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Old 17th June 2012, 04:08 PM   #1
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Default Transformer impedance calcs from windings

Is anyone very handy with the theory and calculations behind transformer windings, specifically an output transformer for an amp? *I'd love to know the primary side reflected impedance (or impedance ratio) for the following transformer if anyone is knowledgeable enough to know how to calculate it from the winding information on the second picture. *I wouldn't really know where to start!


Click the image to open in full size.
Click the image to open in full size.

Winding 1 - 2 layers (37 + 37 winds), wire diameter 1mm.
Winding 2 - 2 layers (94 + 110 winds), wire diameter 0.3mm.
Winding 3 - 7 layers (each 118 winds), wire diameter 0.3mm.
Winding 4 - 1 layer (43 winds), wire diameter 0.9mm.
Winding 5 - 7 layers (each 118 winds), wire diameter 0.3mm.
Winding 6 - 2 layers (110 + 94 winds), wire diameter 0.3mm.
Winding 7 - 2 layers (37 + 37 winds), wire diameter 1mm.

Thanks!
Jonathan
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Old 17th June 2012, 05:04 PM   #2
Funker is offline Funker  Germany
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Quote:
Originally Posted by composition4 View Post
Is anyone very handy with the theory and calculations behind transformer windings, specifically an output transformer for an amp? *I'd love to know the primary side reflected impedance (or impedance ratio) for the following transformer if anyone is knowledgeable enough to know how to calculate it from the winding information on the second picture. *I wouldn't really know where to start!


Click the image to open in full size.
Click the image to open in full size.

Winding 1 - 2 layers (37 + 37 winds), wire diameter 1mm.
Winding 2 - 2 layers (94 + 110 winds), wire diameter 0.3mm.
Winding 3 - 7 layers (each 118 winds), wire diameter 0.3mm.
Winding 4 - 1 layer (43 winds), wire diameter 0.9mm.
Winding 5 - 7 layers (each 118 winds), wire diameter 0.3mm.
Winding 6 - 2 layers (110 + 94 winds), wire diameter 0.3mm.
Winding 7 - 2 layers (37 + 37 winds), wire diameter 1mm.

Thanks!
Jonathan

Hi Jonathan,
the impedance ratio is square of the winding ratio.

zprim/ zsec= wprim/wsec

example:

A given opt has 4000 turns plate to plate. The speaker winding has 200turns .
The speaker impedance is assumed 8 Ohms. What impedance reflects to the primary winding?

Zprim= (wprim/wsec) x Zsec = (4000turns/200turns ) x 8 ohms = 3200 Ohms.


Now you can calculate your xmfr.

73
Wolfgang
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Old 17th June 2012, 08:09 PM   #3
tomchr is offline tomchr  United States
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Apply an AC voltage to the primary (blue to orange on your trafo) for example by connecting it to a variac and slowly dialing up the voltage. Measure the AC voltage on the secondary (black-green and black-yellow).

The turns ratio is then N = Vprimary/Vsecondary.

The impedance is transformed by the square of the turns ratio. I.e. Zprimary = Zsecondary*N^2.

In your case, windings 1 and 7 are in parallel on the secondary. The primary is windings 2+3+5+6. So the turns ratio for the black-green secondary tap should be N = 2060/74 = 27.84. For the black-yellow secondary, N = 2060/(74+43) = 17.61.

If you load black-yellow with 8 ohms, you should get Zprimary = 8*17.61^2 = 2480 ohm. Loading the black-green secondary with 8 ohms yields, Zprimary = 8*27.84^2 = 6200 ohm.

I'm guessing one secondary is a speaker winding and the other a feedback winding. 6200 ohm... Is this for a 6L6 amp by any chance?

It looks like the taps on the primary are at 20 % of the winding rather than the more common 40 % UL taps.

~Tom
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Old 18th June 2012, 02:01 PM   #4
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Rather than using a Variac that most people don't have. Use a little wall-wart AC output power supply/transformer. A wall-wart with an output of about 10 Volts AC would be great. Then use Tom's math.
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