Elenco 130-in-1 Electronics Playground - Manual (Page 96)

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In this experiment, we will make a constant current
circuit, using an operational amplifier and a
transistor. This circuit maintains a constant current
even when the input voltage is changed, because
more energy is used up in the circuit.
View the schematic. When the current is modified,
the voltage across R1 is also modified. The output of
the operational amplifier changes corresponding to
the feedback signal from R1. This output from the
amplifier controls the base voltage of transistor Q1
allowing it to maintain the continual current.
First set the switch to position A, and press the key
while monitoring LED 1. When the key is pressed it
gets dimmer. This occurs because both LED 1 and
LED 2 are in the circuit when the key is closed. The
total current through the circuit is the same, but now
it is split between LED 1 and LED 2, so LED 1 gets
dimmer.
Set the switch to position B with the key off. Do you
notice any changes in LED brightness from position
A to position B? Setting the switch to B modifies the
supply voltage from 9V to 6V. However, the current
remains constant again, so the LED brightness is not
affected.
Notes:
EXPERIMENT #77: STABLE-CURRENT SOURCE
Schematic
Wiring Sequence:
31-132-137
32-35-47
34-138
46-67
48-68-75
63-131-122
69-119-124
76-70-121
123-133