Describe the diode as a one-way component, state its forward voltage, and explain half-wave rectification of an alternating supply

What this dot point is asking

SEAB wants you to describe a diode as a component that conducts in one direction only, to state its forward voltage drop, and to explain how a diode rectifies an alternating supply into a one-way (direct) current. The central insight is that a diode is a one-way valve for current, which is exactly what is needed to convert the back-and-forth of alternating current into the one-way flow that electronic circuits require.

The answer

The diode as a one-way component

A diode allows current to pass in one direction (forward) and blocks it in the other (reverse). Its symbol is a triangle pointing towards a bar; conventional current flows in the direction the triangle points. The lead at the triangle end is the anode and the lead at the bar is the cathode, usually marked with a stripe on the body.

Forward and reverse bias

• Forward bias: the anode is made positive relative to the cathode. Once the voltage is large enough, the diode conducts and current flows.
• Reverse bias: the anode is made negative relative to the cathode. The diode blocks, and almost no current flows.

This one-way behaviour is why a diode does not obey Ohm's law: it is a non-ohmic component.

The forward voltage drop

A silicon diode does not conduct until the forward voltage reaches about $0.7\ \text{V}$. Below this it stays off; above it, it conducts and holds roughly $0.7\ \text{V}$ across itself. In any calculation you must subtract this $0.7\ \text{V}$ from the supply before working out the current in the rest of the circuit. (A light-emitting diode has a larger forward drop, around $2\ \text{V}$.)

Half-wave rectification

An alternating supply reverses direction every half cycle. Place a single diode in series with the load and it conducts only when its anode is positive, passing the positive halves of each cycle and blocking the negative halves. The result is a series of one-way pulses: the current now flows in one direction only. This is half-wave rectification, the simplest way to turn alternating current into direct current.

Smoothing with a capacitor

The rectified output is bumpy, not steady. A large capacitor connected across the load charges up on each pulse peak and discharges into the load between pulses. This holds the voltage up in the gaps and reduces the ripple, producing a much steadier direct voltage. The diode and capacitor together form the heart of a simple power supply.

Examples in context

Example 1. A phone charger. Mains electricity is alternating, but a phone needs direct current. The charger uses diodes to rectify the supply and a capacitor to smooth it, converting the back-and-forth mains into the one-way voltage the phone expects. The humble diode is what makes this everyday conversion possible.

Example 2. Reverse-polarity protection. A diode placed in series with a battery-powered toy lets current through only when the battery is inserted the right way round. If the battery is reversed, the diode blocks, protecting the circuit from damage. Here the one-way behaviour is used as a safety guard rather than a rectifier.

Try this

• Cue. State the forward voltage of a typical silicon diode and what happens below it. About $0.7\ \text{V}$; below this voltage the diode does not conduct and almost no current flows.

• Cue. A silicon diode is in series with a $470\ \Omega$ resistor across a $3.7\ \text{V}$ supply, forward biased. Find the current. Subtract the drop: $3.7 - 0.7 = 3.0\ \text{V}$, so $I = 3.0/470 = 6.4\ \text{mA}$.

• Cue. Explain why a single diode alone cannot supply a smooth direct voltage. It only passes one-way pulses with gaps between them; a smoothing capacitor is needed to fill the gaps and reduce the ripple.