Transformer rewinding question. - UK Vintage Radio Repair and Restoration Discussion Forum

Station X · 13th May 2019, 11:46 am

Having constructed a bobbin I now have to rewind a small mains transformer which forms part of a vintage battery eliminator.

The mains voltage selector has two settings labelled 200-220 volts and 230-250 volts.

What voltages should I use when calculating the position of the tap on the primary winding? Mid range ie 210 volts and 240 volts, or top of the range ie 220 volts and 250 volts?

The original winding had a total of about 1713 turns with at tap 88 turns from the end.

Thanks.

Radio Wrangler · 13th May 2019, 12:43 pm

240 * (1713-88)/1713 = 227.6v so the turns ratio maps the mid point of the upper range to a bit above the top end of the bottom range.

The ranges are quite narrow... +/-4% so I'd stick to 1713 turns for 240v +/- 10v and think of making the 220v tap at 1570 turns .... IE 143 turns before the end.

Should be OK if you're sticking to the original laminations. An old battery eliminator isn't likely to be working too hard and the HT voltage won't be terribly critical.

DAvid

13th May 2019, 11:46 am	#1
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Transformer rewinding question. Having constructed a bobbin I now have to rewind a small mains transformer which forms part of a vintage battery eliminator. The mains voltage selector has two settings labelled 200-220 volts and 230-250 volts. What voltages should I use when calculating the position of the tap on the primary winding? Mid range ie 210 volts and 240 volts, or top of the range ie 220 volts and 250 volts? The original winding had a total of about 1713 turns with at tap 88 turns from the end. Thanks. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

13th May 2019, 12:43 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Transformer rewinding question. 240 * (1713-88)/1713 = 227.6v so the turns ratio maps the mid point of the upper range to a bit above the top end of the bottom range. The ranges are quite narrow... +/-4% so I'd stick to 1713 turns for 240v +/- 10v and think of making the 220v tap at 1570 turns .... IE 143 turns before the end. Should be OK if you're sticking to the original laminations. An old battery eliminator isn't likely to be working too hard and the HT voltage won't be terribly critical. DAvid __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done