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Circuit 16: Threshold voltage infrared diode
Setup information
Install the diode as shown in Fig. 2.16. Install the two resistors. Now it is time for the jumpers. Connect the
moving-coil instrument. Check the placement on the board again and only then connect the battery.
Experiment
If everything is wired correctly, the moving-coil instrument will show a medium deflection. This indicates
the so-called threshold voltage of the diode. This voltage must at least have forward current applied, so that
current can flow through the diode. At lower voltage, neither forward nor reverse current is flowing. In our
case, we are measuring the threshold voltage of an infrared diode, which is typically 1.35 V.
You can use this circuit to determine the type of an unknown diode. Write down the above measured value
of the moving-coil instrument. If the unknown diode shows the same value, we have an infrared diode.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.16.
Components required:
1 x Infrared diode, 2 x 1 kΩ
resistors, 1 x moving-coil
instrument, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.16: Threshold voltage
infrared diode
41
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