Elenco Electronics EP-130 - Changing Input Voltage

A
f
ter you
f
inish the wirin
g
, set the switch to position B
.
LEDs 1 and 2 indicate the output voltage of th
e
operational amplifier I
C
. An LED li
g
hts if it is
c
onnecte
d
to 1.5
V
or
high
er.
I
n t
hi
s exper
i
ment, w
e
c
onnect t
h
e two
LED
s
i
n ser
i
es, so t
h
ey on
l
y
light
when connected with about 3V. When they are o
ff,
the output voltage o
f
the operational ampli
f
ier mus
t
b
e
l
ess t
h
an 3
V.
Vi
ew t
h
e sc
h
emat
i
c
di
agram.
Wi
t
h
t
h
e sw
i
tc
h
a
t
position B, the 1.5V battery volta
g
e is connected to
two 10k
Ω
k
k
resistors, with the positive terminal o
f
th
e
operational ampli
f
ier connected between th
e
resistors. These two 10k
Ω
k
k
resistors divide the 1.5
V
supply volta
g
e in hal
f
. This si
g
ni
f
ies the positive inpu
t
terminal receives an input volta
g
e o
f
only 0.75V
.
To total the output volta
g
e of the operational amplifie
r
you multiply its input voltage by the ampli
f
ication
factor (R1/R2) + 1. So, the output volta
g
e is 0.75V x
((220k
Ω
k
k
/ 100k
Ω
k
k
)
+ 1
)
= 2.4V
.
Slide the switch position A. This eliminates the 10k
Ω
k
k
resistors from the circuit, so the am
p
lifier’s
p
ositiv
e
input terminal receives the full 1.5V input voltage.
Usin
g
the above equation, you can see that th
e
output voltage o
f
the operational ampli
f
ier is no
w
1.5V x
((
220k
Ω
/
100k
Ω
)
+1
)
= 4.8V. Because the
volta
g
e supplied to them is more than 3V, the LED
s
li
g
ht dimly
.
Let’s alter the amplification factor.
S
lide the switch t
o
position B again and press the key. This adds the
47
k
Ω
res
i
stor to t
h
e 100
k
Ω
res
i
stor
i
n para
ll
e
l,
making total resistance of R2 about 32k
Ω
k
k
.
N
ow t
he
output voltage is 0.75V x ((220k
Ω
k
k
/ 32k
Ω
k
k
)
+1
)
= 5.9V
,
enou
g
h to li
g
ht the LEDs bri
g
htly
.
Now slide the switch to position A again
(
to connec
t
1
.5V to the amplifier’s positive
(
+
)
input terminal
)
, and
press the key. The LEDs light brightly. Calculating th
e
output volta
g
e
g
ives 1.5V x ((220k
Ω
/
3
2k
Ω
)
+1) =
11.8V. However, the actual output volta
g
e will b
e
limited by the available battery volta
g
e, which is 1.5
V
+ 3.0
V
+ 3.0
V
= 7.5
V
.
N
o
t
es:
EXPERIMENT #71: CHANGING INPUT VOLTAG
E
W
iring Sequence
:

31-67-92

32-34

8
1-89-88-70-36-12
1

6
3-122

6
8-90-91-138

69
-
132

82
-
84
-
133

8
3-131-120

8
7-
13
7

119
-
124
-
90
-
Schematic