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Revision as of 22:34, 30 August 2010

I've done a substantial rewrite of Diode bridge; at least two remaining items on my agenda for it:

• Rectifier is a redirect to here, but it should be a disamb between the sense of Rectifier diode (as opposed to e.g. laser diodes and LEDs) and the sense of Rectifier circuit; the latter is what should be linked from here.
• The attractive diagrams are not ideal, and my text tries to compensate for that on one hand, and to anticipate the two kinds of changes that could be made to them:
  • If they are edited without being completely redone, the letters "AC" should be removed (twice) in each diagram except the last.
  • If they are redrafted, putting the diodes one to an arm of a diamond-shaped figure, the inputs at opposite sides, the positive output at the top, and the negative output at the bottom would be preferable. (Should color be made so crucial to understanding? How about making only what is colored now, in the diagrams that have color, leaving the others, where no current flows, dotted. Not a convention i recall ever seeing in electronics, but effective w/o color.) The text description of the flows would change for that, and the wording would be much clearer and less awkward.

I'd also like to see half-wave rectification described, tho maybe it is just as well in Rectifier circuit. I'm not sure about voltage doubler and tripler circuits, as to inclusion at all, or whether they belong with the bridges. --Jerzy 05:37, 2004 Jan 18 (UTC)

I would like to see an extension for a three phase AC rectifier (with 6 diodes) and a reference to usage in automobile alternators, and a reference to the inventor, mr. Graetz. MH 213.51.209.230 19:00, 24 May 2004 (UTC)

Changed the diagrams to the new design, please check the text to be sure it matches. I went with colours over the monochrome, because I couldn't get the dotted diodes to look right. Let me know if you need any tweaks to the diagrams. -- DrBob 22:05, 5 Mar 2004 (UTC)

I have another suggestion. There really ought to be an entry for the general concept of the "bridge" ciruit, and I do not think there is one yet, so that readers can understand the meaning that the name diode "bridge" conveys. The current "electronics" entry is really to a networking meaning, not the circuit meaning. Bridge circuits are an extremely important general concept, in both a practical and a historic sense, and are closely tied to the idea of differential measurements. This would help to separate the rectification function from this particular kind of rectifier that uses a bridge configuration. By the way, this circuit has other uses. For instance, if you can stand the extra diode drop, a diode bridge is a way to build a circuit that does not care which way the battery is connected, so it is sometimes used even when AC is not involved. Also, diode bridge configurations are used in switches and modulators. AJim 16:57, 12 Mar 2004 (UTC)

Great, AJim; Edit boldly. The 'graph following the second color diagram does mention the reverse-connection aspect, but ignores the forward drops. But i don't think i ever asked "why 'bridge'?" and you've already motivated that term significantly for me on this talk page.

Let me know if you haven't already noticed our Redirects and Disambiguation pages (Dabs); if you're interested in Rectifier or Rectifier circuit, they're relevant, & i'll point you to descriptions somewhere. --Jerzy(t) 18:26, 2004 Mar 12 (UTC)

Hey, I created an image to show the effects of a filter capacitor but I can't figure out how to upload the image so it can be displayed right. I was going to place the image right before the paragraph that started with, "The capacitor and the load resistance have a typical time constant τ = RC where C and" in the "Output smoothing" section. I uploaded the image to image shack in the hopes that some one else will have better luck uploading the image. "http://img162.imageshack.us/img162/2181/filtereffectsmj1.png" - Bob Leny

Poly Phase Rectification

I really thnk that three phase and polyphase rectification should be in a separate article and not included under diode bridges. Anyone else agree? Light current 06:40, 1 August 2005 (UTC)

Maybe it's just me, but I really liked all this information under rectifier. After all, a bridge rectifier is considered a full wave rectifier. This would also explain what to do with all the other rectifiers, like the half wave and the polyphase rectifiers.... Bob Leny

Re: "As long as the load resistor is large enough ..., the above configuration will produce a well smoothed DC voltage across the load resistance. In some designs, a series resistor at the load side of the capacitor is added. The smoothing can then be improved by adding additional stages of capacitor–resistor pairs, often done only for sub-supplies to critical high-gain circuits that tend to be sensitive to supply voltage noise."

Sometimes it is hard to see what something is used for. Is this how a UPS supplies a perfect sign wave for delicate electronics like computers?

not an article about power supplies

The extension of the article from diode bridge to DC power supplies that use diode bridges is really incomplete. Maybe this topic should be moved to a power supply article? There are a number of important topics that an encyclopedic power supply article could discuss about this circuit configuration, such as the potential of inrush current to damage the diodes, and the harmonic distortion that such a supply can impose on the AC source (to the extent that many supplies now include "power factor correction" circuits), the need for "bleeder" circuilts, and other practical information, most of which is not about diode bridges.

AJim 05:07, 7 July 2007 (UTC)

other uses of diode bridges

they are more important than just rectifiers (as I mentioned earlier)

• switches

AJim 05:07, 7 July 2007 (UTC)

Indeed, I've also seen it used in an application with DC input, but a non-guaranteed polarity. (The plug fits either way). In that case, the bridge simply ensures a consistent polarity is provided to the rest of the circuit. (This is actually very smart use of the bridge, as it can prevent somebody from frying the circuit by plugging it in backwards). 66.254.241.199 (talk) 23:24, 30 January 2008 (UTC)

Editing "Output smoothing"

As mentioned above, this section probably belongs in an article about power supplies. Nevertheless, as I judged this section to be quite confusing to those unfamiliar, I've prepared a fairly extensive re-write to add clarity and content.

Consider-

"pulsating magnitude"

Surely it should read "amplitude"; and "pulsating" seems an inaccurate description of what I believe is correctly called "AC ripple".

"One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor."

First of all, the first sentence isn't "One explanation", it's an incomplete explanation. And "In less technical terms" isn't less technical, it's the back half of the rest of the story - the discharge of the capacitor. What about charging the capacitor?

"Also see rectifier output smoothing."

See it where? There's no link.

There's a lot more confusion, but I don't want to be too harsh on those who obviously put in a lot of effort to create a what is overall a fairly informative article.

I'll post the re-write below, for comments prior to the actual edit. (As you may have noticed if you've checked the "History", I mistakenly saved a major edit earlier - apologies all around.)

== Output smoothing ==

A bridge rectifier supplies an output voltage of fixed polarity but varying amplitude, and consequently varying current amplitude, with the resulting waveforms exhibiting alternating peaks and valleys. These peaks and valleys occur at twice the frequency of the AC input. (see diagram above). This regular variation in amplitude is usually referred to as "ripple", and can often be heard as a low level hum from the speakers of AC-powered electronics when no source audio is present, for example, during the gaps between tracks on an audio compact disc (CD). (A fault in such equipment, and more commonly in the interconnecting cables, may produce a very loud hum; however, the presence of a loud hum rarely indicates a serious malfunction or failure.) For electronic circuits and some other applications, the addition of a power supply filter is required to lessen the variations in amplitude of the DC voltage and current present at the bridge output - to "smooth" the output. A simple filter may consist of a single capacitor (see diagram below), and is thus called a capacitor filter.

The function of the capacitor, known as a reservoir or "smoothing" capacitor, is to lessen the variation in voltage of the DC output of the bridge - to "smooth" the waveform. The capacitor provides a low impedance path for the varying output voltage of the bridge, reducing peak voltage across, and thus peak current through, the destination load - the device the bridge is meant to power. The bridge output also "charges" the capacitor as its voltage rises. As the voltage falls, the capacitor discharges, resulting in a higher voltage at the load than is present at the bridge output. Thus, the drop in the output voltage and current of the bridge is compensated for by the loss of charge in the capacitor. This charge flows out as additional current through the load. In this manner, the variations in bridge output voltage and current are reduced, and their waveforms smoothed.

The simplified circuit shown has a well deserved reputation for being dangerous, because in some applications, the capacitor can retain a lethal charge after the AC power source is removed. If supplying a dangerously high voltage, a practical circuit should include a reliable way to safely discharge the capacitor. If the normal load can not be guaranteed to perform this function, perhaps because it can be disconnected, the circuit should include a bleeder resistor connected as close as practical across the capacitor. This resistor should consume a current large enough to discharge the capacitor in a reasonable time, but small enough to avoid unnecessary power waste.

Because a bleeder sets a minimum current drain, the regulation of the circuit, defined as percentage voltage change from minimum to maximum load, is improved. However in many cases the improvement is of insignificant magnitude.

The capacitor and the load resistance have a typical time constant: ${\displaystyle \tau =RC}$, where C and R are the capacitance and load resistance respectively. As long as the load resistance is large enough such that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a smoothed DC voltage across the load.

In some designs, a series resistor on the load side of the capacitor is added. The smoothing can then be improved by adding additional stages of capacitor and resistor pairs, often done only for sub-supplies to critical high-gain circuits that tend to be sensitive to supply voltage noise.

The idealized waveforms shown above are seen for both voltage and current when the load on the bridge is purely resistive. When the load includes a smoothing capacitor, it is a partially () load, and both the voltage and current waveforms are greatly changed. While the voltage is smoothed, as described above, current will flow through the bridge only when the input voltage is greater than the capacitor voltage. For example, if the load draws an average current of n Amps, and the diodes conduct for 10% of the time, the average diode current during conduction must be 10n Amps. This non-sinusoidal current leads to harmonic distortion and a poor power factor in the AC supply.

In a practical circuit, when a capacitor is directly connected to the output of a bridge, the bridge diodes must be sized to withstand the current surge that occurs when the power is turned on at the peak of the AC voltage while the capacitor is fully discharged. Sometimes a small series resistor is included before the capacitor to limit this current, though in most applications the power supply transformer's resistance is already sufficient.

Output can also be smoothed using a choke and a second capacitor - an inductance-capacitance or LC filter. (In electrical and electronic circuits, "L" represents inductance.) A choke tends to smooth current more than the voltage. However, due to the relatively high cost and weight of an effective choke, compared to a resistor and a capacitor, this method is not employed in modern equipment.

Some early console radios generated the speaker's magnetic field with the output of the radio's high voltage ("B +") power supply, which was then routed to the other circuits. (Permanent magnets were considered too weak for good performance.) The speaker's field coil thus performed two functions simultaneously: it acted as a choke, filtering the power supply, and it produced the magnetic field for the speaker.

It needs a little more work. I'm going to sit on it for a while - see if I draw any comments.

Cheers!

Rico402 (talk) 00:03, 24 June 2008 (UTC)

Editing "Basic operation"

I've prepared an extensive re-write of this section which I've posted below for comments prior to the actual edit. (After considerable editing, I believe this is my final draft.)

Cheers, Rico402 (talk) 16:24, 7 July 2008 (UTC)

==Basic operation==

Free electrons in a conductor are draw to the positive pole of an electrical source, such as a battery or a DC generator, causing an electric current to flow through the conductor from the negative (-) terminal to the positive (+) terminal of the source. This is actually contrary to the conventional model of current flow originally established by Benjamin Franklin, and still followed by many engineers today, in which current appears to flow from the positive pole to the negative pole. In most applications, the actual direction of current flow is irrelevant. However, in the discussion below, current flow is referenced to the actual direction in which electrons move within a conductor, rather than the conventional model.

In the diagram below, the AC voltage on the upper input terminal, which is connected to the left corner of the diamond, is on the positive half-cycle of the "sinusoidal wave", and the voltage on the lower input terminal, which is connected to the right corner of the diamond, is on the negative half-cycle. Current flows from lower input terminal and follows the blue (lower) path to the negative output terminal, and from the positive output terminal returns via the red (upper) path to the upper input terminal. (Regardless of polarity or direction of current flow, in the accompanying diagrams the terminals on the left are always input, and the terminals on the right are always output.)

In the next diagram, the voltage on the upper input terminal is now on the negative half-cycle of the wave, and the voltage on the lower input terminal is now on the positive half-cycle. Current flows along the blue path to the output, and returns via the red path.

Note that current passes through only two diodes in the bridge at any given time, and that the diode symbols always point in the direction opposite to that of actual current flow. (In the conventional model, the diode symbols point in the hypothetical direction of current flow.

In both states shown in the above diagrams, the upper output terminal is always positive, and the lower output terminal is always negative. Since this is true whether the input is AC or DC, this circuit not only converts AC to DC, but can also provide "reverse polarity protection". That is, it permits normal functioning of a DC-powered device when the input polarity is reversed, such as when batteries are installed backwards or a DC power supply is improperly connected, and protects the device from any damage that may result.

Prior to the introduction of the integrated circuit or "IC", diode bridges were constructed from individual or "discrete" components. Since about 1950, single four-terminal components containing four diodes in a bridge configuration have been available for most applications. Diode bridges are now available in a variety of voltage and current ratings.

As a mechanical engineer, I am not an expert, but when I learned the 'conventional' notation, it was not in reference to the charge, but rather voltage. The positive side of a battery has the higher voltage, even though the charge is negative, and free electrons flow TO the positive terminal, not from. The reason I bring this up is because I don't necessarily believe the discussion on 'conventional' vs 'actual' current flow in the "basic operation" section is necessary. Again, just my opinion as a mecahnical engineer. Mcanadian (talk) 22:34, 30 August 2010 (UTC)

1. Stutz, Michael (stutz@dsl.org), 2000 "Conventional versus electron flow", All About Circuits, Vol. 1, Chapter 1