Elenco Electronics EP-130, Manual, Page: 91 / 160

In this project, we will make a constant current
circuit, using an operational amplifier and a
transistor. This circuit maintains a constant current
even when the source voltage changes, because
more energy is used up in the circuit.
Look at the schematic. When the current changes,
the voltage transmitted through R1 changes. The
output of the operational amplifier changes
according to the feedback signal from R1. This
output from the amplifier controls the base voltage of
transistor Q1 allowing it to maintain the constant
current.
Let’s get to the experiment. First set the switch to
position A, and press the key while watching LED 1.
It is dimmer when the key is closed. This is because
both LED 1 and LED 2 are in the circuit when the key
is closed. The load – the amount of energy used by
the LEDs in this circuit – increases, but the current
remains constant. So the LED becomes dimmer.
Now, set the switch to position B with the key off. Do
you see any change in LED brightness from position
A to position B? Setting the switch to B changes the
supply voltage from 9V to 6V. But the current
remains constant again, so the LED brightness does
not change.
Notes:
EXPERIMENT #72: CONSTANT-CURRENT SOURCE
Schematic
-91-
Wiring Sequence:
o
31-132-137
o
32-35-47
o
34-138
o
46-67
o
48-68-75
o
63-131-122
o
69-119-124
o
76-70-121
o
123-133