Transformer dot notation

What I am confused about is fig4s current direction in relation to the dots (I know its open circuit but assume it?s terminated) compared to the forward converter (fig5)"

OK. You have the dots drawn correctly.

"I reckon the transformer is the same and connected in the same way, if this is the case then why when the current moves from 1 to 2 on both figures 4&5 primary?s, does the current in the forward converters secondary move in the opposite direction from that of the standard transformer?"

The dot convention tells you where a positive voltage will "show up" when you feed positive voltage in on another winding with the dot being the positive end. It also tells you that flux will be produced in the core in the same direction *if* you were to *feed* current into that end of the winding.

*However*, what "normally" happens is that you *provide* a voltage on one coil (the primary) -- hence creating flux in one direction --, the voltage appears in phase at the other winding and (if you connect a load) current flows *out* of the other winding (the secondary) *thus producing flux OPPOSITE to that produced by the primary!* The goal in building a perfect transformer is to get these fluxes to completely *cancel*, so that a load gets "reflected" from the secondary back to the primary with nothing more than a "turns ratio square" multiplier.

In figure 4, (assuming Ip and Is are positive currents), both the primary and secondary are creating flux in the transformer in the *same* direction -- each winding will "see" the other winding as simply another generator (voltage source), and potentially try to fight one another if the applied voltages are out of phase. (Think about how transformers meant for either 120V or 240V have their primaries wound: Either in series or in parallel, but such that the current enters the windings to create flux in the same direction. This is essentially what you're drawn in figure 4: Two windings meant to be *driven* in parallel.)

The confusion here might stem from the fact that the transformer itself doesn't care which way current flows through the windings -- with the appropriate sources, you can force current to flow either way through any winding; all the transformer does is to take whatever currents are flowing in the windings and create flux internally. (Faraday's law then tells you what voltages to expect for those un-driven windings where you didn't already know it!) Since there's a fixed *voltage* ratio between the windings, if you connect a *load* to a winding, the turns ratio and the load then determine the current direction as well. (...which, again, will always so as to *oppose* the flux generate by the primary.)

"what is really bothering me is the secondary current direction found in multiple textbooks describing transformers."

What they're descrbing is meant to tell you how to determine the dot convention: Using the right-hand rule, force current through a given winding, and see which way the resultant flux points. In your forward converter, what's going on is "transformer action," wherein a load at one winding will have current flow through it in such a way as to *oppose the flux generated by another winding*.

Does this help any?

---Joel

Content-Transfer-Encoding: 8Bit

reggie wrote:

Yes. And Figure 3 is the same electrically but you have just used fewer turns.

The schematic representations in figures 4 and 5 are not pictures of the physical core/winding layout, and more than the schematic representation of a resistor implies some kind of zigzag inside or the schematic representation of a capacitor implies two plates instead of dozens of interleaved plates. You could renumber the terminals or move the dot to the bottom and it the same basic schematic symbol would describe a a different physical core/winding layout. That's why we call them "symbols."

--
Guy Macon

That's a lot of words, a splash ad site, and a mystery download.

A simple question would get more replies. An ASCII drawing would be even better.

Here's a diagram to help you along (fill in the details):

T1 D1 Vcc>--------. .----|>|---+----> Vout o ) || ( o | ) || ( C1 ) || ( | ) || ( === .---' '---. GND Q1 | | |/ === ----| GND |>. | | === GND

Cheers, James Arthur

I have just been reviewing some magnetic stuff and come across some discrepancies in dot notation between different documents.

I have sketched out a diagram as it is probably too hard to describe, it?s a small JPG file so don?t worry there are no virus issues in downloading this file.

a93d0fcaf3c71e8dd.html

Fig1 and fig 2 are the same electrically but I have just moved the secondary round the core to see it in my minds eye.

What I am confused about is fig4s current direction in relation to the dots (I know its open circuit but assume it?s terminated) compared to the forward converter (fig5)

I reckon the transformer is the same and connected in the same way, if this is the case then why when the current moves from 1 to 2 on both figures 4&5 primary?s, does the current in the forward converters secondary move in the opposite direction from that of the standard transformer?

I know current can only flow from pin 4 to pin3 in the forward converter due to the top diode blocking current flow in the opposite direction. I also know that pin 3 will be positive and thus forward bias the top diode, what is really bothering me is the secondary current direction found in multiple textbooks describing transformers.

I was wondering if there was some good information on the web about dot notation relating to transformer physical construction, or can someone please explain the discrepancies between current directions. (I am assuming conventional current flow from +ve to -ve).

I know its basic, but its something I want to clear up,

Thanks,

Reggie.

The dots on a transformer drawing usually indicate the start or beginnings of the windings. They could also indicate the end of the windings. The point is that all windings are dotted the same. All starts or all ends on a given transformer. They are not mixed.

This means that the instantaneous polarity is the same on all dots. If the polarity of the primary dot is driven positive, all other winding dots are also positive in the same instant.

If current is flowing into a primary dot it must flow out of all other dots (secondaries). Likewise if current is flowing out of a primary dot it must flow into all other dots. The primary is the sink of current and the secondaries are the source of current.

Mystery? It's a JPG, like the man said. I'm viewing it in L-View Pro right now. I'm supposing you did too...

The transformer is backwards for such a flyback converter. But that doesn't matter, because he drew a forward converter, which uses two diodes and a choke (except for the cheapass / ignorant "engineers" that do them without chokes... eugh...).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

right

sn't

Guy, I think you have miss understood what I was asking, probably my long winded explanation=85 Sorry=85

Joel thanks,

Your reply made sense to me. I hadn=92t thought of the possibility of two drive circuits like a split primary mains transformer. I thought I=92d always understood dot notation until I came across some lecture notes from MIT (Massachusetts institute of technology) where the current directions didn=92t make sense when applied to forward or fly back circuits.

I last studied magnetic=92s back in the early 90s, so just wanted to be sure I wasn=92t loosing my mind!!

Your response was very helpful, thanks again!

James,

I take your point about downloading =93mystery files=94 But as Tim said it was a JPG so no virus issues. Sorry for being critical but you looked at it and I have managed to get a good answer; so I shall not be restricting myself to ASCII drawings in the future!

Engineers like diagrams! Why not move with technology, throw caution to the wind and give a video clip in future! Engineers communicate in diagrams so why not on line?

Bob, I agree with what you have said, thanks!

Thanks all again,

Reggie.

Nope. I saw a bunch of ads & didn't care to click any further.

A courteous stranger ought not require of us our trust and inconvenience in addition to our advice. Besides, ASCII art means these things get archived, for future seekers.

Umm, that's a forward converter, which is what the OP meandered on about.

True, it could use a catch diode and an extra inductor if the OP really meant "forward converter," but I wasn't sure.

Since description was lacking, I offered a compromise meant as a starting point--one which would adapt easily to suit either case--and a gentle intro to ASCII art in the bargain. "Fill in the details" was supposed to encourage the OP to modify it to his purpose.

Cheers, James Arthur

Sure, we like diagrams, no question about that. I did not download or view yours, but I'm glad you got the answers you wanted from others who did.

Personally, I prefer pencil and paper.

Cheers, James Arthur

..

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This it true, but that=92s like saying one shouldn=92t move from 3.5 inch floppy disk to DVD. I believe engineers should move with the times and try and always better the tools they use.

We are in fact communicating via the internet with a VAST amount of people, something that is impractical with pencil and paper alone. I did in fact use pencil and paper to produce my drawing, I just scanned it so all you people could see it.

I prefer forums that enable one to upload diagrams, but I cannot seem to find any with power supply engineers in.

I apologise for the adverts on the file sharing site, but when I need a diagram in the future I will be using the same method.

Each to their own and all that...

Since you don't know me, I've added a smiley face to my paper-and-pencil comment above. :-)

Here we post pictures to alt.binary.schematics.electronics, then alert folks to this in our posts to sci.electronics.design. That's not perfect though, since many people can't get binary newsgroups.

No need to apologize.

That site you used was just a little pushy. It immediately pops up and prompts with a download window. That's actually convenient, but speaking of keeping up with the times, surely you understand that thanks to Bill Gates, following a link to a new site full of ads that immediately wants to download something--even a JPEG--to your computer is reason for pause?

For example,

Anyway, it turns out you're a good guy, it's a decent website, etc., etc.

I'll happily download your stuff in the future.

Best wishes, James Arthur