Elenco Electronics EP-130 - Stable-Current Source

-
96-
In this experiment, we will make a constant curren
t
circuit, usin
g
an operational amplifier and
a
trans
i
stor.
Thi
s c
i
rcu
i
t ma
i
nta
i
ns a constant curren
t
even when the input voltage is changed, becaus
e
more ener
g
y is used up in the circuit.
View the schematic. When the current is modified
,
the voltage across R1 is also modified. The output o
f
the operational ampli
f
ier changes corresponding to
the
f
eedback si
g
nal
f
rom R1. This output
f
rom th
e
amplifier controls the base volta
g
e of transistor
Q1
allowin
g
it to maintain the continual current.
First set the switch to position A, and press the ke
y
while monitorin
g
LED 1. When the key is pressed i
t
gets
di
mmer.
Thi
s occurs
b
ecause
b
ot
h
LED
1 an
d
LED 2 are in the circuit when the key is closed. Th
e
total current through the circuit is the same, but no
w
it is split between LED 1 and LED 2, so LED 1
g
et
s
di
mmer.
Set the switch to position B with the key off. Do yo
u
notice any changes in LED brightness
f
rom position
A to position B?
S
ettin
g
the switch to B modifies th
e
supply voltage
f
rom 9V to 6V. However, the curren
t
remains constant again, so the LED brightness is no
t
a
ff
ec
t
ed.
N
otes:
EXPERIMENT #77:
S
TABLE-
CU
RRENT
SOU
R
CE
Schematic
Wiring Sequence:

3
1-1
3
2-1
3
7

32
-
3
5-
4
7

3
4-138

46
-
6
7

48-68-75

63
-
131
-
122

69
-
119
-
124

7
6-70-121

123
-
133