What is "rectifier"

Rectifier \Rec"ti*fi`er\ (r?k"t?*f?`?r), n.

1. One who, or that which, rectifies.

2. Specifically:

   1. (Naut.) An instrument used for determining and rectifying the variations of the compass on board ship.

   2. (Chem.) A rectificator.

1610s, agent noun from rectify.

n. 1 Something that rectifies. 2 A device that converts alternating current into direct current; often a diode. 3 (context nautical English) An instrument used for determining and rectifying the variations of the compass on board ship.

1. n. electrical device that transforms alternating into direct current

2. a person who corrects or sets right; "a rectifier of prejudices"

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-arc valves, copper and selenium oxide rectifiers, semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches. Historically, even synchronous electromechanical switches and motors have been used. Early radio receivers, called crystal radios, used a " cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector".

Rectifiers have many uses, but are often found serving as components of DC power supplies and high-voltage direct current power transmission systems. Rectification may serve in roles other than to generate direct current for use as a source of power. As noted, detectors of radio signals serve as rectifiers. In gas heating systems flame rectification is used to detect presence of a flame.

Because of the alternating nature of the input AC sine wave, the process of rectification alone produces a DC current that, though unidirectional, consists of pulses of current. Many applications of rectifiers, such as power supplies for radio, television and computer equipment, require a steady constant DC current (as would be produced by a battery). In these applications the output of the rectifier is smoothed by an electronic filter (usually a capacitor) to produce a steady current.

More complex circuitry that performs the opposite function, converting DC to AC, is called an inverter.

The word rectifier refers to the general act of straightening. It may refer to:

• Rectifier, a device for converting alternating current to direct current
• Rectifier (neural networks), an activation function for artificial neural networks
• Rectifier, a guitar amplifier manufactured by Mesa Boogie.

In the context of artificial neural networks, the rectifier is an activation function defined as

f(x) = max(0, x), 

where x is the input to a neuron. This is also known as a ramp function and is analogous to half-wave rectification in electrical engineering. This activation function has been argued to be more biologically plausible than the widely used logistic sigmoid (which is inspired by probability theory; see logistic regression) and its more practical counterpart, the hyperbolic tangent. The rectifier is, , the most popular activation function for deep neural networks.

A unit employing the rectifier is also called a rectified linear unit (ReLU).

A smooth approximation to the rectifier is the analytic function

f(x) = ln(1 + e), 

which is called the softplus function. The derivative of softplus is fʹ(x) = e/(e + 1) = 1/(1 + e), i.e. the logistic function.

Rectified linear units find applications in computer vision and speech recognition using deep neural nets.