Help:- Isolation Transformer 240 - 110V

Hello Group.

I need advice on isolation transformers!

Are the yellow 240V-110V building site transformers isolated?
I have purchased a 110V variac and wish to supply it with isolated
power.
This would be fed to tv / monitor / smps
Oscilloscope will be attached etc etc for repair work and I am
concerned with safety.

TIA
Steve.

 

Hi Steve,
your question raises a few points, firstly are you in the uk? i assume
you are judging by your email NTL. i ask this as i cannot understand
why you have purchased a 110 V variac, when the UK supply is 230 V.

the yellow site transformers that you refere to are safety isolating,
but they have a center tapped secondary (output) winding that is
earthed, giving 55V from earth. which makes them not suitable for your
application.

the normal thing to do in a workshop enviorment is to have an earth
free zone, a safety isolating transformer with no earth connection on
the secondary. i refere you to the following

www.hse.gov.uk/pubns/indg354.pdf
www.hse.gov.uk/pubns/eis36.pdf

RS and Farnell supply suitable transformers for this use, but account
needs to be taken of the earth free zone recommendations.
im sorry if this is a bit long winded but it is a complex subject
regards
bob

 

it is common cortesy to reply

 

You should be able to buy a 120/120V isolating transformer. 0.5 to 1KVA
capacity should do you.

I am puzzled by "Yellow 240/110V transformers" You often have transformers
which are rated for 240V and center tapped- these are not isolation
transformers.

 

hi don,
the yellow transformers are a U.K thing, they are used to run 110V
power tools, the mains voltage here is 240V. but on building sites it
is required to use power tools that are 110V.
see my first reply to the op for more info.
regards
bob

 

Sight unseen, I'd still bet money that they're autotransformers.
They're way cheaper.

And not isolated, if you're completely new to the field.

Cheers!
Rich

 

hi rich,
the bet is lost, these transformers are 240V to 110V, with the
secondary winding center tapped to earth, therebye giing 55 V max to
earth. which is the reason that they are not suitable for the use that
the original poster asked., see
http://www.justgenerators.co.uk/pages/transformers.htm the ones that
we are talking about are the yellow ones on this page. (110V)
these units are a UK thing.
i am not the original poster the thread got lost somewhere along the
line
regards
bob

 

Thanks for the info- sorry I missed it before.