# Half wave vs full wave rectifier [duplicate]

I know that a rectifier converts AC voltage into pulsating DC voltage.

However, when would you use a half wave rectifier as opposed to a full wave rectifier ?

Can you give an example of some appliances that use half wave and full wave rectification.

• It's cheaper, I guess if you want to save money on diodes and don't need the extra power or DC smoothness. Less diodes also means more efficient because less diode voltage drop which matters more if your AC is a very low voltage. 0.7V per diode lost matters a lot more at 5VAC than at 120VAC. Don't forget there is also a bridge rectifier. Half-wave = 1 diode, Full-wave = 2 diode (center-tap required), bridge rectifier = 4 diodes. THey are a – DKNguyen Jan 22 '20 at 20:45
• @DKNguyen 2 diodes don't make a full wave rectifier. It's one diode or four. – Sod Almighty Jan 23 '20 at 20:47
• @SodAlmighty 4 diode is specifically a bridge rectifier. There is ambiguity in the term "full-wave rectifier" and no seemingly no agreed upon name for 2 diodes with a center-tap. – DKNguyen Jan 23 '20 at 20:48

The 4 diode bridge is a full-wave voltage doubler and achieves 200% of the voltage and 50% of the current of the centre-tapped 2 diode full wave version. Thus the tapped version has lower impedance.

Some users might consider the tapped 2 diode version as a 2 phase each half wave combined to make a full wave. But in fact, the secondary is only a split-single phase.

A half-wave bridge is a single diode version is used when less voltage and current is needed and thus the longer charge interval is adequate.

• Simple answer which I understand thanks :) – Dan Khan Jan 22 '20 at 22:09
• How does a bridge count as a "voltage doubler"? Surely 100v peak-to-peak still ends up as 49v DC ? – Mike Brockington Jan 23 '20 at 10:03
• @MikeBrockington If you're thinking of it as a pure AC wave oscillating about the zero point, then the "ground" output of a full-bridge rectifier is at -50v DC, resulting in a 100v difference between +v and -v. (- the diode drops) – Tyzoid Jan 23 '20 at 17:10

With a full-wave bridge rectifier you have four diodes, resulting in two diode drops, and the capacitor value is roughly half what you need for a half-wave rectifier with the same ripple. If you have a center-tapped transformer you can use two diodes to get full-wave rectification with only a single diode drop, but the extra transformer tap and associated connections are not free. For rectifying mains voltage or voltage from a typical AC-output wall plug adapter you don't have the option of a center tap (usually).

The four-diode bridge uses a transformer capacity most efficiently, center-tapped (two diodes) full-wave is better than half-wave but worse than the four diode bridge for voltages much higher than a diode drop.

With a half-wave rectifier you have only one diode, and only one diode drop, but you need a larger capacitance (typically physically about twice the volume). It's the least efficient way to use a transformer (as a single rail anyway).

Most switching power supplies use full-wave rectification on the input. On the output they may use half or full wave as the higher frequency means the capacitor size is not as important. With a flyback converter you don't have a choice- it has to be half-wave.

Half wave rectification is sometimes used with an AC-output wall plug adapter to produce balanced +/- rails for audio. I have a hydrogen-oxygen electrolysis welder (US made ca. 1968) that uses half-wave rectification probably because the optimum voltage for the gas production cell is only a couple volts and two fat stud-mount silicon rectifier drops would be less efficient and more costly even though the current is many amperes (no filtering required in that application). These days we'd probably use a switching supply with Schottky diodes at the output.

The half wave rectifier uses one diode, does not need center tapped transformer. The full wave one uses two diodes and needs center tapped transformer. Efficiency for both cases are the same. At the same load with the same output filter design, full wave rectifier has less ripple. From another perspective, with the same ripple requirement, filter design is easier for full wave rectifier with less output capacitance needed.

For the very light load design, half wave rectifier is more suitable, simple and cheaper.

• Why would you need a center tapped transformer for a full-wave rectifier? That's only if you make a 2-diode version, a 4-diode version (bridge rectifier) doesn't need any transformer. – Mast Jan 23 '20 at 8:59
• @Mast, I assume there are three types, half wave rectifier (one diode, xformer w/o center tap), full wave rectifier (two diodes, xformer w/ center tap) and bridge rectifier (full wave, four diodes, xformer w/ or w/o center tap depending on single ended or differential output) – X J Jan 23 '20 at 13:23

Half-wave rectifiers are mostly a relict from electron tube times.

Making circuits with electron tubes is tricky as the cathode and anode are not symetric ; the cathode has a heating circuit which needs to be a cathode potential. For implementing a bridge rectifier, you needed seperate heating windings on the transformer for the high-side diodes. Also, having four diode tubes or two double diode tubes is costly and creates another point where maintenance was needed.

It's simpler to either use a transformer with an air gap, so half wave rectification does not saturate it, and a single diode tube and a bigger capacitor and smoothing coil or to put a symetric wiring with center tap on the transformer core and use a dual-diode tube.

Nowadays, you will find half-wave rectifiers in the output circuit of flyback converters. The transformer has an air gap anyways, and the high switching frequency makes bigger capacitors and smoothing coils small again. Three additional ultrafast recovery diodes cost more.