Zener diode as a shunt voltage regulator — line and load regulation
Zener diode voltage regulator experiment: reverse characteristic, choosing the series resistance, line and load regulation measured from your own readings, the knee and maximum Zener current, and viva questions with answers.
Aim
To plot the reverse characteristic of a Zener diode, to use it as a shunt voltage regulator, and to determine the line regulation and load regulation of the regulator together with the limits of load current over which regulation holds.
Apparatus required
| Apparatus | Specification | Qty |
|---|---|---|
| Regulated DC power supply | 0–30 V, 1 A | 1 |
| Zener diode | 1N4733A, 5.1 V, 1 W | 1 |
| Series resistor | 220 Ω, 2 W | 1 |
| Load resistance / decade box | 100 Ω to 10 kΩ | 1 |
| Digital multimeters | DC volts and DC milliamps | 2 |
| Breadboard and patch cords | — | 1 set |
Theory
A Zener diode is a heavily doped junction designed to be operated in reverse breakdown. Below the breakdown voltage it conducts almost nothing; at breakdown the reverse current rises very steeply while the voltage across it stays nearly constant. That steep region — the knee and beyond — is what makes the device useful as a voltage reference, and the small slope of the characteristic there is its dynamic resistance r(z), typically a few ohms to a few tens of ohms.
In the shunt regulator the Zener sits across the load with a series resistor R(s) between it and the unregulated input. The series resistor carries the whole input current: I(s) = (V(in) − V(z))/R(s). That current divides between the diode and the load, I(s) = I(z) + I(L). When the load draws more, the Zener simply passes less, and the sum stays put — which is how the output holds steady.
The design constraint follows directly. The Zener must keep enough current to stay past the knee at the worst case, which is the lowest input voltage with the heaviest load; and it must not exceed its power rating at the other extreme, the highest input voltage with no load at all, where the diode carries everything. So R(s) has both a maximum and a minimum, and a workable regulator needs I(z,min) ≤ I(z) ≤ P(z)/V(z) across the whole operating range.
Two figures describe how well it performs. Line regulation is the change in output per unit change in input, usually as a percentage, and it is set by the divider formed by r(z) against R(s): ΔV(out)/ΔV(in) ≈ r(z)/(R(s) + r(z)). Load regulation is the change in output between no load and full load, and it is governed by r(z) alone. Both improve as r(z) falls, which is why a Zener of a few volts, where r(z) is lowest, makes the better reference.
The regulator fails in two ways worth seeing deliberately. If the load draws so much that I(z) falls to zero, the Zener drops out of breakdown, the circuit becomes an ordinary divider, and the output collapses and follows the input. If the input rises far enough with a light load, the Zener exceeds its power rating and overheats.
Circuit connections
Check every point below against your board before switching on. There is no diagram here on purpose — a wrong diagram is worse than none, and this is the list a demonstrator actually walks through with you.
- Supply positive to one end of the series resistor R(s); the other end of R(s) to the Zener cathode — the banded end — which is the positive output rail.
- Zener anode to the supply negative, which is the common rail. The diode is reverse-biased: cathode positive.
- Load resistance in parallel with the Zener, between the output rail and common.
- Voltmeter across the Zener to read V(out); milliammeter in series with the Zener branch to read I(z), or in series with the load for I(L).
- Set the supply to minimum before switching on, and raise it only after the connections have been checked.
Procedure
- 1Identify the Zener's cathode by its band and confirm the device with a multimeter diode test: it behaves like an ordinary diode in the forward direction.
- 2Connect the circuit with the load disconnected, supply at minimum, and have it checked.
- 3For the reverse characteristic, raise the input in small steps and record the voltage across the diode and the current through it at each step. Take closely spaced readings around the knee, where the curve turns.
- 4For line regulation, connect a fixed load and vary the input voltage from below breakdown to the maximum safe value, recording V(in) and V(out) at each step. Note the input at which the output first becomes constant.
- 5For load regulation, hold the input fixed at a value comfortably above breakdown and reduce the load resistance in steps, recording V(out) and I(L) each time, until the output begins to fall.
- 6Continue reducing the load resistance until regulation is clearly lost, and record the load current at which that happens — this is the maximum load the regulator can hold.
- 7Plot the reverse characteristic, V(out) against V(in), and V(out) against I(L), and compute the line and load regulation percentages.
Work out your readings
Type in the numbers off the meters. This fills the tabular column, works the calculation through step by step, plots the characteristic — and tells you when a reading cannot physically be right, which is the part a manual can't do. Everything stays on this device, and it works with the network off.
Nameplate and machine data
Fill in the machine data to see the results, the worked calculation and the curve.
Precautions
- Check the Zener power rating before raising the input. With no load the diode carries the entire series current, and a 1 W device at 5.1 V is destroyed above about 196 mA.
- Keep the series resistor within its wattage: the power in R(s) is (V(in) − V(z))²/R(s) and it is largest at maximum input.
- Connect the Zener reverse-biased, cathode to the positive rail. Forward-biased it is just a diode and the regulator does nothing.
- Start with the supply at minimum and raise it gradually rather than switching on at full voltage.
- Keep the ammeter in series and the voltmeter in parallel, and do not leave a milliammeter connected across the supply.
- Let the diode cool between runs if it has been operated near its rating; the breakdown voltage drifts with temperature.
Sources of error
Every record asks for these, and every record gets the same three lines copied from the one before. These are the errors this particular experiment actually has.
- The dynamic resistance of the Zener is not zero, so the output is never perfectly constant; the residual slope is what the regulation figures actually measure.
- Self-heating shifts the breakdown voltage. Zeners below about 5 V have a negative temperature coefficient and those above it positive, so a warm diode reads differently from a cold one.
- Voltmeter loading and the burden voltage of the milliammeter, which are significant when the currents are small and the series resistance large.
- The tolerance of the series resistor and of the Zener itself — a 5.1 V device is typically ±5 %, so the nominal and measured breakdown voltages differ before any other error.
- Reading the knee too coarsely: with steps that are too large the transition looks like a sharp corner rather than the gradual bend it is.
Viva questions with answers
What is the difference between a Zener diode and an ordinary rectifier diode?
Both conduct in the forward direction alike, but a Zener is doped heavily so that reverse breakdown occurs at a low, sharply defined and non-destructive voltage, and it is designed to be operated there continuously. An ordinary rectifier diode is meant to block in reverse, and driving it into breakdown normally destroys it.
Explain the difference between the Zener effect and avalanche breakdown.
The Zener effect is direct field ionisation across a very narrow, heavily doped junction and dominates below about 5 V, with a negative temperature coefficient. Avalanche breakdown is impact ionisation by carriers accelerated across a wider junction and dominates above about 5 V, with a positive coefficient. Near 5 V both act, which is why devices around that value drift least with temperature.
How do you choose the series resistance in a shunt regulator?
It must be small enough to keep the Zener above its minimum current at the lowest input with the heaviest load, and large enough that the Zener does not exceed its power rating at the highest input with no load. Those two conditions give an upper and a lower bound, and any value between them works; the usual choice is near the middle for margin.
What happens to the output if the load current exceeds the maximum the regulator can supply?
The Zener current falls to zero, the diode leaves breakdown, and the circuit degenerates into a plain resistive divider of R(s) and the load. The output voltage drops below the Zener voltage and then simply follows the input, so regulation is lost entirely rather than degrading gracefully.
Define line regulation and load regulation for this circuit.
Line regulation is the change in output voltage caused by a change in input voltage, with the load held constant, usually expressed as a percentage of the output per volt of input change. Load regulation is the change in output between no load and full load at a fixed input, expressed as a percentage of the full-load output. Lower values are better in both cases.
Why is the shunt regulator inefficient, and where is it still used?
The series resistor carries the full current at all times and the Zener wastes whatever the load does not take, so the circuit dissipates most heavily when the load is lightest. It is still used where the current is small and constancy matters more than efficiency — voltage references, bias networks, clamps and protection across sensitive inputs.
What is dynamic resistance and how would you find it from your readings?
Dynamic resistance is the slope of the reverse characteristic in the breakdown region, r(z) = ΔV(z)/ΔI(z), taken between two nearby points past the knee. It is a few ohms to a few tens of ohms, and it is the parameter that sets both the line and load regulation of the circuit.