# About CT secondary winding on toroidal transformer

I am done primary winding today of my toroidal core. Attached photo of transformer below. 755 turn of 0.914 mm magnet wire. Core size before winding OD-100mm, ID-60mm, Ht-50mm (M4,CRGO). Sir when I test for secondary voltage I get 2.7 volt on 10 turn of magnet wire. why I get less volt per turn. Because before making the primary winding I done partial test on core, wind 10 turn for secondary, I get 3.2 volt. And I am winding center tap for this transformer of 35-0-35 using dual winding method (2 wire at same time, bifilar.) If I want lower voltage from same winding can it be done? e.g for 25-0-25?

• A CT usually has 1 or 2 turns on the primary. Can you explain your question to make it easier to follow? Basic punctuation will help a bit. Commented Aug 3, 2016 at 11:32
• What happens if you apply (say) 1/2 the input voltage with your 10-turn secondary? Maybe it is saturating because you don't have enough primary turns. Your proposed bifilar winding of a centre-tapped secondary should be practical at that voltage level. Commented Aug 3, 2016 at 11:38
• Ok I am wind primary of toroidal transformer with 755 turn of 0.914mm magnet wire. But before primary winding I test core with sample winding with old transformer which have 12 volt secondary. so on toroidal core I have did winded up 37 turn for primary and 20 turn for secondary I got 6.66 volt on out. but after actual wind full 755 turn on primary i test secondary voltage again with 20 turn this time I got 5.5 to 5.7 volt. why volt per turn is droping? Commented Aug 3, 2016 at 11:45
• What voltage did you apply to the 755 primary? 240V or 220V? Commented Aug 3, 2016 at 12:04
• 230 volt And get 5.6 volt on 20 turn secondary Commented Aug 3, 2016 at 12:10

But before primary winding I test core with sample winding with old transformer which have 12 volt secondary.

With 12 V and a turns ratio of 37:20 you should expect about 6.5 V. You measured 6.66 V and this sounds OK.

after actual wind full 755 turn on primary i test secondary voltage again with 20 turn this time I got 5.5 to 5.7 volt

755:20 with 5.6 volts on secondary implies a primary voltage of 211 volts.

Maybe you thought you were feeding 240 V but you only have 210 V.

• No sir I tested mains voltage before feeding it was in between 230 to 234 volt Commented Aug 3, 2016 at 12:18
• Then maybe you have miscounted the turns or your turns are not coupled magnetically as well as you expected. Did you have a load on the output? Commented Aug 3, 2016 at 12:20
• but I did series bulb test for primary bulb is fully power off. can i add more turn on primary. Commented Aug 3, 2016 at 12:24
• I don't know what the series bulb test is. Measure the mA taken by the primary when connected to 240 V. Commented Aug 3, 2016 at 12:26
• Series bulb test is give mains supply to primary winding with indusind bulb (who can oprate on mains voltage) in series. If bulb is stay off means turn is ok if bulb is dim on means more turn required Commented Aug 3, 2016 at 12:31