Design the simplest of all voltage regulators — a shunt regulator built from just a series resistor and a zener diode. Adjust the input voltage, zener voltage, series resistor and load resistance to see the output voltage, branch currents and power dissipation update in real time, and build a stable reference.
Parameters
Input voltage V_in
V
The variable input voltage fed to the regulator
Zener voltage V_z
V
Rated breakdown voltage of the zener diode
Series resistor R_s
Ω
Current-limiting resistor between input and output node
Load resistance R_L
Ω
Load across the output. Smaller means heavier load
Results
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Output voltage V_out (V)
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Series-resistor current I_total (mA)
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Load current I_load (mA)
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Zener current I_zener (mA)
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Zener power P_zener (mW)
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Regulation status
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Zener regulator circuit — current-flow animation
The total current from V_in through the series resistor R_s splits at the node into the load current and the zener current. The zener shunts the surplus to ground, holding the output at V_z. The flowing dots show the size of each current.
Zener current I_zener and zener power dissipation P_zener. The zener shunts the surplus current to ground to hold the output constant.
$$V_{out}=V_z \quad (I_{zener}\ge I_{z,\min})$$
The output V_out is pinned to V_z only while the zener current stays above its minimum holding current I_z,min (1 mA in this tool). Below it the zener leaves breakdown and the output falls.
What is the Zener Voltage Regulator Simulator?
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How is a "zener diode" different from a normal diode? I heard you use it with the voltage reversed.
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Roughly speaking, a zener is a diode deliberately engineered to "break down" cleanly in reverse. A normal diode blocks reverse voltage, but a zener, once you exceed a certain reverse voltage, suddenly "breaks down" and conducts heavily. That breakdown voltage is built to a precise target value, and — this is the key part — while it is in breakdown the voltage across it barely moves even as the current changes a lot. That "voltage that won't move" property is what makes it the star of a voltage regulator.
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I see. So how do you use that property to hold a voltage steady?
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Just two parts: a series resistor R_s and the zener. Connect R_s from the input, and put the zener at the far node, reverse-biased toward ground. The load hangs across that same node. R_s takes up the entire difference between the variable, higher input voltage and the fixed output, while the zener sits on the output node and pins the voltage. Move the input-voltage slider V_in on the left — you will see the output voltage V_out stay locked at the 5.1 V zener voltage.
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It really does — V_out stays at 5.10 V no matter the input! But where does the surplus current go?
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That is the zener acting as a "shunt" — diverting current sideways. Of the total current I_total through R_s, whatever the load does not take, the zener dumps to ground. That is I_zener = I_total − I_load. If the load suddenly goes light and stops drawing much, the zener simply absorbs that surplus. If the input voltage rises and the current increases, the extra is again absorbed by the zener. So in both cases the output barely moves. Move R_L on the "zener current vs load resistance" chart below and you can watch how it absorbs.
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Sounds perfect then! ... or is there a catch?
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There are two big weaknesses. One is poor efficiency. Both R_s and the zener constantly pass current and waste power, and even with no load the zener absorbs all the current and turns it into heat. The other is design difficulty: R_s has conflicting constraints. You want R_s small enough that, in the worst case (lowest input, heaviest load), the zener still gets its minimum holding current. But you must not make R_s so small that, in the opposite worst case (highest input, no load), the zener exceeds its power rating. You choose R_s in that tug-of-war. So a zener shunt regulator is great for "building a reference" or "powering a light load", but if you need efficiency or high current, a three-terminal regulator or a switching supply is the standard answer.
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When I lower the input voltage too far, the status changes to "loss of regulation". What is happening there?
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As the input voltage V_in falls, I_total decreases. Once it drops below the load current I_load, there is no surplus left for the zener and I_zener goes to zero or negative. When that happens the zener can no longer stay in breakdown, so it can no longer pin the voltage. The output falls below the zener voltage V_z and the circuit behaves like a plain resistive divider. That is "drop-out — loss of regulation". In practice you choose R_s so that even at the lowest moment of input voltage, the zener current still exceeds the holding current. That is the single most important point in designing a zener regulator.
Frequently Asked Questions
While the zener diode is in reverse breakdown, the output voltage is pinned to its zener voltage V_z. The series resistor R_s takes up the difference between the input voltage V_in and the output, and the zener sits across the output and clamps it. So the output voltage is essentially V_z. This only holds while the zener carries at least its minimum holding current (1 mA in this tool); if the input drops too low or the load gets too heavy, the zener cannot stay in breakdown and the output falls below V_z.
The series resistor carries I_total = (V_in − V_z)/R_s. The load draws I_load = V_z/R_L. The current the zener carries is the difference: I_zener = I_total − I_load. In other words, the zener shunts to ground exactly whatever current the load does not take, keeping the output node pinned at the zener voltage. When the load current rises, the zener current drops by the same amount; when the load is light, the zener absorbs the surplus.
The series resistor R_s has two conflicting constraints. First, it must be small enough that the zener still receives its minimum holding current in the worst case of lowest input voltage and heaviest load — otherwise the output collapses. Second, it must not be so small that the zener exceeds its power rating in the opposite worst case of highest input and lightest (or no) load, where the zener absorbs all the surplus current. Pick R_s within the window that satisfies both worst cases.
Its biggest weakness is poor efficiency. The series resistor and the zener always dissipate power, and even with no load the zener absorbs all the current, so power is continuously wasted. It also cannot supply much output current, and design gets hard when the input voltage range is wide. It is ideal for building a stable reference voltage or powering a light load, but for efficiency or high current a three-terminal regulator or a switching supply is the usual choice.
Real-World Applications
Generating a reference voltage: The most classic and important use of a zener shunt circuit is building a stable reference. A/D converters, comparators and the bias points of analog circuits all need a "voltage that does not move", and for a light load one series resistor and one zener give a reference of adequate accuracy. When higher precision is needed, the zener is replaced with a temperature-compensated (bandgap) type, but the circuit topology is still the shunt regulator.
Auxiliary supply for microcontrollers and small-signal circuits: For light loads such as a low-current microcontroller, a sensor or a single op-amp, a zener shunt circuit can serve directly as the power supply. It uses few parts and is cheap, and it offers the flexibility of producing any output as long as you can pick the zener voltage — even at a voltage where no three-terminal regulator IC is available.
Overvoltage protection and voltage clamping: The zener's "conduct heavily once a rated voltage is exceeded" property is also widely used in reverse, as a clamping element that protects downstream circuits from surges and overvoltage. Placed in parallel on a signal or supply line, the moment the rating is exceeded the zener absorbs current and caps the voltage. This too is the same "shunt" action as in this tool.
Learning circuit design and quick estimation: The zener circuit is the most direct way to experience Ohm's law and Kirchhoff's current law for DC circuits. With an estimate like this tool you can size up the swing of zener current and power dissipation against input and load variation, and confirm at the design stage that the ratings have margin and that the circuit will not drop out. It also works as a sanity check before a detailed SPICE analysis.
Common Misconceptions and Pitfalls
The biggest misconception is assuming that the output voltage always equals the zener voltage. The output is pinned to V_z only while the zener can stay in breakdown — that is, while the zener current is above the minimum holding current. If the input voltage drops too far, or the load gets heavy enough that the load current approaches the series-resistor current, there is no surplus left for the zener and breakdown is released. The output then falls below V_z and the circuit behaves as a plain series-resistor-plus-load divider. Lower the input voltage in this tool and you can see the drop-out point where the output departs from V_z and falls.
Next, the belief that "no load is the easiest case for the circuit". In fact the opposite is true — no load is the harshest condition for the zener. When the load draws no current, the entire current through the series resistor must be carried by the single zener, so the zener power dissipation P_zener = V_z·I_zener is at its maximum. The zener's power rating must be chosen so it is not exceeded in this worst case of "maximum input and no load". Conversely, the holding-current margin is evaluated for "minimum input and heavy load". Always examine both extreme cases separately.
Finally, trying to draw high current without caring about efficiency. A zener shunt regulator is inherently inefficient: the series resistor and the zener always dissipate power, and power is wasted even with no load. To draw a larger output current you must make the series resistor smaller, and the zener loss at no load rises sharply in step. For loads beyond a few tens of milliamps, or battery-powered uses where efficiency matters, the right answer is to add an emitter follower, or switch to a three-terminal regulator or a switching supply rather than a zener alone. Accept that the zener shunt regulator belongs on the "light load, reference voltage, low cost" playing field.
How to Use
Enter input voltage (V_in) between 5–30 V and select zener diode breakdown voltage (V_z) matching your regulation target, typically 3.3 V, 5.6 V, or 12 V for standard applications.
Set series resistor value (R_s) in ohms; start with 470 Ω for 100 mA circuits, then adjust based on load current (I_load) requirements.
Specify load resistance (R_L) in kilohms; the simulator calculates output voltage, current distribution, and zener dissipation to verify regulation stability and thermal limits.
Worked Example
Design a 5 V regulator from 12 V supply with 1 kΩ load: Set V_in = 12 V, V_z = 5.6 V, R_s = 680 Ω, R_L = 1 kΩ. Simulator returns V_out ≈ 5.6 V, I_total ≈ 10.6 mA, I_load ≈ 5.6 mA, I_zener ≈ 5 mA, P_zener ≈ 28 mW. A 500 mW zener handles this safely; increasing V_in to 15 V raises P_zener to 46 mW, requiring temperature derating at 70 °C ambient.
Practical Notes
Minimum series resistance: set R_s high enough that maximum zener current (when load disconnected) stays below the diode's continuous rating, typically 20–50 mA for 1 N4148 types.
Load regulation error increases with R_L; use <500 Ω loads for <5% drift, or switch to active regulators (LM7805) for precision <2%.
Zener power dissipation dominates at light loads; if P_zener exceeds 250 mW, select a higher V_z or increase R_s to reduce wasted current through the diode.