Explain how an LED emits light and why its forward voltage varies by colour.
Size an LED's series resistor from first principles.
Describe Zener breakdown and the Zener diode's reverse operation.
Design a simple Zener voltage reference.
Two special diodes now, both of which you have already used on a board without necessarily knowing why they behave as they do. First the LED. In silicon, an electron falling into a hole sheds its energy mostly as heat. In certain compound semiconductors — gallium arsenide phosphide, indium gallium nitride and their relatives — the same recombination releases a PHOTON instead, with an energy set by the material's band gap. Since photon energy determines colour, the band gap determines colour, and the band gap also sets the forward voltage. That is why an LED's drop is not the silicon 0.7 V, and why it varies by colour.
The circuit from the CA-1 capstone, now with its diode properly drawn: the resistor sets the current, the LED sets the voltage.
An LED's exponential characteristic means you must never set its current with voltage alone — a few tens of millivolts of variation would swing the current wildly, and an LED driven past its rating dies quickly. So you set the current with a series resistor, exactly as you did in CA-1's capstone, and now you can state the rule from the physics rather than as a recipe:
RS=IFVsupply−VFLED series resistor
Now the Zener. Push any diode far enough into reverse bias and it breaks down: at a sharply defined reverse voltage, current begins to flow backwards freely. In an ordinary diode this is a failure. A Zener diode is engineered — by heavy doping — to break down at a precise, stable voltage and to survive doing so indefinitely. Operated deliberately in reverse breakdown, it holds its terminal voltage nearly constant over a wide current range: a voltage reference, and the simplest regulator there is.
A Zener shunt reference: the resistor drops the excess, the Zener holds the output at V_Z. Note the Zener is REVERSE biased — cathode to the positive rail.WORKED EXAMPLE
Sizing an LED resistor, all three colours
Target 10 mA from a 3.3 V rail. Red LED, VF=2.0 V: RS=(3.3−2.0)/0.010=130 Ω.
Green, VF=2.1 V: RS=1.2/0.010=120 Ω.
White, VF=3.2 V: RS=0.1/0.010=10 Ω — and now notice the danger: a 0.1 V margin means a ±0.1 V variation in VF (well within normal part spread) changes the current by 100% or shuts the LED off entirely.
Conclusion: on 3.3 V, run white LEDs from 5 V if available, or accept poor current control.
130 Ω / 120 Ω / 10 Ω — and the white case is a design warning, not a design.WORKED EXAMPLE
A Zener reference under both extremes
Design a 5.1 V reference from an input that varies 8–12 V, load 0–20 mA, using a 5.1 V Zener needing at least 5 mA.
Worst case for Zener starvation: Vin=8 V, load at 20 mA. Resistor current must be at least 20+5=25 mA, and it has 8−5.1=2.9 V across it: RS≤2.9/0.025=116 Ω. Choose 100 Ω.
Worst case for Zener power: Vin=12 V, load 0 mA. Resistor current: (12−5.1)/100=69 mA, all of it through the Zener. Power: 5.1×0.069≈0.35 W.
So specify a Zener rated at 0.5 W or more. Both constraints satisfied; the design is honest at both ends.
RS=100 Ω with a 0.5 W Zener
PRACTICE
Q1
Size the series resistor for a green LED (VF=2.1 V) at 15 mA from a 5 V supply.
Q2
Explain why a blue LED has a higher forward voltage than a red one.
Q3
A 3.3 V Zener with a 220 Ω series resistor runs from a 9 V input with no load attached. Find the Zener current and its power dissipation.
Q4
Two conceptual checks: (a) why must an LED never be connected directly across a voltage source at its rated forward voltage? (b) in a Zener reference, why is the NO-LOAD case the dangerous one for the Zener?