Step-up transformer design

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Hi,

my ESL-project is now in the "transformer"-phase. I have one ESL assembled, and tested with various toroids and EI transformers 230v/6V as discussed in other threads.

I have been playing with the idea of making my own transformers and have looked into toroids; they are not suitable for manual winding. It will take ages to wind a toroid by hand.

Sourcing new parts to built a EI transformer is possible; RS online sells ferrit cores but they are expensive.

So; I have dissasembled a 50VA laminated EI transformer, and sourced several rolls of insulated copper wire in different sizes.

I also have a shipment of suitable bobbins in the mail, so I can try several configurations and easily swap, test and measure.

Also, I have built a jigg for winding copper wire onto the bobbin complete with a digital counter. Picture will be posted later.

The bobbin for this transformer is separating primary and secondary windings - I am not sure if this is what I want?

Has anybody here tried a similar approach, and are any advice? Wire gauge primary/secondary, what insulation, insulation between layers or not?

I will play with different transformer designs for a while, and if not happy I will order a pair of transformers from Sowther.

Regards,
Bent
 
Hi,

I don't think ferrite cores would be suitable. Ferrite has a way too high coercivity, the core losses would be large.

Also, I've never seen separated pri and sec windings side-by-side on audio transformers. They are always wound interleaved. I think, with separate windings, the coupling may be too low, leading to increased leakage inductance and less bass response.

Interleaving has its problems, too: you need better isolation, and the high frequencies may suffer due to capacitive coupling.

I would recommend reading the description of the Williamson output transformer, to get an idea of the interleaving technique.
Another good article: http://www.geofex.com/Article_Folders/xformer_des/xformer.htm

Kenneth
 
To answer some more questions-

Enamel wire has sufficient insulation to the adjacent wires, but insulation between layers is absolutely necessary.

Wire gauges determine the I2R losses (copper losses), so (after determining the number of turns you need,) choose the biggest wire that will fit on the bobbin. Don't forget that the insulation layers take up space too.
 
Hi, thanks for your replies!

The core I will use for my experimenting is a laminated core, not ferrite.

And Yes - I have thought about rewinding the secondary of a toroid. But I will first have a go at the EI-transformer it should give a faster result.

But if I aim at having 8ohms at the primary it will require 90m of 24 awg if my understanding is correct.
 
[...]

But if I aim at having 8ohms at the primary it will require 90m of 24 awg if my understanding is correct.

Bent,

don't confuse impedance with DC resistance. The length and gauge of wire will determine the DC resistance, which you should try to minimize. The 8 ohms you mention is AC impedance, which determines the turns ratio in a normal transformer. For an ESL transformer, the design procedure is a little different:

0. Choose a core size based on the desired audio power at the lowest desired frequency ("guesstimating")
1. Figure out what peak-to-peak drive voltage you want on the ESL and what the maximum peak-to-peak output voltage of your amp is.
2. These two values determine the turns ratio N you need for your transformer.
3. Determine the number of primary turns Np you need, based on amp output voltage, core size, and core material.
4. Determine number of sec. turns: easy, Ns = Np * N
5. Figure out how to interleave the windings (this is the most difficult part, and, together with step 0, will determine the bandwidth of the transformer!!!)
and how thick all the inter-layer insulation will be.
6. Based on 3-5, figure out how thick your wire can be before you run out of bobbin space!
7. Measure & listen
8. Refine
9. Goto 7 :)
 
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I know you are talking about transformers but wanted to toss this in. SimiSouth has
some new silicon carbide power jfets that handel up to 1700v! With these a direct
drive electrostatic amp might be realy easy to build. True scary as heck with all that
voltage though.
 
Hi,

@Woody
1700V -of which maybe 1400V are safely useable- is not enough but just for small HF-Panels and Headphone-panels. You need at least something with >2.5-3kV to drive a medium sized hybrid panel. That asks for cascading of transistors -in which case You don´t need those expensive power-JFETs or You use Power MOSFETs or IGBTs with sufficient high breakdown voltages (see IXYSS semi).
The Semisouth JFETs are different anyway, as they are enhancement types with a positive Ugs while typical n-channel JFETs are depletion mode types with negative Ugs values.

jauu
Calvin
 
re:repost, the higher the voltage(Vds) the better off you are. a typical micro wave oven full wave rectified is about 3120v ,cascading 2 fets at 1700v would give you 3400v it would take 4 1000v fets to safely handle 3120v without the risk of breakdown also causing a 100% increase of parts count.when your dealing high power h.v. the more parts the more problems.not to mention cost (which can be questionable)and reliability. jer
 
Hi, is there a formula that can be used to determine how much wire of a given gauge a coilformer with dimensions X*Y*Z can hold?

I have simulated this in Excel presuming that the coilformer crosssection grows by 2*wire gauge for each layer but there must be a simpler way?

regards,
Bent
 
i have done much research on transformer design and in doing searchs online i hace found many calculators here is one that you might find useful Transformer Calculation Help File i also have several well written spreadsheets and other url's and progrms also but this one determins turns and wire size wire for a user selected core size.see if this helps you.if not i have more.also i inclued a zip of another one. jer
 

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Hi, is there a formula that can be used to determine how much wire of a given gauge a coilformer with dimensions X*Y*Z can hold?

I have simulated this in Excel presuming that the coilformer crosssection grows by 2*wire gauge for each layer but there must be a simpler way?

regards,
Bent
Sure. There is so called space ratio: total wire cross area to former (core) window area.
Max recommended is 0.4. If you need some additional isolation layers ratio will suffer... Do not forget the swelling - the farther from the core the more round the turn gets.
Alex
In real life you can't make fully filled former from the first iteration, i.e. by making transformer/coil. The conservative approach lives you with some space on avail. Optimistic lives you with incompleted primary or secondary.
 
i have not tested that one,jonas,if i had one i would certainly test it for you and post the results. however i did find a few more trnasformers laying around today to mess with,1# a toriodial somewhwere between 100 to 150 watts i'm guessing because it is in a steel sheilding case and is about the same outside dimensions as the previous one i tested, the results were roughly the same almost identical except the resonate peak was at 840khz and less ringing on a square wave which resulted in a much cleaner waveform up to 120 khz.2# an e-i tube type power transformer which failed miseribly, it looked like a bandpass filter tuned to peak of 225hz not even at the 60 hz it was designed for and would not produce a square wave. 3# e-i tube type output for push pull el-34's(6ca7) showed good square wave from 10hz to 600hz with much much more ringing at 175khz ,sine wave test was flat 10hz to 2.2hz with a consistant slope from 2.2khz to +8db at 20khz continuoisly rising onto 100khz and up.which tells me why this amp sounded harsh and trebley,yet a warm low end, when i used it for my guitar (which i liked, but not for music)this is the same transformer i used to drive my mini esl driver when i discribed what i heard (i don't remember if it was this thread or another)coming from it.all of the test made no diffirence in response by swapping the primary for the secondary or vice versa except for the loading of the signal generator level which remained constant reguardless of frequency,changing only the voltage ratio.once again a toroidial power transformer wins hands down and unmodified. jer
 
Hi all, ant thanks for all information sharing regarding transformers.

I am ready to (re)wind my first transformer now, and wonder how does one test a transformer?

I have the winding jigg set up, and a idea of how the transformer should be wound.


The measuting instruments I have are:

- Ears
- Measuring microphone, Behringer MC6000 with mic-amp and various software
- Oscilloscope, 1-channel PicoScope for PC
- Simple but accurate multimeter.

Regards

Bent
 
basicly you have to have signal generator that produces a sine wave(square is optional but can tell you alot about the integraty of the transformer you are testing) and scope and/or a vom.if you have sound card and any decent amplifier can work as a signal generator,i use audacity software it is a great little package simple to use and it is free.set it up for the highest sample rate and resolution you can.i see you have pico probe listed i have never used it so i don't know the spec's.but make sure you use a resistor divider on the input of it when measuring high voltages off of your transformer so as not to blow out the input of the device.
 
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