Submission: Power Supply Circuits

Discussion in 'Feedback and Suggestions' started by Wendy, Jan 27, 2010.

  1. Wendy

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    OK, I'm going to finish the power supply circuits section of the AAC book.

    I'll be using the following outline as my guide. Out of respect for the previous authors I'll leave as much in as I can, but we have different writing styles, so there will be some changes.

    1. Introduction
    2. Unregulated
    .a. Diodes and their characterstics (as applied to PS)
    .b. Transformers, ratings and limitations
    .c. Single Diode
    .d. Dual Diode (CT Transformer)
    .e. Diode Bridge
    .f. ± Power Supplies
    .g. Calculating Ripple
    .h. Wall Warts
    3. Linear Regulated
    .a. Voltage
    .. I. Protections
    ... a.Crowbar
    ... b. Foldback
    ... c. Current Limiting
    .b. Current
    .c. Tracking Power Supplies
    4. Switching (SMPS)
    .a. Ripple Regulated
    .b. Buck
    .c. Buck Boost
    .d. SEPIC
    e. Current vs. Voltage

    I still don't understand where Tony was going with the ripple regulated. I'm merging it with the SMPS class, since I don't think it needs a separate designation.

    Some SMPS will take rectified line voltage and use it straight. Most computer power supplies do this to a degree, transformers have weight. In the last year or so we have been cracking down on dangerous projects, such as LEDs being run off line current. This is the time for feedback on the issue, otherwise I will cover it as I see it.
     
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  2. Wendy

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    Status: Work In Progress. If you see something that is not right feel free to comment, but as a work in progress it may be I haven't corrected something that I will be getting to. If you have a suggestion please make it.

    There are three major kinds of power supplies: unregulated (also called brute force), linear regulated, and switching. All of them have various advantages and disadvantages, which will be covered in a case by case basis. Much of the material has been covered in depth elsewhere in this book. This section will show how it fits together as a whole.

    Unregulated

    An unregulated power supply is the most rudimentary type, consisting of a transformer, rectifier, and low-pass filter. This type of power supply can exhibit a lot of ripple voltage (i.e. rapidly-varying instability) and other AC "noise" superimposed on the DC power. If the input voltage varies, the output voltage will vary by a proportional amount. The advantage of an unregulated supply is that its cheap, simple, and efficient.

    Power supplies as a class try to emulate batteries. They have some unique characteristics such as ripple, but some of the basic concepts, such as internal resistance, still apply.

    Diodes are one constant in almost all power supplies, at least those that take AC from line voltage and convert it into DC. The drop for a silicon diode is around 0.6V at minimum currents, but as the current goes up so does the voltage drop. A 1N4001 power diode will drop around 1.1V at 1Amp, its maximum rating. This drop must be accounted for whenever designing a power supply. All of the following configurations have been covered in detail in Chapter 3, Diodes and Rectifiers. If you have a question about the following circuits you may need to review that section.

    While it is possible to make a power supply without a transformer, it is extremely hazardous, the nature of which can not be understated. Only a transformer provides decent isolation from the mains. There are very few ways a power supply without a transformer can be made to operate safely around people.

    A transformer will drop the voltages of a power supply to much safer levels, as well as isolating the user from the AC power. Power transformers are not treated the same as impedance matching transformers, though the theory of operation is very similar. Transformers are usually rated in VA of output, or as AC volts out and a current rating. The VA rating determines what the maximum current transferred through the transformer, You can calculate the fuse or breaker required by dividing the primary voltage by the VA rating. So if a transformer is a 120VAC on the primary side, and the VA rating is 120VA, then you would use a 1 amp fuse.

    Note that transformers in general will produce their rated voltage at their rated current. Low current models tend to change more than high current versions, since a higher percentage of loading will be used. With no load you can have substantially higher voltages out. Since it is partly a quality issue, a perfect transformer would not do this, it can be hard to find a specification on it, so the user has to be aware of the fact.

    Impedance matching transformers have to handle a wide range of frequencies, while a power transformer is optimized for only one frequency. Many cases a 50Hz transformer can be used for a 60Hz, but there will usually be a small detrimental change in performance, in efficiency and heat produced. Transformers are one of the few man made devices that can exceed 95% conversion efficiencies.

    Another bit of math common to all power supplies is line voltage is given in RMS values, while power supplies will translate this into peak values. The conversion for this is 1.414 (square root of 2). So the output of a power supply will be the output of the transformer times 1.414 minus the diode drop(s).


    Basic Power Supply Configurations

    Different configurations of power supplies will have dramatically different characteristics. The following design uses only one half of a cycle. It's Effective DC voltage (defined as measured by a meter without filtering) is around 31.8% of the AC output voltage. This design, while simple, wastes much of the power possible from the transformer as well as allowing excessive ripple due to fact that there is no power being delivered to the load 50% of the time, and tries to use the capacitor to make up for the deficit. This circuit is referred to as a Half Wave Rectifier power supply.

    [​IMG]
    ......................................................Figure 1

    Note that the fuse is the first thing connecting the power supply from line current. If it blows the entire circuit will go dead. The switch is optional, but recommended. These are standard features for any power supply.

    The excessive ripple can be handled simply in several ways. Increasing the capacitance works, but large value capacitors tend to be physically large. Large capacitors also appear as a dead short for the transformers/diodes while they charge resulting in high surge currents. In extreme cases it can damage or burn out the diodes.

    If the transformer has a center tap then it can be used for full wave rectification as shown. The effective DC voltage for the circuit below is around 63.6%, but since only one half of the transformer the voltage will be half of the transformers AC output. This circuit is referred to as a Full Wave Center Tap power supply.

    [​IMG]
    ......................................................Figure 2


    The full power of the transformer can be squeezed out of it by using the following full wave bridge rectifier. Its effective DC power rating is around 90%, but it uses the full AC voltage of the transformer. This circuit is referred to as a Full Wave Bridge power supply.

    [​IMG]
    ......................................................Figure 3


    There are many circuits out there that need a ± power supply, notably op amps. With a simple center tap on the transformer you can generate a decent power supply for that application. This circuit is referred to as a Dual Complimentary Rectifier power supply.

    [​IMG]
    ......................................................Figure 4


    Ripple

    Ripple is a serious problem with all these designs. 50 Hz is slightly worse than 60 Hz, since the lower frequency requires a larger capacitor to compensate, but ripple should be planned and calculated for any design you make. It is a problem in RC discharge curves. Even if you are planning on using a regulator to clean up the output (which will be covered later) you must get the ripple low enough for the regulator to do its job. The greater the load the larger the capacitor must be to compensate, since the discharge of the capacitor is that much faster.

    While ripple is very undesirable, it is also unavoidable. The first thing you have to define is the peak to peak value of ripple you are willing to accept. In audio and RF it can cause hum, where the line frequency can be heard riding on the signal. In digital circuits it can cause false triggers of gates if it is bad enough, though in general digital circuits are less sensitive to ripple since they are a go/no go system. It is important to figure out how much ripple can be lived with and still allow the circuit being powered to function correctly.

    A capacitor on a power supply with no load will do something odd. It will charge up to the peak value, and stay there. Ripple will be almost none existent, since there is no load resistance to discharge the capacitor between charge cycles. It is the combination of the line frequency, load resistance, and capacitance that ultimately define how much ripple is on the power supply.

    The math for calculating ripple can be quite complex, so over time it has been broken down to several rules of thumb. In the beginning it was done with tables and charts, but math is probably an easier tool to use for the job, understanding that it is somewhat simplified.

    Take a power supply that outputs 12 volts, and we have decided that 100mv ripple at 1 amp is the most ripple the circuit being fed by the power supply can take. The line voltage is using 60Hz. Remember, the fundamental frequency of the line input will affect our math.

    Starting with the half wave rectifier shown in Figure 1 the charge cycle will be once every 60th of a second, or 16.7ms.
     
    Last edited: Dec 5, 2011
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  3. Wendy

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    Wall Warts

    When electronics was new if you wanted a power supply you had to mount a transformer, diodes, and capacitors into a chassis box. However, during the 1970 and beyond another way of providing power has been created. Basically the power supply plugs into the wall, and it was nicknamed a wall wart due to its appearance on the wall. Wall warts are considered components, but have no real standards. They can be a simple transformer, an unregulated DC power supply, or a fully realized regulated power supply with advanced circuitry. This means you can have a AC output to a regulated DC value. In most cases they are simple unregulated power supplies, providing DC to whatever equipment they are meant to power. The power jack has no polarity standards. Most wall warts have their technical information molded into the case or printed on a label.

    An unregulated power supply only provides the rated voltage when they are given their rated load. This is an important fact, as the voltage output by an unload wall wart can be 50% or more than the voltage written on the case. Many of them have the fuse built in, but are meant to be thrown away if the fuse blows.

    Wall Warts have several major advantages over the older methods.

    1. They are safe, as they remove any high voltage from inside the equipment they are powering, and if properly designed isolate this equipment from the line voltage. A wall wart will allow someone new to electronics to have ready DC power without exposing them to the hazards of line voltage.

    2. They remove a major source of heat from the equipment, which allows the equipment to run much cooler.

    3. They simplify the interior of the equipment, allowing the equipment to have a much smaller footprint and be much lighter.

    4. They also help remove electronic noise from what could be sensitive electronics. A properly designed transformer will contain its magnetic field, but not all parts are designed well.

    There are other example of external power supplies which are not wall warts, such as laptops, printers, and other equipment. The power ratings can be larger for these applications. External power supplies is a trend that will probably accelerate over time for more applications.


    Voltage Vs. Current Regulation

    The most common regulator by far is the Voltage Regulator, but it is not the only kind that may be needed. Another kind of regulator is a Current Regulator, often referred to as a Constant Current Source or Constant Current Sink. A constant current source can be very similar to a constant voltage source, with a minor adjustment to the feedback mechanism.


    Linear regulated

    A linear regulated supply is simply a "brute force" (unregulated) power supply followed by a transistor circuit operating in its "active," or "linear" mode, hence the name linear regulator. (Obvious in retrospect, isn't it?) A typical linear regulator is designed to output a fixed voltage for a wide range of input voltages, and it simply drops any excess input voltage to allow a maximum output voltage to the load. This excess voltage drop results in significant power dissipation in the form of heat. If the input voltage gets too low, the transistor circuit will lose regulation, meaning that it will fail to keep the voltage steady. It can only drop excess voltage, not make up for a deficiency in voltage from the brute force section of the circuit. Therefore, you have to keep the input voltage at least 1 to 3 volts higher than the desired output, depending on the regulator type. This means the power equivalent of at least 1 to 3 volts multiplied by the full load current will be dissipated by the regulator circuit, generating a lot of heat. This makes linear regulated power supplies rather inefficient. Also, to get rid of all that heat they have to use large heat sinks which makes them large, heavy, and expensive.

    Generally a transistor is used as the series pass element. It serves as a variable resistor that is used to drop the excess voltage and keep the output voltage constant. Because this is an analog device, it generally runs much quieter than other types. For this reason it is prefered for many amplifiers and oscillators, such as those used for RF circuits. A linear voltage regulator almost always use negative feedback to control the output voltage, so they have to be very fast to correct voltages that are trying to flucuate rapidly.


    Switching

    A switching regulated power supply ("switcher") is an effort to realize the advantages of both brute force and linear regulated designs (small, efficient, and cheap, but also "clean," stable output voltage). Switching power supplies work on the principle of using transistors either on or off, basically a digital mode. These two modes allow a transistor to disappate the least amount of wattage, a transistor that is fully on or fully off does not get hot. This is because power equals voltage times current, if one of these numbers is zero then there can not be heat generated. Any heat generated by the transistor is while it is switching between the two states.

    A typical switching power supply uses a diode, coil, and capacitor to recover the maximum amount of power during the transistions. It is one of the main reason these circuits tend to be so efficient,

    rectifying the incoming AC power line voltage into DC, re-converting it into high-frequency square-wave AC through transistors operated as on/off switches, stepping that AC voltage up or down by using a lightweight transformer, then rectifying the transformer's AC output into DC and filtering for final output. Voltage regulation is achieved by altering the "duty cycle" of the DC-to-AC inversion on the transformer's primary side. In addition to lighter weight because of a smaller transformer core, switchers have another tremendous advantage over the prior two designs: this type of power supply can be made so totally independent of the input voltage that it can work on any electric power system in the world; these are called "universal" power supplies.

    The downside of switchers is that they are more complex, and due to their operation they tend to generate a lot of high-frequency AC "noise" on the power line. Most switchers also have significant ripple voltage on their outputs. With the cheaper types, this noise and ripple can be as bad as for an unregulated power supply; such low-end switchers aren't worthless, because they still provide a stable average output voltage, and there's the "universal" input capability.

    Expensive switchers are ripple-free and have noise nearly as low as for some a linear types; these switchers tend to be as expensive as linear supplies. The reason to use an expensive switcher instead of a good linear is if you need universal power system compatibility or high efficiency. High efficiency, light weight, and small size are the reasons switching power supplies are almost universally used for powering digital computer circuitry.


    Ripple regulated

    A ripple-regulated power supply is an alternative to the linear regulated design scheme: a "brute force" power supply (transformer, rectifier, filter) constitutes the "front end" of the circuit, but a transistor operated strictly in its on/off (saturation/cutoff) modes transfers DC power to a large capacitor as needed to maintain the output voltage between a high and a low setpoint. As in switchers, the transistor in a ripple regulator never passes current while in its "active," or "linear," mode for any substantial length of time, meaning that very little energy will be wasted in the form of heat. However, the biggest drawback to this regulation scheme is the necessary presence of some ripple voltage on the output, as the DC voltage varies between the two voltage control setpoints. Also, this ripple voltage varies in frequency depending on load current, which makes final filtering of the DC power more difficult.

    Ripple regulator circuits tend to be quite a bit simpler than switcher circuitry, and they need not handle the high power line voltages that switcher transistors must handle, making them safer to work on.
     
    Last edited: Dec 16, 2011
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  4. Wendy

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    Last edited: Feb 8, 2010
  5. beenthere

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    Due to our policy about unsafe and dangerous circuits, I wonder if the first example could be simply eliminated?
    It is strange to illustrate such a lethal circuit and then have to tell people not to use it.
     
    Last edited: Jan 29, 2010
  6. Wendy

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    I suspected you might notice that. :rolleyes: Problem is, it is used in real engineering on SMPS supplies, notably computer power units. I would rather mention a subject, and explain why it is a bad idea rather than pretend it doesn't exist. When I was starting my experiments, before I had any school other than hanging around the electronics lab at my high school, I built this and worse. It is something that will naturally occur to anyone who is interested, much like the LEDs circuits that keep coming up. BTW, the LED AC light bulbs are becoming more common as real world appliances.

    I am not wedded to it. I will remove it from this section if you prefer, but it will come up later.

    Reading what Tony has already written in the power supply article I suspect he is of the thought that more than a few SMPS units use this as a basic DC conversion. I don't believe that either, but we probably need to talk about it, either here or in the E-Book forum.

    From my point of view this is one of the rare cases we can influence what is in the book (other than the writer obviously). Frankly, I'm hoping for all the help I can get when I'm further along.

    Is this a college level text book, or a high school level text book? There are going to be circuits that are naturally dangerous, is it better to edit them out entirely or teach the dangers of these circuits?
     
    Last edited: Jan 29, 2010
  7. Wendy

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    I think I mentioned the story about my brother, who followed behind my footsteps. We had talked about using something like that with a Xenon strobe lamp, which Radio Shack sold (both the trigger transformer and the lamp). He made a suicide circuit using a lamp and the high voltage section on his lap and in front of him with no one the wiser. Problem was, he didn't put a resistor in the circuit, so when the lamp fired it stayed on. Light poured out of every crack and keyhole, much like a close encounters movie, and there he was, blinded by the light and groping for the power plug, hoping he didn't find something else instead. We laughed about it, but I got to chew butt about it later. I don't think my Dad ever caught on to what he was really doing.

    Then there was the time I was 12 and didn't have any batteries to launch my model rockets. I figured the house outlet would suffice. It did, but I also flickered the lights in the house, which brought my Dad out. I managed to look innocent and conceal the evidence in time, but figured out I would get my butt busted if he caught on to what I did.

    Ah, being young and stupid.

    My point is we need to make sure people know it is a bad idea, many of them will go off half cocked anyhow.
     
  8. t06afre

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    A very interesting topic is how to dimension the transformer for a specified DC max output current. With different rectifier/filter types.
     
  9. Wendy

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    I remember a thread between several folks, including Electrician, that covered a lot of ground. The part that really stuck for me was the output impedance of transformers. I'll be giving it one paragraph, maybe one sentence. :rolleyes: Sometimes these kinds of books don't do the subject justice. The trick is giving enough information that either the student understands the basics or can figure out where to find more information.

    What I'm writing will be one small subsection (probably bigger than the original author had intended) in a much larger chapter. Being a tech I like examples of circuits.
     
  10. beenthere

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    You're correct about the bridge feeding DC to a SMPS transformer. That is hardly a project for a newbie or even a hobbyist, though.

    The concern is that somebody wants to save a couple of bucks by eliminating the transformer, but has no understanding of the hazard created. That is a long way from an experienced design engineer putting together a switcher.

    We seem to have this tension generated by the level of information presented. A struggle between beginner and EE levels. I would be happier to leave this out, but I am not the only party involved. It may be that I am sensitized, as we have just had a number of questions involving transformerless circuits and/or generating lethal voltages.
     
  11. Wendy

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    Maybe a sticky, or an addition to the forum rules of what we will not allow in the way of circuits? I don't expect it will help much, these folks will go off half cocked much like me and my brother did. Like the chemistry sets we used to be able to buy it is hard to make everything 100% safe.

    It is a fine line and hard to define. Someone with a perceived tech level can ask about and get help help repairing a xenon strobe, while other (obvious) noobs are closed quickly, especially when they make it plain they intend their stuff for human use. In this case the circuits are the same, but the applications is... different.

    I don't have an easy answer. As with the LEDs (which may have to be expanded in the book showing other AC circuits as the technology progresses) we have to handle it on a case by case basis. If it is out there we need to teach theory, at least in the book.

    Since there is a bit of disconnect between the two, my first idea (info about what will not be allowed on the site) may have to serve as a dividing line.

    How about I remove the schematic but leave a blurb about the dangers of a transformerless power supply?
     
  12. beenthere

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    You might wait and see if some others weigh in on this.
     
  13. StayatHomeElectronics

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    I just looked at another often referenced book on electronics and here is how they dealt with the issue...

    "Never build an instrument to run off the power line without a transformer! To do so is to flirt with disaster." The Art of Electronics, Section 6.12

    They have a small explanation of where and why they are used, cheap, but do not show any diagrams, just as you seem to have decided.
     
  14. beenthere

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    I like that.
     
  15. redderek

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    I would like to see some re-phrasing of the first paragraph under Switching power supply description. The current paragraph makes inference to a switching supply that acts from the ac input. I would suggest something such as the following.
    =====================
    A switching regulated power supply ("switcher") is an effort to realize the advantages of both brute force and linear regulated designs (small, efficient, and cheap, but also "clean," stable output voltage). Switching power supplies work on the principle of chopping up a dc source voltage into small energy packets that are sent through a transformer or inductor to either kick up the voltage or drop it down. The energy packets are rectified and filtered to generate a dc output, or the packets are filtered to generate an ac output. Thus the phrase “switching” refers to the chopping and transferring of energy packets by controlling a switching device on and off. Switching supplies are ac-to-dc, dc-to-dc, or dc-to-ac, or ac-to-ac. When the input is ac, the input is typically rectified and filtered to generate a dc input to the pulse-with-modulator (PWM) controller.

    Regulation of the output is through a feedback circuit that looks at the output and sends a voltage or current signal back to the PWM controller. The PWM controller will control the amount of energy switched through the supply; typically pulse width modulated for a fixed frequency controller, or by frequency modulated with a variable frequency controller.

    The size of the power supply can vary greatly depending on the number of outputs, features, frequency, and power rating. Obviously, adding additional outputs and features, and increasing power rating would increase the power supply volume. But the size can be reduced by increasing the operation frequency, which allows for smaller magnetics; magnetics tend to be the largest component of a typical power supply.

    The general categories for switching power supplies are buck, boost, flyback, forward, and sepic. Each of these basic supplies have advantages and disadvantages. Brief description of each are as follows.

    * Buck – Input voltage is higher than the output voltage. This is a non-isolated supply whereby the input ground is at the same reference as the output ground. An inductor is the primary form of energy transfer. The input current to this supply is switched on and off and thus good input filtering is required.

    * Boost – Input voltage is lower than the output voltage. This is also a non-isolated supply. An inductor is the primary form of energy transfer. Input current is switched similar to the buck.

    * Flyback – Input voltage can be higher or lower than output. The output could be isolated from the input, but feedback needs to be isolated as well. A transformer is the primary form of energy transfer. Here the switching element turns on and charges up the transformer. When the switching element is turned off, the energy in the transformer “flys back” to the output. Input current is switched similar to the buck.

    * Forward – Input voltage can be higher or lower than the output. This is similar to the flyback, but the switch operates in the manner that energy is passed to the output when the switching device is on; thus passing energy “forward” to the output. Input current is switched similar to the buck.

    * Sepic – Input voltage can be higher or lower or equal to the output. This supply is more different because the main energy transfer is a capacitor and not a magnetic device. The main advantage of this supply is that the input and output current are much more linear and looks more like dc. The magnetics portion of the circuit is typically an inductor, but coupled inductors – similar to a transformer – can be used to help control output voltage and transfer energy.

    There are other types of supplies that take on a hybrid form of the above. For example, there is the buck-boost converter that uses one inductor, but switching elements are positioned and controlled to allow the supply to look like a buck converter or a boost converter.
    ===============

    From here I’ll have to find some example schematics to attach to each of the basic power supply types. Let me know if you like this. I could go on quite extensively on power supplies, but that information can be gathered from other locations as well.
     
  16. Wendy

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    You are dealing with the section I haven't written. Far as I know this was done by the original author. You can go to the original book on top of the page to see the base line.

    I don't mind help, but keep in mind we also need illustrations. They need not be detailed, but enough to show a beginner how something like this might work. I notice you are using the rough layout I posed, which is good. This is only a guide, if you think anything has been left out or need rearranged feel free to suggest it.

    I'm currently plowing through one of my other articles, LEDs, 555s, Flashers, and Light Chasers. My forum version is where I make changes (link LEDs, 555s, Flashers, and Light Chasers). I'm currently plowing through illustration for this article, it's taking longer than usual.

    I'm also building a capacitance box to verify some math for the bulk power supply section, which is why the article came to a stop. I think it was our moderator beenthere said something about anyone in this hobby has several projects going simultaneously, I'm very much in that mode.

    If you want to sketch some ideas for the illustrations I'll be glad to formalize them (redraw them and clean them up). If you are a fair to middlin technical artist you can do this yourself, there are some restrictions (mostly size and file types, they need to be as small as possible while being clear). They have a preferred package to used for illustrations, but I don't use it myself. It is mostly Linux, but there is a Windows version. I think it is called XCircuit.

    There are other things, but I don't want to get too deep into this right now. If you are interested in helping out there is lots of work to be done. The power supply article is good as any, but it is only the tip of the iceburg. This is a good starting point.

    http://www.allaboutcircuits.com/l_contribute.html

    If you just want to help with this section that is fine too.

    I post in this section so everyone can have an input while it is being written, to head off mistakes I might be making early. I don't claim perfection, but I'm trying to do a good job.
     
  17. Norfindel

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    Mar 6, 2008
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    Transformer limitations would be a good place to say that the power rating is calculated with a resistive load, supplying current in senoidal form. It can be quite of a shock to realize your 3A transformer can supply much less current when using diodes and a capacitor to rectify the AC.
     
  18. guru200773

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    Apr 26, 2010
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    Thanks for a good info friend.. I also thought to make transformer less power supply but after the explanation of some experts in this forum i learnt Safety is first than the money and size...
     
  19. Wendy

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    I'm going to use this specific post to link to other threads that have useful information. If you see one that hasn't been mentioned please print the link.

    I am currently in need how to calculate the ripple for set capacitance. I have a half built capacitance box I'll be working on as an experimental tool to verify some of the math.

    I also need to collect data on diode surge currents when the capacitors are discharged, and how to design around them.

    For those who understand rectifiers
     
    Last edited: Dec 5, 2011
  20. MrChips

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    Can you post the link to this?
     
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