A
diode bridge is an arrangement of four (or more)
diodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s in a
bridge circuitA bridge circuit is a type of electrical circuit in which two circuit branches are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate...
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 currentIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) input into
direct currentDirect current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
a (DC) output, it is known as a
bridge rectifierA rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
. 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
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
with a
center-tappedIn electronics, a center tap is a connection made to a point half way along a winding of a transformer or inductor, or along the element of a resistor or a potentiometer....
secondary winding.
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 is also known as the
Graetz circuit after its inventor, physicist
Leo GraetzLeo Graetz was a German physicist. He was born in Wrocław , Poland as the son of the historian Heinrich Graetz.Graetz was one of the first to investigate the propagation of electromagnetic energy...
.
Basic operation
According to the conventional model of current flow originally established by
Benjamin FranklinDr. Benjamin Franklin was one of the Founding Fathers of the United States. A noted polymath, Franklin was a leading author, printer, political theorist, politician, postmaster, scientist, musician, inventor, satirist, civic activist, statesman, and diplomat...
and still followed by most engineers today, current is
assumed to flow through
electrical conductorIn physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
s from the
positive to the
negative pole. In actuality,
free electronsIn solid-state physics, the free electron model is a simple model for the behaviour of valence electrons in a crystal structure of a metallic solid. It was developed principally by Arnold Sommerfeld who combined the classical Drude model with quantum mechanical Fermi-Dirac statistics and hence it...
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.
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 via 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
upper supply terminal to the right along the
red (positive) path to the output, and returns to the
lower supply terminal via 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 what is sometimes called "reverse polarity protection". That is, it permits normal functioning of DC-powered equipment when
batteriesAn electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
have been installed backwards, or when the leads (wires) from a DC power source have been reversed, and protects the equipment from potential damage caused by reverse polarity.
Prior to the availability of
integrated circuitAn integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s, 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
voltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
age and current ratings.
Output smoothing
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
capacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
may be desired because the bridge alone supplies an output of
pulsed DCPulsed DC or PDC is the form of wave produced from a half-wave rectifier or a full-wave rectifier. Full wave rectified ac is more commonly known as Rectified AC. PDC has some characteristics of both alternating current and direct current waveforms...
(see diagram to right).
The function of this capacitor, known as a
reservoir capacitorA reservoir capacitor is a capacitor that is used to smooth the pulsating DC from an AC rectifier.-Performance with low impedance source:...
(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
impedanceImpedance may refer to:*Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit**Characteristic impedance of a transmission line...
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 canceled 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 simplified circuit shown has a well-deserved reputation for being dangerous, because, in some applications, the
capacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
can retain a
lethal charge after the AC power source is removed. If supplying a dangerous voltage, a practical circuit should include a reliable way to discharge the capacitor safely. If the normal load cannot be guaranteed to perform this function, perhaps because it can be disconnected, the circuit should include a
bleeder resistorA bleeder resistor is a resistor placed in parallel with a high-voltage supply for the purposes of discharging the energy stored in the power source's filter capacitors or other components that store electrical energy when the equipment is turned off....
connected as close as practical across the capacitor. This
resistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
should consume a current large enough to discharge the capacitor in a reasonable time, but small enough to minimize unnecessary power waste.
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
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
'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, 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
chokeA choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
. 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.
Nowadays with the wide 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 pretty much eliminate ripple. Some of these chips have fairly impressive power handling; in case this is not sufficient, they can be combined with a power transistor.
The idealized waveforms shown above are seen for both voltage and current when the load on the bridge is resistive. When the load includes a smoothing capacitor, both the voltage and the current waveforms will be greatly changed. While the voltage is smoothed, as described above, current will flow through the bridge only during the time when the input voltage is greater than the capacitor voltage. For example, if the load draws an average current of n Amps, and the diodes conduct for 10% of the time, the average diode current during conduction must be 10n Amps. This non-sinusoidal current leads to harmonic distortion and a poor
power factorThe power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
in the AC supply.
Some early console radios created the
speakerA loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
's constant field with the current from the high voltage ("B +") power supply, which was then routed to the consuming circuits, (permanent magnets were then too weak for good performance) to create the speaker's constant magnetic field. The speaker field coil thus performed 2 jobs in one: it acted as a choke, filtering the power supply, and it produced the magnetic field to operate the speaker.
Polyphase diode bridges
The diode bridge can be generalized to rectify
polyphaseA polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...
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.
External links