Lab-report On Dc Power Supply.pdf

EXPERIMENT 1

POWER SUPPLY Objectives; 1. To observe the wave form of a half wave and full wave rectified alternating current. 2. To observe the effect of filter on the wave form and average value of dc output. Equipment; The equipment used for the experiment included; an ac generator, Multimeter, Cathode ray oscilloscope, A set for half wave rectification diode, A set of two diodes full-wave rectification equipment, Capacitor, Resistor and Centre-taped transformer.

Theory Electrical Rectification can be defined as the process of converting an alternating current into a direct current using sets of diodes otherwise known as rectifiers. There are two forms of rectifier circuits, that is, full-wave rectifier circuit and half wave rectifier circuit. In half wave rectification, a single diode connected in series with the alternating current power supply and the load through a transformer is used. The current is only conducted during a positive half cycle since the diode is forward biased while the negative half cycle is suppressed sine the diode becomes reversed biased. This makes the current to flow only in one direction through the load though after every single full-wave cycle. The reslting pulsating current can then be smoothen using filter circuit componets such as capacitors and inductors. The graph of input and the output voltage against time can be illustrated as shown; Before rectification;

After rectification

After filtering

The frequency of the out put is equal to the frequency of the input power though the negative parts are suppressed. Rectification efficiency can be given by; dc power out put

n = out put ac power =

2 I ( m ) ×RL

π Im 2 ( ) ×(rf +RL ) 2

× 100.

Where Im is the current through thesecondary winding of the transformer, RL the load resistance and rf the diode resistance. The maximum efficiency can be obtained when the resistance through the diode is reduced to zero. Therefore; nmax =

2 I ( m ) ×RL π

2 I ( m ) ×(RL )

× 100 = 40.6%.

2

Full wave rectifictation involve the flowing of current in one direction for both positive and negative half wave cycles. There are two means of achieving this that is centre taped full wave rectification and fullwave bridge rectification. In centre tapped full wave rectification, the circuit employs two diodes with a center tapped secondary winding transformer conectd as shown in the circuit diagrams so that for each half wave cyle one diode is forward biased while the other is reverse biased and both are connected to the load. This makes the load to receive both the half waves in one direction just as direct curent. In bridge rectification four diodes are arenged in a certain maner so that for a single haf wave cyle two diodes becme forward biased while the other two become reversed biased and the vise verser when the cycle changes the direction. Like in half wave rectification, the repals obtained can also be smothen by filters. The graph of voltage against time can be as shown; Before rectification

After rectification

After filtering

The out put frequency is double the inptut frequency since in every input cycle the output cyle repeats the same patern two times. Rectification efffiency is also give by; n=

power outpu power input

× 100 =

2 2I ( m ) ×RL

π Im 2 ( ) ×(rf +RL ) √2

× 100.

Where Im is the current through thesecondary winding of the transformer, RL the load resistance and rf the diode resistance. The maximum efficiency can be obtained when the resistance through the diode is reduced to zero. Therefore; nmax =

2 2I ( m ) ×RL

π Im 2 ( ) ×(RL ) √2

× 100 = 81.2%.

But it should be noted that this only applies when we assume that ressitance through the diodes is zero which can not be the case in reality. Though it may be reduced to a very low value

but it must exist and this explains the reason why two diodes fullwave rectifiaction is more efficient that bridge rectification. Procedure; i.

ii.

iii.

iv.

First, the input power supply was connected directly with the CRO and and the input voltage taken and recorded. It was then measured directly using multimeter and the reading recorded too. Half wave rectifier circuit was then connected and the out put voltage reading from the CRO and multimeter were taken and recorded, starting with when the capacitor surpassed and when capacitor was incoporated. Binary fullwave rectifier circuit was then connected instead of hafl wave rectifier. Both input and output voltage from both cro and multimeter were taken and recorded at first without capacitor and when capacitor was incoporated. For every stage the power supply and rectification output wave forms were drawn and the results were fed in the tables bellow.

Circcuit Diagram Results; i.

Half wave rectification; Input supply

Output supply without capacitor

Output supply with capacitor incoporated

7.6cm × 2v/cm =15.2 v

3.8cm × 2v/cm = 7.6 v

3.6cm × 5v/cm = 18 v

13.66 v

6.06 v

18.35 v

Power wave form Calculated voltage Measured voltage

ii.

Fullwave rectification; Input supply

Output supply without capacitor

Output supply with capacitor incoporated

Power wave form Calculated voltage Measured voltage

7.4cm × 2v/cm = 14.8 v = 13.6 v

1.8cm × 2v/cm = 3.6 v = 5.83 v

1.8cm × 5v/cm =9v = 8.68 v

Conclussion; It was confirmed that half wave rectifiction gives a power out put that though moves in one direction but the negative half cycles are suppressed. The resulting wave form are ripals equivalent to half cycle that comes as one after every single full wave cycle of the input. This can be illustrated as shown in the talbe above. Incoporetion of capacitor in the circuite resuted to a single horintal line on CRO screen thus confirming the filtering effect of capacitor. On the other hand, fullwave rectification produces a current that moves in one direction with both the negative and positive half wave cycles rectified. The resulting wave form is made of two positive ripals fro every single wave cycle of the input supply which is illustrated in the table above. Like in half wave rectification, the incopapration of capacitor reslted in a constant output supply that fromed a single straight line on the CRO out put.