Rectifier
A rectifier is a circuit or device that converts alternating current, which reverses direction many times a second, into current that flows in one direction only. Nearly every electronic gadget runs on direct current while wall outlets deliver alternating current, so a rectifier, usually followed by a smoothing stage, is the first thing inside a charger, television or computer power supply. The word also names the device that does the work, which is today almost always a diode or a bridge of four diodes.
A half-wave rectifier uses a single diode and discards half the waveform. On the positive half-cycle the diode conducts and passes current to the load; on the negative half-cycle it blocks. The output is a series of positive humps separated by gaps, at the same frequency as the mains, 60 Hz in North America and 50 Hz in much of the world. Half the input energy is wasted and the average output is low, so half-wave rectification is used only for small, cheap, low-power jobs such as a signal detector or a simple bias supply.
Full-wave rectification flips the negative half-cycle instead of throwing it away, in two ways. A center-tapped transformer with two diodes gives each diode half of the secondary winding in turn, and that was the standard arrangement in vacuum-tube equipment. The bridge rectifier uses four diodes arranged in a diamond so that two conduct on each half-cycle and the load always sees the same polarity, needing no center tap. The ripple frequency doubles to 120 Hz on a 60 Hz supply (100 Hz on 50 Hz), which makes smoothing easier. A bridge pays for two diode drops in series, about 1.4 volts with silicon, which matters when the output is only a few volts.
The smoothing capacitor is what turns pulsating current into something close to steady direct current. The capacitor charges to the peak voltage while the diodes conduct, then supplies the load alone as the rectified voltage falls, and is recharged on the next peak. The ripple voltage is roughly the load current divided by the product of the ripple frequency and the capacitance, so doubling the capacitance or the load resistance halves the ripple. The consequence is that current flows from the line only in short, sharp pulses near each voltage peak, which is why simple power supplies create harmonic distortion on the mains and why modern supplies include power-factor correction circuits.
Rectifier technology passed through several materials before silicon took over. Mercury-arc rectifiers, introduced by Peter Cooper Hewitt about 1902, handled industrial currents in electric railways and plating. Vacuum rectifier tubes such as the 5Y3 and 5U4 powered radios and amplifiers for decades. In the 1920s, metal-oxide rectifiers appeared: the copper-oxide rectifier, worked out by Lars Grondahl and Paul Geiger at Westinghouse, and the selenium rectifier, built from the light-sensitive element known since the 1870s and 1880s, stacked in plates for chargers and radios, tolerating overloads but bulky and aging with time. Their physical principle was a barrier at a metal-semiconductor boundary, the same family as the cat-whisker detector. Germanium and then silicon junction rectifiers from the 1950s cut the forward drop and the size drastically, and a one-ampere silicon diode in a small plastic package now costs a few cents.
Modern rectification increasingly uses transistors instead of diodes. In synchronous rectification a MOSFET is switched on during the half-cycle when current should flow and acts as a very low-resistance path, with a drop of a few tens of millivolts instead of the 0.7 volts of a silicon diode. A laptop or phone charger first rectifies the mains to high-voltage direct current, chops it at tens or hundreds of kilohertz, steps it down through a small transformer, and rectifies it again with Schottky diodes or synchronous switches. The same logic runs in reverse in solar inverters and electric vehicles, whose motor drives are, structurally, rectifier bridges built from controlled switches.
A rectifier does not produce clean direct current by itself, and a bridge does not change the voltage. Rectified output peaks at the input peak, which is about 1.41 times the root-mean-square value on the label, minus the diode drops, so a 12-volt winding gives roughly 15 volts with a smoothing capacitor and no load. A larger capacitor is not automatically better, because it draws a larger charging pulse and heats the diodes and transformer. Rectifiers also tend to be confused with regulators: regulation, holding the voltage steady as load and input change, is a separate stage that follows.
Read rectifier as the gatekeeper between the alternating world of power lines and the direct-current world of electronics: a diode arrangement that straightens the wave, a capacitor that fills the gaps, and a lineage of materials from mercury vapor to silicon that follows the history of solid-state electronics.