A diode bridge is a bridge rectifier circuit of four diodes that is used in the process of converting alternating current (AC) from the input terminals to direct current (DC, i.e. fixed polarity) on the output terminals. Its function is to convert the negative voltage portions of the AC waveform to positive voltage, after which a low-pass filter can be used to smooth the result into DC.
When used in its most common application, for conversion of an alternating-current (AC) input into a direct-current (DC) output, it is known as a bridge rectifier. A bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower cost and weight as compared to a rectifier with a three-wire input from a transformer with a center-tapped secondary winding.
Prior to the availability of integrated circuits, a bridge rectifier was constructed from separate diodes. Since about 1950, a single four-terminal component containing the four diodes connected in a bridge configuration has been available and is now available with various voltage and current ratings.
Diodes are also used in bridge topologies along with capacitors as voltage multipliers.
History
The diode bridge circuit was invented by Karol Pollak and patented in December 1895 in Great Britain and in January 1896 in Germany. In 1897, Leo Graetz independently invented and published a similar circuit. Today the circuit is sometimes referred to as a "Graetz circuit" or "Graetz bridge".
Current flow
According to the conventional model of current flow, originally established by Benjamin Franklin and still followed by most engineers today), current flows through electrical conductors from the positive to the negative pole (defined as positive flow). In actuality, free electrons in a conductor nearly always flow from the negative to the positive pole. In the vast majority of applications, however, the actual direction of current flow is irrelevant. Therefore, in the discussion below the conventional model is retained.
The fundamental characteristic of a diode is that current can flow only one way through it, which is defined as the forward direction. A diode bridge uses diodes as series components to allow current to pass in the forward direction during the positive part of the AC cycle and as shunt components to redirect current flowing in the reverse direction during the negative part of the AC cycle to the opposite rails.
Rectifier
In the diagrams below, when the input connected to the left corner of the diamond is positive, and the input connected to the right corner is negative, current flows from the upper supply terminal to the right along the red (positive) path to the output and returns to the lower supply terminal through the blue (negative) path.
When the input connected to the left corner is negative, and the input connected to the right corner is positive, current flows from the lower supply terminal to the right along the red (positive) path to the output and returns to the upper supply terminal through the blue (negative) path.
In each case, the upper right output remains positive, and lower right output negative. Since this is true whether the input is AC or DC, this circuit not only produces a DC output from an AC input, it can also provide reverse-polarity protection; that is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the leads from a DC power source have been reversed, and protects the equipment from potential damage caused by reverse polarity.
Alternatives to the diode-bridge full-wave rectifiers are the center-tapped transformer and double-diode rectifier, and voltage doubler rectifier using two diodes and two capacitors in a bridge topology.
