Inductors & Transformers - Page 1
Forum section: electronics / beginners

author: olsenn
author_rank: Frequent Contributor
timestamp: March 13, 2012, 10:02:03 pm
content: After getting by without ever really needing to know anything about the use of inductors (except for filters), I have begun trying to learn how to build a simple transformer, as well as a boost (step-up) converter... after doing some prototype work I realize my theory doesn't amount to much, because nothing is working.First of all, if I want to build a transformer to step-up 12V to 120V, could I just use a 80uH inductor and an 800uH inductor? Do the values matter, or only the ratio? Secondly, will a 1KHz wave suffice, and does frequency matter? Lastly and most importantly, should the ground terminals of the two inductors be tied together, or should I just place the inductors close to each other? I am currently using through hole inductors (look like resistors) and no iron core... I'm looking at very low power use here.Please forgive my ignorance with this stuff!

author: Psi
author_rank: Super Contributor
timestamp: March 13, 2012, 11:00:30 pm
content: Quote from: olsenn on March 13, 2012, 10:02:03 pmAre you wanting to use a transformer or an inductor to do the stepup?Quote from: olsenn on March 13, 2012, 10:02:03 pmHow do you mean? If you put two inductors near each other the air gap will make it more an RF circuit than a transformer.RF is much harder to work with than a simple iron or ferrite transformer.Quote from: olsenn on March 13, 2012, 10:02:03 pmYes, the frequency has to match the material that the transformer core is made from. There are many different kinds of materials, iron core , ferrite core etc.. and they all have a frequency range that they work for.Iron for example is good for low frequency stuff like 50Hz and there are lots of different types of ferrite for high frequencies.Quote from: olsenn on March 13, 2012, 10:02:03 pmNow we're getting into RF and i'm no RF engineer.Placing two inductors near each other is more like trying to build a transmitter and receiver than a transformer.Quote from: olsenn on March 13, 2012, 10:02:03 pmme too, placing two separate inductors near each other and getting them to couple is way outside my experience

author: amspire
author_rank: Super Contributor
timestamp: March 13, 2012, 11:39:50 pm
content: In a transformer, the primary and secondary windings have to share the same magnetic field so they have to be on the same core.From your question, I couldn't work out what you are trying to do. Turn 12V DC into 120V dc, or do you want 120V AC? If so must it be a sinewave, or can it be a square wave? What power?Basically transformer design is a great thing to learn, but it is not trivial. you have to find a core with the right permeability, you have to calculate the maximum flux strength in the core to make sure you are not saturating the core. You have to have enough turns in the windings to ensure that the current due to the inductance of the windings is much lower then the transformed current. You have to make sure the winding resistance is not too high. You also have to look at the maximum frequency capability of the core material. For higher frequencies, capacitance between windings becomes a factor.An 80uH primary inductance for the 12V winding would cause an inductive current of 23A with no load on the secondary winding. You would normally want a primary with an inductance in the high mH to several H for a 1KHz transformer. But it does depend on the application.

author: olsenn
author_rank: Frequent Contributor
timestamp: March 14, 2012, 12:17:48 am
content: Thanks for all the responses guys; I appreciate the help.QuoteThe 12 - 120 volt conversion was just an example, I am really trying to learn in the most generic way possible how to build a transformer. Essentually thou, I would like to pass an AC signal (sine, square or whatever) into an inductor and receive the same signal across the other inductor, but with a much larger amplitude.QuoteSo what confuses me here is how images I see online show two sepserate coils wrapped around an iron core, as can be seen below, but without a common node between the two coils. However, in the schematic below, which represents what I am trying to do, the two inductors are in fact tied together?png image hostingimage hosting gif

author: Psi
author_rank: Super Contributor
timestamp: March 14, 2012, 12:35:29 am
content: Quote from: olsenn on March 14, 2012, 12:17:48 amHaving a common for both sides of the transformer is optional, It really doesn't matter from the transformers perspective.That sparkplug circuit bothers me, it just doesn't seem right to do it that way.

author: IanB
author_rank: Super Contributor
timestamp: March 14, 2012, 12:57:44 am
content: Quote from: olsenn on March 13, 2012, 10:02:03 pmThere are many tutorials and explanations on the web describing how transformers work. Before you try to build one, you should really seek out some of these tutorials to learn what you are trying to do. Theory always comes before practice, although practice can help a lot to understand theory.In really simple terms, a transformer has two or more coils (inductors) joined together or coupled by a magnetic field. There is in effect a magnetic circuit connecting the two sides of the transformer. Without this magnetic coupling a transformer cannot work. So simply putting two inductors near each other does not make a transformer--the magnetic field of the two inductors is not joined together unless the coils share a common core.

author: IanB
author_rank: Super Contributor
timestamp: March 14, 2012, 01:01:09 am
content: Also note that building a simple transformer for learning is easy enough, but building agoodtransformer is quite hard. Don't expect to do so without a solid understanding and a good bit of skill.

author: gregariz
author_rank: Frequent Contributor
timestamp: March 14, 2012, 01:18:22 am
content: Quote from: Psi on March 14, 2012, 12:35:29 amTypically you'll be driving a sparkplug with a high voltage pulse, as a result you'll need a pulse transformer rather than an AC transformer. The principal difference is that you do want the core to saturate.With regard to what impedances to choose, it is critical to choose them based on both their reactance and the turns ratio.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 14, 2012, 01:23:24 am
content: Quote from: olsenn on March 14, 2012, 12:17:48 amThe ignition circuit is a special case---in fact,auto people don't call it a transformer for that reason,it is an "ignition coil".Under normal conditions,T3 is conducting,so dc current is flowing through the primary,storing energy in the resultant magnetic field.To make a spark, T3 is turned OFF, the primary magnetic field collapses,causing (by transformer action),a high voltage to appear across the output of the secondary,in series with the dc "ignition"supply.The multi winding "chokes" in many switchmode supplies operate in a similar manner---they are energy storage devices,whereas a normal transformer is not.My suggestion is to look at as many real transformers as you can,to "get a feel for"what is involved.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 14, 2012, 02:01:20 am
content: Further to the above,it is easy when looking at an ignition system,for those of us brought up on linear transformers to think:"Input switched so it looks like AC,then transformed to high voltage".You can live with this theory,until you start seriously looking into ignition circuits,when the subleties of the old "Kettering ignition system" become apparent!.

author: G7PSK
author_rank: Super Contributor
timestamp: March 16, 2012, 06:43:21 pm
content: Tie the two wingdings on a transformer and it becomes an auto transformer, you can see that if you imagine the wingdings in your drawing as being end to end with the tied ends as a center tapping. The step up or down is directly linked to turns ratio.

author: robrenz
author_rank: Super Contributor
timestamp: March 17, 2012, 01:32:25 am
content: Quote from: G7PSK on March 16, 2012, 06:43:21 pmG7PSk, I know you know this, but for those who may not think of it. When you turn it into a autotransformer you loose all isolation properties of the oringinal transformer