Linear power supply basics tutorial


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Linear power supply basics tutorial

  1. 1. Linear Power Supply Basics Tutorial - summary or tutorial about the basics of linear power supplies, their design, operation and advantages and disadvantages. In this section • Linear power supply basics • Shunt voltage regulator • Series voltage regulator Linear power supplies are widely used because of the advantages they offer in terms of overall performance. Linear power supplies are often used in exacting situations where the regulation and removal of noise is paramount. While linear power supplies may not be as efficient as other types of power supply technology, they offer the best performance and are therefore used in many applications where noise is of great importance. Often audio amplifiers and many other items of electronic equipment use linear power supplies to obtain the best performance. Linear power supply basics Linear power supplies gain their name from the fact that they use linear, i.e. non-switching techniques to regulate the voltage output from the power supply. The term linear power supply implies that the power supply is regulated to provide the correct voltage at the output. Sometimes the sensing of the voltage may be accomplished at the output terminals, or on some occasions it may be achieved directly at the load. In terms of the overall make-up of a linear power supply, it can be split into several blocks as detailed below. Block diagram of linear power supply architecture The main elements of the linear power supply are: • Input transformer: As many power supplies take their source power from an AC mains input, it is common for linear power supplies to have a step down or occasionally a step up transformer. This also serves to isolate the power supply from the mains input for safety. • Rectifier: As the input from an AC supply is alternating, this needs to be converted to a DC format. Various forms of rectifier circuit are available. Note on Diode Rectifiers: Diode rectifiers are an essential element within many circuits including power supplies and RF detectors. They convert the AC signal into DC. Various forms are available from single diode half wave rectifiers to multiple diode full-wave rectifiers. The type chose will depend upon the application and its requirements. Click for more information on Diode Rectifiers Even for DC powered regulators, a rectifier may be placed at the input to guard against inverse connection of the supply. • Smoothing: Once rectified from an AC signal, the DC needs to be smoothed to remove the varying voltage level. Large reservoir capacitors are used for this.
  2. 2. Note on PSU Smoothing Circuits: In order to remove the undulations on the voltage produced after a diode circuit has rectified an AC or RF signal, it is necessary to smooth the voltage. This is generally accomplished using a capacitor reservoir capacitor. Click for more information on PSU Smoothing Circuits • Linear regulator: Once a smoothed supply is available, this can then be applied to the linear regulator. This will provide a properly regulated output. Linear power supply regulators There are two main types of linear power supply: • Series regulator: This is the most widely used format for a linear power supply. As the name implies a series element is placed in the circuit, and its resistance varied via the control electronics to ensure that the correct output voltage is generated for the current taken. Read more about the Series regulator • Shunt regulator: The shunt regulator is less widely used as the main element within a voltage regulator. For this form of linear power supply, a variable element is placed across the load. There is a source resistor placed in series with the input, and the shunt regulator is varied to ensure that the voltage across the load remains constant. Read more about the Shunt regulator Both of these types linear regulator used in power supplies has its uses and can be used in different situations. Linear power supply advantages / disadvantages The use of any technology is often a careful balance of several advantages and disadvantages. This is true for linear power supplies which offer some distinct advantages, but also have their drawbacks. Linear PSU advantages • Low noise: The use of the linear technology without any switching element means that noise is kept to a minimum and the annoying spikes found in switching power supplies are now found. • Established technology: Linear power supplies have been in widespread use for many years and their technology is well established and understood. Linear PSU disadvantages • Efficiency: In view of the fact that a linear power supply uses linear technology, it is not particularly efficient. Efficiencies of around 50% are not uncommon, and under some conditions they may offer much lower levels. • Size: The use of linear technology means that the size of a linear power supply tends to be larger than other forms of power supply. • Heat dissipation: The use of a series or parallel (less common) regulating element means that significant amounts of heat are dissipated and this needs to be removed. Despite the disadvantages, linear power supply technology is still widely used, although it is more widely used where low noise and good regulation re needed. One typical application is for audio amplifiers. By Ian Poole Shunt Voltage Regulator
  3. 3. - overview and theory of the shunt regulator used in many linear power supply circuits. In this section • Linear power supply basics • Shunt voltage regulator • Series voltage regulator Although the shunt voltage regulator is not widely used to provide the main regulation in many applications, it nevertheless finds uses in many other areas of circuitry. A simple Zener diode circuit provides a prime example of a shunt voltage regulator. As such the shunt voltage regulator is an essential element within linear power supply technology. Shunt voltage regulator basics the basic operation of a shunt regulator can be seen from the diagram. Essentially the load is operated with a resistor in series with the voltage source and the shunt regulator then in parallel with the load. In order to keep the voltage across the load constant, a level of current must be drawn through the series resistor to maintain the required voltage across the load. The load will take some and the remaining current is drawn by the shunt voltage regulator. The circuit is designed so that at maximum load current the shunt regulator draws virtually no current and at minimum load current, the shunt voltage regulator passes the full current. As a result, it can be seen that shunt regulators are inefficient because maximum current is drawn from the source regardless of the load current, i.e. even when there is no load current. Zener diode shunt regulator One of the most common and simple forms of shunt regulator is the simple Zener diode regulator circuit shown below. Its operation is very straightforward. Once over its small minimum current, the Zener diode maintains an almost constant voltage across its terminals. The series resistor drops the voltage from the source to the Zener diode and load. As the Zener diode maintains its voltage, any variations in load current do not affect the voltage across the Zener diode. It takes up the current variations required to ensure the correct drop across the series resistor. Zener diode shunt regulator circuit In this shunt voltage regulator circuit, the Zener diode must be capable to dissipating the power from the maximum amount of current it is likely to handle. This is most likely to be a little more than the maximum current supplied to the load as the Zener diode will need to pass all the current when load current is zero. Thus the total maximum current that will be passed by the diode is the load current plus an allowance for current to maintain the reference voltage when the load is taking its maximum current.
  4. 4. It should also be noted that for the shunt regulator circuit, the series resistance is comprised of the series resistor value, plus any source resistance. In most cases the value of the series resistor will dominate and the source resistance can be ignored, but this may not always be the case. Shunt regulator with feedback loop The basic shunt voltage regulator above does not have any feedback, i.e. it runs in an open loop manner. As imagined, the performance of this form of shunt regulator is sufficient for many applications, but much higher levels of performance can be achieved by providing feedback based on the output voltage of the shunt voltage regulator and feeding this back into the system to ensure that the required output voltage is accurately maintained. Block diagram of shunt voltage regulator with feedback Using a shunt voltage regulator with feedback as shown above, the output voltage is sensed and the voltage compared to a reference. The level of the shunt current is then altered to return the output voltage to the required level. Full shunt regulators are not widely used because shunt regulators offer a low efficiency level. Series regulators are widely available and offer higher efficiency levels, although they are not as high as switch mode supplies. By Ian Poole Series Voltage Regulator (Series Pass Regulator) - overview and theory of the series regulator or series pass regulator used in many linear power supply circuits. In this section • Linear power supply basics • Shunt voltage regulator • Series voltage regulator The series voltage regulator format or as it is sometimes called the series pass regulator is the most commonly used format for providing the final voltage regulation in a linear voltage regulator circuit. As the name suggests, the series voltage regulator or series pass voltage regulator operates by using a variable element in series with the load. In this way a series voltage regulator provides an effective form of voltage regulation within a linear power supply.
  5. 5. Series voltage regulator basics The series voltage regulator or series pass voltage regulator uses a variable element placed in series with the load. By changing the resistance of the series element, the voltage dropped across it can be varied to ensure that the voltage across the load remains constant. The advantage of the series voltage regulator is that the amount of current drawn is effectively that used by the load, although some will be consumed by any circuitry associated with the regulator. Unlike the shunt regulator, the series regulator does not draw the full current even when the load does not require any current. As a result the series regulator is considerably more efficient. Concept of the series voltage regulator / series pass regulator Simple emitter follower voltage regulator One of the simplest implementations of this concept is to use a single pass transistor in the form of an emitter follower configuration, and a single Zener diode drive by a resistor from the unregulated supply. This provides a simple form of feedback system to ensure the Zener voltage is maintained at the output, albeit with a voltage reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor It is a simple matter to design a series pass voltage regulator circuit like this. Knowing the maximum current required by the load, it is possible to calculate the maximum emitter current. This is achieved by dividing the load current, i.e. transistor emitter current by the Β or hfe of the transistor. Simple emitter follower series pass regulator The Zener diode will generally need a minimum of around 10mA for a small Zener to keep its regulated voltage. The resistor should then be calculated to provide the base drive current and the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the current required. [ (Unregulated voltage - Zener voltage ) / current ]. A small margin should be added to the current to ensure that there is sufficient room for margin when the load, and hence the transistor base is taking the full current.
  6. 6. The power dissipation capacity for the Zener diode should be calculated for the case when the load current, and hence the base current is zero. In this case the Zener diode will need to take the full current passed by the series resistor. Series pass regulator with feedback In order to provide improved levels of performance it is possible to add a more sophisticated feedback network into the regulator circuit. Using feedback within a voltage regulator enables the output to be sampled, and compared with a stable reference voltage. The error is then used to correct the output voltage. In this way, a far higher level of performance can be obtained in terms of the required output voltage as well as ripple and spikes. Series pass voltage regulator with feedback It is possible to use a simple two transistor circuit for a series pass regulator with voltage sensing and feedback. Although it is quite straightforward to use an operational amplifier, which will provide higher levels of feedback, and hence better regulation, this two transistor circuit illustrates the principles well. Simple two transistor series pass regulator In this circuit TR1 forms the series pass transistor. The second transistor, TR2 acts as the comparator, feeding the error voltage between the reference diode and the sensed output voltage which is a proportion of the output voltage as set by the potentiometer. The resistor, R1 provides the current for the collector of TR2 and the reference diode ZD1. Voltage reference Any linear voltage regulator can only be as good as the voltage reference that is used as the basis of the comparison within the system. While a battery could in theory be used, this is not satisfactory for most applications. Instead Zener diode based references are almost universally used. Integrated circuit regulators and references use sophisticated on-chip combinations of transistors and resistors to obtain temperature compensated and precise voltage reference sources.
  7. 7. The voltage reference must be driven from the unregulated supply. It cannot be taken from the regulated output as there are start-up issues. At start-up there is no output and therefore the reference output will be zero and this will be maintained until the reference starts-up. Simplified reference source for series pass voltage regulator Often the output from the reference source is fed via a potential divider. Not only does this reduce the output voltage which is normally very useful, but it also allows a capacitor to be added to the output to help remove any ripple or noise that may be present. The reduced voltage is also useful because the minimum voltage output is governed by the reference voltage. Output sampling The simple emitter follower series voltage regulator circuit directly compared the output with the voltage reference. In this way the output voltage was equal to that of the reference, neglecting the base emitter voltage drop. However it is possible to sample a proportion of the output voltage and compare this to the reference. If this is done, then the output voltage becomes greater than the reference voltage as the negative feedback in the circuit fights to keep the two compared voltages the same. If for example the reference voltage is 5 volts and the sampling or potential divider provides 50% of the output voltage, then the output voltage will be maintained at 10 volts. Series pass voltage regulator with sampled output The potential division or sampling can be made variable, and in this way, the output voltage can be adjusted to the required value. Normally this method is only used for small adjustments as the minimum output level obtained by this method is an output equal to the reference voltage. It should be remembered that using a potential divider has the effect of reducing the feedback loop gain. This has the effect of reducing the loop gain and thereby reducing the regulation performance. Normally there is sufficient loop gain for this not to be a major problem except when only a very small proportion of the output is sampled. Care should also be taken not to increase the voltage of the output to a point where the regulator does not have sufficient drop across it to regulate the output voltage sufficiently. Low drop out series voltage regulators One of the considerations of any regulator is the voltage that must be placed across the series pass element. In addition to the drop across the regulator itself, there must be sufficient voltage to run the drive circuitry. In some circuits, a low drop out regulator is important - i.e. where the
  8. 8. level of voltage drop available across the series regulator element is limited. This minimum drop out voltage can be important and is often a specified parameter in many integrated regulator chips. While the circuits shown here are simple transistor circuits, the same principles are used in larger circuits and also within integrated circuits. The same series pass regulator concepts as well as the reference diode circuits, sampling and other areas all use the same elements. By Ian Poole Capacitor Smoothing Circuits & Calculations - notes, details & calculations for smoothing capacitor circuits used with rectifiers with details of ripple voltage and ripple current. In this section • Diode rectifier circuits • Half wave rectifier circuit • Full wave rectifier circuit • Two diode full wave rectifier • Bridge rectifier circuit • Rectifier capacitor smoothing circuit Rectifiers are normally used in circuits that require a steady voltage to be supplied. To provide a steady DC output. The raw rectified DC requires a smoothing capacitor circuit to enable the rectified DC to be smoothed so that it can be used to power electronics circuits without large levels of voltage variation. Capacitor smoothing basics The raw DC supplied by a rectifier on its own would consist of a series of half sine waves with the voltage varying between zero and √2 times the RMS voltage (ignoring any diode and other losses). A supply of this nature would not be of any use for powering circuits because any analogue circuits would have the huge level of ripple superimposed on the output, and any digital circuits would not function because the power would be removed every half cycle. To smooth the output of the rectifier a reservoir capacitor is used - placed across the output of the reciter and in parallel with the load.. This capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge. Smoothing action of a reservoir capacitor It should be remembered that the only way discharge path for the capacitor, apart from internal leakage is through the load to the rectifier / smoothing system. The diodes prevent backflow through the transformer, etc.. Smoothing capacitor value The choice of the capacitor value needs to fulfil a number of requirements. In the first case the value must be chosen so that its time constant is very much longer than the time interval between the successive peaks of the rectified waveform:
  9. 9. Rload * C >> 1 / f Where: Rload = the overall resistance of the load for the supply C = value of capacitor in Farads f = the ripple frequency - this will be twice the line frequency a full wave rectifier is used. Smoothing capacitor ripple voltage As there will always be some ripple on the output of a rectifier using a smoothing capacitor circuit, it is necessary to be able to estimate the approximate value. Over-specifying a capacitor too much will add extra cost, size and weight - under-specifying it will lead to poor performance. Peak to peak ripple for smoothed diode rectifier circuit The diagram above shows the ripple for a full wave rectifier with capacitor smoothing. If a half wave rectifier was used, then half the peaks would be missing and the ripple would be approximately twice the voltage. For cases where the ripple is small compared to the supply voltage - which is almost always the case - it is possible to calculate the ripple from a knowledge of the circuit conditions: Full wave rectifier Vripple = Iload / 2 f C Half wave rectifier Vripple = Iload / f C These equations provide more than sufficient accuracy. Although the capacitor discharge for a purely resistive load is exponential, the inaccuracy introduced by the linear approximation is very small for low values of ripple. It is also worth remembering that the input to a voltage regulator is not a purely resistive load but a constant current load. Finally, the tolerances of electrolytic capacitors used for rectifier smoothing circuits are large - ±20% at the very best, and this will mask any inaccuracies introduced by the assumptions in the equations. Ripple current Two of the major specifications of a capacitor are its capacitance and working voltage. However for applications where large levels of current may flow, as in the case of a rectifier smoothing capacitor, a third parameter is of importance - its maximum ripple current. The ripple current is not just equal to the supply current. There are two scenarios: • Capacitor discharge current: On the discharge cycle, the maximum current supplied by the capacitor occurs as the output from the rectifier circuit falls to zero. At this point all the current from the circuit is supplied by the capacitor. This is equal to the full current of the circuit.
  10. 10. • Capacitor charging current: On the charge cycle of the smoothing capacitor, the capacitor needs to replace all the lost charge, but it can only achieve this when the voltage from the rectifier exceeds that from the smoothing capacitor. This only occurs over a short period of the cycle. Consequently the current during this period is much higher. The large the capacitor, the better it reduces the ripple and the shorter the charge period. In view of the large currents involved, care must be taken to ensure that he ripple current does not exceed the rated values for the capacitor. By Ian Poole 1. linear power supply Presentation Transcript • A presentation of • LINEAR Block diagram and functions of a transformer, rectifier, POWER SUPPLY Types of rectifier, filter, voltage regulator and voltage divider. filter and regulator circuits A presentation of • Power For All electronic circuits need a power source to work. Supply electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific A battery is a common value. DC voltage source for some types of electronic equipment especially portables like cell Most non-portable equipment uses phones and iPods. power supplies that operate from the AC power line but produce one or more DC outputs. A presentation of • Power The input is the 120 volt 60 Hz AC Supply Characteristics power The power supply converts the AC line. into DC and provides one or more Some modern electronic circuits DC output voltages. need two or A good example of a modern power more different voltages. supply is the one inside a PC that furnishes 12, 5, 3.3 and 1.2 volts. A presentation of • Main circuits in most power supplies. Components of a Power Supply A presentation of
  11. 11. • Transformer A transformer is commonly used to step the input AC voltage level down or up. Most electronic circuits operate from voltages lower than the AC line voltage so the transformer normally steps the voltage down by For example, a transformer its turns ratio to a desired lower level. with a turns ratio of 10 to 1 would convert the 120 volt 60 Hz input sine wave into a 12 volt sine wave. A presentation of • The rectifier converts the AC sine wave Rectifier into a pulsating There are several forms of rectifiers used DC wave. but all are Rectifier types and operation will be made up of diodes. covered later. A presentation of • The Filter rectifier produces a DC output but it is pulsating rather than a constant steady value over time like that A filter is from a battery. used to remove the pulsations and create a The most constant output. common filter is a large capacitor. A presentation of • The regulator is a circuit that helps Regulator maintain a fixed or Changes in the load or the constant output voltage. AC line voltage will cause the Most electronic output voltage to vary. circuits cannot withstand the variations since they are designed to work properly with a fixed The regulator fixes the output voltage. voltage to the desired level then maintains that value despite any output or input variations. A presentation of • How The simplest form of rectifier is Rectifiers Work the half wave Only the transformer, rectifier rectifier shown. diode, and load (RL) are shown without the filter and other The half wave components. rectifier produces one sine pulse for each cycle of the input sine When the sine wave goes wave. positive, the anode of the diode goes positive causing the diode to be forward biased. The diode conducts and acts like a closed switch letting the positive pulse of the sine wave to appear across the load resistor. A presentation of • When How Rectifiers Work (continued) the sine wave goes negative, the diode anode will be negative so the diode will be reverse biased and no current No negative will flow. voltage will appear across the load. The load voltage will be zero during the See the waveforms that time of the negative half cycle. show the positive pulses across the load. These pulses need to be converted to a constant DC. A presentation of • Another widely used rectifier is Bridge Rectifier the bridge rectifier. It uses four This is called a full wave diodes. rectifier as it produces an output pulse for each half cycle of the input On the positive half cycle of the sine wave. input sine wave, diodes D1 and D2 are forward biased so act as closed switches appearing in On the negative half cycle, diode series with the load. D1 and D2 are reverse biased and diodes D3 and D4 are forward biased so current flows through the load in the same direction. A presentation of • A How the Filter Works large capacitor is connected across the load resistor. This capacitor filters the pulses into a When the diode conducts, more constant DC. the capacitor charges up to the peak Then when the of the sine wave. sine voltage drops, the charge on the capacitor remains. Since the capacitor is large it forms a long time constant with the load resistor. The capacitor slowly discharges into the load maintaining a more constant The next positive pulse comes output. along recharging the capacitor and the process continues. A presentation of • Most regulators are ICs The Regulator These are feedback control circuits that actually monitor . the output If the output varies, for whatever voltage to detect variations. reason, the regulator circuit automatically adjusts the output back to the set Since Regulators hold the output to the desired value. value. ripple represents changes in the output, the
  12. 12. regulator also compensates for these variations producing a near constant DC output. A presentation of • RC pi Filterii) RC pi Filter•C1 performs the same function that it didin the single capacitor filter. It is used toreduce the percentage of ripple to arelatively low value.•C2 offers infinite impedance (resistance)to the dc component of the outputvoltage. Thus, the dc voltage is passed tothe load, but reduced in value by theamount of the voltage drop across R2.However, R2 is generally small compared A presentation of eSyst.orgto the load resistance. Therefore, the drop • RC pi Filter• C2 offers very low impedance to the ac ripple frequency. Thus, the ac ripple senses a voltage divider consisting of R2 and C2 between the output of the rectifier and ground. Therefore, most of the ripple voltage is dropped across R2.• The RC filter has some disadvantages, however. First, the voltage drop across R2 takes voltage away from the load. Second, power is wasted in R2, R1 and is dissipated in the form of unwanted heat.• The input capacitor (C1) has the greatest pulsating voltage applied to it and is the most susceptible to voltage surges. As a result, it is frequently subject to voltage breakdown and shorting. The shunt capacitor (C1 and C2) in the filter circuit is not subject to voltage surges because of the protection offered by the series filter resistor. A presentation of • Definition The amount of ripple factor of the full wave rectified of Ripple signal is smaller than the half wave signal and provides a better The amount of ripple factor of the full wave rectified filtered signal. signal is smaller than the half wave signal and provides a better filtered signal. • •As is known, in an inductor filter,INPUT (L-C RL but decreases in THE CHOKE ripple increases with FILTER)a capacitor filter.•The combination of L and C filter makes the ripple independent of RL a) Shows the filter circuit. b) The voltage variation. • •The LC input filter is one of the most commonlyused filters. CLC or Pi Filter•The input capacitor C1 is selected to offer very lowreactance to the ripple frequency. Hence, major partof filtering is done by C1. Most of the remainingripple is removed by the combined action of L andC2.•L is a large value iron-core inductor (choke.) It hasa high value of inductance and, therefore, a highvalue of XL, which offers a high reactance to theripple frequency. At the same time, C2 offers a verylow reactance to the ac ripple. L and C2 form an acvoltage divider and, because the reactance of L ismuch higher than that of C2, most of the ripple • • Aside from the voltage divider effect, the inductor improves filtering in another way. You should recall that an inductor resists changes in the magnitude of the current flowing through it. Consequently, when the inductor is placed in series with the load, the inductor tends to hold the current steady. This, in turn, helps to hold the voltage across the load constant.• Generally, this resistance is very low and the dc voltage drop across the coil is minimal. Thus, the LC filter overcomes the disadvantages of the RC filter.• The LC filter has two disadvantages. The first is cost. The LC filter is more expensive than the RC filter because its iron-core choke costs more than the resistor of the RC filter. The second disadvantage is size, since the iron-core choke is bulky and heavy. Thus, the LC filter may be unsuitable for some applications but is still one of the most widely used. A presentation of • a)ZENER DIODE AS VOLTAGE REGULATOR•A zener shunt regulator is thatthe diode dissipation is toolarge in some application. • b) SERIAL TRANSISTOR•The way to reduce the diode zener shunt power Q β = 50dissipation is called an amplifier zener regulator. Transistor Q1 is the seriesOperation control element.•Zener diode DZ provides the reference voltage •If the output voltage decreases, the increased base-emitter voltage causes transistor Q1 to conduct more.
  13. 13. •Thereby raising the output voltage, maintaining the outputconstant. •If the output increases, the decreased base-emittervoltage causes transistor Q1 to conduct less, reducing theoutput voltage maintaining the output constant. A presentation of • A simple regulator consists of a sampling circuit, an error amplifier, a conduction The sampling regulator element, and a voltage reference element. circuit (voltage divider) monitors the output voltage by feeding sample The reference voltage element voltage back to the error amplifier. (zener diode) acts to maintain a constant reference voltage that used The error amplifier’s output is then fed to the by the error amplifier. current-control element (transistor), which used to control the load current. • Negative-feedback•Transistor Q1 acts like an emitter follower. voltage regulator.Transistor Q1 provides voltage gain in anegative-feedback loop.•Suppose the load voltage tries to increase. Thefeedback voltage VF will increase. Since theemitter voltage Q1 is held constant by the Zenerdiode, more collector current flows through Q1and through R3.•This reduces the current throughQ1 and R3.The higher voltage at the base of Q2 increasesthe emitter voltage of Q2, and this almostcompletely offsets the original decrease in loadvoltage. A presentation of • Voltage regulator•The positive voltage regulator LM78 “xx” andnegative voltage regulator LM79xx digits representthe output voltage such as 7805 (5V), 7806 (6 V),7909 (-9V) etc.•Can handle a maximum output current of 1.5A ifproperly heat-sink.•To remove unwanted input or output spikes/noise,capacitors can be attached to the regulator’s inputand output terminals, as shown on figure above. A presentation of • SIMPLE POWER SUPPLY • In All electronic circuits and equipment need a power supply, Summary usually A battery is a one that supplies are very specific DC voltage. near perfect DC supply but it is used mainly in portable In most AC Most equipment uses an AC to DC power supply. applications. to DC supplies, the 120 volt AC line is first filtered then stepped up or down to the desired voltage level then rectified into pulsating DC, then filtered to a constant DC. A regulator holds the output to a desired level. A DC-DC converter may also be used to generate another The two most common rectifiers are the single diode half DC voltage. wave rectifier and the four diode full wave bridge rectifier.