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7th February 2013, 07:13 PM  #11 
diyAudio Member
Join Date: Jun 2004
Location: ExEx Yu,very far from Ei factory

Can anybody help me with the formulas?
Or simply suggest where I should search for the answer? Regards, Yugovitz
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28th March 2013, 03:18 AM  #12 
diyAudio Member
Join Date: Nov 2006

Andrew T gave some advice, but you would need to know the particular coregeometry those formulars are intended to use with.
I have seen (over?)simplified formulas on the web (sorry, I dont remember where I stumbled over them) that at least take core area and window area into account. I would look fore specific data in one of the many core manufactures catalogs. 
28th March 2013, 03:48 AM  #13 
diyAudio Member
Join Date: Nov 2006

If I have a core with unobtainable data I just measure all dimensions and calculate area of core and window, average the magnetic path length. I then use the OPT calculator the DIY member Yvesm has kindly provided. I plug in the cores data and, depending on the application, choose either a suitable current density or a suitable regulation percentage. All you need, except temperature rise, can be simulated. Temperature is what usually sets the powerlimit.
If used in a enclosure cramped with other "heathers" and/or heath sensitive parts nearby then I usually use the very conservative rating of 10mW/sqr cm averaged over the total surface area. This will ensure the transformer runs cool and anything nearby will be happy too. Manufacturers may use 2550mW/sqarecm (or even higher), but that is theyr choice... . Last edited by gorgon53; 28th March 2013 at 04:16 AM. 
28th March 2013, 02:49 PM  #14  
diyAudio Member
Join Date: Jun 2010
Location: Near the Mountain

Quote:
If you look at Andrew formula that for flux @ 1T If you have different core of course you need to change the flux density to suit the core you have. @ 1T i guess it for E I core. The key is you must know the core can be use @ what flux density. Once you know that you can start your trafo build. See this Iron Core Transformer Formula but I'm not finish anything yet hehe... Regards
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29th March 2013, 11:17 AM  #15 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

plot the Iprimary vs Primary supply voltage.
You will see when the Iprimary starts to become excessive. That should be a good clue to what number of turns are required for your worst case supply voltage. Then you could work back to determine the Limiting Flux for that core. Is this feasible?
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regards Andrew T. 
4th April 2013, 08:41 PM  #16 
diyAudio Member
Join Date: Jun 2004
Location: ExEx Yu,very far from Ei factory

Thank you very much for your replies.
The problem began when I salvaged some transformers from an old colour Grundig TVs which had very interesting C cores and bobbins without side sheets similar what Lundahl transformers are winded.I hope the link John and instruction Andrew recommend will help me to calculate the right power for those cores. Best regards, Yugovitz
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5th April 2013, 02:32 AM  #17  
diyAudio Member
Join Date: Nov 2006

Quote:
Here you can find formulas taking not only core area and fluxdensity, but also the window area and wire current density into account Electronics/Transformer Design  Wikibooks, open books for an open world 

5th April 2013, 02:49 AM  #18 
diyAudio Member
Join Date: Nov 2006

If you strive for highest core efficience find the voltage/turn that givs the highest inductance.

5th April 2013, 09:17 AM  #19 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Can you explain the procedure for us?
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regards Andrew T. 
10th April 2013, 02:51 AM  #20 
diyAudio Member
Join Date: Nov 2006

The core must have as little airgap as possible and a winding with a known number of turns on it. Use a variac to feed the coil. Start at some low voltage and measure voltage and current. Take notes and calculate the impedance. Increase the voltage stepwise and caculate the impedances. With increased voltage the impedance will increase until it reaches a maximum. This is the point where the core has the highest impedance. Now divide this voltage with the known turns and you have the V/turn that givs the highest impedance/turn

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