How a Zener Diode Holds a Voltage Steady While the Supply Wobbles

A diode lets current through in one direction and blocks it in the other. That single property converts AC into DC, and small changes to the junction turn it into a light source, a light detector, a power source or a voltage guard.

This lesson covers diode characteristics, rectifiers, the LED, photodiode and solar cell, and the Zener diode as a voltage regulator.

In forward bias a diode conducts very little until the voltage passes its threshold, about 0.7 V for silicon, and then current rises steeply; in reverse bias only a tiny saturation current flows until breakdown.

Forward bias — p-side to the positive terminal. The applied voltage lowers the barrier, letting majority carriers cross.

Reverse bias — p-side to the negative terminal. The barrier grows, so only minority carriers drift across, giving a tiny current that barely changes with voltage.

Dynamic resistance. $r_d=\frac{\Delta V}{\Delta I}$ on the steep part of the curve.

Worked example 1. Raising the forward voltage from 0.70 V to 0.80 V raises the current from 10 mA to 30 mA:

$$r_d=\frac{0.80-0.70}{(30-10)\times 10^{-3}}=5.0\Omega$$

Worked example 2. A silicon diode and a $430\Omega$ resistor are in series with a 5.0 V supply:

$$I=\frac{5.0-0.70}{430}=0.010\text{ A}=10\text{ mA}$$

An everyday example. A home inverter's charging circuit uses diodes so that the battery cannot push current back into the circuit when the mains fails.

The substance. A diode does not obey Ohm's law — its resistance depends on the voltage, so there is no single value of R for the whole curve.

A half-wave rectifier uses one diode to pass only the positive half-cycles of AC, while a full-wave rectifier uses two diodes with a centre-tapped transformer, or four in a bridge, to send both half-cycles through the load in the same direction.

Half-wave. The diode conducts in each positive half-cycle and blocks each negative one. The output pulses at the supply frequency, 50 Hz.

Full-wave. In a centre-tap circuit, one diode conducts in each half-cycle; in a bridge, two diodes conduct at a time. The output pulses at twice the supply frequency, 100 Hz.

Worked example. A transformer gives 12 V RMS, and the diodes are treated as ideal:

$$V_0=12\times \sqrt{2}=17.0\text{ V}{V}_{\text{half-wave}}=\frac{V_0}{\pi }=5.4\text{ V}{V}_{\text{full-wave}}=\frac{2V_0}{\pi }=10.8\text{ V}$$

Smoothing. A capacitor across the load charges at each peak and discharges slowly between them, filling in the gaps.

An everyday example. The adapter of a Wi-Fi router uses a bridge rectifier and a smoothing capacitor to turn mains AC into steady low-voltage DC.

The substance. Rectified output is DC but not steady — it flows one way yet still pulses, until a filter smooths it.

An LED emits light when forward-biased electrons and holes recombine, a photodiode is reverse-biased so that light-generated carriers raise its current, and a solar cell uses light-generated carriers to produce an emf with no bias at all.

LED. Recombination releases a photon of energy close to the band gap, so the material sets the colour:

$$\lambda =\frac{hc}{E_g}=\frac{6.63\times 10^{-34}\times 3.0\times 10^8}{1.9\times 1.6\times 10^{-19}}=654\text{ nm, red}$$

A 2.8 eV gap gives 444 nm, which is blue.

Photodiode. Photons with $hf>E_g$ create electron-hole pairs in the depletion region. The field sweeps them apart, so the reverse current rises with light intensity.

Solar cell. A large-area p-n junction separates light-generated carriers, making the p-side positive. Its V-I curve lies in the fourth quadrant, since it supplies power. Light of 500 nm carries $\frac{6.63\times 10^{-34}\times 3.0\times 10^8}{5.0\times 10^{-7}\times 1.6\times 10^{-19}}=2.49$ eV, well above silicon's 1.1 eV gap. Each silicon cell gives about 0.5 V, so a panel of 36 cells in series gives $36\times 0.50=18$ V.

An everyday example. A TV remote's signal is caught by a photodiode in the television, while LED street lights and rooftop solar panels use the other two devices.

The substance. The same junction physics runs in opposite directions — an LED turns current into light, while a photodiode and a solar cell turn light into current.

**A Zener diode is heavily doped so that in reverse bias it breaks down sharply at a fixed voltage $V_z$, and the voltage stays nearly constant over a wide range of current, so placing it across a load holds the load voltage steady.

The regulator circuit.** A series resistor $R_s$ connects the unsteady supply to the Zener, and the load is connected in parallel with the Zener. Any change in supply voltage changes the current through $R_s$, and the Zener absorbs the extra current while its voltage stays at $V_z$.

Worked example. A 12 V supply, $R_s=100\Omega$, $V_z=6.0$ V and a $600\Omega$ load:

$$I_s=\frac{12-6.0}{100}=0.060\text{ A}{I}_L=\frac{6.0}{600}=0.010\text{ A}{I}_z=0.060-0.010=0.050\text{ A}$$

If the supply rises to 14 V, $I_s=0.080$ A, and the Zener current rises to 0.070 A, dissipating $6.0\times 0.070=0.42$ W, while the load still receives exactly 6.0 V.

An everyday example. Power supplies inside set-top boxes use Zener diodes to hold steady reference voltages for their sensitive circuits.

The substance. **A Zener regulates only while the supply stays above $V_z$** and the current stays within its rating — below $V_z$, it simply stops conducting.