Mains transformer insulation - UK Vintage Radio Repair and Restoration Discussion Forum

mpegjohn · 18th Aug 2016, 10:57 am

Hi,
I want to start winding transformers for valve HT supplies.
In the past such transformers were would as primary, then secondary on top.

I see a lot of examples today with split primary and secondaries.
Bobbins I have seen, have an option of split and split with shroud.

My question is if, as an example, I have a secondary as 250-0-250 volts 100mA, is it still acceptable to wind sec over pri, and if so what do I need to use as inter winding insulation, Mylar tape?

Is it better to use split bobbins, and what is a shrouded split bobbin?

Many thanks,
John

trobbins · 18th Aug 2016, 1:07 pm

You need to fully appreciate the applicable standards requirements.

Andrew B · 18th Aug 2016, 4:47 pm

I like to use split transformers, keeps the mains further away.

The worst that can happen with a split transformer is that your newly wound secondary lets out it's magic smoke.

250-0-250 @100mA (50w) would make a good first attempt, bearing in mind that will be at least 1500 turns.

I cheat a bit and mount the bobbin in a lathe at about 100 RPM.

The man to ask about transformers is Ed Dinning, he helps me with my rewinds.

The transformer below was a 240-->24v @ 4A, now it's a 240-->120 @ 150mA and 6V @2A

Andrew

18th Aug 2016, 10:57 am	#1
mpegjohn Hexode Join Date: Aug 2013 Location: Chichester, West Sussex, UK. Posts: 270	Mains transformer insulation Hi, I want to start winding transformers for valve HT supplies. In the past such transformers were would as primary, then secondary on top. I see a lot of examples today with split primary and secondaries. Bobbins I have seen, have an option of split and split with shroud. My question is if, as an example, I have a secondary as 250-0-250 volts 100mA, is it still acceptable to wind sec over pri, and if so what do I need to use as inter winding insulation, Mylar tape? Is it better to use split bobbins, and what is a shrouded split bobbin? Many thanks, John

18th Aug 2016, 1:07 pm	#2
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Mains transformer insulation You need to fully appreciate the applicable standards requirements.

18th Aug 2016, 4:47 pm	#3
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Re: Mains transformer insulation I like to use split transformers, keeps the mains further away. The worst that can happen with a split transformer is that your newly wound secondary lets out it's magic smoke. 250-0-250 @100mA (50w) would make a good first attempt, bearing in mind that will be at least 1500 turns. I cheat a bit and mount the bobbin in a lathe at about 100 RPM. The man to ask about transformers is Ed Dinning, he helps me with my rewinds. The transformer below was a 240-->24v @ 4A, now it's a 240-->120 @ 150mA and 6V @2A Andrew Attached Thumbnails __________________ worried about the electrons entering the circuit and the smoke leaving Andrew Last edited by Andrew B; 18th Aug 2016 at 5:01 pm.