Zener Diode Regulators
A voltage regulator is a circuit that holds its output voltage steady regardless of changes in the load drawing current from it or variations in the supply voltage feeding it. You have already seen how a pass transistor can do this job in a linear regulator. The zener diode gives you an even simpler way to achieve the same goal — using just one diode and one resistor. It is not the most efficient solution, but it is the easiest to understand, the cheapest to build, and perfectly adequate for low-current applications like bias supplies and voltage references in ham radio equipment.
Shunt Regulators vs Series Regulators
In a series regulator — the type you studied in the previous lesson — the regulating element (a transistor) sits in series with the current path between the supply and the load. The transistor acts like a variable resistor that adjusts itself to drop more or less voltage, keeping the output constant. All the load current flows through the transistor.
In a shunt regulator, the regulating element is connected in parallel with the load — it shunts current away to ground rather than blocking it in series. A fixed resistor sits between the supply and the output node, and the shunt element (the zener diode) draws whatever current is needed to keep the voltage at the output node constant. If the load draws less current, the zener draws more. If the load draws more current, the zener draws less. The two always sum to the fixed current flowing through the series resistor.
The key difference in behaviour: a series regulator wastes power in proportion to the difference between supply and output voltage, but it adapts gracefully to a wide range of load currents. A shunt regulator wastes the most power when the load current is lowest (all the current goes through the zener), which makes it less efficient for varying loads but perfectly fine for steady, low-current applications.
The zener shunt regulator: Rs carries the total current Is, which divides between the load (IL) and the zener (Iz). The zener clamps the output at Vz.
View LargerHow a Zener Diode Works
An ordinary rectifier diode conducts in the forward direction and blocks in the reverse direction. If you keep increasing the reverse voltage, at some point the diode will break down and conduct a large reverse current — this is normally a catastrophic failure in a signal diode. However, zener diodes are specifically designed and manufactured to break down at a precise, well-controlled reverse voltage and to do so repeatedly without damage, as long as the power dissipation stays within the rated limit.
This controlled breakdown is caused by two distinct physical mechanisms depending on the breakdown voltage. Below about 5 volts, breakdown is dominated by the Zener effect (quantum mechanical tunnelling of electrons directly across the junction). Above about 7 volts, breakdown is dominated by avalanche multiplication (high-energy carriers knock further carriers loose in a chain reaction). Between 5 and 7 volts, both mechanisms contribute. The part is called a "zener" regardless of which mechanism dominates — the name has stuck.
The most important characteristic of a zener is its breakdown voltage Vz — the reverse voltage at which it clamps. Standard values range from 2.4 V to 200 V, with common ham radio values including 3.3 V, 3.6 V, 4.7 V, 5.1 V, 6.2 V, 9.1 V, 12 V, and 15 V.
The I-V Characteristic Curve
The I-V curve of a zener diode has three regions. In the forward region (positive voltage applied), it behaves like an ordinary silicon diode — it conducts with a forward drop of about 0.6–0.7 V. In the normal reverse region (small reverse voltage, below breakdown), it passes only a tiny leakage current measured in microamps. In the breakdown region, once the reverse voltage reaches Vz, the current can increase enormously while the voltage stays nearly constant. This near-vertical characteristic in the breakdown region is what makes the zener useful as a voltage reference.
The steeper the breakdown knee, the better the regulation. A zener with a very sharp knee will hold its voltage more precisely as current varies. The sharpness of the knee is related to the dynamic impedance (also called zener impedance Zz), which is the slope dV/dI at the operating point. A lower Zz means better regulation. Typical values range from 1Ω for a high-current 5.1V zener to 100Ω or more for a low-power device at a low current.
The zener I-V curve: breakdown at Vz is steep and nearly vertical — small changes in current produce very small changes in voltage. This is the regulation region.
View LargerTemperature Coefficient
The breakdown voltage of a zener changes with temperature. This is an important consideration for precision references. The temperature coefficient (TC) is measured in mV/°C or ppm/°C. There is a fortunate crossing point: zeners with a breakdown voltage near 5.1 V have a near-zero TC because the positive TC of the avalanche mechanism and the negative TC of the zener tunnelling mechanism almost cancel. The 1N4733A (5.1 V, 1W) is a classic choice when temperature stability matters. Devices with Vz below 5 V have a negative TC (voltage decreases as temperature rises); devices above 7 V have a positive TC.
Voltage Tolerance
Standard zener diodes have a 5% voltage tolerance. A 5.1V zener may actually break down anywhere between 4.85 V and 5.36 V. For precision references, 1% tolerance zeners and dedicated voltage reference ICs (such as the LM336 or REF02) are available. For most ham radio shunt regulator applications — bias supplies, AGC references — 5% tolerance is perfectly adequate.
The Shunt Regulator Circuit
The complete zener shunt regulator has just three components: the supply voltage Vs, a series resistor Rs, and the zener diode from the output node to ground. The load RL connects in parallel with the zener, also between the output node and ground.
Current flows from Vs through Rs to the output node. At that node, the current splits: some flows through RL (this is the useful load current IL), and the remainder flows through the zener to ground (this is the zener current Iz). The total current through Rs is therefore Is = Iz + IL.
The zener clamps the output node to approximately Vz. The voltage drop across Rs is therefore Vs − Vz, and the current through Rs is fixed by Ohm's law: Is = (Vs − Vz) / Rs. Because Vs, Vz, and Rs are all fixed, Is is fixed. This is the key insight: the total current through Rs is essentially constant, and the zener adjusts its own current to make up the difference whenever the load current changes.
How Regulation Actually Works
Imagine you are operating the regulator and you suddenly connect an additional load — perhaps you plug in another circuit that draws 10 mA more. Without regulation, the output voltage would fall because the increased load current would increase the voltage drop across Rs. But the zener has a steep I-V curve near Vz. As the output tries to fall below Vz, the zener current drops sharply — the zener naturally gives up 10 mA of its own current to supply the new load. The total current through Rs stays almost the same, the voltage drop across Rs stays almost the same, and therefore the output voltage stays almost the same.
This is passive negative feedback. No active components are needed. The zener's steep characteristic provides the "error correction" automatically: any tendency for the output to drop causes the zener to release current, and any tendency for the output to rise causes the zener to absorb more current.
The regulation is not perfect because the zener curve is not truly vertical — there is a small but finite dynamic impedance Zz. Every milliamp change in the total zener current produces a small voltage change of Zz × ΔIz at the output. For a zener with Zz = 10Ω and a current swing of 20 mA, the output voltage changes by 10 × 0.020 = 200 mV. This is much worse than an IC regulator but adequate for many applications.
Calculating the Series Resistor
Choosing Rs correctly is the critical design step. You need to satisfy two simultaneous constraints:
Constraint 1 — Maximum load current: When the load draws its maximum current IL_max, the zener must still conduct at least its minimum current Iz_min to stay in the breakdown region and hold the output voltage. If the zener current falls to zero, regulation is lost and the output will drop below Vz. A safe minimum zener current is typically 5 mA for standard zeners.
Constraint 2 — No-load condition: When the load is disconnected (IL = 0), all the current through Rs flows through the zener. This must not exceed the zener's maximum current rating, which is set by its power rating: Iz_max = Pz_rated / Vz.
The design formula for Rs comes from Constraint 1. At maximum load, the minimum total current through Rs must be Iz_min + IL_max. Using Ohm's law:
Rs = (Vs − Vz) / (Iz_min + IL_max)
Where all currents are in amperes and the result is in ohms.
Worked Example: 5.1V Regulator for a Bias Supply
Suppose you need a 5.1 V regulated supply from a 12 V source, with a maximum load current of 50 mA. You choose a minimum zener current of 5 mA to keep it safely in regulation.
Step 1 — Calculate Rs:
Rs = (12 − 5.1) / (0.005 + 0.050) = 6.9 / 0.055 = 125.5 Ω
Nearest standard value: 120 Ω (slightly lower, which ensures the zener always gets its minimum 5 mA even at full load)
Step 2 — Verify no-load zener current:
Iz_max = (Vs − Vz) / Rs = 6.9 / 120 = 57.5 mA
Step 3 — Check zener power at no load:
Pz = Vz × Iz_max = 5.1 × 0.0575 = 0.293 W
Use a 1W zener (e.g. 1N4733A). The 0.5W version would be marginal — always use at least a 2× derating factor, meaning the 1W part is the right choice here.
Step 4 — Check Rs power at no load:
PRs = (Vs − Vz)2 / Rs = (6.9)2 / 120 = 47.61 / 120 = 0.397 W
Use a 1W resistor for adequate derating.
Zener Regulator Series Resistor Calculator
Zener Regulator Series Resistor Calculator
Enter your supply voltage, zener voltage, maximum load current, and minimum zener current. The calculator gives you the required series resistor value, its power rating, the maximum zener current at no load, and the zener power dissipation.
Current Budgeting in the Shunt Regulator
The single most important concept for designing a zener shunt regulator is understanding the current budget. The series resistor Rs sets a fixed total current Is that flows from the supply to the output node. This current is fixed because Vs, Vz, and Rs are all fixed. At the output node, this current divides between two paths:
Where: Is = (Vs − Vz) / Rs = constant
When IL increases, Iz decreases by the same amount. When IL decreases, Iz increases. The zener absorbs the "slack" in the current budget.
Regulation fails if the load tries to draw more current than the total Is minus the minimum required zener current. In our worked example, Is = 57.5 mA. If Iz_min = 5 mA, the maximum permissible load current is 57.5 − 5 = 52.5 mA. If the load tries to draw 60 mA, the zener current would have to go negative — impossible. The zener turns off, the output voltage falls, and regulation is lost.
At the other extreme, when IL = 0 (no load), all 57.5 mA flows through the zener. This is the maximum zener current condition and sets the power dissipation rating you need. A supply that is designed for 50 mA of load current but draws only 5 mA most of the time will have the zener conducting 52.5 mA most of the time — nearly at its maximum. This is the inherent inefficiency of the shunt regulator: it wastes the most power when it is least needed.
This also explains why you should match the design load current to the actual load current. If the load is always drawing close to IL_max, the shunt regulator is relatively efficient. If the load current varies widely, a series regulator or IC regulator is a better choice.
Selecting the Right Zener
Choosing the correct zener for your application involves four parameters: voltage, power rating, tolerance, and temperature coefficient.
Voltage and Standard Values
Zener voltages are available in the standard E24 series (similar to resistors) with preferred values at 5% spacing. Common values you will encounter for ham radio work: 2.4 V, 2.7 V, 3.0 V, 3.3 V, 3.6 V, 3.9 V, 4.3 V, 4.7 V, 5.1 V, 5.6 V, 6.2 V, 6.8 V, 7.5 V, 8.2 V, 9.1 V, 10 V, 11 V, 12 V, 13 V, 15 V, 18 V, 20 V, 22 V, 24 V, 27 V, 30 V.
Power Rating
The most common power ratings in through-hole packages are 0.4 W (glass DO-35 package, e.g. BZX79 series), 0.5 W (DO-35), 1 W (DO-41, e.g. 1N4728–1N4764 series), and 5 W (DO-201, e.g. 1N5333–1N5388 series). For most shack projects drawing up to 100 mA from a 12 V supply, the 1 W series is the workhouse choice. Always derate to 50% of the rated power in practice, meaning a 1 W zener should not dissipate more than 0.5 W in a warm environment.
| Series | Power Rating | Package | Voltage Range | Typical Use |
|---|---|---|---|---|
| BZX79 | 0.5 W | DO-35 (glass) | 2.4–75 V | Low current references |
| 1N4728A–1N4764 | 1 W | DO-41 | 3.3–100 V | General purpose shunt regulators |
| 1N5333B–1N5388B | 5 W | DO-201 | 3.3–200 V | Higher current shunt regulators |
| BZX85C | 1.3 W | DO-41 | 2.7–75 V | General purpose, slightly higher rated |
Tolerance
Standard zeners carry a 5% voltage tolerance (suffix "A" in some designations). Precision zeners with 1% or 2% tolerance are available but cost more. For a voltage reference feeding an ADC in an SDR front end, 1% or better is worthwhile. For a bias supply where exact voltage is not critical (within 10% is acceptable), 5% is fine.
Temperature Coefficient
If your circuit will experience a wide temperature range — say, a portable station operating from −10°C to +50°C — the TC matters. The 5.1 V zener family (1N4733A) has the most stable TC because both breakdown mechanisms contribute nearly equally and their TCs oppose each other. A 12 V zener has a positive TC of about +8 mV/°C, meaning a 60°C temperature swing causes the output to shift by almost half a volt — significant for a precision reference, negligible for a transmitter bias supply.
Practical Limitations of the Zener Shunt Regulator
The zener shunt regulator is elegant in its simplicity but it has real limitations you need to understand before deciding whether to use it in a particular design.
Poor load regulation: The output voltage changes with load current because of the zener's dynamic impedance Zz. If the load current changes by 20 mA and the zener has Zz = 15Ω, the output voltage shifts by 15 × 0.020 = 300 mV. An IC regulator would hold the output within a few millivolts over the same range.
Poor line regulation: If the supply voltage Vs changes — say the car battery drops from 14.2 V to 11.8 V under heavy load — the current through Rs changes, which changes Iz, which changes the output voltage slightly (again by Zz × ΔIz). The regulation is imperfect against supply variation as well.
Limited output current: The maximum useful load current is approximately (Vs − Vz) / Rs minus a small margin for Iz_min. For a 12 V supply and a 5.1 V zener, you can only achieve useful regulation up to about 50–100 mA with a practical resistor and a 1 W zener. Anything beyond that requires a higher power zener, a larger resistor dissipating more heat, or a better regulator topology.
Zener noise: The breakdown process in a zener diode generates broadband noise — more than a resistor, and sometimes significantly more in audio and RF circuits. A 5.1 V zener operating at 5 mA in the breakdown region produces noise in the range of several microvolts RMS over a wide bandwidth. For the bias supply of a GaAsFET low-noise amplifier, this noise can degrade the amplifier's noise figure if it is not filtered. Adding a small RC decoupling filter (e.g. 100Ω + 10 µF) after the zener output reduces the noise reaching sensitive circuits.
Efficiency: In the worst case (no load), 100% of the power drawn from the supply is wasted — half in Rs and half in the zener. For battery-powered equipment, this is unacceptable. Use the shunt regulator only where the load is continuous and relatively constant, or where the absolute simplicity of two components justifies the waste.
Ham Radio Applications
Despite these limitations, the zener shunt regulator appears throughout ham radio equipment because of its simplicity and reliability. Here are the places you will actually find it:
GaAsFET LNA bias supply: Low-noise amplifiers for VHF and UHF often use gallium arsenide field-effect transistors (GaAsFETs) that require a very stable gate bias voltage of around −0.5 V to +2 V and a drain voltage of about 3.3 V. The drain supply is often derived from a 12 V rail using a 3.3 V zener (e.g. a BZX79C3V3) with a small series resistor. Because the drain current is typically only a few milliamps, zener noise is the main concern — so an RC filter follows the zener.
ADC voltage reference: Analog-to-digital converters require a stable reference voltage. In older equipment, a 5.1 V zener was a common low-cost reference for 8-bit ADCs where 1% absolute accuracy was acceptable. Modern designs usually use a precision reference IC instead, but you will find zeners in this role in older equipment you are repairing.
ALC threshold reference: Automatic level control circuits in SSB transmitters compare the detected output level against a reference voltage. A zener in a simple voltage divider often sets this threshold. The absolute value of the threshold is not critical — just that it is stable from minute to minute during a transmission session.
Protection clamping: A zener placed from a signal line to ground can protect a circuit against voltage transients. If the signal tries to rise above Vz, the zener conducts and clamps it. This use does not require a series resistor — the source impedance of the circuit limits the current. You will see this at the input of receiver front ends to protect against nearby transmitter pulses.
Simple 9V regulated supply from 12V: A 9.1 V zener with an appropriate series resistor provides a quick regulated 9 V rail for logic or microcontroller circuits drawing up to 20–30 mA from a 12 V vehicle or station supply. This is often simpler than adding an IC regulator for a small sub-circuit in a larger project.
Experiment: Build and Test a 5.1 V Zener Regulator
⚖ Experiment: Zener Shunt Regulator — Output vs Load Current
This experiment builds a simple 5.1 V zener shunt regulator on a breadboard and measures how the output voltage changes as you vary the load current. You will directly observe the imperfect regulation and calculate the effective output impedance (dynamic impedance) of the circuit.
- 9 V battery with clip
- 1N4733A zener diode (5.1 V, 1 W) or equivalent
- 120 Ω resistor, 1 W (for Rs)
- 100 Ω resistor, 0.5 W (for load RL1)
- 220 Ω resistor, 0.5 W (for load RL2)
- 1 kΩ resistor (for load RL3)
- Digital multimeter
- Solderless breadboard and jumper wires
- Connect the 9 V battery positive terminal through the 120 Ω resistor to a node on the breadboard — this is the output node.
- Connect the zener diode from the output node to the negative battery terminal (ground). Important: the zener's cathode (marked band) goes to the output node; the anode goes to ground. This puts the zener in reverse bias as required for operation in breakdown.
- Connect your multimeter (in DC voltage mode) from the output node to ground. Record the no-load voltage.
- Connect the 1 kΩ load resistor across the output. Record the voltage. This is a light load of about 5 mA.
- Replace the 1 kΩ with the 220 Ω resistor (about 23 mA load). Record the voltage.
- Replace with the 100 Ω resistor (about 51 mA load — near the maximum for this design). Record the voltage.
- Calculate the voltage change from no-load to full load. Divide by the load current change to find the effective output impedance in ohms.
At no load the output will be close to 5.1 V (perhaps 5.0–5.2 V depending on your specific zener). As you increase the load current, the output voltage will fall slightly — a few hundred millivolts from no-load to 50 mA is typical. The voltage change divided by the current change gives you Zz + Rs_parallel, the effective output impedance. This will be much larger than a good IC regulator (which is a fraction of an ohm) but small enough for most bias and reference applications. Note that the 100 Ω load test places the regulator near its design limit — if the output drops significantly below 5 V, the zener has come out of regulation.
Frequently Asked Questions
Why does the output voltage change slightly when I vary the load current?
The zener does not have a perfectly vertical I-V curve in the breakdown region. There is a finite dynamic impedance Zz (also called the slope resistance), which means each milliamp change in zener current produces a small voltage change of Zz × ΔI. When load current increases, zener current decreases by the same amount, and the output voltage drops by Zz × ΔIz. A lower Zz (found in higher-power zeners operating at higher currents) gives better regulation.
Can I put two zener diodes in series to get a non-standard voltage?
Yes, stacking zeners in series is a standard technique. The breakdown voltages add together. For example, a 6.2 V and a 3.9 V zener in series (both in reverse bias) give 10.1 V. You can also stack a zener in reverse with an ordinary silicon diode in forward — the forward diode drop of approximately 0.6 V adds to the zener voltage. This is sometimes used to get non-standard values and also to improve temperature coefficient, because the positive TC of many zeners is partially offset by the negative TC of a forward-biased diode.
What is the difference between a zener diode and a dedicated voltage reference IC?
A dedicated voltage reference IC such as the LM336-5.0, REF02, or TL431 provides much better performance than a raw zener. These devices use internal circuitry to temperature-compensate the breakdown reference and provide a much lower dynamic impedance (often below 1Ω). The TL431 is a programmable shunt regulator that uses a band-gap reference and an error amplifier internally — it behaves like a zener but with dramatically lower output impedance and a tolerance of 0.5% or better. For any application where you need better than 1% voltage accuracy or very low noise, use a reference IC rather than a plain zener.
What happens if I omit the series resistor and connect the zener directly across a voltage source?
The zener will be destroyed almost immediately if the supply voltage is close to or above Vz. Without a current-limiting series resistor, the current is limited only by the internal resistance of the supply, which may be a fraction of an ohm. The resulting current will far exceed the zener's rated maximum current, and the device will fail from overheating. The series resistor is not optional — it is the element that limits current and makes the circuit work safely.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.