Transformer "dot" convention - Page 1
Forum section: electronics / general

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 21, 2025, 08:46:45 pm
content: Been racking my head for a few minutes now. It seems to me the transformer "dot" convention is a lie. It doesn't actually matter. All that matters is how you connect the transformer into your circuit.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 21, 2025, 08:52:14 pm
content: Au contraire, mon ami: it matters greatly in this circuit (a Joule thief, of which I've built many):If you reverse one of the windings it just won't work.It's true that where the dot is placed on any one winding is arbitrary, but therelationshipof the dots between windings is crucial.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 21, 2025, 08:53:44 pm
content: But wait, how does this work out flyback and other isolated SMPS topology transformers (or "coupled inductors" if you want to be extra-correct).For example if I made one winding clockwise, and another anticlockwise on the same core, are they 180 out of phase? Or in phase if I connect one of them the other way around!I never expected real world implications to be so confusing. Power electronics is truly fearsome!

author: Analog Kid
author_rank: Super Contributor
timestamp: June 21, 2025, 08:55:14 pm
content: Quote from: BadeBhaiya on June 21, 2025, 08:53:44 pmTrue and true.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 21, 2025, 10:00:34 pm
content: Quote from: Analog Kid on June 21, 2025, 08:52:14 pmI finally get it all now. What threw me off was the fact that the dots in schematics are indeed arbitrary (and windings are just windings, nothing more nothing less) but in reality current flow and magnetic flux exists, which makes it a bit more complicated to visualise. Anyway, just as you said the dot relationship is only relevant when its between windings. For that the direction of current flowing inat leastone winding has to be decided and chosen. After that has been decided, its quite straightforward to figure out the "dot" notation of other windings as long as I remember the "right hand rule".Probably should have stopped after the 4th cup of coffee(its 3am right now)

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2025, 10:21:31 pm
content: Simple meaning of the dot on two windings:If you apply a sinusoidal voltage between the dot and non-dot pins on one winding and connect a two-channel oscilloscope to both windings, the voltages on the “dots” with respect to the “non-dots” will be in phase for both windings.How you connect the polarities depends on your circuit requirement.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 21, 2025, 10:43:43 pm
content: So this raises a question for me:Let's say a guy is winding a homemade transformer on a ferrite rod. He does this like so:so that the second winding starts from the same end but is wound in the opposite direction, counter-clockwise instead of clockwise (hopefully visible in my crappy drawing).Would the two windings be in phase or out of phase? Where would the dots be in this case?And would they (the dots) be the opposite if he wound the second winding in the same direction as the first one?There must be some kind of rule of thumb here.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 21, 2025, 11:00:05 pm
content: Quote from: TimFox on June 21, 2025, 10:21:31 pmDoes this illustrate that accurately?

author: Circlotron
author_rank: Super Contributor
timestamp: June 21, 2025, 11:01:00 pm
content: Quote from: BadeBhaiya on June 21, 2025, 08:53:44 pmReal man's electronics.

author: ledtester
author_rank: Super Contributor
timestamp: June 22, 2025, 12:31:03 am
content: Quote from: Analog Kid on June 21, 2025, 10:43:43 pmTo answer the bolded question...have a look at this video of how an electromagnet is wound:Winding an electromagnet -- Ucraniumhttps://youtu.be/tQY-XjCIFuoThe direction of the first layer is from right to left and the second layer is laid down going left to right and subsequent layers alternate the traversal direction.These alternating layers don't cancel or diminish the strength of each other - i.e. the direction the winding traverses along the core doesn't matter.QuoteI think it's called the "right hand rule" -- which does involve a thumb:https://www.csusm.edu/lts/studentresources/handouts/project2_handouts/phys202_right_hand_rule.pdf

author: Analog Kid
author_rank: Super Contributor
timestamp: June 22, 2025, 01:23:30 am
content: So what then determines winding polarity? Which end you start winding from?

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2025, 01:34:04 am
content: Quote from: Analog Kid on June 21, 2025, 11:00:05 pmYes

author: ejeffrey
author_rank: Super Contributor
timestamp: June 22, 2025, 01:45:16 am
content: Quote from: Analog Kid on June 22, 2025, 01:23:30 amNo, doesn't matter which end you start from. Consider if there was only a single turn. It matters which way it wraps around the core. Pick one end of the core and look at it end on. Imagine watching the wire be wound. From this angle you can't judge depth but you can still see if the winding is going clockwise or counter clockwise.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 22, 2025, 02:22:47 am
content: Quote from: ejeffrey on June 22, 2025, 01:45:16 amThat's what I thought. But @ledtester's reply above seems to contradict this:QuoteNow I'm confused ...

author: Bud
author_rank: Super Contributor
timestamp: June 22, 2025, 05:51:30 am
content: Sometime it matters which end of the winding is closer to the core, so it would be the one you start winding from.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 22, 2025, 07:46:57 am
content: As per my understanding, assuming the current direction in the primary winding, and then using the right hand rule to figure out the flux direction and then finally figuring out the induced current direction from the flux direction is the most robust (although not very straightforward) way to figure out the dot notation in the real worldThis totally falls apart when more complex cores geometries are used in transformers though

author: m k
author_rank: Super Contributor
timestamp: June 22, 2025, 08:58:27 am
content: Quote from: Analog Kid on June 22, 2025, 02:22:47 amToo many directions.Construction time must be excluded.How winding emerges during construction is not relevant.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 22, 2025, 12:16:48 pm
content: Quote from: Analog Kid on June 22, 2025, 02:22:47 amDitto.The problem is different posters are using the word "direction" differently: whether the coil is wound left-to-right or right-to-left when the core is viewed side on; or clockwise or anticlockwise when the core is viewed end on. Only the latter matters when talking about the polarity of the secondary voltage.Talking of dots, I assume this pic is correct?

author: jonpaul
author_rank: Super Contributor
timestamp: June 22, 2025, 12:26:08 pm
content: As a trsf and magnetics since 1970s the dot indcates the wdg start.For example, to null flux, current enters the dot a pri and exits the dot on sec.Jon

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2025, 01:07:23 pm
content: Quote from: SteveThackery on June 22, 2025, 12:16:48 pmWhen using a transformer, those are the relevant drawings.

author: m k
author_rank: Super Contributor
timestamp: June 22, 2025, 07:13:58 pm
content: How are North and South?

author: paul cotter
author_rank: Regular Contributor
timestamp: June 22, 2025, 08:17:26 pm
content: Steve's post #17 gives a clear and unambiguous answer. As is said, a picture is worth a thousand words(something like that, I think!).

author: Analog Kid
author_rank: Super Contributor
timestamp: June 22, 2025, 08:46:48 pm
content: Quote from: paul cotter on June 22, 2025, 08:17:26 pmWait, not so fast there: yes, it's a very good illustration, but the last part still leaves me scratching my head:So if left-right direction of the windings is irrelevant to polarity, then what does determine polarity?Using that picture above, the two windings are wound in opposite directions to each other, one clockwise around the core, the other counterclockwise. So that being the case, does their being "flipped" end-for-end cancel that difference, with the start of each (where the dots are) being the same polarity?I have to admit when I was winding my little transformers for my Joule thieves (and I've made dozens of them), I didn't really know which way was which until I actually tested them in-circuit, at which point I came up with a standard method of winding them so I wouldn't have to test polarity on each one.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 23, 2025, 12:42:17 am
content: Quote from: Analog Kid on June 22, 2025, 08:46:48 pmWhat matters is the direction the wire goes around the core. Indulge me in this: look at the diagram we are discussing, and in your mind's eye rotate the core such that you move the left end of the core towards you (and the right end away from you). Remembering that the current enters the coil at the blob, when you are looking at it end on I think we can agree that the wire (and thus the current) nearest you is going anticlockwise round the core. Do you agree?Keep the core in this position and now consider the winding at the far end. Remember that the current enters at the blob. I hope you will agree that it, too, is going anticlockwise around the core. Thus it is, in fact, the same magnetically.Am I cheating by making the blobs fit my explanation? No! Think of it like this: when winding the first layer, do it like I've drawn on the left, so you start at the left and move to the right. Then, when you have filled the first layer and you are at the right hand end of the core, continue winding, overlapping the first layer and moving to the left. That second layer will look like the winding I drew on the right. Thus the two windings are, in fact, magnetically the same. The blobs accurately represent where the current enters each layer, including when the coil is wound continuously from left-to-right and right-to-left.Yet another way to look at it is this: in both windings, the current in the visible part of each coil is going upwards. Thus they are the same magnetically.Sorry, that's a lot of words! I hope you can make sense of it.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 23, 2025, 12:45:27 am
content: OK, that was a pretty good explanation. I think it makes sense to me now.I think.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 23, 2025, 01:03:12 am
content: I forgot the "right hand grasp" rule.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 23, 2025, 01:07:28 am
content: In both the left and right coils in my diagram the current is going upwards (in the bit you can see), so in both cases the right hand grasp rule means the fingers point upwards and the thumb points to the left.Found a better picture (it needed tweaking).

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2025, 01:16:12 am
content: Very important if you want to connect two secondary windings in parallel for more current or series for move voltage.Get it wrong and you either get 0V for series, or a dead short for parallel

author: Analog Kid
author_rank: Super Contributor
timestamp: June 23, 2025, 03:47:20 am
content: Quote from: Psi on June 23, 2025, 01:16:12 amWait a minute: 0V for series, sure.But by the same token, if you connect two secondaries wrongly in parallel, like thiswon't the transformer just sit there and hum, instead of going boom!, because the anti-phase voltages will just cancel each other out?Asking for a friend before he actually tries this with a real transformer. (Just kidding.)

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: June 23, 2025, 04:46:54 am
content: Quote from: Analog Kid on June 23, 2025, 03:47:20 amthat would be just connecting the two secondaries in series and shorting the output. the resistors are there because LTspice doesn't like to put things like L2 and L3 in parallel, and V1 and L1 in parallel. it calls that an overdefined circuit

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2025, 07:47:43 am
content: well...It's not going to go bang like if you dead shorted the primary.But it won't be happy with its secondary shorted and will get very hot very fast.There will be a lot of humming yes.Eventually there will be a bang when the primary insulation fails from the heatIn fact, one way to find the polarity of multiple unmarked secondary's is to short them like this, polite people add a lamp in series with primary first

author: m k
author_rank: Super Contributor
timestamp: June 23, 2025, 08:58:37 am
content: Easier with unmarked is to put primary and secondary in series.Then supply primary and measure secondary not from the middle.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 23, 2025, 01:58:18 pm
content: I realise I could have made my diagram much easier to interpret just by adding some arrowheads. Sorry I forgot before.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 24, 2025, 10:55:01 am
content: I had a surplus 100w 24v smps using a Fairchild fs7m0880, flyback mode. I was looking for a ~6V 10A supply to run an electrolysis cell so I decided to modify this existing offline smps. Having rewound the transformer secondary and changed components around the 431 I put it to test and the output voltage was in the range required. When I put it on load however both the transformer and the smps ic both became excessively hot very rapidly. Once I scoped it the problem became obvious- I had got the secondary polarity wrong and I was forcing the circuit to operate in forward mode instead of flyback! I remain amazed that the ic survived this treatment.