Isolation for use on old TV chassis  - Page 1
Forum section: electronics / beginners

author: Electronics-Repairman
author_rank: Contributor
timestamp: July 01, 2014, 06:19:56 pm
content: Hi , I'm going to do some work on an old TV, so I want to build a mains isolation box, which I would use on just about anything with line power coming in any way. I have my mind tied up in knots with this , and hoping someone can shed light on what I'm about to explain I'm thinking two transformers the first one 240 primary and 50 secondary , the second one turned around 50volts/ 240 as output , so it would go 240- 50 - 50- 240 ,using the first as step down and the second as step up. hope this is making sense. And would this configuration work.

author: rob77
author_rank: Super Contributor
timestamp: July 01, 2014, 06:23:24 pm
content: it's better to use a 240v/240v transformer - it's called isolation transformer.but if you don't have one and you have 2 identical transformers with a sufficient power rating then you can even use the step down + step up.

author: Electronics-Repairman
author_rank: Contributor
timestamp: July 01, 2014, 06:27:55 pm
content: Quote from: rob77 on July 01, 2014, 06:23:24 pmHi Rob OK, this was the reason for asking I do have 2 transformers rated at 240/100v and 2x 240/50, so I'll use one of the pairs probably the 240/100v pair.Cheers

author: kingofkya
author_rank: Regular Contributor
timestamp: July 01, 2014, 08:37:12 pm
content: You can get medical surplus transformers pretty cheap all you need to do is remove the grounded strap to Neutral, and they are very nicely built inside.I wish could be more help looks like there mostly available in the us. I have a dale/fluke 5amp one that I got for $50 delivered in the us.http://www.ebay.co.uk/sch/i.html?_sacat=0&_from=R40&_nkw=isolation+medical+transformer&LH_PrefLoc=2&_arm=1&_armm=63&_ruu=http%3A%2F%2Fwww.ebay.co.uk%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3Disolation%2Bmedical%2Btransformer%26_arr%3D1

author: bance
author_rank: Contributor
timestamp: July 02, 2014, 08:51:31 am
content: A good inexpensive source in the UK would be builders 110V site transformers, they often have a 240-240 centre tapped transformer....

author: Electronics-Repairman
author_rank: Contributor
timestamp: July 02, 2014, 11:28:00 am
content: Quote from: bance on July 02, 2014, 08:51:31 amThanks Bance I know where there is one, doing nothing, never even give them a thought.

author: Monkeh
author_rank: Super Contributor
timestamp: July 02, 2014, 11:44:32 am
content: Quote from: bance on July 02, 2014, 08:51:31 amUh, no, they're 55-0-55 secondaries.

author: Electronics-Repairman
author_rank: Contributor
timestamp: July 04, 2014, 06:01:00 pm
content: Quote from: Monkeh on July 02, 2014, 11:44:32 amI won't bother to even take the lid off the one in the garage, am I been lazy

author: Monkeh
author_rank: Super Contributor
timestamp: July 04, 2014, 06:02:37 pm
content: Quote from: Electronics-Repairman on July 04, 2014, 06:01:00 pmNot really, they're not much use for isolation. They're 55-0-55 secondaries with the center tap hard wired to earth. Often potted.

author: bance
author_rank: Contributor
timestamp: July 05, 2014, 08:31:57 am
content: QuoteOOPS!