Diodes
Diode symbols, characteristics and properties; half-wave and full-wave rectification; smoothing; clippers and clampers; Zener, Schottky, photodiode, varactor, LED, thyristor and varistor devices.
Diodes, summary notes
- A diode conducts from anode to cathode in the direction of the symbol's arrow. The painted BAND marks the cathode.
- Forward volt drop is about 0.7 V in silicon. PIV (peak inverse voltage) is the maximum reverse voltage the diode can withstand.
- Half-wave rectification uses one diode and produces ripple at the SUPPLY frequency.
- Full-wave rectification, centre-tap (2 diodes) or bridge (4 diodes), produces ripple at TWICE the supply frequency, which is easier to smooth.
- A reservoir/smoothing capacitor across the output reduces ripple by holding charge between peaks.
- Special diodes: Zener (operates in reverse breakdown for voltage regulation), LED (emits light), photodiode (light-sensitive), Schottky (fast switching, low drop), varactor (voltage-variable capacitance), SCR/thyristor (gate-latched), varistor (surge suppression).
- ⚠ Exam trap: bridge and full-wave ripple is 2f; half-wave is 1f. The band marks the cathode. A Zener works in REVERSE breakdown.
- Silicon forward drop
- ≈ 0.7 V per junction
- Half-wave ripple
- f_ripple = f_supply
- Full-wave / bridge ripple
- f_ripple = 2 × f_supply
- Bridge output (ideal)
- V_dc ≈ 0.637 × V_peak (unsmoothed)
A full-wave bridge rectifier is fed from a 400 Hz aircraft supply. What is the ripple frequency?
Full-wave rectification gives ripple at twice the supply frequency: 2 × 400 = 800 Hz.
How many diodes does a bridge rectifier use, and how many conduct at any instant?
Four diodes in a bridge; two conduct in each half cycle, in diagonally opposite pairs, so the load current always flows the same way.
Diode rectifier, half & full wave
Smoothing filter & ripple
Zener voltage regulator
Diode clipper
Diodes concept map
Diodes
Diodes quiz
Diodes, quiz
1. In the diode symbol shown, terminal B is the one connected to the bar. Terminal B is the:
CathodeAnodeGate2. The circuit shown produces the output waveform on the right. The arrangement is a:
Half-wave rectifierFull-wave rectifierVoltage doubler3. The four-diode arrangement shown is:
A full-wave bridge rectifierA half-wave rectifierA diode clipper4. In the circuit shown the zener is connected in reverse across the load. The series resistor Rs is fitted to:
Drop the surplus voltage and limit the zener currentRaise the output voltageRectify the supply5. In the circuit shown, the capacitor C is fitted to:
Reduce the ripple in the outputRectify the AC inputRaise the output frequency6. A silicon diode conducts in series with a resistor across a 12 V supply. Taking the forward volt drop as 0.7 V, the voltage across the resistor is:
11.3 V12.7 V0.7 V7. A single-phase full-wave rectifier is supplied at 50 Hz. The ripple frequency of its output is:
100 Hz50 Hz25 Hz8. A 5.1 V zener diode is fed from a 12 V supply through a 1 kΩ series resistor, with no load connected. The current through the zener is approximately:
6.9 mA12 mA5.1 mA9. The ripple frequency at the output of a full-wave bridge rectifier fed from 50 Hz mains is:
100 Hz50 Hz25 Hz10. On a diode body, the painted band indicates the:
CathodeAnodeGate11. A Zener diode is normally operated:
In reverse breakdownIn forward conductionAt zero bias12. A bridge rectifier uses how many diodes?
FourTwoOne