Diodes, Half-Wave Rectification and Zener Voltage Regulation
How Diodes Control Current Direction
Diodes are two-terminal semiconductor devices that allow current to flow in one direction only. They sit at the heart of rectifier circuits and protection sub-systems, making them one of the most widely-used components in electronic design. In this lesson you will meet three roles the diode plays: passing current in a circuit, protecting inductive loads from voltage spikes, and maintaining a fixed output voltage in a regulator.
The I-V Characteristic of a Silicon Diode
A silicon diode has an anode (positive terminal) and a cathode (negative terminal). When the anode is more positive than the cathode, the diode is forward biased. Current does not flow immediately; instead, the applied voltage must overcome the forward voltage drop before significant current passes. For a silicon diode this threshold is approximately 0.7 V. Below about 0.7 V the diode conducts very little current; above this it conducts strongly. Its forward voltage varies with current and temperature.
When the anode is more negative than the cathode the diode is reverse biased. Virtually no current flows — the diode blocks it. This one-way behaviour is what makes diodes useful for rectification and protection.
A forward-biased silicon diode drops approximately 0.7 V across it when conducting. This value stays nearly constant over a wide range of currents — it does not increase significantly as current rises.
Protection Diodes for Inductive Loads
When an inductive load is switched off, the collapsing magnetic field induces a large reverse voltage spike. This spike can exceed the breakdown voltage of a transistor switch and destroy it in microseconds. A flyback diode (also called a freewheeling or protection diode) is connected in reverse bias across the inductive load. During normal operation the diode is reverse biased and does not conduct. When the load is switched off and the spike appears, the diode becomes forward biased and provides a low-resistance path that safely dissipates the stored energy.
A flyback diode is only needed for electromagnetic (inductive) loads: relay coils, solenoids, and motors. These devices store energy in a magnetic field.
A flyback diode is not needed for resistive loads such as lamps, heaters, or resistors. Resistors dissipate energy immediately and produce no voltage spike when switched off. A common exam error is placing a protection diode around a lamp — this costs marks because it shows a misunderstanding of why the diode is there.
Signal path for a relay circuit with flyback protection: Supply (+) → Relay coil → Transistor collector → Transistor emitter → Supply (−). Flyback diode: cathode to the supply (+) end of the relay coil, anode to the collector end — so it is reverse biased during normal operation.
Half-Wave Rectification
Mains electricity is alternating current (AC) — the voltage repeatedly swings positive then negative. Many electronic circuits require direct current (DC). A single diode converts AC to a pulsating DC by blocking the negative half-cycles and allowing only the positive half-cycles to pass. This is called half-wave rectification.
The output of a half-wave rectifier is a series of positive pulses separated by gaps where the negative half-cycles were removed. The peak output voltage is the AC peak voltage minus the 0.7 V diode forward drop. This pulsating DC is usually smoothed by a capacitor to give a steadier supply, though smoothing is covered in a later unit.
In a half-wave rectifier, the diode passes the positive half-cycles and blocks the negative half-cycles. The output waveform shows repeated positive humps with gaps between them, not a continuous signal.
Zener Diodes and Voltage Regulation
A zener diode is a special type of diode that is deliberately operated in reverse breakdown. It is connected in reverse bias across the output of a power supply. When the reverse voltage reaches the zener’s breakdown voltage (also called the zener voltage, \(V_Z\)), the diode conducts in reverse and clamps the voltage at exactly \(V_Z\). Zener diodes are available in standard ratings — common values include 3.3 V, 5.1 V, 5.6 V, 6.2 V, 9.1 V, 12 V and 15 V.
How the Zener Regulator Works
The regulator circuit consists of a series resistor \(R_S\) connected between the unregulated supply \(V_{in}\) and the zener diode (cathode up to the supply, anode to 0 V). The output is taken across the zener. The series resistor drops the “spare” voltage \((V_{in} - V_Z)\) and limits the current through the zener to a safe value.
The crucial point — and the most common exam misconception — is this: if \(V_{in}\) rises, the output voltage across the zener stays at \(V_Z\). The extra voltage is absorbed by the series resistor. The zener achieves regulation by automatically increasing its conduction current to accommodate the higher input, keeping \(V_{out} = V_Z\) constant. The output does not change when the supply fluctuates — that is the entire purpose of regulation.
A 5.1 V zener diode is used to regulate a 9 V unregulated supply. The maximum zener current allowed is 40 mA. Calculate the minimum series resistance required.
Step 1 — find the voltage across the series resistor: $$V_{R_S} = V_{in} - V_Z = 9 - 5.1 = 3.9 \text{ V}$$
Step 2 — apply Ohm’s law. Use the maximum allowed current to find the minimum resistance: $$R_S = \frac{V_{R_S}}{I_{Z(max)}} = \frac{3.9}{0.040} = 97.5 \text{ }\Omega$$
The nearest standard E24 value at or above 97.5 Ω is 100 Ω, which keeps the zener current within its rating. If the question gives a zener power rating instead of a maximum current, use \(P = V_ZI_Z\) to calculate the maximum zener current.
Error 1 — adding a flyback diode to a lamp: Protection diodes are for electromagnetic loads only. A lamp is resistive; it stores no energy in a magnetic field. Adding a diode around it scores no marks and shows a conceptual error.
Error 2 — thinking \(V_{out}\) changes when \(V_{in}\) changes: The whole purpose of a zener regulator is that \(V_{out} = V_Z\) stays constant. If \(V_{in}\) increases, more current flows through \(R_S\), but \(V_{out}\) remains unchanged.
Error 3 — drawing the half-wave rectifier output incorrectly: Only positive half-cycles appear at the output. The negative half-cycles are absent (not inverted). The positive humps are separated by flat periods at 0 V, not by negative humps.