Rectification: MDCAT Physics notes

What rectification is

Rectification is the conversion of alternating current (AC) into direct current (DC) that flows in one direction only. The circuit is a rectifier, and the component that does the job is the PN-junction diode, because it conducts only when forward biased. A reverse-biased diode has a very high resistance, so it blocks current on the opposite half cycle.

The output of a rectifier is pulsating DC: it never reverses direction, but its magnitude rises and falls. Mains voltage is first reduced with a step-down transformer, because electronic circuits need a low DC voltage.

Half-wave rectifier

Components: transformer, one diode and a load resistance $R_L$. During the positive half cycle the diode is forward biased and current flows through $R_L$. During the negative half cycle the diode is reverse biased (very high resistance) and no current flows. Only one half of each cycle reaches the load.

• Output pulses per second = supply frequency (50 Hz mains gives 50 pulses per second).
• Ripple factor about $1.21$; maximum efficiency about 40.6%.
• PIV across the diode = $V_m$.

Full-wave rectifiers

A full-wave rectifier uses both half cycles and turns them the same way up, so the current through the load is always in the same direction.

Centre-tap rectifier (two diodes)

A centre-tapped secondary feeds two diodes. On one half cycle D1 conducts, on the next D2 conducts: the diodes work in alternate half cycles. The non-conducting diode must withstand the whole secondary, so its PIV is $2V_m$.

Bridge rectifier (four diodes)

Four diodes are arranged like a Wheatstone bridge; two conduct on each half cycle. It is the most widely used rectifier because it needs no centre tap, can use a smaller transformer for the same output, and each diode has a lower PIV ($V_m$). Pakistani textbooks describe full-wave rectification with the bridge, which is why MCQs often key "four diodes".

Feature	Half-wave	Centre-tap full-wave	Bridge
Diodes	1	2	4
Ripple frequency (50 Hz mains)	50 Hz	100 Hz	100 Hz
Ripple factor	1.21	0.48	0.48
Max efficiency	40.6%	81.2%	81.2%
PIV per diode	$V_m$	$2V_m$	$V_m$

The efficiency of a full-wave rectifier is therefore about double that of a half-wave rectifier, and its ripple is much smaller.

Filtering

A filter (usually a capacitor across the load) removes the ripples from the rectified output. The capacitor charges near each peak and discharges slowly through $R_L$ between peaks, giving a smoother DC. Full-wave output is easier to smooth because the gaps between pulses are shorter.

Key formulas

• Ripple frequency: half-wave $f_r=f$; full-wave $f_r=2f$.
• Ripple factor $r=\dfrac{V_{ac,\,rms}}{V_{dc}}$: 1.21 (half-wave), 0.48 (full-wave).
• PIV: $V_m$ (half-wave, bridge); $2V_m$ (centre-tap).

Common MDCAT traps

• The rectifying device is the diode, not the transformer; the transformer only steps the voltage down.
• A full-wave rectifier on 50 Hz gives ripple of 100 Hz, not 50 Hz.
• "Full-wave" can be 2 diodes (centre-tap) or 4 (bridge); if the question says bridge or gives only 4 as an option, choose 4.
• PIV means peak inverse voltage, not "positive inverse voltage".
• The filter removes ripple; it does not rectify.

Quick revision

• Rectification: AC to pulsating DC using diodes.
• Reverse-biased diode: very high resistance.
• Half-wave: 1 diode, ripple factor 1.21, pulses = $f$.
• Bridge: 4 diodes, no centre tap, most common.
• Load current in a full-wave rectifier never changes direction.
• Capacitor filter smooths out ripple.