Capacitors, Diodes, LEDs and Rectifiers – Basic Electronic Components (Senior 3 Physics) | Ganzaa

Basic Electronic Components

Lessons

Senior 3 · Physics

Basic Electronic Components

4 lessons

Lesson 2 of 4

Capacitors, Diodes, LEDs and Rectifiers

30 min

From the socket to the phone

A mains socket supplies alternating current (AC), which reverses direction 50 times a second; you met it in units 6 and 7. A phone battery can only be charged with direct current (DC), which always flows the same way. The charger lowers the voltage with a transformer (unit 6), and then two small components finish the job: diodes, which let current through one way only, and capacitors, which store charge and smooth the result. This lesson explains both, and the light-emitting diode that probably shows when your charger is on.

Definition

Capacitor: a component made of two conducting plates separated by an insulator, which stores electric charge. Capacitance $C$ is the charge stored per volt: $C = \frac{Q}{V}$ . Unit: the farad (F); practical values are in microfarads ( $1\,\mu\mathrm{F} = 10^{-6}\,\mathrm{F}$ ), nanofarads and picofarads. Example: a 470 µF capacitor in a power supply.

Capacitors

When a capacitor is connected to a battery, electrons are pushed onto one plate and pulled off the other, until the voltage across the capacitor equals the battery's. No current passes through the insulator. Disconnected, the capacitor keeps its charge; connected across a lamp, it drives a brief current as it discharges. The charge stored is

Q = C V

Smoothing: in a power supply, a capacitor charges up at each peak and supplies current in the gaps, so the output is nearly steady (see the rectifier below).
Timing: a capacitor charging through a resistor takes a predictable time to reach a set voltage, which is used in timers and flashing lights.
Quick release of energy: a camera flash charges a capacitor slowly from a battery and empties it into the flash tube in a fraction of a second.

Electrolytic capacitors, the little cans with a stripe, give large capacitance in a small size but must be connected the right way round; the stripe marks the negative lead. Connected backwards, or above their rated voltage, they can overheat and burst.

Worked example 1: charge stored

A 470 µF capacitor is charged to 12 V. How much charge does it store?

Q = CV = 470 \times 10^{-6} \times 12 = 5.64 \times 10^{-3}\,\mathrm{C}

Worked example 2: finding a capacitance

A capacitor stores $2.0 \times 10^{-3}\,\mathrm{C}$ when the voltage across it is 5.0 V. Find its capacitance.

C = \frac{Q}{V} = \frac{2.0 \times 10^{-3}}{5.0} = 4.0 \times 10^{-4}\,\mathrm{F} = 400\,\mu\mathrm{F}

Diodes: a one-way valve for current

A diode is made from a semiconductor, usually silicon. It conducts when its anode is more positive than its cathode (forward bias) and blocks current when connected the other way (reverse bias). The triangle in its symbol points the way conventional current can flow, and the bar is the cathode, marked on the real diode by a band.

A silicon diode needs about 0.7 V across it before it conducts well. A light-emitting diode (LED) gives out light when forward-biased; it needs about 2 V (more for blue and white) and always a series resistor (lesson 1). LEDs turn far more of their energy into light than filament lamps do, which is why they now light homes, torches and screens.

Definition

Diode: a semiconductor component that allows current to flow in one direction only. Forward bias: connected so that it conducts. Reverse bias: connected so that it blocks current.

Rectification: the conversion of alternating current into direct current, using diodes.

Rectifiers: AC to DC

A single diode in series with a load lets through only the half-cycles that forward-bias it. This is half-wave rectification: the current flows one way only, but in bursts, and half of the energy is wasted. A bridge rectifier of four diodes uses both halves: whichever end of the supply is positive, current flows through the load in the same direction. This is full-wave rectification. A capacitor across the load then smooths the output to a nearly steady DC, with a small remaining "ripple".

When the top of the supply is positive, current flows through the top-right diode, the load (right to left) and the bottom-left diode. When the bottom is positive, it flows through the bottom-right diode, the load (again right to left) and the top-left diode.

Worked example 3: counting pulses

A 50 Hz supply is rectified. How many pulses of current pass through the load each second with (a) a half-wave rectifier, (b) a bridge rectifier?

(a) One pulse per cycle: 50 pulses per second. (b) One pulse per half-cycle: $2 \times 50 = 100$ pulses per second. With more frequent pulses, the smoothing capacitor has less time to discharge between them, so full-wave output is smoother.

Warning

Common mistakes and safety

Thinking current flows through the insulator of a capacitor. Charge builds up on the plates; current flows only in the wires while it charges or discharges.
Connecting an LED without a series resistor, or backwards. Without a resistor it burns out; backwards it simply stays dark.
Connecting an electrolytic capacitor the wrong way round or above its voltage rating: it may burst.
Large capacitors in mains equipment, such as televisions and chargers, can keep a dangerous charge after the plug is pulled out. Never open them.

Tip

The arrow tells you. In a diode symbol, current goes the way the triangle points and is stopped by the bar. In a bridge, the two cathodes (bars) meet at the + output and the two anodes meet at the − output.

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Practice

State what a capacitor does, and give two uses.
A 1000 µF capacitor is charged to 9.0 V. How much charge does it store?
Which capacitance stores 0.30 C at 6.0 V?
Draw a circuit with a cell, a diode and a lamp in which the lamp lights. Then explain what happens if the diode is turned round.
What is rectification? Sketch the output of a half-wave rectifier and of a full-wave rectifier, starting from an AC input.
Why does a bridge rectifier need four diodes, and why does it give a smoother output than a single diode?
Explain how a capacitor smooths the output of a rectifier.

Show answersHide answers

It stores charge. Uses (any two): smoothing power supplies, timing circuits, camera flashes, filtering signals.
$Q = 1000 \times 10^{-6} \times 9.0 = 9.0 \times 10^{-3}\,\mathrm{C}$ .
$C = \frac{0.30}{6.0} = 0.050\,\mathrm{F}$ (50 000 µF).
The diode's triangle must point away from the positive terminal round the circuit. Turned round, it is reverse-biased, no current flows and the lamp goes out.
Changing AC into DC. Half-wave: only the positive halves remain, with gaps. Full-wave: every half-cycle appears above the axis, with no gaps.
Two diodes conduct on each half-cycle, so both halves of the AC are used, always sending current the same way through the load. The pulses come twice as often (100 per second from 50 Hz), so there is less time for the output to fall between them.
The capacitor charges up to the peak voltage on each pulse, and between pulses it discharges slowly through the load, keeping the voltage up. The output only falls a little (the ripple) before the next pulse recharges it.

Key points

A capacitor stores charge: $Q = C V$ ; unit farad, usually µF. Uses: smoothing, timing, flashes.
A diode conducts in forward bias only; an LED emits light and needs a series resistor.
Rectification turns AC into DC: one diode gives half-wave, a four-diode bridge gives full-wave.
A capacitor across the output smooths it, leaving a small ripple.

Sources and credits

Written by Ganzaa: the lesson text, the three diagrams (the waveforms computed from the circuit behaviour), worked examples and practice exercises.

1From the socket to the phone

2Capacitors

3Diodes: a one-way valve for current

4Rectifiers: AC to DC

From the socket to the phone

Capacitors

Diodes: a one-way valve for current

Rectifiers: AC to DC