DIY isolation transformer

Discussion in 'Power Electronics' started by BarryTron, Dec 6, 2018.

  1. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    I have been toying with an idea of building an isolation transformer. I have a UPS battery backup and a Microwave that friends are willing to sacrifice for my project. The idea was to take the transformer out of the existing appliance instead of buying one.

    What are your thoughts about building an isolation transformer? What other options do I have besides a microwave and battery backup? I have been looking for other affordable options but it looks like most DIY flocks use a transformer from a microwave.
     
  2. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Well it depends how you intend to wind the transformer. If you plan to wind one winding on top of the other then maybe it would work, but if you just want to wind the secondary where the old secondary was then there may be too much leakage inductance. Leakage inductance means that you dont just get a transformer out of it, you get a transformer with an inductor in series with one of the windings. How big that inductance turns out to be with the given steel and winding arrangement will determine how bad it gets when you turn something on or off and how much voltage drop (regulation) you get.
     
  3. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    Thank you MrAl for the answer.
     
  4. DickCappels

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    Aug 21, 2008
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    thedoc8 likes this.
  5. BarryTron

    Thread Starter Member

    Nov 18, 2018
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  6. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Just a quick note, all of the microwave transformers i have seen in the past (about 5) all of them were welded already.
    That makes getting them completely apart probably impossible without ruining the steel core. Any small gaps in the steel core where the E laminations meet up with the "I" laminations means more leakage inductance, which can get pretty large with even small gaps. Industry keeps these gaps small by using precision milling operations which would be hard to duplicate without a milling machine.
     
  7. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    you have a good point. i have a grinder and a welder. is it possible to evaluate the leakage inductance (before and after).
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The 2 I took apart must have had high inductance leakage then. While they were the E/I type the E's were all on one side and the I's were on one side, not interleaved like a normal transformer.
     
  9. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    did you try welding it back to see if that fixed the leak?
     
  10. shortbus

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    Sep 30, 2009
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    No I ended up throwing them away without using them. They were welded but only the outside edges, the center of the E couldn't get welded, so it would always have a gap, no matter what.
     
  11. recklessrog

    Well-Known Member

    May 23, 2013
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    The cheapest way here in the U.K is to buy a couple of second hand 240-110v power tool site transformers and connect the output 110v from one to the 110v winding on the other, that way you get 240 out again double isolated. be careful that you fit an appropriate 240v output socket on the second one instead of a a plug with live pins exposed.
    I have seen them on flea bay rated at 3kW for around £25.00 each.
     
  12. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    I know I can always buy things, but that defeats the objective of building and learning.
     
  13. MrAl

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    Jun 17, 2014
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    Hi,

    Not sure what you mean by having a grinder and a welder.
    Yes you can evaluate leakage inductance with some measurements. Might be better off looking on the web for this info, but we can discuss it here too if you like.
    One of the ways we used to do it was to short the secondary and apply an AC current to the primary. It's not the best way though.
     
  14. BarryTron

    Thread Starter Member

    Nov 18, 2018
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    I was only referring to the tools that are available to me.
     
  15. MrAl

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    Jun 17, 2014
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    Hi,

    Oh ok thanks for clearing that up. I guess you would need at least an AC current meter and AC volt meter so you can measure both of these things. Indirectly maybe set up a DC power supply and apply some load and note the decrease in voltage. May not be easy to do.
    If you had a variac you could apply a current but that might not be something you have either.
     
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