Iron Core Transformer Formula - diyAudio
 Iron Core Transformer Formula
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 20th November 2011, 11:31 AM #1 diyAudio Member     Join Date: Jun 2010 Location: Near the Mountain Iron Core Transformer Formula Do you know the formula? to get the best numbers of turn to this kind of transformer / trafo? I have over simplified formula, but maybe some other formula to compare with This my formula for EI trafo for 50 Hertz frequency primary winding n = 45 * V / A secondary winding n = 49.5 * V / A n = number of winding turn V = voltage input or output (volt) A = core area (cm square) Maybe someone help to add another formula here, please... I want to make right trafo this time Thanks Last edited by JOHN BALI; 24th January 2012 at 05:30 PM.
 20th November 2011, 11:37 AM #2 diyAudio Member   Join Date: Jul 2004 Location: Gone on holiday again. back soon. the secondary shows a +10% correction for loaded output voltage. I suspect this is a guesstimated value for regulation of the final transformer build. Regulation can vary from <=3% to >=30%. A fixed turns ratio correction of +10% is not appropriate to all transformers. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 20th November 2011, 11:44 AM #3 diyAudio Moderator     Join Date: May 2003 Location: Palatiw, Pasig City are you winding on an EI core or torroid? __________________ the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
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Join Date: Jul 2004
Location: Gone on holiday again. back soon.
Quote:
 Originally Posted by JOHN BALI This my formula for EI trafo for 50 Hertz frequency
Quote:
 Originally Posted by Tony are you winding on an EI core or toroid?
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

 20th November 2011, 11:51 AM #5 diyAudio Member     Join Date: Nov 2011 Location: UK This is OVERSIMPLIFIED. The important thing to learn is the magnetic properties of the transformer that you are trying to wind. The first formula is A = Square Root (W) .......----------------...= mm2 ................K Where W is the Secondary Power + 10% K = 0.0086 for standard E & I laminated sheet 0.0150 for T & U laminations. Once you've got the correct iron work for the transformer. Turns / Volt = 1 x 10e9 ..................------------ ...................K x B x f x A Where K = 4.44 for a SINE wave, 4.00 for a SQUARE wave. B = Max Flux Density - 780mT is good for reliabilty, hoever this can be pushed to 1350mT f = Frequency 50Hz or 60Hz (Unless you are design SMPS then its kHz) A = The Core Area as discussed above. Once you know the Turns / Volt its a simple case of working out the Primary and Secondary Turns. Last edited by KatieandDad; 20th November 2011 at 11:54 AM.
 20th November 2011, 12:22 PM #6 diyAudio Member   Join Date: Jul 2004 Location: Gone on holiday again. back soon. Is the first formula? A = sqrt(W) / K where W is the input power in Watts and K is 0.0086 for EI. The answer in mm^2 (sqmm) Is the second formula? Turns/Volt = 10^9 / [ K' * B * f * A] where K' = 4.44 for sinewave, B = 780mT for EI to stay cool and avoid the saturation knee on high mains voltage, f = mains frequency, A = core area in sqmm Inserting values into that first equation gives the same answer as: Watts = 31 * [Core Area (in square in)]^2 using 100W and 1160sqmm = 1.8sqin Inserting values into the second formula for 4.44, 780mT, 50Hz, 1160sqmm, gives 5T/V resulting in 1145T for 230Vac @ 100VA Whereas, John's first formula result in 892T for 230Vac with 1160sqmm. This seems to confirm that Johns' first formula has assumed B=1000mT = 1T Have I got this correct? __________________ regards Andrew T. Sent from my desktop computer using a keyboard Last edited by AndrewT; 20th November 2011 at 12:49 PM.
 20th November 2011, 12:57 PM #7 diyAudio Member     Join Date: Nov 2011 Location: UK 1000mT is fine for commercial design. If you've got the core material I would reduce the Magnetising Force (B). You can ALWAYS increase the Turns / Volt, NEVER reduce them. 780mT is what the MoD employ. It also allows for Mains Transients of over 100%, not that the windings wouldn't take that for a short time anyway. For SMPS designs the B figure needs to be MUCH lower. Last edited by KatieandDad; 20th November 2011 at 01:03 PM.
 20th November 2011, 01:00 PM #8 diyAudio Member   Join Date: Jul 2004 Location: Gone on holiday again. back soon. But, have I interpreted your formulae correctly and drawn the correct conclusions? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 20th November 2011, 01:07 PM #9 diyAudio Member     Join Date: Nov 2011 Location: UK Looks fine to me.
 20th November 2011, 01:46 PM #10 diyAudio Member   Join Date: Jul 2004 Location: Gone on holiday again. back soon. I have only done it once so far, but would testing for the knee in the B vs V curve give confidence in having guessed at the correct identification of the core material? I found that plotting for Vac from 10Vac to 260Vac showed that the tiny 3VA transformer was being run at far too high a voltage. 160Vac would be far cooler than running it at a nominal 240Vac, even though it is rated as 220/240Vac. Lies!! or the cheapskates thought I couldn't care about being burnt by the surface of the EI core __________________ regards Andrew T. Sent from my desktop computer using a keyboard

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