# Transformer coil design?

#### electronice123

Joined Oct 10, 2008
339
I am in need of a transformer with the following characteristics:

Turns ratio 1:8
Primary: 12V square wave @ 100mA (120 Ohms impedance)
Secondary: 96V square wave @ around 1mA (9600 Ohms impedance)
Operating Frequency 6.5kHz-20kHz

The transformer will have 2 secondary coils wound together. Each secondary coil needs to output 96V @ around 1mA, only 1 coil will be used at a time.

Is there any easy way to design such a coil?......I contacted an engineering company and they want 1 grand just to start the project....

The coil is not for anything important, just for hobbies so I don't need exact specs, just something that will work!

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#### wayneh

Joined Sep 9, 2010
17,493
Is there any easy way to design such a coil?......
Sounds like you already did. What questions do you need to answer? One place I'd start is with a wire table. The dimensions and properties of magnet wire will define the size of your coil, how much length you'll get for however many turns you want, and so on. The calculations can be a challenge.

Winding your own coils is tedious and, IMHO, not much fun. On the scale of things to make-versus-buy, a transformer is squarely in the buy column. Is there some reason you don't want to just find one ready made?

#### SgtWookie

Joined Jul 17, 2007
22,230
I'll take a stab at it.

You'll need a ferrite toroid to wind the transformer on.
https://www.amidoncorp.com/items/18
FT-140-77
That's a ferrite toroid made of material 77 (Al~=2520, ui~=2245)
The size and material type is important.
For the primary, wind on 25 turns of AWG-24 or AWG-26 magnet wire.
Now, you'd think that all you'd need to do is multiply by 8 to get the number of turns for the secondary winding. However, at 12kHz, the secondary will have an impedance of around 7.6k Ohms. If you have a 1mA current draw, the 7.6k Ohms will drop 7.6v across it - so you'd get 88.4v @ 1mA instead of 94V like you wanted.

So, I'll ballpark this here. 219 turns will have an impedance of ~9.1k, and about 105.1v @ no load. With 1mA load, it'll drop to about 96v, which is what you want.

You'll need about 9.5 yards of AWG-32 or finer for each secondary. It will probably be a pretty tight squeeze.

I used the freeware program "Mini-Ring Core Calculator" to get these numbers.
Your mileage may vary, but I think this will work pretty well. You should at least be "in the ballpark". Let us know how close it worked out.

If your frequency is significantly different than 12kHz, your winding XL will change significantly. If the output voltage is too high with a 1mA load, take a few turns off the secondary windings.

#### electronice123

Joined Oct 10, 2008
339
Thanks SgtWookie,

How did you determine the coil material?

Also, is there a way to determine core area in relation to the VA rating?

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#### SgtWookie

Joined Jul 17, 2007
22,230
How did you determine the coil material?
The core material 77 included your desired frequency range, and the AL was high enough that you would not need lots of turns to get your desired primary impedance, and the core is large enough to be reasonably easy to wind. If I'd chosen a smaller core, you would've had to use smaller gauge wire, increasing copper losses, and it would have been much harder to wind.
Also, is there a way to determine core area in relation to the VA rating?
Yes there is, but I don't recall it at the moment, and much of my reference material is on another computer that's in temporary storage. However, this toroid is maximum overkill for your VA needs. You shouldn't have much in the way of losses.

Keep in mind that changing the frequency you use on the primary will change the apparent impedance of the primary. At 6kHz, it'll be ~60.3 Ohms; at 12kHz, ~120.6 Ohms; 20kHz, ~201 Ohms. 11.937kHz will be right around 120 Ohms.

Keep in mind that all this has simply been calculated. Your mileage may vary due to variances in individual toroids. 25 turns for the primary should measure right at 1.6mH/1600uH. The secondary at 219 turns should be right around 122.7mH.