Instructions

134
Circuit 109: Constant current source 5.7 mA
Setup information
Install the two transistors and the four resistors as shown in Fig. 2.109. The eight jumpers are next. Check
the placement on the board again and only then connect the battery.
Components required:
2 x 2N3904 transistors,
1 x 47 Ω resistor, 1 x 10 kΩ
resistor, 1 x 47 kΩ resistor,
1 x resistor (see Experiment
for value), 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.109: Constant current
source 5.7 mA
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 5.7 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 (5.7 mA), which we already know.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.109.