Introduction
The diode is a basic circuit element that possesses nonlinear IV characteristics. In contrast to a simple resistor, doubling diode current does not necessarily double the voltage across the diode. In order to visualize this nonlinear relationship, it is common to plot diode current vs. voltage on a 2D graph. This graph can also be used at a later point to perform “load line” calculations on circuits with diodes.
In this exercise, the experimenter will construct a simple circuit to measure the IV characteristic of a diode. The Low Cost USB DAQ will be used as a programmable voltage source as well as an analog measurement device. The experimental data collected can be compared to expected theoretical data to draw conclusions regarding measurement noise, model shortcomings, and data acquisition.
Pre-Lab Assignment
- Research the exponential diode model using textbooks, the internet, or any other source. This model is more commonly referred to as the “ideal diode model”. What other diode models can you find?
- What formula(s) can you find relating diode current and voltage? Note that the exponential, or “ideal diode model” should have an exponential term.
- Search Google for “Phillips 1N914” and open up the pdf datasheet. Find the graph of diode forward current vs voltage and note your observations. Does the forward IV curve look exponential? How does the reverse IV curve look?
- Use a simulation package such as Multisim to obtain the IV characteristic of a simulated diode. Do this by sweeping either the diode current or voltage, and measuring the other quantity.
- Become familiar with the National Instruments USB 6008 and 6009 data acquisition devices. These datasheets are available on www.ni.com.
Theory
One commonly used method of modeling a diode is the “ideal diode model”. Using this model, diode voltage and current are related according to the following formula:
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Since Is is typically very small (1E-12), this equation is commonly simplified to:
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Note that in the formula, Is stands for the saturation current which is around 1E-12 A, and Vt is a temperature dependent constant equal to 0.0259 V at 300 degrees K. Id and Vd represent diode current and voltage respectively.
Therefore, a theoretical diode IV curve can be plotted using LabVIEW:
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Hardware and Software Required
- 10 Ohm resistor
- Diode
- National Instruments Low Cost USB DAQ
- LabVIEW 8.20 software (LabVIEW 7.1 or 8.0 will work as well)
Labratory Procedure
1) Construct the following circuit and connect to the USB 6008/6009. By outputting a certain voltage at AO 0 and measuring the voltage at AI 3+, the experimenter can obtain a data point containing diode voltage and current. The diode voltage is simply the voltage at node AI 3+, and the diode current is identical to the resistor current (Ohm’s Law).
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2) Use a DMM to measure the actual resistance used for R1 in the circuit above (take R1 out of the circuit for measurement). This will help to obtain a more accurate current reading.
3) Using the DAQ Assistant Express VI, construct the following LabVIEW block diagram. Note that the user will be able to select the number of IV data points to collect, as well as the ending sweep voltage for AO 0. Select this voltage low enough to ensure that resistor R1 does not exceed its power specification.
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4) Run the VI several times using different values for the number of data points and sweep ending voltage. Record your observations and print out three diode IV curves with different numbers of data points.
Post-Lab Questions
- Did the measured diode characteristic curve closely resemble what you expected? How closely did this curve match the datasheet, simulation, and theoretical data you researched earlier?
- The measured IV curve may have contained some noise. Elaborate on some possible sources of this noise (think about your surroundings). Can you think of a way to eliminate some of the undesired noise from the signal? Would you do this in hardware or software?
- What real-world situations would require a designer to measure the diode characteristic curve?
- What quantities could you measure for a bipolar junction transistor (BJT) using the low cost USB acquisition device? Explain how you would make your measurements.
"This set of six student labs are designed to allow anyone to recreate simple experiments at home to explore electronics and electrical engineering concepts. The software and hardware utilized is […]"