Diode bridge rectifier

Rectifier circuits are classified into two major groups i.e., single phase and three phases. In both cases they are again classified into three main categories those are uncontrolled, half controlled and fully controlled. If we use diode to convert this voltage we can call that as uncontrolled, instead if we use power electronic components like SCRS we can call as controlled rectifiers. We can control half wave or full wave according to the application dependency.

The main difference between conventional rectifier and bridge rectifier is that it produces almost double the output voltage as a full wave center-tapped transformer rectifier using the same secondary voltage. The advantage of using this circuit is that no center-tapped transformer is required. In center tapped rectifier each diode uses only one-half of the transformer secondary voltage, so the DC output is comparatively small, also it is difficult to locate the center-tap on secondary winding of the transformer and the diodes used must have high Peak-inverse voltage.

Bridge Rectifier

Bridge Rectifier Circuit and the output waveform

During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2switch “OFF” as they are now reverse biased. The current flowing through the load is the same direction as before.

The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage. Generally for DC power supply circuits the smoothing capacitor is an Aluminum Electrolytic type that has a capacitance value of 100uF or more with repeated DC voltage pulses from the rectifier charging up the capacitor to peak voltage. However, there are two important parameters to consider when choosing a suitable smoothing capacitor and these are its Working Voltage, which must be higher than the no-load output value of the rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage.

5 Advantages of bridge rectifier

  • The rectification efficiency of full-wave rectifier is double of that of a half-wave rectifier.
  • Higher output voltage, higher output power and higher Transformer Utilization Factor in case of full-wave rectifier.
  • The ripple voltage is low and of higher frequency in case of full-wave rectifier so simple filtering circuit is required
  •  No center tap is required in the transformer secondary so in case of a bridge rectifier the transformer required is simpler. If stepping up or stepping down of voltage is not required, transformer can be eliminated even.
  • For a given power output, power transformer of smaller size can be used in case of the bridge rectifier because current in both primary and secondary windings of the supply transformer flow for the entire ac cycle

2 Disadvantages of Bridge Rectifier

  • It requires four diodes.
  • The use of two extra diodes cause an additional voltage drop thereby reducing the output voltage.

An Application – Converting AC power to DC using a Bridge Rectifier

Regulated DC Power supply is often required for many electronic applications. One of the most reliable and convenient way is to convert the available AC mains power supply into DC supply.  This conversion of AC signal to DC signal is done using a rectifier, which is basically a system of diodes. It can be a half wave rectifier which rectifies only one half of the AC signal or a full wave rectifier which rectifies both cycles of the AC signal. The full wave rectifier can be a centre tapped rectifier consisting of two diodes or a bridge rectifier consisting of 4 diodes.

Here the bridge rectifier is demonstrated. The arrangement consists of 4 diodes arranged such that the anodes of two adjacent diodes are connected together to give the positive supply to the output and the cathodes of other two adjacent diodes are connected together to give the negative supply to the output. The anode and cathode of other two adjacent diodes are connected together to the positive of AC supply whereas the anode and cathode of another two adjacent diodes are connected together to the negative of the AC supply. Thus 4 diodes are arranged in a bridge configuration such that in each half cycle two alternate diodes conduct producing a DC voltage with repels.

The given circuit consists of a bridge rectifier arrangement whose unregulated DC output is given to an electrolyte capacitor through a current limiting resistor. The voltage across the capacitor is monitored using a voltmeter and keeps on increasing as the capacitor charges till the voltage limit is reached. When a load is connected across the capacitor, the capacitor discharges to provide necessary input current to the load. In this case a lamp is connected as a load.

A Regulated DC Power Supply

A regulated DC power Supply consists of the following components:

  • A step down transformer to convert high voltage AC to low voltage AC.
  • A bridge rectifier to convert the AC into pulsating DC.
  • A filter circuit consisting of a capacitor to remove the AC ripples.
  • A regulator IC 7805 to get regulated DC voltage of 5 V.

The step down transformer converts the AC mains supply of 230V to 12V AC. This 12V AC is applied to the bridge rectifier arrangement such that the alternate diodes conduct for each half cycle producing a pulsating DC voltage consisting of AC ripples. A capacitor connected across the output allows the AC signal to pass through it and blocks the DC signal, thus acting as a high pass filter. The output across the capacitor is thus an unregulated filtered DC signal. This output can be used to drive electrical components like relays, motors, etc.  A regulator IC 7805 is connected to the filter output. It gives a constant regulated output of 5V which can be used to give input to many electronic circuits and devices like transistors, microcontrollers etc. Here the 5V is used to bias a LED through a resistor.

Photo Credit:

  • Bridge Rectifier Circuit and the output waveform by  neilorme

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