EE 2115 - Lec 05 - 2026_09_02

Overview

Hiren Trada introduces voltage-resistor-diode (VRD) circuits and compares diode-equation calculations with LTspice simulations. The lecture works through a 10 V LED circuit designed for about 50 mA, iterative analysis of a 270 Ω circuit, and piecewise-linear diode models used to predict voltage transfer and clipping behavior.

Key takeaways

For a 10 V LED circuit targeting 50 mA, the diode-equation estimate gives 163 Ω, so a 160 Ω standard resistor produces about 49 mA in LTspice.
A diode model's 6 Ω internal resistance adds about 0.3 V at 50 mA, explaining why the simulated LED drop is about 2.14 V instead of the 1.86 V idealized estimate.
A 10 V source with a 270 Ω resistor yields about 30.2 mA by iterative diode analysis, within roughly 2% of LTspice's 29.6 mA result.
The ideal diode switches at 0 V forward bias, while the silicon CVD model typically switches near 0.7 V; that threshold sets a clipper's output limit.
A +5 V bias on a diode cathode shifts its positive clipping threshold to 5 V for an ideal diode or about 5.7 V for a 0.7 V CVD diode.
Reversing a clipper diode changes which sine-wave half-cycle is removed, and using two oppositely oriented diodes can limit both positive and negative excursions.

Chapters

0:00 VRD Circuits and the Nonlinear Diode

A VRD circuit combines a voltage source, a current-limiting resistor, and a diode.
Unlike linear passive elements such as resistors, capacitors, and inductors, a diode has a nonlinear voltage-current relationship.
LTspice examples are intended for students to rebuild and practice outside class.

6:30 Sizing an LED Resistor with the Diode Equation

The example uses a 10 V source and an LTspice LED model, with a target current of approximately 50 mA.
The diode equation uses saturation current, diode voltage, thermal voltage of about 25.8 to 26 mV, and ideality factor n, typically between 1 and 2.
Using the model's parameters gives an estimated 1.86 V diode drop; the remaining 8.14 V across the resistor implies about 163 Ω.
A 160 Ω standard resistor is selected, and students are challenged to calculate and simulate three LEDs in series.

13:30 LTspice Verification and Iterative Current Analysis

The 160 Ω LED circuit simulates at about 49 mA, close to the 50 mA design target.
LTspice reports about 2.14 V across the LED rather than the estimated 1.86 V because the model includes 6 Ω of internal series resistance.
For a 10 V source and 270 Ω resistor, KVL combines resistor drop and diode voltage into a transcendental equation that cannot be solved with basic algebra.
Iterating diode voltage and current converges quickly: the manual estimate reaches about 30.2 mA, compared with LTspice's 29.6 mA, an error of roughly 2%.

21:00 Ideal and Constant-Voltage-Drop Diode Models

The ideal diode model turns on at zero forward voltage and is off under reverse bias.
The constant-voltage-drop (CVD) model uses a fixed forward threshold, commonly 0.6 to 0.7 V for silicon and 0.2 to 0.3 V for germanium.
CVD and ideal models approximate the nonlinear diode curve as separate linear operating regions, making them piecewise-linear analysis tools.
Use 0.7 V for a silicon diode when a problem does not specify a different forward drop.

25:30 CVD Transfer Characteristics and Positive Voltage Limiting

For a diode connected across the output, the CVD model keeps the diode off while input voltage is at or below 0.7 V, so Vout follows Vin.
Above 0.7 V, the diode conducts and holds the output near 0.7 V, producing positive-side voltage limiting.
The LTspice verification uses a 1N4001 diode, a 200 Ω resistor, and an input sweep from -5 V to +5 V in 0.05 V steps.
The simulated 1N4001 turn-on is approximately 0.66 V, with a slight curve that the fixed-drop model does not capture.

31:00 Diode Clippers on Positive and Negative Sine-Wave Peaks

A diode voltage limiter is also called a clipper, and its orientation determines which half-cycle is suppressed.
With the diode oriented to conduct on positive peaks, the output is clipped once the input exceeds the forward threshold, around 0.6 to 0.7 V.
Reversing the diode switches the clipping action to the negative half-cycle.
The LTspice examples contrast transient sine-wave behavior with the earlier DC input sweep.

35:00 Biased and Dual-Diode Clipper Transfer Exercises

With a diode cathode biased at +5 V, the ideal model conducts when Vin exceeds 5 V; the 0.7 V CVD model raises the threshold to 5.7 V.
In the dual-diode exercise, D1 limits the positive side near +5.7 V under the CVD model.
D2 is oriented for negative-side conduction: its CVD threshold is reached when Vin falls below about -0.7 V, while the ideal model switches at 0 V.
Students are asked to sketch transfer characteristics for both ideal and CVD models.

40:00 Output Across the Resistor and Midterm Practice

The final VRD variation changes the output measurement from across the diode to across the resistor.
Students must determine the resistor output for diode-on and diode-off conditions, using the chosen ideal or CVD model.
The lecture recommends reviewing the slides, simulations, and exercises individually or in study groups.
VRD circuit analysis is identified as material for the first midterm.

Transcript excerpt

0:03 Good afternoon. Lecture five. Today we are going to look at VRD circuits. VRD, as the acronym suggests, is uh three components: voltage source, resistance, and diode. Let's see. Right there. VRD circuit. Okay. Okay.

0:55 We're going to look at a simple circuit, a diode with a current-controlling resistor, and a voltage source that is applied. Now, if you recall, the three primary components we study in this course, or in this class, is are the resistor, capacitor, and inductor. All three of them are linear passive elements. The diode is the other two-terminal element we are going to study in this course, but it is a non-linear element. Non-linear because the relationship between the voltage and the current is not linear. And uh we saw in one of the previous lectures that the diode voltage and current have this kind of relationship.

1:46 Right? Now, the first thing we should understand about non-linear uh elements is that trying to predict the outcome um of a non-linear element in a circuit is very difficult. We try to obtain an accurate model for the non-linear device. But there are there's no direct solution that we work with. There are iterative solutions or recursive solutions that we try to try to use. Okay. So, let's look at the circuit. And this circuit is created in Spice. Now, anytime you see a Spice circuit in the lecture notes, please have it understood that students

2:33 should practice outside of class using this exact same circuit. Okay. Circuit's easy enough to build. For this model, we are using an LED. And see. Right there. And typically an LED should have a symbol that looks like this. It's an arrow pointing outwards. Could be a single arrow like this or it could simply be a arrow pointing out like this.

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Related notes

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Hiren Trada.