-2-
VOLTAGE REGULATOR
The MC7805 (U2) is the voltage regulator which is
used to supply a constant voltage of about 5V to the
circuit.
TEMPERATURE SENSOR
The Temperature Sensor is an NPN transistor
(2N3904). The NPN transistor has three
terminals: the collector,base,and emitter as
shown in Figure 1. When the collector and base of
this transistor are shorted together as shown in
Figure 2, they behave exactly like a diode, which is
atemperature-sensitive device. At a constant
current, the voltage across the base to emitter
junction decreases by approximately 1mV for
every 1OFincrease in temperature.
OP-AMP
The op-amp (LM1458) has twoinput terminals and
one output terminal. Figure 3 shows the op-amp
symbol. Terminals 1 and 2 are the input terminals,
and terminal 3 is the output
terminal. Many op-amps use
twoDC power supplies. The
twoterminals, 4 and 5, are
connected to a positivevoltage
(V+) and negative voltage
(V–), respectively. The TA-
12/K-40 uses a single supply
with ground tied to the V–
input.
The op-amp senses the difference between the
voltage signals applied at its two terminals (that is,
the quantity v2 - v1), multiplies this by the open loop
gain A, and causes the resulting voltage A(v2 - v1)
to appear at output terminal 3 as shown in Figure 4.
In an ideal op-amp,
the input impedance
is infinite, the output
impedance is zero
and the open loop
gain A is very large
and ideally infinite. In
practice,the open
loop gain is usually
greater than 100,000.
In order to control the gain of our circuit we use
feedback to close the loop around the op-amp as
shown in Figure 5. This circuit consists of an op-
amp and two resistors R1 and R2. Resistor R2 is
connected from the output terminal of the op-amp,
terminal 3, backto the negativeinput terminal,
terminal 1. We speak of R2 as applying negative
feedback; if R2 were connected between terminals
3and 2, we would call this positive feedback. Note
that R2 closes the loop from the output terminal
backaround to the input terminal of the op-amp. In
this circuit, the closed loop gain (G) of the op-amp,
from the input Vi to the output terminal, depends on
the ratio of R2 to R1.
For example, if R2 = 100 and R1 = 10, the gain G =
R2/R1 = 100/10 = 10. Thus, the output voltage V0
at terminal 3 would be equal to –10(Vi). The (–) sign
indicates that the output and input voltages are of
opposite polarity.
The TA-12/K-40 Temperature Adapter Kit is used in
conjunction with a digital multimeter to read
temperature. The Temperature Adapter consists of
three main sections: • Voltage Regulator
•Temperature Sensor
•Op-Amp
INTRODUCTION
THEORY OF OPERATION
Figure 1
Collector
Emitter
Base 2N3904 NPN
Transistor
Figure 2
2N3904 NPN
Transistor
Base and
Emitter
Junction
Figure 3
V+
V–
4
5
3
1
2
–
+
Figure 4
V+
V–
4
5
3
1
2
–
+
i1 = 0
A(V2 - V1)
+
–
V2
+
–
V1 i2 = 0
Figure 5
R2
1–
+V0 = –(R2/R1) Vi
or
V0 = – GVi
+
–
Vi R1 2+
–
3
