IV characteristics of diodes

This week I finally got to play around a little with some electronic components I ordered and test them before using them on side projects. My goal was to find the IV curve of a 3V 0.5W zener diode I need to use on a project. Since I had all the instruments set up, as a bonus I decided to find the IV curve of a blue LED as well.

Test circuit and instruments setup.

The test circuit I used for the measurements is the following

Since my voltage source V1 can only output positive voltage (it’s a unipolar voltage source), to measure the characteristic of the diode I had to put it in reverse in the circuit. Of course to measure the direct characteristic I simply reversed the diode again. Generally speaking, the reverse characteristic is the one that matters for zener diodes.

The circuit was also used for measuring the IV characteristic of the LED but I considered only the direct characteristic since the LED is not used in the negative part of the characteristic.

The test circuit is made of the following:

1. A unipolar voltage source.
2. A 1k Ohm 0.5 W 5% resistor.
3. The device under test.
4. The voltmeter (not shown in figure) is set to measure the voltage drop across the diode.
5. The current meter (not shown in figure) is in series with the power supply.

Test conditions

The test was performed at an ambient temperature of about 30 °C.

Devices under test

The devices tested where the following:

1. 3V 0.5W zener diode BZX55C3V0.
2. 3mm blue LED.

Test results

This is the IV curve of the zener diode

The dotted points are actual measurements while the line is an interpolation. This diode should provide a 3V more or less stable reverse breakdown voltage but as you can see this is true only at a specific value of current. My idea was to use this diode as a voltage reference to an operational amplifier and by using this curve I can size the resistor in series with the diode to the appropriate value. It can be noted than when forward biased, the diode shows a pretty stable forward voltage drop.

For the blue LED, the direct characteristic is the following

It can be seen that the LED starts to turn on at about 2.5 V (at 2.47 V it is very dim, to be precise). Then at around 5 mA the LED is almost at full brightness. It might be interesting to do a comparison with LEDs of other colors.