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Transformer secondary winding question

Could some electronics guru please shed some light on this. I am helping a colleague of mine wind a step down transformer, as a weekend project. We have done all the calculations, The secondary is center tapped, and my understanding is that a "pig-tail" has to be created, which serves as the center tap. The secondary coil is made of two coils, such that the end terminal of one coil is attached to the start terminal of the next coil, -- the "end-start" connection being the center tap. Is this correct ? Obviously the two coils making up the secondary must have the same number of turns. But the question is: suppose the output is 9 - 0 - 9 i.e., +/- 9 Volts with a center tap, for output of 18 Volts. Now would each secondary coil have the total number of windings for 9 Volts ? All hints. suggestions would be greatly appreciated. Thanks in advance for your help.

Reply to
dakupoto
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  • -----YES-------^

  • Naturally, that is the original specification.

Be advised that if a +/- DC supply is needed, that the RMS AC voltage will different,depending on the type of filtering and regulation used. Ripple actor and capacitor charging current must be taken into consideration.

Reply to
Robert Baer

** Yep.

** AC supply transformers are a easy to design once you know and use the " turns per volt " for the particular core being used.

This number depends on the supply frequency, grade of steel used in the laminations and the cross sectional area of the centre limb for an e-core or the wound ring for a toroidal. The larger the core, the less turns per volt are needed.

Eg. Say you have a 1 square inch core section and the laminations are of grain oriented silicon steel and the AC supply is 60Hz. Then you need to wind about 4.5 turns for each volt - making the primary 540 turns.

Secondaries can be calculated the same way but you must allow extra turns to compensate for the drop in voltage under load - called the regulation factor.

Another rule is that the primary should occupy half of the available space on the core and all secondaries the rest.

Calculating the wire gauges to use for best fit is the hard part and may take a bit of cut and try.

... Phil

Reply to
Phil Allison

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