isolation transformer needed

[sbnjhfty]

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.

 

[N_Cook]

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.

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

 

[GregS]

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.

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

 

[Adrian C]

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?

Seems to be a US safety requirement to ground one leg.
http://en.wikipedia.org/wiki/Isolation_transformer

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.

 

[GregS]

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.

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

 

[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 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"
http://www.tripplite.com/en/products/model.cfm?txtSeriesID=325&EID=13703&txtModelID=230
Also look through
http://www.tripplite.com/en/keyword-search.cfm?q=isolation transformer
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

 

[GregS]

Seems to be a US safety requirement to ground one leg.
http://en.wikipedia.org/wiki/Isolation_transformer

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

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.

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

greg

 

[[email protected]]

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.

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

 

[GregS]

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.

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

 

[Adrian C]

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

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.

 

[Baron]

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?

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) !

 

[mm]

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,

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.

 

[mm]

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.

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

 

[sbnjhfty]

So what do you want it for, to cut down on noise when you contact the "live"
when working on the PSU or to avoid shocks whilst working on the gear ??

Regards......... Rheilly P

To avoid shocks.

 

[bz]

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.

I have a 'short extension cord with a built in GFI'. I don't remember where
I bought it but you should be able to find such.
A google search for
extension cord with GFI
brings a bunch of hits.

 

[Adrian C]

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.

This must probably again be a voltage US/UK thing - or maybe me - but if
I was working on 240V live equipment I'd rather it was ALL floating
through an isolation transformer than rely on the complexities and
effectiveness of a GFI/RCD to prevent shocks at the bench.

 

[Adrian C]

To avoid shocks.

In which country are you?

 

[sbnjhfty]

In which country are you?

USA

 

[[email protected]]

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.

Well, the OP said 'Power supplies'. I have a bad habit of taking
people at their word. The last time I worked on an amplifier I noted
the power supply was an classic 'heavy iron' design, with the AC line
feeding a transformer. Still, I wouldn't be surprised to find a SMPS
in something rated at a pretend 1000 watts.

IMHO, it's a fool who repairs a power supply, then does not test it
with a dummy load BEFORE hooking it up to it's intended load. And
again, the OP has to decide if he wants an isolation transformer
capable of handling the full rated load. Personally, that is always
my recommendation. It avoids nasty surprises when one is in a hurry
to test and forgets to switch the power cord to an isolated source.

PlainBill

 

[mm]

Yes, of course; more important, one can get a GFI socket and
wire it into a handy box (or a string of boxes with multiple
plugs) and make a power distribution panel. There are
(expensive, alas) sockets that offer 'isolated ground' so that
the metal case(s) of the distribution panel can be grounded
while the special sockets can be floated. Thus, you can
wire sockets with ground-optional to the isolation transformer
and have only the distribution panel on the workbench (the
transformer can live on the floor or wherever).

Very good. Thanks to you and bz.