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Circuit 90: Constant current with bipolar transistor
Setup information
Install the two transistors, the LED diode and the silicon diode as shown in Fig. 2.90. Then install the resis-
tor. The seven jumper wires come next. Check the placement on the board and only then connect the bat-
tery.
Experiment
Although everything is wired correctly, nothing will happen when you connect the battery. Up to now you
still don't know the value of the LED. You can use any LED, whether red, yellow or green. Current will always
be high enough to deliver a bright LED light. Substantial current flows through the LED in our circuit. Even if
battery voltage drops, the LED will always shine with the same brightness.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.90.
Components required:
1 x J111 FET transistor,
1 x 2N3904 transistor,
1 x LED (see Experiment
for value), 1 x 1N4148
silicon diode, 1 x 1 kΩ
resistor, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.90: Constant current
power source with bipolar
transistor
115
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