New idea on re-winding a MOT with readily available materials - Aluminum foil!

Discussion in 'General Electronics Chat' started by RogueRose, May 4, 2017.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
    I am in need of rewinding a MOT to the lowest possible voltage - basically 1 - 1.3 turns - with as thick a cable/conductor as possible. The problem is that most wires of large size also have a lot of insulation taking up a lot of room and running at maybe 1.4-2.2vac, I don't think significant insulation is needed - so maybe wrapping bare wire with electrical tape, overlapping by 50%, so 2 layers are formed.

    It may be difficult to get proper wire sizing but I just thought about using aluiminium foil either rolled or folded in the the proper thickness/size needed to fit through the opening.

    Finding out how much a one ft length weights, a similar sized piece of wire could be found to see what gauge this corresponds to.

    Is there any reason to think that Al foil would not be adequate for this - and I'm not talking about challenges of rolling it to proper size, or knowing what gauge it is - but for the use of the material for this.
  2. wayneh


    Sep 9, 2010
    Is there a reason you're avoiding magnet wire?
  3. Reloadron

    Distinguished Member

    Jan 15, 2015
    Why not just use Magnet wire or enameled wire which comes with an enamel insulation? If you want some additional insulation look at using Kapton Tape which should work well or just plain cut strips of Kapton? As to weight per unit of length? Any decent wire gauge chart will give you all of that information.

    Wayneh got it before I did. :)

  4. SLK001

    Senior Member

    Nov 29, 2011
    I doubt that aluminum foil will work well - it is way too thin. Why don't you try some copper shim stock instead. It comes in rolls and also in various thicknesses. Wrap it with kapton tape for longevity.

    You need a wide conductor, since you will only have 1-2 turns. The "wideness" allows for intersecting as many magnetic flux lines as possible.
  5. #12


    Nov 30, 2010
    This is news to me. Would you like to educate me (and the T.S.)?
    I thought it was impossible to defeat the magnetic coupling for any wire inside a transformer window.
  6. RogueRose

    Thread Starter Member

    Oct 10, 2014
    Yes, availability mainly and getting it into the space required. I can make what is basically a square wire with the foil pretty easily and the connection is pretty simple with some custom aluminum blocks made to compress the ends of the foil wrap.

    The foil is readily available and can be made to any gauge needed to fit in the space available. The foil allow to maximize taking advantage of the square corners where round wire doesn't get into these areas. It can be made to whatever length needed and cost is neglegible. I can get a 50 ft long x 12" roll of heavy duty (1.75x thicker than regular) for $2. A comperable piece of ooooo0 orr 00 gauge would be close to $10+ per ft (plus shipping - never found stranded locally of this gauge) where I can get about 2.5ft of equal amperage capacity for $2. MOST importantly, it is available to anyone anywhere basically immediately.

    I researched Al foil and it is one of the purest aluminium alloys available being over 99% pure -, much in the 99.5 - 99.8% (the cheaper store brand is actually more pure than name brand which has some more things like SiO2 in it where the store often doesn't have the SiO2 and just very small Cu and Sn (copper/tin) in it.

    It makes insulating it easier as electrical tape works and with 1.4-2.2 vac, I think this tape should be fine and heat dissapation should be good as well with thinner insulation. Worst case scenario an enamal/varnish spray could be used on the outer layer and then wrap (or heat shrink if necessary, but that size may be difficult to find).
  7. RogueRose

    Thread Starter Member

    Oct 10, 2014
    Thanks for the info and suggestion guys - this gives me some things to look into. I am going to make a 1ft long roll of what would be 8 gauge equivialance and do some testing - using a wire distribution block to connect on each side and then hook a 4g wire on both sides and then maybe a 2g on each side, and then so some testing to see how the foil reacts to the current passing through it. I want to see if I can get it to melt or at least loose physical integrity so it sags - and see how things react in all.

    My question for this testing is whether 12vdc is adequate to test the Al roll. I have some power supply units which are 1200w (I have 6) and I have 16 750w 12vdc supplies - all run off 240. The 1200w all run at 12.01-12.03vdc as well as the 750's - they are server power supplies. They run in parallel no problem but IDK if that small voltage difference would cause any issues. I also have 2 large car batteries each able to put out 1100 CCA or 900 A normal - 2 in parallel could work I would think.

    My load is going to be an electrolysis cell I made which I have run with one of these batteries and it was pretty amazing!!!! Which would be better for the testing, the PSU's or batteries?

    As far as saftey, I know this is really dangerous with that much current, so I'm using a bunch of circuit breakers I have for high amperage car audio stuff - 300A & 500A breakers and maybe some 250A fuses if I need to add more but I'm wondering if they will interfere with trying to test the aluminium roll. I plan to make all the connections with the wires/breakers/dist-blocks, electrolysis unit and batteries/cell and then close the switch on the breakers at once (teamed breakers - to throw numberous ones at once).

    I just think that if Al foil can be used for this, it may be ideal when very low voltage is needed in something like this, but I do use copper wire when I need anywhere from 3+ turns on the transformers.
  8. tcmtech

    Distinguished Member

    Nov 4, 2013
    At that low of voltage the breakers are unnecessary. Amps without volts are basically harmless unless you put a lot of effort into doing something wrong on purpose.

    I've a worked on large commercial units that could do the high side of 20,000 amps at ~ 2 volts. Same thing. Unless you make an effort to short things out with a large heavy conductor not much will happen beyond what happens when you short out good D battery.

    If you need a fuse or circuit breaker on the system put it on the primary side.
  9. MaxHeadRoom


    Jul 18, 2013
    I remember someone bringing me a small welding transformer with no longer any output.
    I found that the secondary was aluminum foil type.
    On removing the winding, I found most of it had various holes where the aluminum had melted in spots, until one point where it severed completely.
  10. RogueRose

    Thread Starter Member

    Oct 10, 2014

    Well that is good to know and is the main reason I'm going to be testing some pieces before any use. I just rolled/folded 5 pieces (60" total width of heavy foil) to 18" and it is equal in weigh to 6g Al 18" wire and it would wrap much easier where I need it. I think A LOT would depend on a competent rolling/folding job where there are no kinks/bumps/ridges/etc - keep it smooth and all in contact with the next layer thus there should be no space for odd voids within the wrapping. Another option would be to make 5 seperate "rolls" and see if that might not allow for better air flow or cooling within the pieces.

    This is just an idea to see if something like this might be workable and giving it a shot shouldn't be too dangerous if a close eye is kept on it and saftey precautions are taken.

    I found that 1000 ft of 8g bare copper wire is 50lbs and aluminium 15.2lbs

    Copper Wire specs (weight of bare wire per 1000 ft) - Southwire - PDF

    Aluminum wire size
    Aluminum wire size - PDF file

    EDIT: I just found the references I was using for the weight of Al wire in my above posts was wrong - I was using about 48.2lbs per 1000ft - which I found really odd - I thought it was just much larger in size thus making up for nearly the same weight, so that 6g wire I have is actually like 3 6gauge wires and it is pretty small since it is flat. Looks promising!
  11. tcmtech

    Distinguished Member

    Nov 4, 2013
    At the low voltages you are working with common printer paper is plenty thick enough to work as winding insulation and will take heat way better than electrical tape will.

    As for conductor sizing 4/0 (0000) gauge is as large as the standard gauge values go. Above that the common reference is the MCM designation which goes into the thousands until you reach the point you are working with solid buss bar material which goes by common Width and Thickness inch or MM dimensional designations. wire sizes.htm

    Now the biggest issue with working with high current at a very low voltage is system resistance.
    You don't want any which means if you are planning to go to a kilowatt or more ar less than 2 volts you will need multiple 4/0 conductors in parallel.
    By MCM rating a 6 ga conductor is a 26 and a 4/0 is a 212 and for the low voltages you are working with you would need something in the 1 MCM per amp equivalent.

    I don't know what your intended electrolysis cell voltage and current are but given what you're implying repurposed MOT's, aluminum foil and 6 ga wire are about two magnitudes of order short of doing what you want.

    Realistically to handle 1000 amps with aluminum foil assuming it's the common .001" thick stuff you would need a cross sectional area of ~2 square inches or about 1000 layers 2 inches wide.

    At those current levels you would be money and fabrication time ahead to go to your local scrap yard and metals dealer and buy new or good used aluminum bar stock. At ~ $1 a pound for good scrap aluminum it's not that expensive and if you can't afford that you obviously can't afford anything else in the project either.

    Now as for repurposable transformers MOT are junk. What you would want are common HID ballast transformers. They are overbuilt and very efficient compared to a MOT of equivalent VA capacity yet are as easy or easier to work with.

    Also they're dirt cheap too. Stop by any scrap yard or electrical shop and ask for some. They take them out of commercial street and building lighting fixtures all the time. Figure they are worth about $1 per 100 watt ballast rating so 1000 watt ballast might cost you $10.

    Lastly, I am assuming you need DC power which add more losses being each diode will have a ~ .4 - 1 volt forward drop depending on whether it's a germanium or silicon type which again at 1000 amps that a 400 - 1000 watt power loss just in converting the low voltage AC into DC.
    Last edited: May 5, 2017
    RogueRose likes this.
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    I was wondering how TS would connect the AL to his electrolysis cell or what ever.
    Al foil is quite fragile.
    shortbus likes this.
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    The best way to use that MOT is 8 gage wire wrapped 7 to 10 turns, depends on what fits.

    Then put electrolysis calls in series to utilize the higher voltage. 10 volts would drive 4 or 5 cell in series quite well.
    RogueRose likes this.
  14. RogueRose

    Thread Starter Member

    Oct 10, 2014
    TCMtech - thanks for the excellent explination! The size of the foil is about the same calculation that I had 2" x 1". I haven't done a lot of rectification and always forget about the forward voltage drop being such a factor in something like this. I think I may try to work with some ballasts instead.

    I have a bunch of 250w units. They are the long style (about 15" including capacitor) and have what looks like tar around the outside of the windings (and cap). I did put it near a heater outside and allowed it to drip off but IDK if it can all be removed. IDK at what temp it melts and if it can be removed with something else. I was thinking about freezing it or something, but that seems that it may be more trouble than it is worth.

    These were built since 2000 in the US so there shouldn' tbe any PCB's or anything in the tar.
  15. RogueRose

    Thread Starter Member

    Oct 10, 2014
    Well I wired up a 25A full wave bridge rectifier and will be doing some testing as it looks like the voltaage drop and amount of heat dissappaation would be insane if running at 1.6-2v.

    As I said before this is for an electrolysis unit to produce H2 and O2. I'm not crazy about having the gases mixed, but that might be useable for some applications, but I'm looking for a better method of doing this - so I'm starting a new thread. This thread was for the power supply of the electrolysis unit, so it is the same project.
  16. tcmtech

    Distinguished Member

    Nov 4, 2013
    PCB's were in the capacitors not the tar and they haven't been around since the 60 - 70's. and even then a intermittent exposure to the stuff is harmless.

    As for ballasts I prefer the open type not the potted ones like you have just for the ease of working with them.

    Now for removing the tar tear the bottom cover off and set the ballast with that side down and start heating it with a good heat gun until the guts fall out then just keep heating that until the majority of the tar has came off. After that a good soaking and brush scrubbing with gasoline will dissolve everything else.

    Its messy and a bit tedious but the iron core and windings can be salvaged in working order from one of those with some work.

    Really for the power levels you are wanting to work with I would recomend tracking down some commercial 1000 or 1500 watt open frame type HID ballasts. That and designing your system to run multiple cells in series to increase the system voltage while reducing the overall amperage.