Designing an Interstage Transformer

[TheGimp]

After doing more research I found this paper:

http://www.intalek.com/Index/Project...rmeability.PDF

Figure 1.17a on page 3 shows permeability peaking about at the middle of the BH curve slope.

I measured the current and voltage through the secondary with an adjustable resistor in series with the transformer. I adjusted the resistor to have a voltage drop equal to the voltage drop across the transformer, and used this to calculate the inductance.

The peak inductance should coincide with the peak permeability. No?

If so, then the peak permeability occurs with an input voltage of 11.3V with a current of 78mA for an inductance of 0.477H.

From this and the number of turns I can calculate Mu.

Is this the proper point at which to calculate Mu? Or does this only tell me the peak flux density at saturation?

[iko]

What's the window area? As defined by D x F in the attached image.

It so happens that I've become interested in winding my own inductors too, but won't start with an interstage transformer. Will probably do filter chokes first. We could exchange notes if you'd like.

[iko]

These are the formulas I found for the design of an inductor with DC, and using these the relative permeability is not needed.

N = (L*Ipk*10^4)/(Ac*Bm)

where
N = the number of turns
L = desired inductance
Ipk = peak current = Idc + Iac/2
Ac = effective core area
Bm = desired max flux density, usually 1.2T

Assuming Ac = 5.68 cm^2, Idc = 10mA, Iac = 4mA, L = 44H, and Bm = 1.2T, gives

N = 774 turns.

Next would be to evaluate the max wire size that would fit so many turns in your winding area, or window area Wa.

The wire are in cm^2 is given by

Aw <= Ku Wa / N

Ku = window utilization factor (constant), which for laminates like you have is 0.48.

Hence my question for your core Wa.

Next, calculate the gap length, in cm:

lg = (0.4*pi*N*N*Ac*0.00000001)/L

In your case lg = 0.001198 cm, which is equivalent to just butting the ends together with not paper between them, roughly. One sheet of fish paper is about 7 mils, which is about 0.01778 cm.

We should probably take into account the fringing flux

Frf = 1+(lg/SQRT(Ac))*LN(2*D/lg)

where D is the window height as seen in the image I attached previously.

Knowing F you can recalculate the number of turns:

N' = =SQRT((lg*L)/(0.4*pi*Ac*Frf*0.00000001))

There's more, I'm working on a spreadsheet too.

[ErikdeBest]

Hi theGimp and Ikoflexer!
Winding own transformers is another of my long term goals, so I don't mind if you keep exchanging notes on diyaudio so I can pick up some information as well.
Erik

[AJT]

Originally Posted by 316a

Hammond 126C and 126B , to me sound superb for a £30 a piece lump of iron . These bifilar 1:1 IT are a bargain , probably the biggest bang for buck out there

316A

thanks for your input, did you try to measure frequency response? i thought that winding biffilar increases winding capacitance to affect hf?

[sjs]

Originally Posted by Tony

thanks for your input, did you try to measure frequency response? i thought that winding biffilar increases winding capacitance to affect hf?

this may be worth a read: Audio Note Kits - Interstage Coupled Amps

[sjs]

and here are some typical FR for some bifilar ITs from back in the late 90s with -3dB beyond 120KHz
http://www.sjselectroacoustics.co.uk/images/IT102.GIF
and
http://www.sjselectroacoustics.co.uk/images/IT103.GIF

[TheGimp]

I've been interested in winding transformers for many years but never seemed to have the time to study it sufficiently to understand what I'm doing.

ikoflexer, window measurements are 1.1cm X 3.33cm for an area of 3.663 sq-cm

The calculations for the 1:1 interstage transformer should be very much like those for an inductor.

If Bi-Filar wound, construction is the same, except two wires are feed at the same time.

Eric, this is an ope forum so every one is welcome to participate or just follow along as they wish.

Tony, bi-filar does increase coupling capacitance, apparently this is not always bad.

sjs, thanks for the links.

I still wish to measure the characteristics of these cores so I can use those values in my calculations. So if anyone can give me some guidance I would appreciate it.

[6L6]

Originally Posted by TheGimp

Tony, bi-filar does increase coupling capacitance, apparently this is not always bad.

Over in the Pass Labs forum we are using a 1:1 to 1:1 quadra-filar interstage transformer as an interstage transformer/phase inverter for the F6 amp, the capacitance of the windings is not an issue.

I also don't think it's avoidable, so no worries...

[iko]

This is what I get for your core. This calculates one winding, assuming it occupies half the total winding area. It also assumes standard industry silicon steel core and a target working Bmax of 1.2 tesla. The method used is called the area product method (temperature rise) from the book Transformer and inductor design handbook by Col. Wm. T. McLyman.