Quick Transformer Question - Page 1
Forum section: electronics / projects

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 01, 2010, 05:23:12 pm
content: Toroidal power transformers ... How (electrically) do they differ from regular old iron laminate core transformers?I'm looking at this one ...http://www.allelectronics.com/make-a-store/item/TTX-128/12VCT-8A-TOROIDAL-TRANSFORMER/1.html

author: TechGuy
author_rank: Regular Contributor
timestamp: October 01, 2010, 06:10:32 pm
content: Quote from: JohnS_AZ on October 01, 2010, 05:23:12 pmRule #1: There is no such thing as a "Quick Transformer question"Electrically there is no significant difference. EMI is different as toroidal cores usually emit more EMI than E-Core transformers. They leak a lot of magnetation energy from the donut hole. Toriod cores may have better efficiency and better magnetic coupling between the primary and secondary(s), depending on how they are wound. Toriod have superior thermal dissapation than E-Cores. as the provide more exposed surface area for the windings.60/50 hz transformers use laminated plates whether they are toriod or e-core. Some high efficiency 60/50hz transformers will use tape wound foil instead of plates to reduce eddy current lossesA e-core transformer is a a pair of toriod cores, that are joined in the center. The difference is that all of the winding is in a limit area instead of all around like in a toroid.Personally I prefer ecore transformers over toriods they are easier to mount and much easier to wind (when I make my own custom transformers). I do use toroid power inductors in non-isolated DC-DC converters, as they are better than rod core inductors, and have good thermal disspation. The only time I would buy a toriod 60hz transformer, is if I couldn't find a compatiable e-core transformer that meet the output voltage and power requirements.

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2010, 06:34:35 pm
content: Quote from: TechGuy on October 01, 2010, 06:10:32 pmAre you sure they're more leaky?Surely if the coupling then there should be less leakage?I prefer toroidal transformers because of the reasons you describe. I agree they're harder to wind though so the only sane option is to but one ready made. As far as mounting is concerned:I find it easier than E core transformers. Some have a resin filled hole with a screw thread insert which makes it easy to fix it to either the case or PCB for smaller units which can also be PCB mounted by threading thick wire through the and soldering it to the PCB but make sure there's not electrical contact between either ends of the wire.

author: Rhythmtech
author_rank: Regular Contributor
timestamp: October 01, 2010, 06:52:17 pm
content: Your question is kind of a generalized design question. If the question were contextualized in terms of application and maybe which two transformers you were looking at, it would be easier to give a good answer. If one were always better than the other in every application out there it wouldn't make much sense to keep both designs around right? Conversely if the stand iron core is 1/10th the cost and would do your application just well as the toroidal then why pay more or vise versa... Picking out components is a lot more fun than just "which is best."

author: KTP
author_rank: Frequent Contributor
timestamp: October 01, 2010, 07:24:34 pm
content: Quote from: TechGuy on October 01, 2010, 06:10:32 pmhmmm, so why did they go with a toroid 60hz transformer in a medical laser unit I scrapped for optics parts? It says primary 100-120V 50/60hz 2549VA (really? not 2550?) secondary 206V 2400VA. I was going to throw it away, but now I am curious what the core looks like...Unless somebody wants it? It is about 20 pounds...

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 01, 2010, 07:46:28 pm
content: It's for the motor (DC, not stepper) supply section of what is essentially a 4' by 8' flat bed plotter. This is a personal project for my brother, not a commercial product. The controllers will run off of an isolated SMPS. 8 amps is a bit more than I need, but that transformer would fit handily into a 2U rack enclosure.I ordered a couple of these motors, but may ultimately choose something else.https://www.surpluscenter.com/item.asp?item=10-2221&catname=electric

author: Time
author_rank: Frequent Contributor
timestamp: October 01, 2010, 08:39:48 pm
content: Toroidal power transformers are more efficient at the lower VA ratings. They also produce less EMI due to the way its constructed. The cores field is shielded - which is a big plus and might explain why you commonly find them in medical equipment. They are just a pain in the butt to make.

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2010, 09:07:41 pm
content: I've never completely unwound one but as far as I'm aware the core is just a piece of transformer steel wound in a spiral, like a snail's shell. It'll be coated in a soft resin lacquer to protect it from corrosion and the enamelled wire from damage when it's wrapped round it.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 01, 2010, 09:20:19 pm
content: Wow. I always figured that they were a big donut of ferrite or something like that.

author: Time
author_rank: Frequent Contributor
timestamp: October 01, 2010, 09:31:33 pm
content: They are a doughnut ferrite usually. There are different kinds of core materials and methods to their construction. Its broad area so it depends on things like application and budget. Look at a toroidal inductor which doesn't completely cover the core and you can get a good idea what they typically look like.I have constructed a few high voltage pulse transformers based on cores made by a company called metglas (2kV up to 60 kV <100 ns pulse widths, I feel like this kind of application is a very good test of a cores ability). They handle the extremely fast HV rise times without saturating too quickly. Their performance totally sold me so I try to always use their nanocrystalline cores when budget permits.http://metglas.com/products/page5_1_6.htm

author: TechGuy
author_rank: Regular Contributor
timestamp: October 01, 2010, 10:09:26 pm
content: Quote from: Hero999 on October 01, 2010, 06:34:35 pmYes:http://www.diyaudio.com/forums/power-supplies/89051-toroid-ee-ferrite-core-best-240v-mains-smps.html#post1040250Quote from: Hero999 on October 01, 2010, 06:34:35 pmNormally e-core transformers use split bobbin so that the primary and secondary are physically separated, this results in poor coupling. If the primary and secondary winding were to overlap in a e-core, as they do with typical toroid windings, then the coupling is very good.However, in low power, Mains transformer, poor coupling is desired to reduce capacitance coupling between the primary and secondary which increases the noise from the primary to be transferred to the secondary windings. Low 50/60hz transformers have excessive number of turns which increases the amount of interwinding and coupling capacitance.All my custom e-core transformers designs use winding interleaving that provides exceptional coupling. Since these are operate atl high frequencies, the number of turns is small and interwinding capacitance and coupling isn't a problem. For very low noise applications, use of an electrostatic shield is include in the transformer design.Quote from: Hero999 on October 01, 2010, 06:34:35 pmThe problem is connecting them directly to PCBs. When mounting a Toroid horizontially (flat) One lead of a winding is always in the inside and the other is on the outside and it round. This become difficult to get the thru-holes lined up. especially with multple turns. Ideally mounting a Toriod horizontially is bad because it leaks a lot of EMI into any nearby traces. Vertical mounting becomes a challenge when you're dealing with a low profile case and if you have several secondaries. When using a flexiable bundled cable (pigtail) it sticks out and can become snagged in the case. The pig tail also acts as a EMI antenna, especially bad when it operated in high frequency. its also easy to kink the copper in the pig tail causing a high resistance point.Ecores are fairly easy to mount to PCBs since the connections can all be lined up and face the same side. Nice a square profile on the PCB too. ECores are easy to gap if you run into a core saturation problem. and the emit less emi when mounted horzontally (flat).FWIW: I never use linear power supplies so I have no use for premade toriod transformers. 99% of my designs using transformers is for high frequency isolated DC-DC applications. For Power supplies I rely on SMPS since can provide far better voltage regulation, high efficiency and lower weight and smaller size than with a typical linear power supply. The only toriod devices I use are PCB mount vertical inductors for low power, point of use DC-DC conversion.

author: TechGuy
author_rank: Regular Contributor
timestamp: October 01, 2010, 10:17:57 pm
content: Quote from: JohnS_AZ on October 01, 2010, 09:20:19 pmFerrites materials are used for high frequency applications. They can't be used in 60/50hz transformers because they saturate too easily. Ferrite material is used for high frequencies to keep the eddy current to a minimum. A steel laminate driven with a high frequency source will heat the metal red hot in a matter of minutes (google induction heating)A typically 50hz/60 hz transformer will have a primary inductance measure in the millihenryes to 1 henry. A typical ferrite, high frequency transformer (50-100 khz) will have a primary inductance measured in microhenries.See, I told you there was no such thing as a quick transformer question!

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2010, 08:08:49 am
content: Some PCB mount toroidal transformers sold by my local supplier.http://docs-europe.origin.electrocomponents.com/webdocs/0080/0900766b80080147.pdf

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 02, 2010, 09:06:42 am
content: Another practical difference is toroids can draw a pretty big surge on power-up, so you may need a higher mains fuse rating than with an equivalent square transformer. I think this is to do with demagnetizing the residual field left from when it was last turned off.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: October 02, 2010, 10:05:20 am
content: A word of warning! Becareful how you mount these things, I was working on a machine one day that had just had its mains transformer changed (Toroid) and found that it had gone again.After much checking and head scratching I could find no fault and decided to just change the transformer again it was then I found that the rubber insulators were missing from under the fixing discs.According to the sheet supplied with the new tramsformer these are to prevent a magnetic short when mounting with the central fixing bolt.Fitted new one and ran ok.You can clearly see the rubber washer in the pic above.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2010, 10:52:52 am
content: Yes, either ends of the mounting bolt must not be allowed to make electrical contact with each other. It's not a magnetic short but an electrical short circuit which is caused by a parasitic short circuit winding being formed when either end of the bolt is in electrical contact with each other. A large current flows through the bolt and the metal short circuiting it which loads the primary more causing it to fail.