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isolation transformer needed

Discussion in 'Electronic Repair' started by sbnjhfty, Dec 2, 2009.

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  1. sbnjhfty

    sbnjhfty Guest

    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.
  2. N_Cook

    N_Cook Guest

    Try builders supplier for "site transformer" and also get a variac and a
  3. GregS

    GregS Guest

    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.

  4. Adrian C

    Adrian C Guest

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

    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.
  5. GregS

    GregS Guest

    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.

  6. GregS

    GregS Guest

    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"
    Also look through 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.

  7. GregS

    GregS Guest

    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.

  8. Guest

    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

  9. GregS

    GregS Guest

  10. Adrian C

    Adrian C Guest

    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.
  11. Baron

    Baron Guest

    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) !
  12. mm

    mm Guest

    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.
  13. mm

    mm Guest

    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.

  14. sbnjhfty

    sbnjhfty Guest

    To avoid shocks.
  15. bz

    bz Guest

    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.
  16. Adrian C

    Adrian C Guest

    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.
  17. Adrian C

    Adrian C Guest

    In which country are you?
  18. sbnjhfty

    sbnjhfty Guest

  19. Guest

    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.

  20. mm

    mm Guest

    Very good. Thanks to you and bz.
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