Upcycle An Isolation Transformer

There are several reasons you should have an isolation transformer. They can prevent ground loops and also prevent a device under test from having a DC path to ground (or isolate an oscilloscope from DC ground, which can be dangerous in its own right, but that’s another discussion). [Tanner_tech] noticed that finding ballast transformers for sodium vapor street lights is getting easier as more street lights move to LED technology. What to do with these transformers? Build an isolation transformer, of course.

Of course, your dumpster transformer might be a little different than the one shown in the post (and the video, below). [Tanner] shows how to work out the leads you need. A little wood work and a PC power supply case finished the project.

Judging from the comments, some people take [Tanner’s] talk about safety as an implication that a transformer makes working on mains safe. It doesn’t. It makes it safer if you know what you are doing. Working with high voltage isn’t a place to learn by doing.

If you want some practical advice, [Jenny List] has a good read for you. You probably also ought to invest an hour in watching this video that has a lot of practical advice.

40 thoughts on “Upcycle An Isolation Transformer

    1. Careful, the step up and step down 115/230 transformers are usually autotransformers. That means no isolation. If you’re not familiar with the difference, Google it, and then you can use a multimeter on the resistance setting to check.

    2. Or connect 2 microwave ovens back to back after removing the magnetic shunts.
      A 1kW micro wave usually has a 1400W transformer.
      You can do this with any 2 transformers with the same winding ratio.
      If the ratio between the transformers is slightly different you can use that to compensate for losses.

  1. he has a good idea. but a few problems with his design, first no fuse on input, the on/off switch is quite undersized maybe good for 5-7 amps, and from the diagram on the transformer its designed for 100w usage so he may burn up the transformer first. also i hope he remove the ignitor from the transformer.

  2. Most, if not all, hid lighting transformers are auto transformers. Isolation limits some hazards, but also, if not protected properly can create other hazards. This a poorly executed hack job that could hurt somebody, or atleast give ’em a good shock.
    Why do people insist on mounting main voltage devices to wood?
    Totally unnecessary.

    1. According to the schematic, this transformer is configured as auto-transformer. But due to the resonant voltage regulation it has 2 windings (separated by the magnetic shunt). So you have to break this connection between pri and sec. If you ground the core, then there is no risk, if not, you need at least a double insulation fault (primary and secondary)

  3. just finished a “isolation transformer” 2 old 1000va ups transformers with the low side connected back to back in an old (metal) ups case (case is ground referenced), still need to mount a appropriate small fuse 1 or 2 amps maby, but for now the old ups breakers will have to do (20A input, 10 A output breaker wayyyy to high !!!!)

    1. This. The cool thing about using back to back UPS transformers is that those transformers typically have many taps. With different combinations you can either buck the output voltage a bit lower, or boost it higher if desired. Some of the larger rack mount APC UPSs even have two matched transformers in them.

  4. This transformer is quite useless as an isolation transformer. He doesn’t seem to understand what the transformer was needed for in the first place, even though he uses the term “ballast”.

    Notice the huge piece of iron between the two coils? That’s a magnetic shunt. The two coils are not on a shared leg of the transformer core, and there only is a loose magnetic coupling between them, or, in other words, there is a very large leakage inductance (uncoupled flux).

    Everything will seem just fine with only a multimeter attached, but the voltage will sag quickly when a load is applied.

  5. While we’re on the topic – lol – can anyone suggest a way to ground the outlets in my house? This is in Mexico and the contractor quit while the house was half built. The electrician put in the wiring – again – but he didn’t put in a third wire. All of our sockets are three-hole prong capable, but i’m afraid that middle prong isn’t connected to anything. Is there any way i can ground each outlet without having to run a third wire? There is no room anyway in the plastic tubing they used to route the wires. This is a concrete block construction so there’s not actually much worry about fire hazard, but any appliance that’s plugged in is not grounded of course.

    Have a great evening! :)

    1. If the guy didn’t put any grounds in I wouldn’t trust that he did any of the rest of the work properly either. That being said, I dunno what the code is in Mexico but back in the day people would use the plumbing system (metal pipes) for electrical ground (this is no longer allowed in the US but you still see it in old construction.)

    2. If the pipes for your wiring are anything like the normal pipes for electricity there is plenty room for the 3rd wire.
      The way to go is to first pull out the 2 wires, add the 3rd wire and then put them back in the pipes together again.

      You could run the 3rd wire outside the pipes, since it’s not carrying any voltage. But this has issues. Main issue that it’s probably out of regulations (lose insurance if the house burns down?) and with reliability.
      But I guess its better than having 3-prong sockets with an open earth connection.

    3. The wire used in most of the US is typically NM-B, or Non-Metallic sheathed cable. Otherwise known as Romex. Romex is double insulated.

      That wire comes with the ground inside already, there is no adding of ground.

      Previously used wires were “BX” or “AC” (Armored Cable.) Newer Armored cabled includes a separate ground wire. Back a long time ago it includes a thin metal strip that would be wrapped around the metal jacket and in contact with the metal strain relief clamp.

      In that case there was no separate ground wire, the metal jacket was the wire.

      If rigid metal conduit is used, as is the case in some cities or in, cement construction, perhaps Chicago, the metal conduit itself is suitable for use as the ground conductor.

      what type of outlets and wiring do you have? Is it in a conduit? take a picture.

  6. First, the use of the made up word “upcycle”

    …the word is recycle.

    “There are several reasons you should have an isolation transformer. They can prevent ground loops and also prevent a device under test from having a DC path to ground (or isolate an oscilloscope from DC ground, which can be dangerous in its own right, but that’s another discussion). ”

    Draw out the circuit, including the electrical panel, DUT and oscilloscope. Don’t forget that the oscilloscope alligator clip lead is connected to the scope’s chassis (earth) ground.

    Now trace that all the way back to the electrical panel. There the earth ground is connected to AC mains neutral through the back of the sheet metal electrical panel.

    Now either your DUT or your oscilloscope needs an isolation transformer to Galvanically separate it from the AC Mains Line and Neutral.

    If you don’t do this your oscilloscope probe “ground” alligator clip will short out whatever non-isolated (AC Mains powered) circuit it is connected to (60 times per second) or (50 times per second,) and will trip your hopefully functioning circuit breaker.

    The isolation transformer allows the DUT to be powered, but to “float” to the oscilloscope probe alligator clip lead reference point (earth ground / neutral.)

    Your wording is a little funny “DC Ground.”

    1. ‘Up-cycle’ is certainly a word, albeit mostly frowned upon, used at least for the past decade or so, with distinctions from the word ‘recycle’. (In layman’s terms, it differentiates from unprocessed – post manufactured components put back in to service, and processed – materials recovered to their primary types and reused in manufacture)
      To ‘really’ nit-pick – most [English] words are ‘made up’ anyway.

      For the oscilloscope, i’d suggest a suitable ‘scope or probes (isolated or differential).
      I do not suggest unsafe methods (like isolating the ‘scopes ground from the mains) – it’s not just failure mode it protects, but the chassis now has a ground ref of the probe ground.

    2. First, all words are made up, get over it, you unovergetter.

      Second, if you think the point above is wrong. Why the hell are you using words unknown to Shakespeare, (Who couldn’t spell worth a damme.) like oscilloscope?

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