Question about isolation transformers

Discussion in 'General Electronics Chat' started by jodo, Dec 9, 2008.

  1. jodo

    Thread Starter Member

    Dec 9, 2008
    15
    0
    Greetings from a n00b.

    I'm about to get me a bench isolation transformer to do some of my first AC line powered tests (for repairs).

    Being very concerned about personal safety, I have read up on this topic in various internet and book sources. It's not a widely discussed topic, in my experience (hopefully because it's trivial). I have understood the gist; that it would protect a person by eliminating the current path between AC common and ground.

    I'm left with these questions:

    1. Is it enough to power the tested apparatus through the iso. tranfo? Or is it relevant to power scope, meters and/or solder station through that as well?
    2. I read that sometimes the earth ground is passed by some kind of filter before connecting it to the isolated side. What purpose has such a filter?
    Lastly, I'd like to ask for a recommended supplier/online retailer to get a good quality unit for hobbyist use, at a reasonable price. Also, links to any good documentation relevant to the topic are very welcome. Thanks a lot!
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Mains powered instruments will contain their own isolation transformers.

    The purpose of a bench isolation transformer for repair/service is as much to protect you own mains supply and other connected equipment as personal safety, in the event of a faulty appliance being connected, it may still be powered and run to determine the fault, without blowing all the breakers.

    Obviously with live apparatus connected in this way you, the repairer must be extra careful. It is good practice to keep an unoccupied hand in a pocket to prevent inadvertant contact. Also avoid ties, loose chains etc and wear safety glasses.

    Many repairers have a 'variac' (variable transformer) they can run equipment slowly up to voltage on. However not all equipment can be tested in this way.
     
  3. theamber

    Active Member

    Jun 13, 2008
    318
    0
    Just plug in the DUT (device under test) which is the one you are going to have expose open.
    I think the filter is a line tammer to reduce voltage spikes and line transients.
    I recomend you a Sencore unit PR57 like this:
    http://cgi.ebay.com/Sencore-AC-Powe...5|66:2|65:12|39:1|240:1318|301:1|293:1|294:50
    Sencore also has the Powerite II:
    http://www.sencore.com/products/general-test-measurement-equipment/122
     
  4. jodo

    Thread Starter Member

    Dec 9, 2008
    15
    0
    Excellent responses, thanks again!
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    "Mains powered instruments will contain their own isolation transformers."

    You've obviously eluded the era of the "All American Five" table radio.....where it was standard practice to attach one side of the power line DIRECTLY TO THE CHASSIS! These ubiquitous radios were affectionately known as "widow makers"...and were largely responsible for the United States achieving ZPG. a few years after the baby boom :)

    Eric the survivor.
     
  6. theamber

    Active Member

    Jun 13, 2008
    318
    0
    I am not sure that there are many Test equipment that have isolation transformers even today. What I noticed is that many are powered by batteries and the chassis are made of pastics trying to advoid exposed metal parts but the bare necesity, just in case one AC line get in contact with the instrument chassis making it a hot chassis. But as long as noone messed with the inside there is no reason for accidental contatcs.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Yes I'm a mere stripling.
    But I have read Scroggie. He used to recommend supply such equipment through a mains light bulb, to control any excess inrush current. Another good trick.

    Why would the OP ask about powering these from his isolation TX?

    Let's face it. Using the European standard 230 volts a typical 250 to 500 watt bench isolation transformer can supply 1 to 2 amps. Taking a typical human resistance of 10K or less the bench transformer will still put out enough poke to kill you.

    What it will do is prevent a faulty apparatus with a dead short to ground or neutral from blowing any fuses other than local ones in circuit, the moment you connect.
     
Loading...