RS Isolating Transformer

[Bushbaby]

Hi All,
I need some help with regards to the wiring of an old RS isolating transformer 500w 2A that was kindly given to me by my good friend and our colleague Kevin Hoyland to operate my American test gear safely in my workshop.
Can anyone clarify the best manner to wire this for 2 x output sockets please. Futhermore, what is the best way to add an earth connection to the input side (primary) of this transformer?

Thanks in advance

Rob

   
   

[Skywave]

The primary side may have a terminal marked "E". Join this to the incoming supply earth. I would also check that the resistance from this terminal to the laminations (if exposed) or the shroud around the windings (if one is fitted) indicates a very low value. If an "E" terminal is not fitted, then I would scrape any paint (or whatever) from one of the transformer mounting lugs and by use of a suitable solder tag, use that as a connection point for the incoming supply earth.

As for the secondary, I assume that your American test equipment is 110-v. and that the transformer sec'y. can supply this. (I may be wrong here, of course, since I use a lot of American test equipment that requires 230-v.a.c.: Tektronix and Hewlett-Packard gear for example). It may be that your transformer has a 0v - 110v - 230v secondary. You also state that you want two outlet sockets. Now it gets a bit complicated since I am not sure what is required. You could configure the secondary to provide two outlets at 230-v. each, or both at 110-v., or one socket at 110-v and one at 230-v. As for earthing on the sec'y side, this really depends on if you are likely to be using that transformer - or any equipment on your bench - that is of the a.c. / d.c. configuration.

At this point I'll stop, since there are clearly a few bits of information from you that I need before I can sensibly continue.

Al. / Dec. 25th. //

[pwdrive]

Hello Rob, Ideally I would say that the primary of the isolating transformer needs to be hard wired to a switch fused outlet plate.
For normall bench use the arrangements that I have used in the past are that the PE (protective earth) from the electricity supplier is connected to the screen and/or frame of the isolating transformer and that same PE is brought out to a suitable terminal somewhere on the bench where a seperate earth lead is then connected from that terminal to the chassis of the equipement under test/repair. The secondary of the Isolating transformer being then connected to a suitable standard outlet socket but without the earth. This is done to prevent a possible shock if there is an earth/neutral reversal in the plug of the DUT (device under test) ie: someone has wired the plug incorrectly as in a lot off cases the DUT would still work but it's chassis/case would become 240 ac above the PE's earth potential so to speak.
Before acting on any of this info I will try and post a link to an article that can probably explain this a little better, then with that and any others relavent postings from other members you might be able to make an informed choice. Disclaimer: I am not a qualified Electrician (But I can repair radios pretty good)

Lawrence.

[pwdrive]

The link pertaining to my first post:

http://www.bbc.co.uk/safety/pdf/safety-e...v2-1nm.pdf

Appologies Al I must have been writing as you posted, no pre empt intended.

Lawrence.

[Skywave]

Hi Lawrence - these things happen. Better to get two 'overlapping' replies than none at all!

Afterthought.
Whilst we are waiting for Rob to reply, I am a bit concerned about why the transformer is needed. If, and in retrospect, the need is for testing a.c. / d.c. radios, then using the transformer to power one radio - and not the test equipment - is the appropriate way to use this transformer. But I jump the gun - I'll come back later when Rob has found the time to reply.

Al.

[Bushbaby]

Hi Al & Lawrence,
Realistically the transformer only needs to supply one outlet to suit American voltages (I assume this to be 110 - 115v) in all honesty. I don't recall seeing an E terminal on the Primary side.
I suppose in all honesty I'm look on this as a step down transformer to power American equipment rather than as isolating transformer for use with radios. Does this give a better description/indication of what I am seeking?

Thanks for your continued help

Rob

[Skywave]

Hi Rob - thanks for returning; hope your Christmas day was all that you wished for.

For safety reasons, you must connect all the case, laminations, etc. of that transformer to supply earth. I would also run that earth connection through to the secondary winding outlets so that your test equipment has its exposed metalwork connected to supply earth as well: safety considerations, plus that also puts true earth / 0v. on the 'earths' of your test equipment.
You don't actually need an isolation transformer to produce the 110v. / 115-v that you need, but it will do the job - provided that it is big enough in terms of its V-A.

Having said that, if you do plan to work on any a.c. / d.c. equipment ~ such as a DAC90 ~ you will need another isolating transformer with only one outlet to connect only that one item to it for a power source. Then you can use your test equipment with it safely, since there is then only one earth connection.

You may be better advised to reserve that isol. transformer for that latter use, and obtain another transformer (isolating or auto.) as a source of 110-v / 115-v.

HTH

Al.

[Yorkie]

The link pertaining to my first post:

http://www.bbc.co.uk/safety/pdf/safety-e...v2-1nm.pdf

Appologies Al I must have been writing as you posted, no pre empt intended.

Lawrence.

A very useful post Lawrence.

The diagram on page eight of that most informative paper from an authoritative source makes interesting reading. Apart from the use of an isolating transformer for step-down purposes, simply as an isolating transformer as a safety precaution, it seems to me that reading the paper, there are as many potential hazards introduced to the unwary as we're trying to prevent. Earthing arrangements are nothing like as straightforward as we might imagine, if for example, we wish to use a scope or sig genny on an AC/DC radio.

[Mark Hennessy]

Also, thanks from me for posting that link - I'd forgotten that BBC EGNs were available externally. Whenever the subject of isolation transformers comes up, I find myself repeating the same old story - now I just need to give that link

Looks like a useful transformer, with two separate secondary windings, meaning you could have two outputs that are isolated from the mains and each other. No need for any of this isolation in your case, as already discussed, but still an interesting option for anyone who collects US gear.

In your position, I might be tempted to hang on to this transformer for use as a genuine isolation transformer, and wait for an auto-transformer. These tend to be physically smaller and cheaper than isolation transformers as they need rather less copper. Of course, any mains transformer with dual 120V primaries can be used as an auto-transformer - just disregard the secondaries (or use them for pilot lights, cooling fans or fine-tuning of the output voltage).

Cheers,

Mark

[pwdrive]

For what it's worth, my own take and probably the take of others is attention to the earthing arrangements, from what I understand in a workshop type of environment, it's either totaly earth free (including human/floor interface) or everything is strapped to earth. Am I correct in assuming this ?

Lawrence.