Zener regulator
A 1N4733A holding 5 V out of a 12 V supply, with the load line that says where it settles and what the load costs. There is no microcontroller in it: the simulator solves the 3 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $3.18. On Mokxi they're free.
What you'll learn
- How a Zener diode clamps a voltage
- Reading a load line: where two curves cross
- Why a series resistor sets the current
- Dynamic resistance as a measure of regulation quality
- Power in a Zener: volts times amps
- Difficulty: Beginner
- Time: About 10 minutes
- Parts: 5
How it works
A 12 V supply feeds a 220 ohm resistor. At the other end of that resistor sits a 1N4733A Zener diode to ground, with a 1k ohm load in parallel. The Zener does nothing until its voltage reaches about 5.1 V. Then it conducts just enough to hold it there.
The resistor can only pass what is left of the supply, so its current is (12 - Vout) / 220. The Zener has its own nearly vertical curve at 5.1 V. The circuit settles where the two lines cross. With nothing attached that is about 31.9 mA at 4.98 V.
The 1k load takes about 4.9 mA, so the Zener gets that much less. Its curve is not perfectly vertical, so the output drops by roughly 7 ohms times 4.9 mA, about 34 mV, to 4.94 V.
Parts list
5 parts, plus the jumper wires. They retail for about $3.18; on Mokxi they're free.
- 1 × Half breadboard
- 1 × Power, 12 V
- 1 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Zener diode
- 1 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Resistor, 220 Ω pin 1 | 12 V |
| Zener diode pin A | Ground |
| Resistor, 1k Ω pin 2 | Ground |
| Zener diode pin C | Resistor, 220 Ω pin 2 |
| Resistor, 1k Ω pin 1 | Resistor, 220 Ω pin 2 |
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// A Zener regulator, and the load line that explains it.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit.
//
// WHAT IT IS. A 12 V supply, 220 ohms in series, and a 1N4733A Zener across
// the output with its band to the positive side. The Zener does nothing at all
// until the voltage across it reaches about 5.1 V, and then it conducts as hard
// as it has to in order to keep it there.
//
// THE LOAD LINE, which is the whole lesson. The resistor can only deliver
// what is left of the supply:
//
// I = (12 - Vout) / 220
//
// That is a straight line on a graph of current against output voltage: 54.5 mA
// if the output were shorted to ground, nothing at all if the output were the
// full 12 V. The Zener's own curve is the other line: nearly vertical at 5.1 V.
// The circuit sits where the two cross. Here that is 31.9 mA at 4.98 V with
// nothing on the output.
//
// NOW THE LOAD. The 1 k across the output takes about 4.9 mA for itself, so
// the Zener gets 4.9 mA less than it would with nothing on it. The resistor's
// line has not moved, so the crossing slides down the Zener's curve, and
// because that curve is not perfectly vertical, the output falls. It falls by
// exactly the datasheet's dynamic resistance times the current that moved:
//
// 7 ohms x 4.9 mA = 34 mV
//
// With the load on, 4.94 V; take it off (set the load to 1000000 in the
// properties panel) and it is 4.98. That 7 ohms is "Zzt" on a 1N4733A
// datasheet, quoted at the 49 mA test current, and it is the number that says
// how good a regulator this is. It is why nobody builds a supply this way for
// anything that draws real current, and why it is perfect for a reference a
// few milliamps wide.
//
// THINGS TO TRY. Turn the supply down to 6 V and watch the Zener give up: below
// about 5.3 V there is no current left to spare and the output is just the
// divider of 220 ohms over 1 k. Swap the Zener for a 1N4740A (10 V) in the properties panel and the
// output follows. Or drop the series resistor to 47 ohms and watch the Zener
// take 145 mA, three quarters of a watt, which on a real 1 W part is where it
// starts to smell.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The output is near 0 V or follows the supply This project
Check the Zener's direction. The band (cathode) must face the positive side, with the anode to ground. - The output falls well below 5 V at a lower supply This project
If the supply is below about 5.3 V, there is no current left for the Zener to use, so the output is just the 220 ohm and 1k divider. Raise the supply back to 12 V. - The Zener gets very hot with a small series resistor This project
A smaller resistor lets more current through. At 47 ohms the Zener takes about 145 mA, around three quarters of a watt, which is too much for a 1 W part. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure. - A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground. - The Zener does not hold the voltage, or it burns out. Zener diode
Use a series resistor to limit the current. Size it as (supply minus Zener voltage) divided by the current, and keep the power under the diode's rating, such as 0.5 W. - The Zener acts like a normal diode. Zener diode
Put it in reverse: the cathode (band) goes to the positive side. Forward it conducts at about 0.7 V. The regulated voltage also shifts a bit with current and temperature.
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