Smoothing DC (Direct Current)

Smoothing is performed by a large value electrolytic capacitor (polorised capasitor) connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.



Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.

Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor. A larger capacitor will give less ripple. The capacitor value must be doubled when smoothing half-wave DC.

Smoothing capacitor for 10% ripple, C = 5 × Io
Vs × f

C = smoothing capacitance in farads (F)
Io = output current from the supply in amps (A)
Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC
f = frequency of the AC supply in hertz (Hz), 50Hz in the UK

Rectifier

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is the most important and it produces full-wave varying DC. A full-wave rectifier can also be made from just two diodes if a centre-tap transformer is used, but this method is rarely used now that diodes are cheaper. A single diode can be used as a rectifier but it only uses the positive (+) parts of the AC wave to produce half-wave varying DC.

Bridge rectifier
A bridge rectifier can be made using four individual diodes, but it is also available in special packages containing the four diodes required. It is called a full-wave rectifier because it uses all the AC wave (both positive and negative sections). 1.4V is used up in the bridge rectifier because each diode uses 0.7V when conducting and there are always two diodes conducting, as shown in the diagram below. Bridge rectifiers are rated by the maximum current they can pass and the maximum reverse voltage they can withstand (this must be at least three times the supply RMS voltage so the rectifier can withstand the peak voltages). Please see the Diodes page for more details, including pictures of bridge rectifiers.



Bridge rectifier
Alternate pairs of diodes conduct, changing over
the connections so the alternating directions of
AC are converted to the one direction of DC.



Output: full-wave varying DC
(using all the AC wave)

Single diode rectifier
A single diode can be used as a rectifier but this produces half-wave varying DC which has gaps when the AC is negative. It is hard to smooth this sufficiently well to supply electronic circuits unless they require a very small current so the smoothing capacitor does not significantly discharge during the gaps. Please see the Diodes page for some examples of rectifier diodes.



Single diode rectifier



Output: half-wave varying DC
(using only half the AC wave)

Transformer

Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC.

Step-up transformers increase voltage, step-down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK and 240W in Malaysia) to a safer low voltage.

The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.

Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.

The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

turns ratio = Vp = Np and power out = power in
Vs Ns Vs × Is = Vp × Ip

Vp = primary (input) voltage
Np = number of turns on primary coil
Ip = primary (input) current Vs = secondary (output) voltage
Ns = number of turns on secondary coil
Is = secondary (output) current

September 13 2009 - Activity

Type of Diodes

Diodes can be classified by the functions of the circuit in which it is being used, or more commonly, by the shape that is demanded by the size of the products in which it will be mounted. The complicated point is that there is no direct relation between the two and you must keep them both in your mind at all times. However, you can think of the function as the base, and since this function can then be supported by many different shapes, it can then be further classified by those shapes.

Zener diode (constant voltage diode)
Since a Zener diode generates a constant voltage from the Zener effect that occurs when a reverse bias voltage is applied, it is also called a constant voltage diode. By using the resulting constant voltage, it can be used in constant voltage circuits and other circuits that require a reference voltage.

Schottky barrier diode
A Schottky barrier diode attaches a Schottky gate electrode directly to a n-type semiconductor and makes use of the fact that reverse bias voltages are prevented from causing current flow across the junction of the metal and semiconductor. Some are for high frequencies and some are for general rectification. Those for high frequencies are often used in high-speed switching for wave detectors and mixers in the UHF and microwave bands. Since the forward voltage is small and the reverse breakdown voltage cannot be made too high (currently, approx. 100 to 200 volts) compared to normal diodes for general rectification, Schottky barrier diodes for general rectification are used for the rectification of power supplies for low voltages and high currents, or power supply switching for the rectification of high frequencies with its small reverse recovery time.
Switching diode
By using the rectifying properties of the pn junction (the extreme difference in the forward and reverse bias resistances), these diodes are mainly used for switching the circuit on and off. Generally, when you speak of switching diodes, it includes Schottky barrier diodes and band switching diodes, but we at Rohm think of them as separate types. Switching diodes are the normal silicon small-signal switching diodes. They are used for demodulation, modulation, switching, mixing, and the like and make up approximately half of the total diodes produced in Japan. Rohm holds the largest share of this market.

Rectifier diodes
In general, these diodes are aimed at current rectification for currents of 1 ampere or more and are used in rectification circuits for power supplies. There are many types from low power to high power with a wide range of different packages. The most common type is the low power 1 ampere class that makes up approximately 70% of the rectifier diodes produced.

Band switching diodes
The band switching diode was developed as a general small signal diode for high frequency use. Since it is used for switching the frequency bands for high frequency tuners, the resistance at high frequencies is small and the capacitance between the electrodes is kept as small as possible.

Monsters, Inc.

It's been several years since Pixar first wowed us with "Toy Story", the first three-dimensional cartoon ever produced. Now we know it as the re-shaping of the animation world as this year alone we have had three 3-D animated films hit the big screen. One of those was the movie "Shrek" an insanely funny animated work from Dreamworks that so far ranks as the funniest movie of the year. Pixar hopes to stand up right next to the giant ogre this weekend with "Monsters Inc", a delightful family film that sadly just falls short of that achievement, but is still worth the trip.

One of the reasons why is for the inventive ideas the story comes up with. It takes place in the city known as Monstropolis, a haven for monsters, which is all powered by the power plant, commonly known as Monsters Incorporated.

The company consists mainly of frightening children, storing their screams, and then later powering the city with them. The number one scarer in the biz is John P. Sullivan, or Sully, (voiced by John Goodman), a hairy, six foot, and blue working class monster with a good heart, but also a real knack for his work. His partner is the always fast talking Mike Wazowski (voiced by Billy Crystal), who is both friend and trainer to Sully.

The two of them are very excited for the reason that Sully is about to break the all time scare record. But one monster that stands in there way is Randall Boggs (voiced as evil as ever by Steve Buscemi), a chameleon-like creature who finally decides to resort to cheating when Sully remains at the top of the standings board at the end of the day.

The company uses an assembly line of closet doors to get to the human world, just they are to be locked away at the end of the day. But when Sully goes back to the office to pick up something for Mike, he sees a door left out, and a little human girl pulling at his fur. A monster touching a human or a human getting through the doors is considered to be a contamination alert, so Sully can do nothing but panic when he sees her.

Soon the entire city is looking for the monsters hiding the little girl, and both Sully and Mike each have to think of a plan before this ruins them both.

"Monsters Inc" is loaded with creative ideas, from the plot all the way down to the objects and animals used as inspirations for each monster. Each character is weird looking in their own way and all of them have personalities that are colorful enough to be entertained by. Billy Crystal's Mike Wazowski is easily the most energetic character in the film, and thus the most entertaining.

Only the main problem I had with the movie is the lack of comedy it is able to create. There are a couple of belly laughs and a handful of sight gags, but there is a big gap between all of them that could seriously use some of that witty dialogue that worked so well in last year's Disney film, "The Emperor's New Groove".

But "Monsters Inc" makes up for a lack of comedy by adding a lot of excitement (the finale features one of the best chase sequences I've seen from a Disney film), and some really touching moments that both parents and kids will like. If you're looking for a family film that's interesting and fun, then "Monsters Inc." will do just fine for the time being.

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Finding Nemo

Actors: Starring the voices of: Albert Brooks, Ellen DeGeneres, William Dafoe, Geoffrey Rush, Allison Janney, Barry Humphries, Brad Garrett, Austin Pendleton, Stephen Root, Vicki Lewis, Joe Ranft, John Ratzenberger, Alexander Gould, Elizabeth Perkins

Director: Andrew Stanton

Special Notes: Here are a couple of fishy fun facts to know -- did you know that 71 percent of the earth is covered by water and that there are 3.7 trillion fish that live in then ocean?

Plot: Marlin (Brooks) is an overly cautious clownfish who lives in the beautiful tropical waters of the Great Barrier Reef. Marlin lives a safe and secluded life with his only son Nemo (Gould), warning him about the dangerous world beyond. But like all young fish, Nemo is eager to explore the mysterious reef surrounding his home and the world beyond. One day, when Nemo knows better, he swims outside the safety of his father and unexpectedly is "fish-napped" and taken far from home. Nemo is placed in a fish tank in a dentist's office and quickly makes friends, but he longs for his father. Meanwhile back at the reef, Marlin has to face his fears and venture out to find his son. On his journey, Marlin's joined by a Regal blue tang fish named Dory (DeGeneres) who has a bad short-term memory but a big heart. As the two search for Nemo, they encounter all sorts of obstacles (hungry seabirds, sewer systems, man) and plenty of new friends and foes [Gill (Dafoe), Bloat (Garrett), Peach (Janney) and Nigel (Rush)], as gradually, Marlin becomes a hero in the eyes of his son.

Good: Hurray for another winner from the folks who brought us "Toy Story", "A Bug's Life" and "Monsters Inc."! Get ready for a new world of computer-animation that is not only breathtakingly beautiful to behold, but will spoil young and old alike with a new standard of animation. The movie's simple but clever premise may be as old as parents' advice, but it still makes for a funny, entertaining and realistic story dealing with issues that parents go through on an everyday basis with their kids (Marlin wants his son to be proud of him despite his fears, Nemo is disobedient and has to deal with going to school for the first time, etc.). Parents, there are several topics that will make for interesting discussions afterwards. This is one of those refreshing stories with a perfectly cast ensemble that definitely has the "awww" factor going for it. DeGeneres is delightful as the forgetful but faithful companion that gently coaxes the overly cautious Brook's to find his son. The witty one-liners and quick-witted script will entertain the mature crowd, while the beautiful colors and catchy score will mesmerize the little ones. In a summer when intense action adventure heroes (and sequels) are dominating the box office, it's nice to see Disney and Pixar come through with another example or fabulous family-friendly entertainment at it's best!

Bad: I can't imagine little ones being scared by this movie, but there are a couple of scenes with creepy looking fish that scare and then chase Marlin and Dory. If I was a kid, I think Bruce the shark would bother me the most.

Bottom Line: Just when you think it's safe to go back into the water … Disney and Pixar make it unforgettable and fun as well. Not only are the underwater scenes brilliant and fascinating, but the adventures on dry land prove to be challenging (rescuing Nemo from a dentist's office) and intense as well. You'll LOVE the ending! I have to admit … I enjoyed DeGeneres in this movie! Not only is her voice perfect for the part, but she's hilarious with her delivery as well. Never again will you look at an aquarium as a "fish tank" or the ocean as simply a place to go fishing!

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Who Am I

Jackie Chan, a top secret militant soldier, crashes into the South African jungle after his mission of kidnapping three scientists (who were experimenting with a powerful mineral) has gone array. Waking up in a village of local natives, Chan has no memory of who he is, thus being addressed as "Who Am I". His journey with aid from two female sidekicks to find out his identity leads him all the way to Rotterdam where he coincidentally discovers the location of the organization that kidnapped the three scientists. With no memory, Chan is thirsty for answers by any means necessary.

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