isolation transformer needed

I'm looking for an isolation transformer for repairing small (less than 1000W) power supplies. I see loads of them on ebay and some are medical grade and inexpensive. Are these usable for what I want? Other than that, any tips on where to get a good deal on one? I don't want to spend a load as this is hobby work only.

Reply to
sbnjhfty
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Do you want to power the 1000 watt supplies with the isolation transformer, or use the transformer to power your soldering iron that you use to repair the supplies?

Reply to
hrhofmann

Try builders supplier for "site transformer" and also get a variac and a RCCD/ELCB/GFCI .

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Site transformers (UK) are 240 to 110 volt. And you pay for the rugged box and outlets - not needed in the workshop.

I'm not sure if a similar thing is used in the US. Where I suppose the OP is. Do wish people gave their location when asking about this sort of thing. ;-)

A good quality 1000va isolating transformer isn't going to be cheap new.

--
*How many roads must a man travel down before he admits he is lost? *

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Just an 'isolation transformer" means NOTHING for what you want. Most all isolation transformers sold have the secondary tied to ground. You need a service isolation transformer. You can usually get inside and remove a secondary "neutral" connection.

greg

Reply to
GregS

That's interesting. I've never seen any transformer with the secondary tied to ground - unless as part of an installation. The word 'isolation' means just that. Sure you're not thinking of an auto transformer?

--
*I\'m already visualizing the duct tape over your mouth 

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Seems to be a US safety requirement to ground one leg.

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I've got here a Farnell 500W 240V/240V isolating transformer that looks and weighs similar to a typical building site transformer, the outputs are floating.

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Adrian C
Reply to
Adrian C

Futher explanation, if the transformer is raw unmounted, its output is not tied to anything. A company who sells a transformer mounted in a box with outlets to consumers, MUST ground one leg of the isolation transformer. This also makes for what the transformer is mostly used, to help cut down on noise, especially noise not in common with ground.

I think you can buy service transformers ungrounded, but I never had one.

greg

Reply to
GregS

Interesting.

Indeed. They used to be very common for feeding things like electric guitars in a TV studio, etc. And grounding one leg of the secondary would negate the whole purpose of them. These days it's more usual to do a quick PAT (test) and supply the mains via an RCD.

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*It\'s this dirty because I washed it with your wife\'s knickers*

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

I have been thinking about it. The MAIN purpose of an isolation transformer is to make it non isolated by making a NEW neutral close to the device being powered. The main purpose of an isolation transformer is noise control.

Now we have us tecks who all our lives have learned isolation and that stuff. I taliked to the people at Tripplite and verified ALL their isolation transformers ARE grounded. Read the paragraph, and yet it says above, "Complete line Isolation"

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Also look through
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The isolator you buy from MCM is very unlikely to have the secondary grounded, but I really can't verify that.

In reading PC Power Protection by Mark Waller, he pointed out their is a NEC requirment for this grounding. I have not found an exact description in the code. There is about 75 references in that book, but no index marks. Man, I should sell that book !!!! Big Bucks.

greg

Reply to
GregS

I just put that there. it was not there this morning.

Thats probably true, and correct. its up to the contractor to use it in the right way.

greg

Reply to
GregS

Spending too little isn't cost effective either if you manage to fry a scope probe or other piece of test gear. Still, the 30 KVa three phase is serious overkill for your needs.

Give a little thought as to how you are going to be using this. If you want to isolate the input side of the power supply from the line so you can look at the control ICs a relatively low capacity isolation transformer will suffice. If you want to isolate 1000 watt power supplies while testing at full load, you'd better have an isolation transformer rated at something over 1000VA.

I would say something rated at 250 VA is the minimum you want. The medical isolation transformers or the BK Precision TR110 would be better.

PlainBill

Reply to
PlainBill47

I think to a technician, ground isolation is whats important or really needed. In some cases it may also include line isolation, but I have to think about that for a long time.

greg

Reply to
GregS

Could perhaps the figure of 1000W be taken from what might be repaired as a hobby interest - surround sound home theater amplifiers, where the actual power draw from the mains is considerably less than that hyped

1000W - and if it's that item, ye wouldn't really (if you value your ears) be testing at full load/volume anyway after fixing something that perhaps failed after moderate use.
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Adrian C
Reply to
Adrian C

A good source of pretty large 1 to 1 isolation trannies is old slot machines. I used to be in that business and still have several. Find a company who rents out gaming machines and ask, they scrap lots of obsolete machines.

On the subject of earthing (grounding) the transformer frame and (if metal) the enclosure is earthed, but neither side of primary or secondary.

Ron(UK)

Reply to
Ron

My bench Isolation transformer, 2KW rating, has a 240v primary with a centre tapped secondary that has provision to tie the centre tap to the case which is also earthed. FWIW it weighs about 100 LB its also marked as (Noise Suppressing) !

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Reply to
Baron

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It could be down to how power is distributed in our different countries. Effectively in the UK as regards power outlets there is only one voltage (240v) and all will be on the same phase. And the neutral is at ground potential. Without any form of protection, touching the neutral is safe. Touching the live isn't. With an isolating transformer, you can touch *either* safely, as there is no potential to ground. The only power outlet allowed in a UK bathroom is a shaver socket - and that is fed via an isolating transformer.

--
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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Yes, they are. In terms of shock hazard, you can use a GFI to the 1000W unit, and a small isolation transformer to run your oscilloscope, and get the same kind of result. The best isolation transformers are made for RF passthrough protection, or medical equipment uses, and will work well for the less-demanding electronics workbench even when they're 40 years old... so buying a used one makes sense.

Reply to
whit3rd

Do they make plug-in GFI's? Like they make plug in surge supressors and plug-in adapaters from 3 pin plugs to 2 slot receptacles, it should be easy to make a plug-in, point of use GFI, but I havent' come across one.

Reply to
mm

Now you've got me going and I want one too, but the biggest I saw today was 385 va, and it's listed as new, not surplus (which is what I thought you meant. They're 23 or 30 dollars plus 12 dollars shipping)

I see one for 8 dollars but it's only 100va.

Maybe I misunderstand your requirements, or my own :) .

I'll wait as long as you tell me to, and I'm not going to bid against you, but you say there are loads of them. Can you give me a url or item number so I can see what you're talking about.

I'd appreciate it.

Mike

Reply to
mm

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