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Two-Wire IV Curve Tracer

Two-Wire IV Curve Tracer

Purpose

The purpose of this lab is to guide you on how to build your own IV Curve Tracer that can analyze the real performance of two-wire components, such as diodes, LED’s, and resistors.

Figure 1.png

Figure 1- IV Curve for an LED

Background

IV Curve Tracers are great for analyzing the performance of various electronic components, but especially diodes.  Components like diodes and LEDs (Light Emitting Diodes) do not have a linear relationship between current and voltage like resistors do.  There are four basic modes of operation for a diode.  To understand these regions, you must first understand that diodes are directional components, meaning that their orientation in a circuit is important.  The IV Curve (refer to Figure 2) does a great job of displaying this characteristic.  The leftmost region is called the “Breakdown” region.  This is where current is flowing against the intended direction.  To generate Figure 2, a Zener Diode was used.  Zener diodes intentionally have a smaller breakdown voltage (Vbr), which allows us to see the breakdown region in Figure 2. The next region is the reverse bias region.  In this region, the voltage across the diode is still negative, but little or no current is flowing through it (depending on the diode).  After that comes the forward bias region.  Here, the voltage is positive, but there is again, little or no current flowing.  Once the voltage across the diode exceeds the diode’s forward voltage drop (Vd), the diode is said to be “turned on.”  The diode offers very little resistance to current in this region.

Figure 2.png

Figure 2- IV Curve for a Zener Diode

Equipment


  • NI myDAQ
  • Large Resistor (~10kΩ)
  • Attachable Probes
  • Electrical component for testing


Note: The Probes do not have to be attachable, but you must have two probes with banana plugs.

Hardware Setup

To build the circuit, follow the steps below.  A circuit diagram can be seen in Figure 3 below.

Figure 3.png

Figure 3- Circuit diagram

1) Connect analog output 0 to your large resistor.  This resistor is here to prevent burning out any sensitive components, such as LEDs.

2) Connect the positive side of the component you are testing (the anode for a diode) to the other side of the resistor.

3) Plug one of your probes into the current measuring port (shown below) of the myDAQ’s DMM.  Connect that probe to the other end of the component you are measuring.

Figure 4.png

Figure 4- NI myDAQ DMM ports

4) Plug the other probe into the DMM COM port (shown above) and connect it to ground.

5) Connect the positive terminal of analog input 0 to the positive side of the component you are testing.

6) Connect the negative terminal of analog input 0 to ground.


Software Instructions

To use this two-wire IV Curve Tracer, first either download the VI or drag the code below into your own blank VI.

Two Wire IV Curve Tracer Snippet.png

Figure 5- LabVIEW Block Diagram

Figure 6.png

Figure 6- VI Front Panel

To setup the VI, select an appropriate starting voltage, voltage step, and final voltage for your test.  Once this is done, click “run” and wait for the VI to finish running.

How it Works

A two-wire IV Curve Tracer requires one analog output and two analog inputs.  The analog output is a voltage output, and it controls the voltage going into the whole circuit.  The first analog input takes in a voltage reading.  The input is connected directly around the component being tested to provide an accurate measurement of the voltage across it.  This is very important, because with the large resistor we added to the circuit, the input voltage will not necessarily be the same as the voltage across our test component.  The second analog input is a current input.  The myDAQ's analog channels are capable of both inputting and outputting analog currents, but cannot when the channels are already being used for voltage signals. To get around this problem, the DMM is used as the second analog input.

If your test component is not reaching a high enough test voltage, it is probably because the large resistor is taking too much of the voltage.  Try increasing the final voltage until satisfactory results are obtained.  If that does not work, replace the resistor you are using with one of less resistance.

Jeff S.
National Instruments
Comments
musician
Member
Member
on

Hi

I find this extremely useful for me as I am trying to measure the diode vi characteristic.

However, can the above experiment be done with USB-6008 instead?

Regards

Jeff.St
Member
Member
on

Hi Musician,

I'm glad that the example can help you! Unfortunately, the USB-6008 isn't going to work though.  You need to be able to measure current and voltage, and looking at the specifications of the USB-6008, it looks like it only measures voltage.

There are plenty of other products available that can measure current though.  You could use one of them with your USB-6008 to perform this example. Also, you may want a device with a larger range for the analog output.

Good luck!

Jeff S.
National Instruments
hyperhyper
Member
Member
on

Hi Jeff,

sorry, but may I correct you in that by simple using Ohm's law, you can well use a USB-6009 to measure current, albeit indirectly. See https://decibel.ni.com/content/docs/DOC-13122#comment-37798

Cheers,

Hyperhyper

Jeff.St
Member
Member
on

Hyperhyper,

You are of course correct.  Thanks for pointing that out!

Best,

Jeff S.
National Instruments
FrederikLeBlank
Member
Member
on

With what application do we have to run the .vi? I'm trying to open teh VI tracer but it doesn't work

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