Electronics I lab. DC Power Supply

Transcription

1 Electronics I lab DC Power Supply

2 Objective Design and construct circuits that transform sinusoidal (AC) voltages into constant (DC) voltages. Evaluate the performance of simple rectifier and regulator circuits. Theory: DC power supplies are the essential part of most of the electronic equipments since most of the equipments need DC power for operation. The power which is readily available everywhere is AC (in Gaza 220v/60Hz). DC power supplies convert this AC power into DC that can be used to operate most of our equipments. The main components of a DC power supply are rectifier diodes and capacitors. As you have seen in the last lab that rectifier diodes under normal condition can conduct only in the forward direction. This property of diodes is used to remove (or invert) the negative cycle of the AC voltage so that we have uni-directional voltage but not a constant one. Further, capacitors are used to smoot h out this varying unidirectional voltage and make it as smooth as possible, since capacitor tries to resist the change in its induced voltage, a suitable value of capacitor or a combination of capacitors are used to smooth out the change in the uni-directional voltage and make it constant. The output of the power supply is a fairly smoot h DC voltage. There are 4 main operat ions to mos t powe r supplies: Transfor mer (step do wn the voltage ) Rectification (convert AC to pulsating DC) Filtering (Smooth pulsating DC to reduce peaks and raise valleys) Regulation (Control output voltage level to make it robust against input voltage fluctuations and load changes)

3 Transformer : The power transformer provides separate windings to isolate the primary and secondary circuits. Isolation is necessary for safety considerations. Bizarre accidents are possible with non-isolated systems. A transformer also provides line voltage step down or up as needed by the DC power supply. Using a transformer, voltages can be transformed from one level to another with minimum power loss. All transformers have a voltage rating, frequency rating, and volt-ampere rating, which is the maximum power (real or imaginary) that the secondary can deliver to a load. This rating is a function of the physical size of the iron core. Ideally, the load is resistive and the volt-ampere rating then represents the power in watts. Rectification The rectification process uses diode s to pe rmit current from the transformer in one direction and block reverse current. The alternating current thus becomes unidirectional current known as DC. At this po int the current is in pulses rather than be ing continuo us and s moot h. But there is only one direction of the current thus it is DC.There are three types of rectifier circuits; half-wave, full-wave center-tap, and full-wave bridge. Although we can 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 soldered directly into a PCB circuit board or be connected by spade connectors. The image to the right shows a typical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead. The ot her two connecting leads are for the input alternating voltage from a transformer secondary winding.

4 The Smoothing Capacitor Pulsating DC voltages are generally not useful for electronics. It is preferred to have a relatively constant DC voltage. The filter is frequently just a large capacitance to smooth

5 the raw voltage fluctuations from the rectifier. More advanced designs will include a series inductor in addition to the capacitor for significantly improved filtering and also for improved efficiency in the rectification process. The filter capacitance is usually of the electrolytic type in order to keep the physical size to a minimum. The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage. Two important parameters to consider when choosing a suitable a capacitor are its Working Voltage, which must be higher tha n the no-load output value of the rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage. Too low a value and the capacitor has little effect. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mV peak to peak. Ripple factor : The ipe factor is an indication of the effectiveness of a filter and is defined as r V = V r ( pp ) DC where ( ) r pp V is the peak-to-peak ripple voltage and Vdc is the dc(average )value of the filter's output voltage.the lower the ripple factor the better the filter.the ripple factor can be lowered by increasing the value of the filter capacitor.

6 V V r ( pp ) p ( rect ) frlc DC 1 ( ) V 1 (1 ) V 2fR C l p ( rect ) Surge Current in Capacitor Input Filter When the power is first applied to a power supply, the filter capacitor is uncharged. At the instant the switch is closed, voltage is connected to the rectifier and the uncharged capacitor appears as a short, An initial surge of current is produced through the forward-biased diodes. It is possible that the surge current could destroy the diodes, for this reason a surge-limiting resistor R surge, is sometimes connected. The value of this resistor must be small to avoid a significant voltage drop across it. The diode must have a forward current rating that can ha ndle the momentary surge of current. This rating Is specified in the datasheet of the diode as I FSM R surge V = p (sec) 1.4 I FSM

7 Voltage Regulator While filers can reduce the ripple from power supplies to a low value, the most effective a approach is a combination of a capacitor input filter used with a voltage regulator.a voltage regulator is connected to the output of a filtered rectifier and maintains a constant output voltage despite changes in the input,the load current,or the temperature.the capacitor-input filter reduces the input ripple to the regulator to an acceptable level. Most regulators are integrated circuits and have three terminal an input terminal,an output terminal,and a reference terminal.the input to the regulator is first filtered with a capacitor to reduce the ripple to <10%.the regulator reduces the ripple to a negligible amount.in addition,most regulators have an internal voltage reference,short circuit protection. Some common voltages that these are available for include +5, +12,+15,-5,-12, and-15.three-terminal regulator s designed for fixed output voltages require only external capacitors to complete the regulation portion of the power supply.filtering is accomplished by a large value capacitor between the input voltage and ground.an output capacitor (typically.1 uf to 1uF)is connected from output to ground to improve the transient response.

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9 Worksheet Study the effect of the capacitor 1-Change the value of C1 :1n,100n,1u,50u,100u,470u. For every value compute the ripple factor and plot the output voltage D1 D2 V VOFF = 0 VAMPL = 17 FREQ = 60 V1 V+ C1 R1 D3 V- D4 100u 1k 0 Study the effect of regulator. Plot the output Note : regulator from OPAMP library D1 D2 U1 LM7805C 1 2 IN OUT GND VOFF = 0 VAMPL = 17 FREQ = 60 V1 C1 C2.33u 3 C3.1u R1 1k D3 D4 100u 0

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