Full wave Rectifier explained


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Full wave rectification

A single-phase full wave rectifier is a circuit that converts AC voltage into DC. the output from a full wave rectifier is Pulsating DC. This article will discuss two types of full wave rectifiers: AC Voltage rectifiers constructed with semiconductor PN junction diodes for single-phase input. One of two types is a bridge rectifier, and the other is a center-tapped transformer full wave rectifier. We will learn the operation of a full wave bridge rectifier with a schematic of a single-phase full wave diode bridge rectifier with RL load.  And we have tried to explain the working of bridge rectifiers with relevant waveforms. Learning to Design and calculate full wave rectifier parameters is the goal of this article. Sometimes, the Bridge rectifier gets hot, or the diode heating problem is ignored by new designers or students in lab experiments. In this section, we will discuss how to estimate the power dissipation of bridge rectifiers. Power loss in rectifiers is converted into thermal energy. how do remove excessive heat when the bridge rectifier or diode is getting hot? In other design calculation lists, we have included a bridge rectifier power loss calculation and the efficiency of a full wave rectifier in this article. To design High power rectifiers heat sink selection is an important task.  the output voltage signal of a bridge rectifier is fluctuating Direct current which is not acceptable for deliciated applications. Because single polarity is not enough to drive electronic circuits. A smooth and regulated DC supply is necessary.

The output waveform of a full wave bridge rectifier without a capacitor filter contains ripples but electronic circuits need a smooth DC voltage. We have added a circuit diagram of a full wave bridge rectifier with a capacitor filter. With a detailed description of the design of capacitor filter for bridge rectifier. Some variations of the filter are also discussed here like smoothing rectifier output with pi section filter, and common mode Chowk filter. And the detailed comparison of full wave rectifier with and without a filter.

Another type is center tapped full wave rectifier. In this type, two diodes are eliminated by tapping a wire from the transformer’s secondary coil. Although the number of diodes used in the canter tap full wave rectifier is only two, it has some merits and demerits to bridge rectifiers. Operation of full wave rectifier circuit with center tap transformer is explained below with capacitor filter.

Single phase Full wave rectifier

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 A full wave rectifier is a circuit that converts Alternating Current into pulsating Direct Current.

Waveform

Single Phase sinewave waveform. radian frequency. input signal of a rectifier
Fig:1 Sinewave rectifier input
 Output Signal waveform after full wave rectification. when input is sinewave. Time domain is in radians.
Fig:2 Full wave Rectified output signal waveform

In the Figure single phase, a sinewave signal is applied to the input of a rectifier. Each cycle of a sinewave is divided into two parts one is positive and another is the negative half cycle. That shows us sinewave alternating current continuedly to switch polarity and magnitude. For a clear and smooth DC voltage, we need to convert this input AC signal into a single polarity. Combining negative and positive half cycles at one side of the zero line is called the rectification process. The rectified output is single polarity and alternating amplitude. This alternating amplitude is called ripples or pulsating DC. A capacitor Filter is used to smoothen rippled output.

Full wave rectifier is categorized by Phases inputs and controlling. Rectifiers are used with single-phase and multi-phase AC inputs. Some rectifiers are controlled by input triggers or gate signals. Most simple and Common rectifiers are constructed with PN junction diodes and have no control inputs. Uncontrolled rectifiers are used in general-purpose DC power supplies. Here we are going to discuss only uncontrolled rectifiers, Bridge, and center tap transformer rectifiers. Medium and high current rating diodes are used for rectifier construction. Rectifier diodes are available with 50V to 1500V maximum reverse voltage. Here, the bridge rectifier is described first, and the Center tapped transformer rectifier is discussed after that. 

 Full wave rectifier average voltage

Vavg = 2Vp / π

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Full wave rectifier RMS Voltage

Vrms = Vp * 0.707

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Full Wave rectifier Peak Inverse Voltage

The reverse breakdown voltage of the diode or bridge rectifier module is peak inverse voltage.

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Full Wave rectifier crest factor

Crest Factor = Vp / Vrms

in the sinewave case crest factor of the full wave is the square root of 2.

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Full wave rectifier Ripple Factor

Without filter full wave rectifier Ripple factor γ is

γ = 0.48

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Full Wave rectifier Efficiency

Bridge rectifier power efficiency is 81.2%. and is a ratio of output DC power and input AC power. A full wave rectifier converts both positive and negative half cycles of input sinewave. for that reason, a full wave rectifier is more efficient than a half wave rectifier.

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Simple Full wave Bridge Rectifier

Full Wave Bridge rectifier DC Power Supply without filter capacitor. constructed with four general purpose silicon diodes. Pulsating DC delivers to RL Load.
Fig:3 Full Wave Bridge rectifier DC Power Supply without filter

To start explaining full wave bridge rectifier ac to dc conversion operation. As shown in the figure the total no of diodes in a bridge rectifier diagram is four.  Input is sinewave and single phase which is fed into a step-down transformer primary coil and downconverted to a required voltage level. Input AC voltage is alternating its polarity and amplitude periodically. To gain a steady state DC voltage we are using a full wave bridge rectifier circuit without a filter or capacitor filter. After understanding, we will use a filter capacitor and different configurations to attain a ripple-free DC output. Each transformer’s secondary terminal is connected to two diodes: anode and cathode. And other terminals of two diodes with the same polarity are connected. One pair of diodes through Positive voltage from transformer secondary coil. At the same time another diode through negative voltage from secondary coil to RL load resistance. In the following full bridge rectifier circuit diagram.

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Working of a bridge rectifier

 we can see positive and negative half cycles of input sinewave are distinguished by orange and green colors. Different colors explain the working of a full wave bridge rectifier and better demonstrate the current flow concerning time and input waveform.  normally shown circuit is used with a domestic electric supply which is 110V/220V AC 60Hz/50Hz pure sinewave. So, this article explained the working of bridge rectifiers with relevant waveform or pure sinewave. In future and advanced topics, we may describe working on a full wave bridge rectifier with other types of input waveform signals. But here we will only discuss sinewave input. The calculation of rectified output with sinewave input is different than other waveform types like square and sawtooth waveform signals. Our AC power sources follow the sinewave so it is standard to study with a sinewave input supply. The diodes D1 to D4 are connected in series with only two diodes forward biased during each half cycle. During the positive half cycle of the supply, diodes D2 and D4 in series forward biased while diodes D1 and D3 are reverse biased and the current flows through the RL or load resistance as described in the figure

Bridge Rectifier two diodes are conducting in positive input cycle.
Fig:4 Bridge rectifier response to Positive half cycle

. During the negative half cycle of the supply, diodes D1 and D3 are in series forward biased while diodes D2 and D4 are reverse biased.

upon negative input cycle two diodes conducts to deliver single polarity voltage at load terminals.
Fig:5 Negative half cycle response of Bridge rectifier

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Full wave bridge rectifier with filter capacitor 

A capacitor is connected across the load. capacitor continuously charges with rectifier output voltage variations. at each peak, the capacitor is charged to that level. when output voltage decreases capacitor starts to discharge current to load. during each period, the capacitor delivers its charged voltage to the load. this voltage also decreases continuously which depends on load resistance and capacitance of the filter capacitor. for high currents and low frequency, high capacitance is required to acquire smooth DC.

bridge rectifier with filter capacitor.
Fig:6 Full wave bridge rectifier with smoothing capacitor

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Bridge rectifier with LC Pi Filter

In figure 7 filter capacitors C1 and C2 are used to store electric charge and provide backup to load when the rectifier output voltage is low. thus these capacitors dampen pulsating DC voltage. An inductor blocks high-frequency noise and attenuates the ripple voltage. After attenuation of the AC component by the inductor, C2 flattens the output waveform.

Bridge rectifier output filtration with LC pi filter. Two capacitors and one inductor.
Fig:7 LC Pi filter to smoothen Rectifier output

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Bridge circuit output filtration with Common Mode Choke 

in a pulsating DC power, AC components always have di/dt equal to a non-zero value. So at the choke or inductor terminal, a L*di/dt AC voltage is induced. in common mode choke voltage induced, across both coils are equal due to equal inductance. but resultant magnetic flux is summed up and a mutual inductance develops a very high inductive reactance against AC voltage. DC voltage is not faced by inductive reactance or high impedance. in figure 8 C1 smooths the pulsating voltage. after passing through a common mode choke, attenuated ripple voltage is further smoothened by a C2 capacitor. common mode choke suppresses high-frequency noise produced by EMI interference. The filter capacitor also absorbs load current transients. current transitions from the load side also induce high-frequency noise. without filtration, noise radiates and injects into a power line. common mode choke also prevents those noise signals to travels the rectifier side.

Common mode choke filter to eliminate EMI interference and smoothen DC output.
Fig:8 Bridge rectifier output filtration with common mode choke and capacitors

      

Fig:8A Capacitor filter , common mode choke and LC Pie filter comparison in full wave rectifier

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How to check a bridge rectifier with a digital multimeter

If output DC voltage is abnormal may be a fault of the bridge rectifier. to Check if the bridge rectifier is not faulty, use a digital multimeter followed by five steps for each diode. If any diode is found faulty, Rectifier will not provide DC output. may pass AC components or may short input source. Finally may damage the input transformer or AC source or destroy the sensitive load circuits. The following steps should proceed for each diode in the bridge.

  1. Separate or unsold bridge rectifier from circuit
  2. Select DMM Diode or continuity function with a healthy battery installed
  3. Pin the DMM’s Positive probe to one AC~ input of Bridge and negative probe to rectifier’s Positive output. if the multimeter reading is between 0.5V~0.9V then the concerned diode’s forward biasing is normal. this voltage is the forward voltage drop at silicon diode terminals. in the case of germanium, it may vary from 0.2V to 0.4V depending on the DMM test current and environmental temperature. if DMM beep or shows OL then the diode is short or open circuit respectively.
  4. Repeat step 3 with reverse probes on the same bridge’s pins. the meter reading should be OL if the diode is not faulty. if the diode shows short or DMM beeps then the diode is faulty. it can be checked with the DMM OHM function to check diode leakage resistance if any.
  5. If all four diodes pass steps 3 & 4 Tests then Bridge is fine. check PCB traces, filter capacitor regulator, and transformer also.
  6. If one or more of four diodes is found not fulfilled in steps 3&4 then, the rectifier has to be replaced.
Red Probe to diode's anode and Black probe to diode's cathode terminal shows 0.684V for a normal forward bias junction barrier voltage
Fig:9 Forward Bias Test of a diode in bridge rectifier with Digital Multimeter
Red Probe to diode's cathode and Black probe to diode's anode terminal shows Over-Load for a normal reverse bias open circuit
Fig:10 Reverse Bias Test of a normal diode in bridge rectifier with Digital Multimeter
Faulty Open circuit diode in forward bias DMM reading is OL. PN Junction is broken or disconnected.
Fig:11 Faulty Diode is open in forward bias
Faulty short circuit diode in reversed bias DMM beeps and reading is 00V . PN Junction burnt.
Fig:12 Faulty Short Circuit Bridge rectifier diode

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Losses of a bridge rectifier

Diode junction barrier voltage is a major loss of rectifier circuits. Power losses are directly proportional to load current. Diode Forward voltage varies between a small voltage span or remains nearly constant.

Diode Power dissipation (W) = Forward Voltage x Current (Ampere RMS)

All the Diode power dissipation converts into heat. each diode in the rectifier heat upon load current rises. in a bridge rectifier, the Forward voltage drop is double due to two diodes being forward biased at any time in series with the load. so that bridge rectifier diode loss is double that of center tap rectifier.

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Heat removal from diodes or bridge rectifier

In four independent diode configurations, each diode’s thermal radiation is separate from the other. Each diode needs a separate heat sink or cooling mechanism. in this configuration thermal radiation is not concentrated like Bridge rectifier in a single package. in a single package, all diodes are not thermally isolated. so, 4 diodes simultaneously radiate heat from a congested place. Temperature goes up more rapidly than in a separate diode circuit. So in these situations, aluminum heat sinks are used to keep cool the bridge rectifier. The bridge rectifier needs more °C/W heat sinks to keep safe diode’s PN junction from melting down. For load current in hundreds of amperes, air ventilation is used to transfer heat from the heat sink. The fan is used for this purpose.

High Current Bridge Rectifier outer body is metallic to exhaust heat due to diode voltage drop
Fig:13 Metallic Case High current Bridge Rectifier

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Center tapped transformer rectifier using two diodes

Center tapped transformer has an extra wiretapped from mid of secondary coil winding. reference to this terminal transformer has two secondary coils with equal inductance value or number of turns. Both Secondary outputs voltages are reversed polarity from each other. In Fig 14 Anodes of D2 and D3 are connected to secondary outputs. and cathodes are connected to load and deliver positive supply. Center tapped terminal is used to provide a returning path to load current. or exhibit as a negative supply terminal or ground. Thus, the center tap transformer converts single-phase AC Voltage into Bi-Phase AC voltage with a 180-degree phase difference.

Full Wave Rectifier constructed with half bridge or two diodes negative output is obtained by tapping from transformer's secondary coil
Fig:14 Center tapped transformer FW rectifier with two diodes

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Working of center tap rectifier               

 in fig 15 positive half cycle of the single-phase sine wave is applied to the input of the center-tapped transformer. The upper coil terminal is positive regarding GND or center tap terminal on the secondary side. D1 is forward biased and provides a positive voltage to RL and RL current is returned to the secondary coil from the center tap terminal. the center tap is working as a negative supply terminal. at this time lower secondary coil terminal voltage is negative so the D2 diode is reversed biased and greyed in figure 15. And D2 is preventing negative supply voltage from injecting into positive output.

Center Tap transformer rectifier operation in positive half cycle. half-bridge rectifier positive half cycle
Fig 15 Center Tapped transform rectifier operation in positive half cycle

After the ending of the positive half cycle. the polarity of input supply voltage is reversed as shown in fig 16. Now voltage on upper sec terminal is negative and at lower terminal voltage is positive with ground reference. So D2 is now reverse biased. And act as a barrier to protect the load from the negative supply voltage. and D2 is forward bias and supplies positive voltage to load from the lower secondary terminal. The center tap connection is working as a negative supply path.

Center Tapped half bridge rectifier operation in negative half cycle. half-bridge rectifier negative cycle.
Fig 16 Center Tapped transform rectifier operation in negative half cycle

in both figures, 16 output waveform is given. Both sinewave cycles are located on the positive side without changing the sinewave shape. we desire a smooth DC output. To remove these bumps filter circuit is used. The filter may consist of a single capacitor or multiple components like choke and inductors.

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Center tap rectifier with filter capacitor

like bridge rectifier filter circuit center tap rectifier output may be smoothened by same circuits. Different filter configurations may be used to acquire a plane DC line on the graph. In figure 17 between rectifier output and load terminal, an LC pie filter circuit is applied. in this filter, capacitors store electrical energy and deliver it when the rectifier output voltage is decreasing. inductor blocks high frequencies noise and attenuates 100 Hz ripple voltage. so overall pie filter works better than a single capacitor filter. for sensitive loads common mode choke is used to remove spikes and external Electromagnetic interference. in Figure 8A effect of filters on full wave rectifier waveform is given. the chart is also applicable to the center tap rectifier. the only difference is the single diode forward bias voltage.

Center tap full wave rectifier with pie filter capacitor.
Fig 17 Center tap full wave rectifier with filter

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Dual DC output with Center Tapped Full bridge rectifier

Some analog circuits require positive and negative DC supply voltage to operate. like operational amplifiers, Analog to Digital converts, and Digital to analog converter circuits. so two DC sources in the series qualify for this requirement. with a small modification in the transformer bridge rectifier circuit, we can achieve our goal. in figure 18 a center tap transformer’s secondary coil is connected to the bridge rectifier. transform tapped terminal is used as a dual supply’s common output. which is act as a negative terminal for a positive supply, and also acts as a positive terminal for a negative supply. from figures 18 and 19 we can learn that for each cycle, two diodes conduct to give current path towards positive and negative supply loads according to input voltage polarity. positive voltage is demonstrated with orange color and negative with green color. D1 and D3 are reverse biased in the positive half cycle and D2 and D4 are forward biased. the grey color shows diode is reverse biased. D4 is conducting positive voltage to Positive load and current is returned from transformer center tapped terminal. and D3 is passing negative supply voltage to the negative supply load, transformer center tapped terminal is also returning path for the negative supply load current.

dual output bridge rectifier supply with filter capacitors. plus minus output rectifier. working in Positive half cycle
Fig 18 dual supply bridge rectifier input positive half cycle
dual output bridge rectifier supply with filter capacitors. Positive and negative output  bridge rectifier circuit. working in Negative half cycle
Fig 19 Dual supply full wave bridge rectifier operation in negative half cycle

When a negative half cycle is introduced to bridge rectifier D1 and D3 conduct, D2 and D4 remain reverse biased. D1 supplies negative voltage to the negative output. D3 supplies positive voltage to positive output. both outputs is equipped with filter capacitor to acquire a smooth DC voltage.

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Comparison of Center tap rectifier and bridge rectifier

Center tapped transformer has two secondary coils in each half cycle of the input waveform. only one coil provides supply current. A transformer with one extra secondary coil makes it heavier and more costly. However only one diode is in load series, Diode forward voltage loss is half that of the bridge rectifier. Center tapped transformer may be used in double output supply. Center tapped transformer secondary coils supply current consecutively and each coil shares the current burden.

Bridge rectifier does not need center tapped transformer. four diodes is disadvantage and major disadvantage of bridge rectifier is double the forward voltage losses as compare to center tapped rectifier supplies. Number of diodes are four is not an issue as bridge rectifiers are widely available in one package with proper mounting and compatibility with heat sinks.

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Advantages of Full Wave rectifier over half wave rectifier

half wave rectifier always skips one-half cycle. so its power efficiency is half that of full wave rectifier power efficiency. full wave rectifier output ripple frequency is double the input frequency. in the case of a half-wave rectifier, the ripple frequency is equal to the input frequency. Due to the double ripple frequency, the half-wave rectifier needs a big filter capacitor. half-wave rectifiers are not used fully for supply-sensitive electronic circuits. how ever simple resistive loads with reduced duty cycle are a major application of single diode rectifiers.

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