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Makezine_COTM_October-PowerSupplyOctober is power supply month! For the rest of the month, we’ll be posting about ways to utilize or create power supplies for your projects, with the most obvious way being an AC-DC power adapter. The following is an excerpt from Charles Platt’s definitive Encyclopedia of Electronic Components, Vol. 1:

What It Does

An AC-DC power supply converts alternating current (AC) into the direct current (DC) that most electronic devices require, usually at a lower voltage. Thus, despite its name, a power supply actually requires an external supply of power to operate.

Larger products, such as computers or stereo equipment, generally have a power supply contained within the device, enabling it to plug directly into a wall outlet. Smaller battery-powered devices, such as cellular phones or media players, generally use an external power supply in the form of a small plastic pod or box that plugs into a wall outlet and delivers DC via a wire terminating in a miniature connector. The external type of power supply is often, but not always, referred to as an AC adapter.

Although an AC-DC power supply is not a single component, it is often sold as a pre-assembled modular unit from component suppliers.

Variants

The two primary variants are a linear regulated power supply and switching power supply.

Linear Regulated Power Supply
eeco_1601A linear regulated power supply converts AC to DC in three stages:
* A power transformer reduces the AC input to lower-voltage AC.
* A rectifier converts the AC to unsmoothed DC. Rectifiers are discussed in the entry on diodes in this encyclopedia.
* A voltage regulator, in conjunction with one or more capacitors, controls the DC voltage, smooths it, and removes transients. The regulator is properly known as a linear voltage regulator because it contains one or more transistors, which are functioning in linear mode—that is, responding linearly to fluctuations in base current, at less than their saturation level. The linear voltage regulator gives the linear regulated power supply its name.

This type of power supply may be described as transformer-based, since its first stage consists of a transformer to drop the AC input voltage before it is rectified.

Because the rectifier in a power supply generally passes each pulse of AC through a pair of silicon diodes, it will impose a voltage drop of about 1.2V at peak current. A smoothing capacitor will drop the voltage by about 3V as it removes ripple from the current, whereas a voltage regulator typically requires a difference of at least 2V between its input and its output. Bearing in mind also that the AC input voltage may fluctuate below its rated level, the output from the power transformer should be at least 8VAC higher than the ultimate desired DC output. This excess power will be dissipated as heat.

The basic principle of the linear regulated power supply originated in the early days of electronic devices such as radio receivers. A transistorized version of this type of power supply remained in widespread use through the 1990s. Switching power supplies then became an increasingly attractive option as the cost of semiconductors and their assembly decreased, and high-voltage transistors became available, allowing the circuit to run directly from rectified line voltage with no step-down power transformer required.

Some external AC adapters are still transformer-based, but are becoming a minority, easily identified by their relatively greater bulk and weight. An example is shown to the right: A relatively old, cheap AC adapter contains only the most rudimentary set of components, and does not supply the kind of properly regulated DC power required by electronic equipment.

Switching Power Supply
eeco_1604Also known as a switched-mode power supply, an SMPS, or switcher, it converts AC to DC in two stages.

* A rectifier changes the AC input to unsmoothed DC, without a power transformer.
* A DC-DC converter switches the DC on and off at a very high frequency using pulse-width modulation to reduce its average effective voltage. Often the converter will be the flyback type, containing a transformer, but the high-frequency switching allows the transformer to be much smaller than the power transformer required in a linear regulated power supply. See the DC-DC converter entry in this encyclopedia for an explanation of the working principles.

A simplified schematic of a switching power supply is shown in picture to the right. It’s a greatly simplified schematic showing the principal components of a switching power supply. Note the absence of a 115VAC power transformer. The transformer that is inserted subsequently in the circuit functions in conjunction with the high switching frequency, which allows it to be very much smaller, cheaper, and lighter.

John Baichtal

My interests include writing, electronics, RPGs, scifi, hackers & hackerspaces, 3D printing, building sets & toys. @johnbaichtal nerdage.net


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Comments

  1. Always remember on switching type supplies. ALWAYS treat the mains side as live, even if it’s not plugged into the wall. the 200-400uF, 250WV capacitors store the energy at full line potential, but in DC! Enough to knock you on your bumm, or worse! they usually ae connected to bleeder resistors, but never trust that they are (a) connected, or (b) internally broken. ALWAYS Wait at least 10 minutes before dismantling a switching supply, and even still, safely bridge the capacitors to drain them.

  2. Ray Jones says:

    I really appreciate the clear explanations. Been tinkering for 40 plus years and you help to make the ‘blue smoke’ understandable…..

  3. chuck says:

    This would be a great time to cover electrolytic rust removal using recycled power supplies. I used a trash scored ATX power supply, some scrap metal, a rubber maid tub and some washing soda to build a machine that saves me a lot of time prepping metal. It’s like electroplating in reverse.

    1. Ray Jones says:

      Details? I am a 62 year old scrapper…..

      1. chuck says:

        Here’s an instructable using a battery charger, but you could swap that out for various recycled power supplies-
        http://www.instructables.com/id/Electrolytic-Rust-Removal-aka-Magic/
        Also check out youtube for videos. Just search ‘electrolytic rust removal’.
        Good luck! This can easily be scaled up for doing large parts.

    2. Interesting… thanks for sharing! I might have to try that someday, I’ve got just about everything needed to do it.

  4. James Bryant says:

    The analysis of the transformer output voltage required by a linear power supply does not take into account that for sine-wave AC the PEAK volage is 1.414 times the RMS voltage, and the capacitor beyond the rectifier holds the highest voltage that it sees. This means that the calculation of excess voltage needed for correct operation is over-generous.

    Note also that it is possible to calculate the ripple if you know the maximum current and the value of the input smoothing capacitor. 3V may be much too large, or much too small depending on circumstances.

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  6. Everything is vrry open with a really clear clarification of the issues.

    It was definitely informative. Your website is useful.

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  7. Daniel Kim says:

    What I’d really like to see is an easy way to reduce the voltage from the 5V that I get from a cellphone charger to the 3V of so that I want for running my MP3 player. Is there a way to do this? I am ignorant, so please use small words.

    1. James Bryant says:

      Simply connect a voltage dropper of some sort between the positive terminals of the two devices (the grounds/negative terminals must also be connected together).

      The simplest dropper is two or three 1N4001 (or 1N4002/3/4/5/6/7) diodes in series. Try three and measure the voltage at the MP3 player input (for details on how to use a digital voltmeter (DVM) see http://www.jbryant.eu/pages/DMM.htm). If it is less than 2.7V use two diodes.

      You may also use a 2V Zener Diode (power rating needed depends on the current consumption of the MP3 Player – if it is under 250 mA then a 500mW Zener will do, otherwise you’ll need a 1W Zener). You can measure the current consumption with a DVM, too.

      But the best option is to use an LDO (Low Drop Out) Regulator such as the Analog Devices ADP124ARHZ-3.0-R7. This will give you an accurate 3V output with varying inputs between 3.4V and 5.5V at currents up to 500mA.

      Diagrams of all these arrangements are at http:www.jbryant.eu/Drop.htm

      Note that in addition to the diodes/zener/LDO Regulator there are also one or two capacitors. These are important, if you leave them out the system may be noisy.

      1. Daniel Kim says:

        Thank you very much for this quick reply. Uh . . . why does the LDO regulator have so many little legs? It looks like pins 5 – 8 are all connected together on one side, and pins 1 – 3 are all together on the other side, and there are two capacitors in there. Well, it looks simple enough, and the components seem pretty inexpensive. I’ll see about giving it a try. I’m grateful for the help.

        1. James Bryant says:

          It needs the legs for the heat to get out. If the supply is 5.5V and the current is 500mA then the device is dissipating 1.25W. This needs to go somewhere and it leaves through the leads and the conducting pad on the bottom. Read the data sheet of the device about how to ensure that there is enough metal on the pins and pad to dissipate the heat. If the device were larger it would not need the leads to lose heat and you would only need four leads (+input, +output, ground, and chip enable [CE - turns the device on and off - in the present case I've hard-wired it on by connecting CE to the +input]).