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Isolation Transformer

 
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Isolation Transformer

Post by mendipviews » Sat Aug 27, 2011 2:53 pm

Having just obtained a dirty great big Isolation Transformer I have a quick query to earthing.
Do I earth the transformer to the mains side and if so where does the output get earthed to.
Just want to clarify before I connect it all up nicely.

Many Thanks
Luke

 
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Post by crustytv » Sat Aug 27, 2011 4:20 pm

Hi Luke,

Only earth the main input i.e where you plug the TX in to the wall. The other end should not have an earth ( the end that you plug a TV radio or scope to. If you did earth that end it would defeat the objective of isolation.

Chris

 
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Post by Michael Watterson » Sat Aug 27, 2011 4:31 pm

The what ever it is you connect it to gets its normally unearthable Live Chassis or live SMPSU negative connected to ground so you can connect test gear.

Otherwise not.

The chassis of the transformer can be connected to earth.

On building sites it's different, the isolation  reduces voltage to 110V, and then the centre is earthed, so no appliance wire is ever more than 55V (nearly safe) if a cable is damaged.

The main reason for isolating transformer with Electronics repair is really only two scenarios:

1) "live chassis" TV or Radio. It may normally be connected to neutral (thus can easily be live or connected to live). Some sets have a bridge rectifier, so the negative rail (chassis) is live no matter which way round mains cable is.

2) Off line SMPSU. These use a bridge rectifier and the negative rail of the switcher is isolated from output via transformers and opto-isolator, no matter which way round it's connected, the -Ve is "live".

So to connect a scope or other test gear running from mains (which should be earthed!) you must not connect either transformer O/P wire to earth, but earth the -ve rail or chassis or other point of exposed live metal work or reference point for tests and then connect the isolated winding to the mains in. Polarity is not important.

The equipment under test or repair isn't now "safe" it's just a chosen known area of it is now earthed. It still has lethal voltages.

 
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Post by mendipviews » Sat Aug 27, 2011 6:04 pm

Thanks Guys thats cleared that up.
Its a bit of a big transformer but it was free, it shouldn't matter if its a high current rated transfomer should it ???

Many Thanks
Luke

 
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Post by crustytv » Sat Aug 27, 2011 6:42 pm

Hi Luke, no it will be fine you should see the beast I have, It was a present from Malc Scott.

Chris

 
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Post by mendipviews » Sat Aug 27, 2011 8:25 pm

Thanks Chris,
                   I will have to get a picture of mine, I found it in a recycling centre, don't know where its from but its in a big metal box and rated at 10kva :D.
I think I will be safe from overloading it. :twisted:

Regards
Luke

 
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Post by GlowingAnode » Wed Aug 31, 2011 9:19 am

Hi Luke, that sounds a real beasty.

There's been a lot of discussion elsewhere about the use of isolation transformers recently, and I feel there are still some important points to be raised.

Trevor made some very good points about the set up at a former place of work, however these may not be appropriate for a typical domestic setup.

Firstly, why do you want to use an isolation transformer at all?
Reasons include;
1)To provide electrical seperation to one item
2)To provide electrical seperation to several items
3)To block any DC apparent on the supply
4)To reduce any common mode noise on the mains supply
5)To convert from one supply system to another eg T-T to T-NS or T-NCS to IT.

Secondly, what kind of setup will you use?
A permanent installation?
A bench fed from a single 13A socket?

Thirdly, what earth do you intend to use as you bench reference?

Do you intend to bring in external earths eg from an antenna bolted to a metal mast.

The prime reason for using an isolation transformer is to reduce the risk of electric shock. This must come first and foremost over any technical reason for using one, eg working on ac/dc sets or live chassis tv's.

If you can provide more detail of your intended setup, then I can advise you better.
Regards,
Rob.

Only

 
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Post by Mark Hennessy » Wed Aug 31, 2011 11:15 am

A 10kVA transformer will have an impressive surge at start-up - you might find problems with fuses blowing. Perhaps a soft-start scheme would be useful? I've found it's bad enough with 500VA transformers, but have no direct experience with anything substantially bigger like your beast  :)

Mark

 
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Post by Michael Watterson » Wed Aug 31, 2011 11:56 am

TBH I'd only ever use one so as to connect test gear to an AC/DC chassis or "live" side of a SMPSU.

If I was working on "live" chassis TVs or Radios, I'd earth the chassis with a clamped earth wire that can't fall off and then power via isolated transformer.

I'd worry about using a 10KVA one though.


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