Diode bridge

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Detail of a diode bridge, rated at 1000 Volts x 4 Amperes
A hand made diode bridge. The wide silver band on the diodes indicates the cathode side of the diode.

A diode bridge is an arrangement of four (or more) diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input.

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 3-wire input from a transformer with a center-tapped secondary winding.[1]

The essential feature of a diode bridge is that the polarity of the output is the same regardless of the polarity at the input. The diode bridge circuit was invented by Polish electrotechnician Karol Pollak [2] and patent was recorded in 14 Jan, 1896 under the number DRP[3] 96564. It was later published in Elektronische Zeitung, vol. 25 in 1897 with annotation that German physicist Leo Graetz also was researching this matter at that time.[4] Today the circuit is still often referred as Graetz circuit or Graetz bridge.[5]

Basic operation[edit]

According to the conventional model of current flow (originally established by Benjamin Franklin and still followed by most engineers today[6]), current is defined to be positive when it flows through electrical conductors from the positive to the negative pole. 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.


Diode bridge alt 1.svg

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 via the blue (negative) path.[7]

Diode bridge alt 2.svg



AC, half-wave and full wave rectified signals.[8]

Prior to the availability of integrated circuits, a bridge rectifier was constructed from "discrete components", i.e., separate diodes. Since about 1950, a single four-terminal component containing the four diodes connected in a bridge configuration became a standard commercial component and is now available with various voltage and current ratings.

Output smoothing[edit]

For many applications, especially with single phase AC where the full-wave bridge serves to convert an AC input into a DC output, the addition of a capacitor may be desired because the bridge alone supplies an output of pulsed DC (see diagram to right).[1][8]

Diode bridge smoothing.svg

The function of this capacitor, known as a reservoir capacitor (or smoothing capacitor) is to lessen the variation in (or 'smooth') the rectified AC output voltage waveform from the bridge. There is still some variation, known as ripple. One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor. This charge flows out as additional current through the load. Thus the change of load current and voltage is reduced relative to what would occur without the capacitor. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current.


The capacitor and the load resistance have a typical time constant τ = RC where C and R are the capacitance and load resistance respectively. As long as the load resistor is large enough so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a smoothed DC voltage across the load.

When the capacitor is connected directly to the bridge, as shown, current flows in only a small portion of each cycle, which may be undesirable. The transformer and bridge diodes must be sized to withstand the current surge that occurs when the power is turned on at the peak of the AC voltage and the capacitor is fully discharged. Sometimes a small series resistor is included before the capacitor to limit this current, though in most applications the power supply transformer's resistance is already sufficient. Adding a resistor, or better yet, an inductor, between the bridge and capacitor can ensure that current is drawn over a large portion of each cycle and a large current surge does not occur.

In older times[when?], this crude power supply was often followed by passive filters (capacitors plus resistors and inductors) to reduce the ripple further. When an inductor is used this way it is often called a choke. The choke tends to keep the current (rather than the voltage) more constant. Although the inductor gives the best performance, usually the resistor is chosen for cost reasons.

Because of the increasing availability of voltage-regulator chips, passive filters are less commonly used. The chips can compensate for changes in input voltage and load current, which the passive filter does not, and eliminate ripple to a high degree.



Polyphase diode bridges[edit]

The diode bridge can be generalized to rectify polyphase AC inputs. For example, for a three-phase AC input, a half-wave rectifier consists of three diodes, but a full-wave bridge rectifier consists of six diodes.

Half-wave rectifier may be considered as wye connection (star connection), because it returns the current through the centre (neutral) wire. Full-wave is more like delta connection, although it can be connected to the three phase source of either wye or delta and it does not use the centre (neutral) wire.

Three phase full wave bridge rectifier.
3-phase AC input waveform (top), half-wave rectified waveform (center), and full-wave rectified waveform (bottom).
Three-phase bridge rectifier for a wind turbine.

See also[edit]


  1. ^ a b Horowitz, Paul; Hill, Winfield (1989). The Art of Electronics (Second ed.). Cambridge University Press. pp. 44–47. ISBN 0-521-37095-7. 
  2. ^ British patent 24398
  3. ^ "D.R.P. and D.R.G.M.". 
  4. ^ Strzelecki, R. Power Electronics in Smart Electrical Energy Networks. Springer, 2008, p. 57.
  5. ^ "Graetz Flow Control Circuit". 
  6. ^ Stutz, Michael (stutz@dsl.org), "Conventional versus electron flow", All About Circuits, Vol. 1, Chapter 1, 2000.
  7. ^ Sears, Francis W., Mark W. Zemansky and Hugh D. Young, University Physics, Sixth Ed., Addison-Wesely Publishing Co., Inc., 1982, p. 685.
  8. ^ a b "Rectifier", Concise Encyclopedia of Science and Technology, Third Edition, Sybil P. Parker, ed. McGraw-Hill, Inc., 1994, p. 1589.