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Old 20th October 2010, 11:58 AM   #11
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there are a lot of folks out there diy'ing their own traffos, i am pretty sure they must be doing it right....
planet10 needs your help: Let's help Ruth and Dave...[B
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Old 20th October 2010, 10:53 PM   #12
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I plugged OP's core area into my spreadsheet and i'd estimate that his core weighs 3Kg so i think he's off by a factor of ten somewhere.
certainly a lot bigger than a 15-30va transformer, which would require about 1200-2000 turns on the primary
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Old 20th October 2010, 11:47 PM   #13
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for low grade cores, 1750 turns per square inch of core area is what i go by.....
planet10 needs your help: Let's help Ruth and Dave...[B
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Old 21st October 2010, 12:18 AM   #14
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Default Excellent Reference Info!

Originally Posted by santonel View Post
So yeah, I'm kind of a newbie but I'm working on rewinding a transformer I had lying around. It's an EI transformer with a plastic split bobbin and it's original output was stated at 12-14.4v at 1.8A. So it's around 20 - 25VA ....snipped....
The information in the link was perfect and I bookmarked it for my future
use, thanks for the link! Your transformer, if stated V and A are correct,
is about 25va. Typical EI cores of that size do need about 10 turns/volt.
The author's technique of overwinding 10 turns, measuring the voltage,
then dividing by 10 to get turns/volt works every time. I tend to be
somewhat ADD, so I check my numbers a few times to be sure. A great
safety procedure is to use highest ohms setting on a multimeter to verify
pri/sec and pri/frame isolation, then apply power to the unloaded
transformer through an incandescent light bulb of equal or slightly lower
watt rating than the va rating of the transformer. The bulb should glow
dimly, if at all. A well-built transformer with a low-loss core will not cause
the bulb to glow visibly, but will almost always warm the bulb's outer
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Old 21st October 2010, 01:12 AM   #15
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measuring the resistance from primary to secondary to core should be in the gigaohm range... or hundreds of megs for larger stuff.


transformer ratings are dependant on both core area and window area.

today it is best to throw most of those old rules of thumb out.

toss a test coil on it, measure the iron loss with a good wattmeter (kill-a-watt meters need to be calibrated, don't take them at face value below 40 watts!) , subtract the copper loss from the readings with a dc test.

core to copper loss ratio should reflect the average power to peak power ratio to minimise cost and and your electric bill, there is no reason to run it 50-50 unless its running full load 24-7
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Old 20th August 2013, 06:02 PM   #16
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sir please help me transformer rewinding in secondary(30/0/30v,5A),(0,14V,5A)and(12/0/12v 1A) primary 230V,50HZ AC how to calculate in primary and secondary turns and core size and bobbin size
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Old 21st August 2013, 02:47 PM   #17
AndrewT is online now AndrewT  Scotland
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wind on 10 turns.
Power up and measure the Vac of those 10 turns.

Then calculate how many turns to get your 30Vac, 14Vac & 12Vac.

Remember that you will be measuring open circuit voltages. The actual voltage under load on each of the windings will vary with the loading.
You decide how many extra turns on each winding to give the required loaded voltages for your circuit loadings.
regards Andrew T.
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