EE 2115-01 - Lec 12 - 2026_09_21

Overview

Hiren Trada finishes voltage-comparator analysis by showing how a 0–10 V comparator responds when a triangular input crosses a fixed 5 V reference, then applies the same switching logic to a low-supply indicator using resistor dividers and diode drops. The lecture introduces Zener diodes as reverse-breakdown voltage references and regulators, covering their symbols, breakdown behavior, and a 10 V supply circuit with a 620 Ω resistor and 4.7 V Zener.

Key takeaways

A comparator’s output is set by its supply rails: it switches to +VCC when V+ exceeds V− and to −VE when V− exceeds V+.
A 30:10 resistor divider sends one quarter of its input voltage to the comparator, so an 8 V supply becomes a 2 V threshold signal.
Two forward-biased 1N914 diodes provide an approximate 1.2–1.4 V reference, allowing the example comparator to change state as its monitored supply falls from 8 V to 4 V.
A Zener diode is designed to conduct in reverse breakdown and maintain a nearly constant voltage, avoiding the number and space required to stack ordinary diodes.
For a 10 V supply, 620 Ω series resistor, and 4.7 V reverse-biased Zener, the output is about 4.7 V and the resistor current is approximately 8.5 mA.

Chapters

0:00 A 0–10 V Comparator Tracks a Triangular Input Across 5 V

The comparator’s inverting input stays at 5 V while its non-inverting input follows a 0–10 V triangular waveform.
A 0.5 ms rise and 0.5 ms fall create a 1 ms triangle period in LTspice.
With 0 V and +10 V supply rails, the output is 0 V while the triangle is below 5 V and +10 V while it is above 5 V.

6:00 Comparator Analysis and the Low-Supply Indicator Application

Analyze a comparator by identifying its power-rail limits first, then finding where the two input voltages cross.
Students are encouraged to vary comparator rails and inputs, solve manually, and check the result in LTspice.
A low-supply indicator uses a comparator to flag when an incoming voltage falls above or below a set threshold.

12:30 Resistor and Diode Dividers Set the Supply-Voltage Threshold

A 30:10 resistor divider places one quarter of its input at the comparator input: an 8 V signal produces 2 V, and 4 V produces 1 V.
Two series 1N914 diodes provide an approximate 1.2–1.4 V reference when forward biased.
At an 8 V input, the resistor-divider input is 2 V while the diode-clamped input is about 1.2–1.4 V, so the comparator selects its negative rail.

18:30 A Falling Supply Reverses the Comparator Output

When the monitored voltage falls to 4 V, the 30:10 divider produces a 1 V comparator input.
The diode pair still clamps the other input near 1.2–1.4 V, making the non-inverting input higher and switching the output to the positive rail.
Comparator thresholds can monitor real system conditions such as inadequate dryer supply voltage or low washer water flow.

24:00 Comparator Inputs Choose Between the Power Rails

A comparator output is either +VCC or −VE; the input signals determine which rail is selected, not the output’s voltage magnitude.
The switching rule is: V+ > V− gives the positive rail, while V− > V+ gives the negative rail.
In the example, the comparator uses 0 V and +10 V rails; the common reference symbol connects circuit points even when no continuous wire is drawn.
The class considers LED indicators for the two output states and recommends checking the input crossing and output transitions in simulation.

33:30 Why a Zener Replaces Stacked Forward-Biased Diodes

A fixed-voltage-drop diode model is only an approximation because a real diode’s forward voltage varies with current.
Stacking ordinary diodes to obtain a larger reference voltage adds parts and consumes circuit-board or chip area.
A Zener diode is designed to operate in reverse bias and hold a near-constant voltage in its breakdown region.

36:20 Zener Breakdown, Reverse Bias, and Voltage Ratings

The Zener symbol has a bent cathode line; in reverse bias, the cathode is positive relative to the anode.
Purpose-designed doping allows Zener breakdown at relatively low voltages, with common devices available from roughly 2 V to 200 V.
After breakdown, a large current change produces only a small voltage change, provided the diode’s current and power ratings are respected.
Ordinary rectifier diodes also have reverse-voltage limits; the lecture cites 50 V for a 1N4001 as an example.

44:20 A 4.7 V Zener Regulates a 10 V Supply Through 620 Ω

When forward biased, the Zener behaves like an ordinary silicon diode with an approximately 0.7 V drop; a 10 V supply and 620 Ω resistor yield about 15 mA.
With the diode reverse biased, a 10 V input exceeds its 4.7 V breakdown rating, so the output across it is approximately 4.7 V.
In the reverse-biased example, the resistor drops about 5.3 V, giving roughly 8.5 mA through the 620 Ω resistor.

48:00 Zener Diodes as Voltage Clamps and Surge Protection

A reverse-biased Zener can clamp a circuit node near its rated voltage when the supply exceeds that threshold.
The lecture connects this clamping role to surge suppressors in power strips, which help protect downstream devices from excessive voltage.
Zener protection depends on selecting a device and current-limiting path that keep the diode within its voltage, current, and power ratings.

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.