MFRs) of 1 per cent tolerance,
as this is an instrumentation applica-
tion. Power supply should be a stable
+5V, -5V supply, for which one can use
7805 and 7905 regulators.
The input terminals TC+ and TC–
should go to a 4-way barrier terminal
block. Two extra terminals are used to
mount TH1 Cu thermistor. This forms
an isothermal block, which is good
enough.
A simple way to make a TH1 Cu
thermistor is to take a 1meg-ohm, 2W
resistor as a former and wind 2 metres
of 46 SWG enameled copper (Cu) wire
(5.91ohm/metre) over it. This gives a 12-
ohm value. Terminate wire ends on re-
sistor leads.
For calibration, you will need a DMM/
DPM and a millivolt source (as shown in
the figure below). First connect source
between terminals TC+ and TC–, then
set source to 0.00 mV (verify with DMM
for zero). The output across +out and –
out termi-
nals must
be in mV
(use DMM),
represent-
ing the
room tem-
perature
(RT). For
example, if
RT is 30
o
C
(use a glass
thermom-
eter), +out
should be
30mV. At
0mV in-
put, adjust
VR1 till
30mV is
read at
+out ter-
minal.
This is
MV Thermocouple Temperature in
o
C
00
2.585 50
5.268 100
10.777 200
16.325 300
21.846 400
27.338 500
33.096 600
Reference junction or cold junction at 0
o
C.
Table I
S.C. DWIVEDI
Remarks
As cold junction is not zero but is at room
temperature (RT), add RT to temperature.
Example
Feed 10.777mV between the TC+ and TC-
terminals. If RT is 30
o
C, reading on 2V DPM
will be 230 counts i.e. 230mV.
ANANTHA NARAYAN
TEMPERATURE MEASUREMENT
INSTRUMENT
186