M V Sainath

Simple Bridge Rectifier Circuit We mainly have two types of voltage types present that are widely used these days. They are alternating and direct voltage types. These voltage types can be converted from one type to another using special cirucits designed for that particular conversion. These conversions happen everywhere. Our main supply which we get from power grids are alternating in nature and the appliances we use in our homes generally require a small DC voltage. This process of converting alternating current into direct current is given the name rectification. Converting AC to DC is preceded by further process which can involve filtering, DC-DC conversion and so on. One of the most common part of an electronic power supply is a bridge rectifier. Many electronic circuits require rectified DC power supply for powering various electronic basic components from available AC mains supply. The simple bridge rectifier is used in a variety of electronic AC based power devices. Another way to look at the rectifier circuit is that, it can be said to convert currents instead of voltages. This makes more intuitive sense, because we are more accustomed to using current to define a component’s nature. Concisely, a rectifier take a current which has both negative and positive components and rectifies it such that only the positive component of the current remains. Bridge rectifiers are widely used in power supplies that provide necessary DC voltage for the electronic component or devices. The bridge rectifier can be constructed with switching devices. The most efficient switching devices whose characterstics are known fully are diodes. In theory any solid-state switch which can be controlled or cannot be controlled can be used instead of the diodes. Based on the type of rectification circuit does, the rectifiers are classified into two categories. • • Half wave rectifier Full wave rectifier Half wave rectifier only converts half of the AC wave into DC signal whereas Full wave rectifier converts complete AC signal into DC. Bridge rectifier is the most commonly used rectifier in electronics and this report will deal with the working and making of one. Simple bridge rectifier circuit is the most popular method for full wave rectification. Types of Bridge Rectifiers Bridge rectifiers are of many types and the basis for the classification can be many, to name a few, type of supply, bridge circuit’s configurations, controlling capability etc. Bridge rectifiers can be broadly classified into single and three phase rectifiers based on the type of input they work on. Both of these types include these further classifications which can be made into both single and three phase rectifiers. The further classification is based on the switching devices the rectifier uses and the types are uncontrolled, half controlled and full controlled rectifiers. Some of the types of rectifiers are discussed below. 1. Single Phase and Three Phase Rectifiers This classification is based on the type of input a rectifier works on. The naming is pretty straightforward. When the input is single phase, the rectifier is called a single phase rectifier and when the input is three phase, it is called a three phase rectifier. The single phase bridge rectifier consists of four diodes, whereas a three phase rectifier uses six diodes arranged in a particular fashion to get the desired output. These can be controlled or uncontrolled rectifiers depending on the switching components used in each rectifer such as diodes, thyristors, and so on. 2. Uncontrolled Bridge Rectifiers A rectifier uses switches to work. The switches can be of various types, broadly, controllable switches and uncontrollable switches. A diode is unidirectional device that allows the current flow in only one direction. The working of a diode is not controlled as it will conduct as long as it is forward biased. With a configuration of diodes in any given rectifier, the rectifier is not fully in the operators control, so these types of rectifiers are called uncontrolled rectifiers. It does not allow the power to vary depending on the load requirement. So this type of rectifier is commonly used in constant or fixed power supplies. 3. Controlled Bridge Rectifier The need for a controlled rectifier is apparent when we look into the shortcomings of uncontrolled bridge rectifier. To make an uncontrolled rectifier into a controlled one we use Icontrolled solid state devices such as SCRs, MOSFETs, IGBTs. We have the full control of when SCRs are switched ON or OFF based on the gate pulses we apply to it. These are generally more preferred than their uncontrolled counterparts. Circuit diagram Components Required 1. Transformer(6-0-6) 230VAC / 6VAC 2. 1N4007 diodes 3. Resistor 1k Transformer (6-0-6) It is a general-purpose chassis mounting mains transformer. Transformer has 240V primary windings and centre tapped second windings. The transformer acts as step down transformer reducing AC 240V to AC 6V. A transformer is an electronic device which is used to regulate AC voltages. A transformer can be designed to either step up or step down a voltage hence, the regulating function. The working of a transformer is simple, there are two windings which are closed placed to each other. Due to the alternating current through them, they produce magnetic field around them. The mutual induction interaction between the two coils is the reason why power transfer is possible in a transformer. This varying magnetic flux induces a varying electromagnetic force or voltage in the secondary winding. One of the most commonly used core for transformers is high permeability silicon steel. The steel has permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. A schematic diagram of the transformer is given below. 1N4007 Diode 1N4007 is a PN junction rectifier diode. These types of diodes allow only the flow of the electrical current in one direction. So, it can be used for the conversion of AC power to DC. 1N4007 has different real life applications, e.g. free-wheeling diodes applications, general purpose rectification of power supplies, inverters, converters etc. The pinout for the given Diode is given below. 1N4007 Diode Pin name Anode cathode Pin no. 1 2 Charge +ve -ve The diagram above shows the symbolic and the actual picture of the 1N4007. The understanding of any component of an electrical circuit is vastly improved when the electrical characteristics of that device is known. The electrical characteristics of the diode 1N4007 is tabulated below. 1N4007 electrical characteristics Parameter Forward voltage at 1.0 A Reverse current at 25°C Total capacitance at 1.0 MHz Maximum full load reverse current at 75° Average rectified forward current Peak repetitive reverse voltage The diode 1N4007 features are as follows: Values- Units V uA pF uA A V • • • Low leakage current Low forward voltage drop High forward surge capability This diode ha a lot of real life applications in embedded systems, a few of the major applications associated with the particular diode are given below: 1. 2. 3. 4. 5. 6. Converters For switching purposes in embedded systems Freewheeling diodes applications Inverters General power rectification of power supplies To avoid reverse current and protecting microcontrollers like Arduino or PIC microcontroller. Working Connect all the components correctly as shown in the circuit diagram. From the circuit diagram it is apparent that the diodes are connected in a particular fashion. This unique arrangement gives the converter its name. In bridge rectifier, voltage that is given as the input can be from any source. It can be from a transformer that is used to step up or down the voltage or it can be from the mains of our domestic power supply. In this article, we are using a 6-0-6 centre tapped transformer for providing AC voltage. In the first phase of working of the rectifier, during the positive half cycle, diodes D3-D2 get forward biased and conducts. Diodes D1-D4 gets reversed biased and do not conduct in this half cycle, acting as open switches. Thus, we get a positive half cycle at the output. Conversely, in the negative half cycle, diodes D1-D4 get forward biased, and start conducting whereas diodes D3-D2 gets reversed biased and do not conduct in this half cycle. Again, we get a positive half cycle at the output. At the end of the rectification process, the negative part of the AC current is converted into a positive cycle. The output from the rectifier is two half-positive pulses with the same frequency and magnitude as that of the input. In contrast to the working of a half-wave rectifier, the full bridge rectifier has another branch which allows it to conduct for the negative half of the voltage waveform which the half-bridge rectifier had no means of doing. So the average voltage at the output of the full bridge rectifier is double than that of the half-bridge rectifier. Although we use four individual power diodes to make a full wave bridge rectifier, pre-made bridge rectifier components are available “off-the-shelf” in a range of different voltage and current sizes that can be used directly to make a working circuit. The output voltage waveform after the rectification is not a proper DC, so we can try to make it more into a DC waveform using a capacitor for filtering purpose. Smoothing or reservoir capacitors that are connected in parallel with the load across the output of the full wave bridge rectifier circuit increases the average DC output level to the required average DC voltage at the output because the capacitor not only acts as a filtering component, but it also periodically charges and discharges effectively increasing the output voltage. Capacitor charge till the waveform goes to its peak and discharges uniformly into the load circuit when waveform starts going low. So when the output is going low, capacitor maintains the proper voltage supply into the load circuit, hence creating the DC. Advantages of a bridge rectifier: 1. Low ripples in the output DC signal 2. High rectifier efficiency 3. Low power loss Disadvantages of bridge rectifier: 1. Bridge rectifier is more complex than a half-wave rectifier 2. More power loss compared to centre tapped full wave rectifier.