Rectification circuit for a dc power supply

1. Introduction

Most of the electronic devices do not power on AC basis, and if so it will conduct a lot of power and will be damaged the first moment it operates. Thus a device that reduces the voltage and correspondingly smoothes the output voltage to become much more stable and reliable to use than the AC which is the DC is needed. A question may arise that asks why do not use batteries to supply a DC voltage. Well, it is a good it a good question, but batteries are expensive and does not handle large electronic devices. Imagine having a desktop on batteries and having to change its battery every half an hour or so, it is a nightmare. As a result, in this case, a linier DC power supply that operates from fixed (ex: at home) AC voltage is needed. A rectification circuit do all that automatically. It smoothes and filters the current and make it a better than a battery DC source.

The report paper will focus primarily on the technical aspects of the Rectification circuit used in the linear DC power supply. Other aspects than mentioned will not be included.

2. Discussion

2.1. Linear power supply

AnACpowered linear power supply usually uses atransformerto convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produceDC, arectifieris used. A capacitoris used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known asripple (Wiki, 2009).

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The voltage produced by an unregulated power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications alinear regulatorwill be used to stabilize and adjust the voltage. This regulator will also greatly reduce the ripple and noise in the output direct current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from over current (Wiki, 2009).

Figure 1 illustrates two common linear power supply circuits in current use. Both circuits employ full-wave rectification to reduce ripple voltage to capacitor C1. The bridge rectifier circuit has a simple transformer but current must flow through two diodes. The centre-tapped configuration is preferred for low output voltages since there is just on diode voltage drop. For 5V and 12V outputs, Schottky barrier diodes are commonly used since they have lower voltage drops than equivalently rated ultra-fast types, which further increase power conversion efficiency. However, each diode must withstand twice the reverse voltage that a diode sees in a full-wave bridge for the same input voltage.

The linear voltage regulator behaves as a variable resistance between the input and the output as it provides the precise output voltage. One of the limitations to the efficiency of this circuit is due to the fact that the linear device must drop the difference in voltage between the input and output. Consequently the power dissipated by the linear device isVi-VoÃ- Io. While these supplies have many desirable characteristics, such as simplicity, low output ripple, excellent line and load regulation, fast response time to load or line changes and low EMI, they suffer from low efficiency and occupy large volumes. Switching power supplies are becoming popular because they offer better solutions to these problems (National, 2002).

2.2. Transformer

Atransformeris a device, figure 2, that transferselectrical energyfrom onecircuitto another throughinductively coupledconductors—the transformer’s coils. A varyingcurrentin the first orprimarywinding creates a varyingmagnetic fluxin the transformer’s core, and thus a varyingmagnetic fieldthrough the secondarywinding. This varying magnetic fieldinducesa varyingelectromotive force (EMF)or “voltage” in the secondary winding. This effect is called mutual induction.

If aloadis connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:

Step down transformers convert electrical voltage from one level or phase configuration usually down to a lower level. They can include features for electrical isolation, power distribution, and control and instrumentation applications. Step down transformers typically rely on the principle of magnetic induction between coils to convert voltage and/or current levels.

Step down transformers are made from two or more coils of insulated wire wound around a core made of iron. When voltage is applied to one coil (frequently called the primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turn’s ratio of the two sets of windings determines the amount of voltage transformation.

An example of this would be: 100 turns on the primary and 50 turns on the secondary, a ratio of 2 to 1.

Step down transformers can be considered nothing more than a voltage ratio device.

With step down transformers the voltage ratio between primary and secondary will mirror the “turn’s ratio” (except for single phase smaller than 1 KVA which have compensated secondary’s). A practical application of this 2 to 1 turn’s ratio would be a 480 to 240 voltage step down. Note that if the input were 440 volts then the output would be 220 volts. The ratio between input and output voltage will stay constant. Transformers should not be operated at voltages higher than the nameplate rating, but may be operated at lower voltages than rated. Because of this it is possible to do some non-standard applications using standard transformers.

Single phase step down transformers 1 kva and larger may also be reverse connected to step-down or step-up voltages. (Note: single phase step up or step down transformers sized less than 1 KVA should not be reverse connected because the secondary windings have additional turns to overcome a voltage drop when the load is applied. If reverse connected, the output voltage will be less than desired).

2.3. Rectifier

Arectifieris an electrical device that convertsalternating current(AC) todirect current(DC), a process known asrectification. Rectifiers have many uses including as components of power suppliesand asdetectorsofradiosignals. Rectifiers may be made ofsolid statediodes,vacuum tubediodes,mercury arc valves, and other components (Wiki, Rectifier, 2009).

While half-wave and full-wave rectification suffice to deliver a form of DC output, neither produces constant-voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing circuit orfilter,is required. In its simplest form this can be just areservoir capacitoror smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of ACripplevoltage where the voltage is not completely smoothed. Sizing of the capacitor represents a trade-off. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases where many rectifiers are loaded onto a power distribution circuit, it may prove difficult for the power distribution authority to maintain a correctly shaped sinusoidal voltage curve (Wiki, Rectifier, 2009).

For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle (National, 2002). The load current and the supply frequency are generally outside the control of the designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design.

A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in very small power supplies. A full wave rectifier achieves two peaks per cycle and this is the best that can be done with single-phase input. For three-phase inputs a three-phase bridge will give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order (Wiki, Rectifier, 2009).

To further reduce this ripple, acapacitor-input filtercan be used. This complements the reservoir capacitor withinductor and a secondfilter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor.

A more usual alternative to a filter, and essential if the DC load is very demanding of a smooth supply voltage, is to follow the reservoir capacitor with avoltage regulator. The reservoir capacitor needs to be large enough to prevent the troughs of the ripple getting below the voltage the DC is being regulated to. The regulator serves both to remove the last of the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller reservoir capacitor (these can be large on high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common strategy. The extreme of this approach is to dispense with the reservoir capacitor altogether and put the rectified waveform straight into a choke-input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse, but instead the current delivery is spread over the entire cycle. The downside is that the voltage output is much lower – approximately the average of an AC half-cycle rather than the peak.

2.4. Regulator

The regulator is an electrical regulator designed to automatically maintain

3. The Proposed Circuit

* The proposed circuit shall have an input voltage equals to 220v.

* A centre-tapped step down transformer with 220v output and 12v output.

* 2 diodes (1N4001) working independently (note: duel regulated) as half-wave rectifiers.

* 2 capacitors (1000u) to smooth the signal.

* 2 voltage regulators (LM7815C) to regulate the coming voltages from the capacitors.

* 2 small values capacitor (10u) for final smoothing for the output voltage.

* The output voltage of 0-5v.

4. Conclusion

One of the best ways to output an efficient and reliable DC power supply is to use linear power supply, which uses a step-down transformer, a rectification circuit, a regulator and a filter.

The step-down transformer takes the voltage down from 220v to 12v, then the rectification circuit smoothes and filters it using capacitors and diodes, as a result, the output DC voltage would be 0-5v with a minimal ripple factor using the proposed circuit as mentioned.

Works Cited

National, S. C. (2002, September 1). Introduction to Power Supplies. Santa Clara, California, USA.

Wiki. (2009, December 10). Power supply. Retrieved December 14, 2009, from Wikipedia, the free encyclopedia: http://en.wikipedia.org/wiki/Power_supply

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Power supply. Retrieved December 15, 2009, from: http://www.kpsec.freeuk.com/powersup.htm

http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

Wiki. (2009, December 12). Rectifier. Retrieved December 15, 2009, from Wikipedia, the free encyclopedia: http://en.wikipedia.org/wiki/Voltage_regulator

AC to DC Cuircts. Retrieved December 16, 2009, from: http://www.discovercircuits.com/DJ-Circuits/acdc1.htm

Power supply. Retrieved December 15, 2009, from: http://www.trcelectronics.com/power-supply.shtml

Step-down transformers. Retrieved December 14, 2009, from: http://www.powertransformer.us/stepdowntransformers.htm

Step Down Transformers. Retrieved December 15, 2009, from: http://www.electricityforum.com/electrical-transformers/step-down-transformers.html