# Full wave bridge w/ only two diodes given center tapped x-former?

Discussion in 'General Electronics Discussion' started by Easymkay, Aug 7, 2012.

1. ### Easymkay

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Aug 3, 2012
I am no longer convinced that you [always] need more than 2 high current diodes for a full wave bridge, to shape the wave-form of a North American 110v wall receptacle from AC into DC. In my case, the output of my transformers secondary is center tapped to negative. Here is a picture of what I am talking about: http://en.wikipedia.org/wiki/Rectifier along with a description of "single phase full wave rectification" with only two diodes and a center tap x-former. Is there a reason this AC to DC circuit wouldn't work this shottky barrier diode, which, if you scroll down on its data sheet here: http://www.sanrex.com/images/PDFs/BKR400AB10.pdf appears to be nothing more than two massive diodes already wired in proper polarity/configuration?? Thanks.

2. ### Alchymist

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Apr 16, 2011
You are confusing the terms full wave and bridge. Full wave uses 2 diodes and a center tapped transformer. A bridge uses 4 diodes and does not need a center tap.

In a full wave each half of the transformer conducts on alternate half cycles. In the bridge, the whole winding conducts on both half cycles. To obtain the same output voltage on a full wave as with a bridge, the full wave winding voltage must be double that of the bridge. The bridge is more efficient on a cost/weight basis than a full wave. I'll let you work on what a full wave center tapped circuit is used for.

3. ### GreenGiant

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Feb 9, 2012
This diode can be used to create the full wave rectifier with the center tap as shown in the picture on Wikipedia, so long as the waveform is less than 200Vp-p and doesn't draw more than 400 Amps

As Alchymist stated (correctly) you are confusing bridge and rectifier

4. ### Electrobrains

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Jan 2, 2012
Those big diodes will work wonderfully in a center tapped rectifier configuration, as long as the specs are not violated!

That means for that Schottky block (amongst other things):
Reverse Voltage< 100V
Average Forward Current< 400A

If the voltage peaks (and surges) from your center tapped transformer (at any time) reach higher voltage than 100V (between center tap and outer winding), you should not use the diodes. Be aware of that the peak voltage of the sine wave is about 1.4 times higher than what you read on your DMM display. (for 110V: approx. 156V).
That gives you a max AC voltage for those diodes of about 70VAC. To that you must add a security factor for surges, power line variations, load dependent changes of transformer etc. I would not use 100V diodes with an AC transformer giving more than 50V output (without load).

Probably true with respect to weight and transformer cost/efficiency, but be aware of that the bridge configuration gives you a double voltage drop over 2 diodes (i.e. double the power loss in the diodes). This will especially be noticeable in the overall calculation of low voltage, high current output circuits. And don't forget the price of the two extra power diodes.

Last edited: Aug 7, 2012
5. ### Electrobrains

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Jan 2, 2012
Hm, that's an interesting idea!
The transformers supplying the half waves one at a time! Funny!
Just some thoughts:
1. The measured output voltage of each transformer must not exceed 70VAC
2. I think the transformers will be very noisy because of the asymmetrical load
3. You will have much power loss in the transformer core, because of the non sinus wave. The one-transformer with the full wave rectifier will still have the magnetic sine field continuously flowing, just loading diffent parts of the secondary winding at different times.
4. It will matter in what order you connect the windings in series of the two transformers (the phase of the sine voltage must be different at the two anodes of the diodes)

6. ### Easymkay

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Aug 3, 2012
Thank you for delineating, Alchymist; clarifying GreenGiant; more facts still, Electrobrains. This is to rectify into DC, an AC "stick" welder made from 2 rewound microwave oven transformers, like this: http://www.instructables.com/id/Build-a-Microwave-Transformer-Homemade-Welder/?ALLSTEPS. Since it has two transformers' outputs wired in series, one might tap between them (it is not currently wired that way, I mis-spoke)...but maybe not at the expense of a division of voltage/efficiency of the 2 diode system, like Alchymist describes. On the other hand, Electrobrains describes downsides to the bridge, too. My guess is that for the weld to start/maintain arc, it simply wouldn't work with less than the ~40 V (DMM measured [x 1.4 = peak VAC according to above post]) equals 56 VAC peak. That's the only measurement I took other than to make sure the two rewound x-formers were not "fighting phase'. If it matters I suppose I could test output current to load with my clamp ammeter function. ???

7. ### Electrobrains

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Jan 2, 2012
Would be interesting to hear the result!

Just a question from somebody not knowing much about welding:
What is the advantage with DC welding over AC?

8. ### Alchymist

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Apr 16, 2011
Not to rain on your parade, but there are a lot of drawbacks to what you are attempting. Not the least is no control over current, a must if you want quality welds. Another is lack of an inductor to smooth the dc. And don't go looking to do any heavy welding - 10 Gauge wire won't handle much over 30-40 amps. There is a reason that real welders have copper cables ranging from #6 on up.....just saying. On the other hand, if you are playing with scrounged parts with little money invested, it's a great learning experience.

9. ### Easymkay

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Aug 3, 2012
Wow, you folks are way more grounded in the physics/theory than me. I don't have the level of understanding of some of you, but I sure appreciate that you do. Here is a picture I took back when I wound one of the two transformers' secondary with #10g copper braided wire. The other one is wound similar, and they are connected in series such that where their two secondaries are connected into one, there is a point that could potentially be used for a center-tap. As of now, that "center-tap", albeit in reality just a tap point between two separate secondaries of two separate cores of two separate x-formers, is what might be used for full wave with only two diodes. As it stands and functions today, that point between the two is connected to nothing other then the other transformer. I am only assuming they are in phase with each other because when I measured them in series their combined voltage seemed double their individual. Hope this helps inform all the more complex concepts you are mentioning. It seems like it should be the bridge option in order to keep final output voltage high enough to strike arc, as first described by Alchymist...on the other hand...???

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10. ### Easymkay

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Aug 3, 2012
Points well taken. That most welders out there will outperform my bush league garage project is true, just not for this price. On the other hand, the authors of that project were at least ostensibly professors of engineering at MIT, and their stated reason theres would outperform commercial cheap AC stick welders is that their design is pure copper. I'm certainly also glad that welders (I'm very novice) take such pride in their work, particularly structural enough for life to depend on it. This is nothing like that. This is more for tinkering and light projects.

11. ### Easymkay

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Aug 3, 2012
Basically, I'm reading that: 1) DC is overall smoother to deal with and better to learn on because, although they are all different, it more closely resembles some of the other modern welding systems; 2) It's supposed to be more versatile because you can choose whether to make the electrode the workpiece or the filler material. In so doing you can weld thin metal better with one polarity, thick better with the other. At least that's what I read...I'm new to all this.

12. ### Easymkay

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Aug 3, 2012
Drawbacks to be sure. If this project-in-evolution works, adding a inductive "reactor" like you describe might be next.