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Electronics

Half-wave vs full-wave bridge rectifiers

A rectifier turns AC, which pushes one way and then the other, into one-way current. A half-wave rectifier is one diode: it passes the positive halves and blocks the negative ones. A full-wave bridge rectifier is four diodes that steer both halves the same way through the load. A smoothing capacitor then fills the dips to make nearly steady DC.

Half-wave: one diode

One diode in series with the load conducts while the AC is positive and blocks while it's negative. The load gets humps with gaps between them, 50 times a second on 50 Hz mains. Half the input is simply thrown away.

Full-wave: a bridge of four

A bridge of four diodes works like a ratchet spanner: whichever way you swing the handle, the bolt turns the same way. On each half of the cycle, two of the four diodes conduct and steer the current forward through the load, so the negative halves come out flipped up. Each path crosses two diodes:

Vpeak out ≈ Vpeak in − 1.4 V

The humps still drop to 0 V, now 100 times a second on 50 Hz mains.

Live experiment

Turn AC into DC

A 12 V peak, 50 Hz supply feeds a little radio that needs one-way current. Try no diodes, one diode, then a bridge of four, and add a smoothing capacitor. The bridge refills 100 times a second and sags about half as far as one diode.

Try it yourself: free lesson

“Rectifiers” is lesson 2 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 29 before it first.

Smoothing and ripple

A big capacitor across the load fills near each peak, in short gulps, then feeds the load until the next one. Between peaks it sags by the ripple, ΔV:

ΔV ≈ I over fr × C

fr is the ripple frequency: 100 Hz for a bridge on 50 Hz mains, but only 50 Hz for a half-wave. So with the same capacitor, a half-wave rectifier ripples twice as much.

Example: a 12 V RMS transformer, 12 × 1.414 ≈ 17 V peak, feeds a bridge and a 220 mA load. The peak is 17 − 1.4 = 15.6 V. For 1 V of ripple:

C = 0.22 A over 100 Hz × 1 V = 2200 µF

The formula errs on the safe side: the real ripple is a little less, because the refill starts before the period ends.

Choosing parts

  • Capacitor: rated well above the peak, 25 V for 15.6 V. A bigger one can't lift the output above the peak; it only fills in the dips.
  • Diodes: rated above the reverse voltage they block, and for the surge. The capacitor refills in a short slice of each cycle, so the diodes' peak current is many times the load current. In a half-wave rectifier with a capacitor, the diode blocks nearly twice the peak.

A centre-tapped transformer can do full-wave with just two diodes, one drop each, at the cost of a transformer with twice the winding.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Rectifiers” is lesson 2 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 29 before it first.

Or start from the very first lesson, no account needed

Work it out

Practice questions

Three quick questions from the lessons. Pick an answer to check it.

  1. Question 1 of 3What does a big smoothing capacitor do in a power supply?
    Show the answer

    Answer: It fills at each peak and feeds the load in the dips. The capacitor tops up near each peak, then feeds the load until the next one, so the dips fill in. It can't lift the output above the peak, and with no diodes it just gets pushed back and forth.

    • Why not “It turns AC into DC on its own”? On its own a capacitor just gets pushed back and forth by AC. The diodes do the rectifying; the capacitor smooths.
    • Why not “It lifts the voltage above the peak”? A capacitor can only charge up to the peak it's fed. It fills the dips; it can't lift the output above the peak.
  2. Question 2 of 3The stripe painted on a diode marks…
    Show the answer

    Answer: the cathode. The stripe matches the bar in the symbol, the cathode. Current flows in at the anode and out at the cathode, the way the symbol's arrow points.

    • Why not “nothing: a diode works either way round”? A diode only conducts one way, so which end is which matters. That's the whole point of the stripe.
    • Why not “the anode”? The anode is the other end. The stripe matches the bar in the symbol, which is the cathode.
  3. Question 3 of 3A 1000 µF capacitor is charged to 10 V. How much charge sits on each plate?
    Show the answer

    Answer: 10 mC. Q = C × V = 0.001 F × 10 V = 0.01 C = 10 mC, +10 mC on one plate and −10 mC on the other.

    • Why not “1 mC”? That's 0.001 F × 1 V. At 10 V the charge is ten times bigger, Q = C × V.
    • Why not “100 mC”? That would need 10 V across a 10,000 µF capacitor. 1000 µF is 0.001 F, and 0.001 × 10 = 0.01 C.
Keep practising: free lesson

“Rectifiers” is lesson 2 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 29 before it first.

Questions people ask

What is the difference between half-wave and full-wave rectification?

A half-wave rectifier, one diode, passes only the positive halves of the AC and wastes the rest. A full-wave rectifier, usually a bridge of four diodes, flips the negative halves up too, so the load gets twice as many humps and, with the same capacitor, half the ripple.

Why does a bridge rectifier drop 1.4 V?

Whichever way the AC pushes, the current passes through two diodes in series, each dropping about 0.7 V. So the output peaks about 1.4 V below the input's peak, and a little more at high current.

How do I choose a smoothing capacitor?

Use C ≈ I ÷ (f_r × ΔV), where f_r is the ripple frequency (100 Hz for a bridge on 50 Hz mains) and ΔV the ripple you'll accept. For 220 mA and 1 V of ripple, C = 0.22 ÷ (100 × 1) = 2200 µF. Rate it well above the peak voltage.

What frequency is the ripple from a bridge rectifier?

Twice the mains frequency: 100 Hz on 50 Hz mains, 120 Hz on 60 Hz. The bridge makes two humps per cycle. A half-wave rectifier ripples at the mains frequency itself.

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