Constant Current Source - Maker Factory ELECTRONICS EXPERIMENTS BOX Manual

2.47 Constant current source

Circuit 107: constant current source 1.2 mA
Setup information
Install the two transistors and the six resistors as shown in Fig. 2.107. Then insert the eight jumpers. 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 1.2 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 and calculate
the resistance value using the current (1.2 mA), which we already know.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.107.
132
Components required:
2 x 2N3904 transistors,
1 x 47 Ω resistor,
1 x 2.2 kΩ 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. 2.107: constant current
source 1.2 mA