Control of Rectified Direct Current Using Low Series Capacitance

Control of Rectified Direct Current Using Low Series Capacitance Parantap Nandi, Department of Electrical Engineering Ideal Institute Of Engineering, Kalyani, West Bengal University of Technology, West Bengal, India. Abstract: A capacitor does not allow passage of D.C. Hence it can not be used in series with a circuit to control D.C current. At first the capacitor behaves as a short circuit and allows full current. But once it is fully charged, it blocks the current. This decay is exponential. In a high voltage D.C supply obtained using a rectifier-filter arrangement, current can thus only be controlled by a resistor in series. But this leads to the loss of power in the additional resistor. Also all resistors are not applicable. For e.g. carbon resistors can t be used to regulate current in case of 220V supply because they get burnt out very easily. Hence high voltage can not be harnessed in industrial applications like electrolysis of molten salts. This is because such a high voltage will cause a highly vigorous reaction causing violent sparks. As a result the reaction will go beyond control. But experiments show that a suitable capacitor put in series between an A.C supply and a bridge rectifier can regulate current. This circuit can provide a constant D.C current at high voltage. Also as a capacitor is used, power loss is minimal. Hence it can electrolyze solutions at a controlled rate. Keywords: Capacitor; A.C; D.C; Power I. INTRODUCTION Unlike A.C, D.C has fixed polarity. If a D.C supply is connected to a resistor and a capacitor in series, the current decays exponentially. Initially the capacitor is uncharged. So it behaves as a short circuit. When it is fully charged, it behaves like an open switch. Hence it can not be used to control current in a D.C circuit. The only way (to regulate current) is to put an additional series resistor of suitable rating. Carbon resistors are unsuitable for this purpose. They get burnt. Also the additional series resistor causes power loss. This is given by P=I 2 R. Inductors can not control D.C. Electrolysis of aqueous solutions and molten salts require application of direct current. If high voltage is applied, for instance220v using a rectifier to molten sodium chloride solution, violent sparks are caused. The reaction becomes so vigorous that it can t be controlled. This will be definitely hazardous. Hence low voltage must be used ranging between 6-12V. This will be available either from a lead acid battery or a step down transformer. Both are highly expensive. Instead of a battery if rectified D.C is connected to a capacitor and the electrolyte in series, initially electrolysis will occur, but it will last only for a few seconds. Once the capacitor is charged the circuit will become open. On the other hand A.C can be passed through a capacitor. This is because A.C alters its polarity. So the capacitor can be repeatedly charged and discharged. But this is no good for electrolysis. But capacitor has an advantage. It causes almost no power loss. This is because power is given by P=VICosф; where V=Voltage, I= Current, ф= Phase angle. As for a capacitor ф=90, Cos ф=0. Therefore P=0. If A.C current is controlled using a capacitor, then it is converted to D.C using a bridge rectifier, and load is connected across the rectifier arrangement, high voltage D.C can be controlled. This circuit can also serve the purpose of electrolysis. In this paper the characteristics of such a circuit have been discussed based on experimental observations. A. Apparatus and Materials: i. 4 IN4007 diodes (for bridge rectifier). ii. 220V A.C supply. iii. Incandescent lamps (Rating 250V 40W& 250V 60W). iv. A strong electrolyte solution e.g. NaOH (Concentrated solution is preferable). v. A.C capacitor rated 2.5µF 440V (4). vi. Electrolyte capacitor rated 22µF 400V, 10µF 250V, 1µF 160V. vii. Ceramic capacitor rated 474pF 400V (3). viii. ix. Digital multi meter having good precision. Good quality thick Cu wires having proper insulation. II. EXPERIMENTAL At first a simple bridge rectifier was constructed using 4 diodes. It was connected to a 220V A.C, 50Hz supply. No series capacitor was used. The output voltage at no load was 218V. It was connected to an incandescent Available Online@ 98

lamp rated 250V 40W. The lamp was chosen as the load because it is cheap, has constant resistance and does not burn out. It glowed with full brightness. The voltage across the lamp was 218V i.e. same as no load voltage. Now the lamp was replaced by a concentrated solution of NaOH because it is a strong electrolyte. Violent golden yellow sparks were observed along with large heat production. Current in the circuit was about 2A. Very soon it reached 2.5A. This is due to the fact that NaOH is an electrolyte and resistance of electrolytes fall with rise in temperature leading to an increase in current. As current increases more heat is produced and resistance drops further. This process goes on and soon the reaction goes beyond control. If a resistor is connected in series it wastes a lot of power. Inductors are very expensive as they require Cu winding. When an ordinary choke (rated 240V, P.F=.5) is connected in series it does not give very effective regulation of current. It requires a much larger one for the purpose. More over the inductor gets heated up. So in this case also there is loss of power. Before starting the experiments the characteristics of the specified diode and lamp were determined by making the lamp glow using a half wave rectifier circuit. The following criteria were noticed: A. A.C supply voltage was 218V. B. Voltage drop across the lamp was 100V. C. Voltage drop across the diode was 100V. D. D.C current was 123±.5mA Similarly a series RC circuit was set up using a 2.5µF capacitor and the 40W incandescent lamp. The recordings are given under: V C =195V V R =125V Cosф=.539 Ф=57.384 A circuit similar to figure 1 used for electrolysis and it gave excellent results. But for convenience we study the circuit characteristics using an incandescent lamp instead of an electrolyte. The reasons for using a lamp are: a. The filament is made of tungsten which has a constant resistance i.e. it does not vary with temperature. b. The filament can withstand high temperature and does not burn out. A circuit was set up as depicted in the diagram below. Figure 1: Circuit Diagram Diagram specifications: A. The circle indicates 220V A.C supply. B. The capacitor connected in series is strictly a capacitor suitable for A.C circuits. This works best for a capacitor rated 2.5µF 440V used in ceiling fans. C. The lines with arrows indicate diodes. D. The points marked D and C represent D.C output. E. The capacitor connected in parallel to the bulb can be any capacitor but for electrolyte capacitor, correct polarity should be maintained. F. The switch is optional. The capacitance connected in series was varied by connecting identical capacitors in series and in parallel. Electrolytic capacitors are totally unsuitable for the purpose. This is because they explode if such a connection is made. The capacitor should have high voltage rating (above 400V) and low capacitance. Capacitive reactance is given by X C =1/2πfC. Hence lower the capacitance, higher the reactance. For a 2.5µF capacitor at 50Hz, X C 1274Ω. If two such capacitors are connected in series the reactance becomes 2548Ω. The incandescent lamp acts as the D.C load connected to the output terminals of the bridge rectifier. The resistance of the bulb is constant given by R=V 2 /P=250 2 /40=1562.5Ω.The switch indicates that Available Online@ 99

capacitors were connected to the load in parallel for smoothening of the output. As 250V A.C was not available, the bulb could not be operated under rated conditions. The supply voltage was 218V as recorded by a multi meter. So at this juncture the power dissipated should be P=V 2 /R=2182/1562.5=30.42W. If a bridge rectifier is operated normally i.e. without a series capacitor, open circuit voltage in the network is 218V and when the lamp is connected, the voltage drop across the lamp remains the same. So there is full power dissipation (30.42W). This occurs because load is not too high. D.C current drawn by the load at this position was 200mA. When the circuit shown in figure 1 is used both the current and voltage decreases. This circuit was tasted with two standard loads i.e. two incandescent lamps one of 40W and the other 60W. When the experiments were over the circuit was tasted with concentrated NaOH solution as the load so as to check whether the circuit was suitable for electrolysis or not. Since supply is A.C and a capacitor is connected in series, the circuit must have a leading power factor (as the circuit is capacitive) less than unity. So the A.C voltage drop across the capacitor and the A.C terminals of the rectifier were recorded and power factor was calculated. This is discussed under the next section. A systematic recording of the voltage drops across the loads as well as currents in the circuits was tabulated. III. OBSERVATIONS When a simple bridge rectifier circuit is constructed (without any capacitor in series) the D.C voltage at no load is 218 V. When a standard load of 40W is connected, the voltage remains 218V. When a capacitor is connected in parallel across the load as filter, the D.C voltage across the load increases. The current in the circuit also increases. This occurs both for D.C current through load and total A.C current in the circuit. The following graph shows the increase in A.C current in the circuit with increase in parallel capacitance across the load. This is obtained when a standard load of 40W is used as load. When the circuit given in figure no.1 is used the current graph shows the following characteristics: The A.C voltage across the load also shows the same characteristics i.e. it decreases with increase in parallel capacitance. This graph was obtained using series capacitor of 1422pF**. **1422pF was obtained by connecting three capacitors of 474pF in parallel. Without any series capacitor the A.C voltage (ripple across the load was plotted as: Available Online@ 100

** For any other series capacitor of small capacitance (2.5µF, 1.25µF e.t.c.), the graph shows same characteristics. The table was tabulated for capacitances in the range of µf only to prove that any such circuit behaves in a similar manner. For the purpose of electrolysis, even capacitances in the range of micro Farads may provide low reactance as a result of which current may not get controlled very effectively. So capacitances in the range of pf were tried. At first the results were tabulated with a standard resistor as the load i.e. a bulb. The table is given below: The variation of D.C voltage across load is given in the table below: Table No 1: Variation of D.C Voltage across Load ****Three capacitors 474pF connected in parallel to get a total capacitance of 1422pF. Just like voltage D.C current also varies with increase in parallel capacitance connected across the load i.e. it decreases. A table is given: ***5µF capacitance was obtained by connecting two 2.5µF capacitors in parallel. To illustrate the drop in voltage graphically the experiment was carried out using series capacitor of1422pf and a load of 60W. Capacitance in series Capacitance connected to load in parallel. D.C current ( I D.C ) 2.5µF 0 151.6mA 2.5µF 2.5µF 138.4mA 2.5µF 5µF 134.3mA 2.5µF 22µF 132±mA 1.25µF 0 86.1mA 1.25µF 2.5µF 83.7mA 1.25µF 5µF 82.5mA ***The previous table was constructed for a series capacitor in the range of pf. This is for µf. Regulation decreases with increase in value of capacitance connected across load for a normal rectifier circuit. Regulation= (V D.C.O -V D.C )/V D.C. Here V D.C.O =Open circuit voltage & V D.C =Voltage under Available Online@ 101

load. So the circuit gets better. This is evident from the graph given under: found to remain constant and of a moderate value. The graph is given under: From this graph it is evident that a 2.5µF capacitor is enough to bring the regulation value to zero. So it is very easy to get zero regulation. But in case of the circuit given in the figure, regulation increases. A graph for such a circuit is given below: Here it is noted that initial current is 91.4mA and final current is 91.3mA. Also the least value is 90mA. There is very little deviation from maximum value. Hence it is quite stable. The voltage across the solution was also plotted. It is given as: So from this point of view, this circuit is not quite suitable for purposes other than electrolysis. For electrolysis both A.C as well as D.C components can be reduced by connection of capacitors. So it proves very effective. Now 100ml. of NaOH was used as the load and three 474pF capacitors connected in parallel were used to provide an overall series capacitance of 1422pF. As NaOH is a strong electrolyte normally the current increases very rapidly on application of high voltage across the electrolyte. But in this case the current was The voltage curve is almost a straight line parallel to the time axis. A rough calculation yields the following results: Average current i.e. I avg =90.88mA Time (t) =10 minutes=3600s Total quantity of charge delivered, Q=It=327.168C Available Online@ 102

So total amount of H 2 liberated= (327.168/96500)*.5=1.695*10-3 moles 38ml. H 2 at S.T.P The corresponding graph is To get this quantity of the gas, the D.C power wasted is P=VI=4.76*90.88*10* -3 =432mW (This calculation was done using average values of voltage and current). When the same experiment is carried out using a 9V 1A transformer and a bridge rectifier the current is 150mA and the D.C voltage across the solution is 8.6V. So the current to voltage ratio is.017 (considering magnitude only). But in case of the above circuit it is.019. In electrolysis magnitude is more important than voltage because it is a charge dependent phenomenon (Q I). So this circuit is suitable for electrolysis. As for the power factor the formula was used: Cosф=V R / (V C 2 +V R 2 ) where V C = Voltage across the capacitor put in series, V R = Voltage across the A.C terminals of the rectifier. The table for lamp as the load and a 2.5µF capacitor in series is given under: Capacitanc e in parallel A.C Voltage across capacito r in series A.C Voltage across rectifier termina l Cosф 0 195V 133V 0.563 55.944 2.5µF 160V 105V 0.548 56.77 5µF 160V 96V 0.514 59.069 7.5µF 160V 90V 0.49 60.659 10µF 190V 90.1V 0.428 64.633 12.5µF 190V 89.7V 4 0.426 9 15µF 190V 88.7V 0.423 17.5µF 190V 88.3V 18.5µF 195V 89.5V 0.421 4 0.417 1 22µF 195V 85V 0.4 ф 64.729 64.975 65.076 65.348 66.421 The Table with NaOH solution as the load and a series capacitance of 474pF is given: Capacitance in parallel A.C Voltage across capacitor in series A.C Voltage across rectifier terminal Cosф 0 225V 22.5V 0.099 84.318 2.5µF 225V 20.5V 0.0907 84.796 5µF 225V 17.6V 0.0779 85.532 7.5µF 225V 19.6V 0.0867 85.026 10µF 225V 17.6V 0.0779 85.532 12.5µF 230V 3.6V 0.0156 89.1061 15µF 230V 3.6V 0.0156 89.1061 17.5µF 230V 3.5V 0.0152 89.1290 20µF 230V 3.5V 0.0152 89.1290 22µF 225V 16V 0.0709 85.934 The corresponding graph is given under: ф Available Online@ 103

**From the table it is clear that the voltages show irregularities. But on the whole it can be concluded that power factor for such a circuit decreases with increase in capacitance put in parallel across the load. IV. RESULTS AND DISCUSSIONS From the above graphs, tables and observations it is clear that a capacitor of suitable rating can be used to regulate amount of current and voltage obtained from a bridge rectifier circuit. The capacitor must have very small capacitance. The results have been summarized as follows: a. A capacitor of very small capacitance can be used in a bridge rectifier circuit to control D.C current through the load. b. The capacitor must be an A.C capacitor with high voltage and low capacitance rating. Electrolyte capacitors are totally unsuitable for the purpose. c. During filtering of the out put voltage obtained from the rectifier, The D.C voltage across load and the current through load both increase with increase in the parallel capacitance connected across the load under normal conditions i.e. when a capacitor is not connected in series with the A.C supply. So the graphs have a positive slope. But when a capacitor is connected in series with the supply, the current through the load and the voltage across the load start decreasing with increase in capacitance connected in parallel with the load. Thus the graphs in this case show a negative slope. d. Not only the D.C components but also the ripple components behave in the same way. e. For temperature dependent resistances, e.g. electrolytes the capacitor connected in series should be in the range of pico Farads. In this case two or three capacitors may be connected in parallel to get a desired current. f. For a capacitor, Cosф 0. Therefore power loss in the capacitor is almost zero. This is unlike resistors and inductors where power is wasted considerably. g. This circuit allows the electrolysis of solutions where low currents close to 900mA can produce desirable effects. For e.g. the electrolysis of alkaline water can be done very effectively. h. During electrolysis voltage drop across the solution and current in the circuit remains constant unlike electrolysis using 220V supply directly i.e. without using series capacitor of low capacitance. i. Regulation of the circuit has a high value i.e. it is not good. j. For electrolysis the D.C voltage as well as the ripple can be reduced to a desired value by connecting capacitors in parallel across the load. k. Using capacitors in the range of pico Farads V D.C can be maintained close to 5V and current of about 91mA. This is highly favorable for H 2 and O 2 production using electrolysis. l. Power factor is very small and hence phase angle is high. Of course phase angle varies with load, capacitance in series, e.t.c. For electrolysis, where capacitance in the range of pico Farad is preferred power factor is very low and phase angle lies between 85to 90. CONCLUSION A capacitor does not allow passage of D.C. On the other hand if a resistor is used to control D.C there is power dissipation in it which is useless. Hence electrolysis can not be done using high voltage D.C (obtained from a rectifier). A bridge rectifier can however be connected to a capacitor in series (as shown in the diagram and current can be controlled. The capacitor consumes very little or no power. The capacitor should be suitable for A.C circuits and must have a low capacitance. Regulation of such a circuit increases with increase in capacitance put in parallel across the load. For a normal circuit regulation decreases with increase in capacitance put in parallel across the load. So this arrangement can be utilized for electrolysis. Alkaline water can be electrolyzed very efficiently. But for this the capacitance put in series with the bridge rectifier must be in the range of pf. This gives a D.C voltage of about 5V across the solution and a constant current of 91±1mA. It is enough for H 2 and O 2 production. The power factor of the circuit was calculated by recording the A.C voltage across the capacitor and the voltage across the A.C terminals of the bridge rectifier. The phase angle was found to be high. It depends on: a. The capacitance in series, i.e. lower the capacitances lower the value of Cosф. b. The value of capacitance put in parallel across the load, i.e. higher the capacitance lower the power factor. Available Online@ 104

Acknowledgements This work has been supported and financed by Mr. P.B Nandi, D.A.O (Grade 1) Public Health Engineering, West Bengal. I am also indebted to Mrs. K. Nandi for allowing me to make this research a success. References 1. The formulae for regulation and power factor have been taken from Basic Electrical & Electronics Engineering-1 by Abhijit Chakrabarti and Sudipta Nath, a publication by Tata McGraw Hill Education Private Limited. Available Online@ 105