part · Level 5 · Control · passive
3.3 V zener diode (1N4728A): resistor math
Zener diode 3.3 V (1N4728) — A diode built to conduct backwards at exactly 3.3 V, so it pins a voltage no matter what the battery does.
What it is
An ordinary diode is a one-way valve: it passes current from anode to cathode and blocks it the other way. Push hard enough backwards and every diode eventually gives in, in a rush called breakdown. For a 1N4001 that happens somewhere past 50 V and may destroy it. A zener diode is built so that breakdown happens at a precise, low voltage and does no harm. The 1N4728 breaks down at 3.3 V.
That turns the diode into a voltage clamp. Wire it backwards, cathode (the striped end) toward +, and nothing flows until the voltage across it reaches 3.3 V. Then it conducts as much as it must to stop the voltage rising any further. Whatever a 9 V battery wants, the point above the zener sits at about 3.3 V.
A zener with a series resistor is the simplest voltage regulator there is. Every regulator chip you will meet later does the same job with more transistors and less waste.
How it works
Forwards it is just a diode: about 0.7 V, like the 1N4001. If you wire it that way round by mistake, that is all you get.
Backwards, below 3.3 V: a few microamps leak through. Call it open.
Backwards, at the knee: the current climbs very steeply. In the datasheet the part holds 3.3 V at its 76 mA test current; in the site's model the current is 1 mA at 3.30 V and grows about 2.7× for every extra 26 mV, so 12 mA at 3.36 V. The voltage hardly moves while the current changes tenfold. That is the whole trick.
The series resistor is not optional. It is what turns the battery's fixed 9 V into a current the zener can share out. In the 3.3-Volt Rule, 220 Ω from 9 V to the 3.36 V node passes (9 − 3.36) ÷ 220 ≈ 25.6 mA. The LED lane (100 Ω + red LED) takes 13.4 mA of that; the zener takes the rest, 12 mA. Close the switch to add a second lane and the lanes take 25.4 mA between them; the zener's share drops to 0.5 mA and the node sags only to 3.28 V. The rule:
I_zener = I_series − I_loads
Keep I_zener above about 1 mA and the voltage stays pinned. Ask the loads for more than the resistor delivers and the zener switches off; the node then falls with the load, unregulated.
Power. The zener dissipates 3.36 V × 12 mA ≈ 40 mW against a 1 W rating. The 220 Ω dissipates 0.0256² × 220 ≈ 0.14 W, more than half of a ¼ W part's rating: warm to the touch, which is the price of this kind of regulator. It burns the same power whether the loads use it or not.
The numbers
| Zener voltage | 3.3 V — 1N4728A, ±5 %, at the 76 mA test current |
|---|---|
| Power rating | 1 W — 3.3 V × 300 mA |
| Forward drop (wrong way round) | ≈ 0.7 V |
| Knee in the site's model | 1 mA at 3.30 V — 12 mA at 3.36 V |
| Dynamic resistance | ≈ 10 Ω — datasheet, at 76 mA |
| Stripe | cathode — toward + — the opposite of a normal diode in a circuit |
See it working
The The 3.3-Volt Rule circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Stripe toward −. Forward-biased: the node sits at 0.7 V and nothing downstream works. Fix: the stripe faces the + side; a zener regulates backwards.
- No series resistor. A 3.3 V zener straight across a 9 V battery conducts everything the battery can give. It smokes. Fix: the resistor sets the current; the zener only shares it.
- Loads heavier than the resistor supplies. The node drops below 3.3 V and the zener stops helping. Fix: a smaller series resistor, or fewer loads; keep at least 1 mA for the zener.
- Expecting exactly 3.300 V. The part is ±5 % and the knee has a slope. Fix: design for "about 3.3 V" and measure.
- A 220 Ω from the AA pack. 4.5 V leaves only 1.2 V for the resistor: 5 mA total, not enough for an LED lane. Fix: the zener project uses 9 V for a reason.
Questions people ask
- What is the difference between a zener diode and a normal diode?
- Forwards they behave the same, about 0.7 V. Backwards a normal diode blocks until a high, uncontrolled breakdown voltage; a zener is made to break down at a precise low voltage, 3.3 V here, and to survive doing so. You use that reverse breakdown on purpose.
- How do I choose the series resistor for a zener?
- Decide the total current: what the loads need plus at least 1 mA for the zener. Then R = (V_battery − V_zener) ÷ I_total. From 9 V to 3.3 V at 25 mA that is 5.7 ÷ 0.025 ≈ 230 Ω, so 220 Ω.
- Can a zener regulator power an Arduino or a sensor board?
- Only something that sips a few milliamps and does not mind the voltage wobbling a little. Boards that draw tens of milliamps need a regulator chip, which is the same idea with a transistor doing the heavy lifting.
Projects that use this part
- ⚖️ The 3.3-Volt Rule — A zener diode holds 3.3 V for the LED no matter what the 9 V battery wants. Flick the switch to add a second load and watch the zener share. L5 · 30 min
- 🔋 The Zener Battery Tester — A zener in series with an LED makes it ignore the first 3.3 V. A fresh 9 V lights it; a run-down battery barely can. A sharper gauge than a bare LED. L5 · 25 min
verified against: Vishay — 1N4728A–1N4761A zener diode datasheet · SparkFun — Diodes · Kuphaldt — Lessons in Electric Circuits, Zener Diodes