Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 131

Circuit 108: Constant current source 2 mA
Setup information
Install the two transistors and the five resistors as shown in Fig. 2.108. The nine jumper wires are next.
Check the placement on the board again and only then connect the battery.
Experiment
A digital multimeter's constant current source ensures that the value of resistors can be measured. In our
circuit you could use any value between 100 Ω and 1 kΩ for R5. The current flowing through this resistor
is always 2 mA. Since the resistance value is calculated by voltage divided by current according to Ohm 's
law, a voltage measurement will do. You can measure the voltage at R5 using a multimeter or a DSO (digital
oscilloscope) and calculate the resistance value using the current (2 mA), which we already know.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.108.
Components required:
2 x 2N3904 transistors,
1 x 47 Ω resistor,
2 x 10 kΩ resistors,
1 x 47 kΩ resis-
tor, 1 x resistor (see
Experiment for value),
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.108: Constant current
source 2 mA
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