Electronics
How does a Zener diode work?
A Zener diode is a diode built to conduct backwards once the voltage across it reaches a set value, its Zener voltage (VZ), and then to hold that voltage almost still however much current flows. Put one across a load, with a resistor in series, and the load gets a steady voltage from an unsteady supply.
A diode that lets go backwards
Forward-biased, a Zener is an ordinary silicon diode that drops about 0.7 V. Reverse-biased, it blocks until the push reaches VZ. There it breaks down and conducts, and the voltage across it barely moves. So in a regulator it sits reverse-biased, stripe toward +.
Think of a tank with a spillway. Whatever pours in, the level can't rise past the lip: the extra spills away. The Zener is the spillway, and a load wired across it sees a steady level.
Live experiment
A chip that wants 5 V
A battery feeds a chip through 220 Ω, with a 5.1 V Zener across the chip. Slide the battery from 0 to 12 V and watch the plot, then take the Zener out, and try it turned round.
“Zener regulators” is lesson 3 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 30 before it first.
Size the series resistor
The resistor feeds both the load and the Zener, so its current splits between them (Kirchhoff's current law). Below a few milliamps a Zener goes soft and stops holding, so give it at least IZ,min, about 5 mA:
Example: 12 V in, a 5.1 V Zener and a 20 mA load:
Round down to 270 Ω. Then 6.9 ÷ 270 ≈ 25.6 mA flows: 20 mA to the load and 5.6 mA to the Zener.
Check the no-load power
If the load is unplugged, the Zener takes the whole 25.6 mA. It must survive that:
That's well under a typical 0.5 W rating. The same reasoning explains the regulator's one weakness: the resistor passes a fixed current all the time, and the Zener burns off whatever the load doesn't use.
Where Zeners are used
A plain voltage divider sags as soon as a load draws real current. A Zener holds its voltage while the load changes, as long as it keeps its minimum current. That makes Zeners good voltage references inside meters and regulators, and good clamps that stop a sensor's voltage from rising high enough to damage a chip's input. For bigger loads, a three-pin regulator such as the 7805 is better: it takes only the current the load needs.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Zener regulators” is lesson 3 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 30 before it first.
Or start from the very first lesson, no account neededWork it out
- Zener diode resistor calculatorThe series resistor for a Zener regulator, and the power ratings it needs.
- Zener diode practice problemsSize a Zener regulator's resistor, split its current and check the Zener's power.
- Common diodes chart1N4148, 1N4007, 1N5819 and the 1N47xx Zeners compared, and which one to use.
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: backwards: cathode toward +. A zener regulates backwards, once breakdown starts. Turned forwards, it's an ordinary diode that drops about 0.7 V.
- Why not “forwards: anode toward +”? Forwards, a Zener acts like an ordinary diode and drops only about 0.7 V. It holds its rated voltage only when wired backwards.
- Why not “either way round”? Like any diode, a Zener behaves differently each way round. Backwards it holds its Zener voltage, and forwards it drops about 0.7 V.
Show the answer
Answer: Limit the current it has to absorb. Without it the spike's current can burn the zener.
- Why not “Raise its voltage”? A series resistor doesn't change the voltage a Zener clamps at: the Zener sets that itself. The resistor drops the rest of the spike.
- Why not “Nothing”? Without a resistor, only the source's own resistance limits the current, and that can be almost nothing, so the Zener can burn out.
- Why not “Make it faster”? A resistor doesn't make the Zener any faster. Its job is to limit the current that flows into the Zener while it clamps.
Show the answer
Answer: The load pulls its output down. A divider acts like a battery behind Rth. A heavy load forms a second divider with Rth, so the output sags. A Zener holds its voltage instead.
- Why not “Dividers only work on AC”? A divider works on DC just as well as on AC: it splits its supply by the ratio of its resistors.
- Why not “It makes too much voltage”? A divider's output is always less than its supply, never more. The trouble is that a heavy load drags the output down, which a Zener prevents.
“Zener regulators” is lesson 3 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 30 before it first.
Questions people ask
What is the difference between a Zener diode and a normal diode?
Both conduct forwards at about 0.7 V. A normal diode is meant to block in reverse. A Zener is built to break down in reverse at a chosen voltage, its Zener voltage, and to hold that voltage steady while current flows. So in a circuit a Zener is fitted reverse-biased, stripe toward +.
Why does a Zener diode need a series resistor?
Above its Zener voltage, a Zener's current climbs very steeply for a tiny rise in voltage. Wired straight across a supply, it would draw as much current as the supply can give and burn out. The series resistor sets the current and takes the difference between the supply and V_Z.
How do I calculate the resistor for a Zener regulator?
R = (V_in − V_Z) ÷ (I_load + I_Zmin), where I_Zmin is a few milliamps to keep the Zener regulating. Round down, not up. Then check the Zener survives with no load: it takes all the current, so P = V_Z × I_R must sit under its power rating.
Related topics
- How does a diode work?A one-way valve for current: anode, cathode, the 0.7 V forward drop and Schottky diodes.
- Voltage divider explainedTwo resistors that split a voltage: the formula, potentiometers, and why a load drags the output down.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.