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Current transformer Wire wrapping

Discussion in 'Electronic Basics' started by Bordon, Sep 1, 2005.

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  1. Bordon

    Bordon Guest

    Im trying to build a current transformer from a U Bolt.

    I am a novice so be gentle.

    I want to detect 120 volts of current in a household line.

    I wrapped a 1/4 inch u bolt in about 300 wraps of 30 Gage Magnet wire.
    I have wrapped about 4 wraps of the house wire as the primary.

    When I test it I only get about 1/2 volt of power out of the secondary.

    I want it to light an led or light. I have one led that is about 9volts.

    It is my understanding that I should be getting around 12 Volts with my set

    One question I have is does it matter what the ubolt is made out of.
    Everybody talks Iron, but of course I assume that this ubolt is made out of
    steel or who knows what, from home hardware. ~ Will this work? (I have no
    Idea if anything is made out of Iron any more)

    It also is zinc plated, and then I covered the bare ubolt in Teflon tape to
    make the wire stay and in case of shorts. - is this a problem (eg Zinc)?

    I wrapped it from the threaded part to the other end and stopped just before
    the other threaded part. and back over the secondary a little because I was
    not finished my wrappings. I tried to be neat but of course its not that
    easy. I put one bolt on each threaded bolt and then the plate and then
    another bolt to hold it on so it didn't move --o|o--- .

    So far do you see any problems? And possibly why IM getting low voltage on
    the Primary. I have tried to test it with a 2 amp drill. The voltage ranges
    from .5 - .8.volts.

  2. Bordon

    Bordon Guest

    Woops mis-spoke on one point.
    What I meant to say was Why am I getting low voltage on the Secondary.
  3. A current transformer transforms current, so the voltage in the wire
    has nothing to do with the output. I perfect current transformer
    outputs (into a short circuit) a current that is proportional to the
    current in the primary by the ration of the primary turns count
    divided by the secondary turns count.
    That gives you a turns count of 4/300=.0133. So, if the core were
    perfect, the transformer would output .0133 amperes for each ampere in
    the 120 volt line.
    Is this open circuit voltage? Current transformers are usually loaded
    with very low resistance loads. What is the current in the line wire?
    Bare (no series resistor) red LEDs usually need about 1.5 volts to
    make them glow.
    Your understanding is pretty weak. ;-)

    It sure does. The core needs to be magnetically soft and of high
    permeability. A steel U bolt is pretty poor on each count. Normally,
    current transformers are tape wound donuts made of silicon transformer
    steel or more exotic alloys ,like permalloy or other nickel iron
    alloys. There are also ones made with interleaved E and I stampings,
    like those used in all sorts of voltage transformers. Here is an example: Magnetics/Web Photos/New Photos/CSE187L.jpg

    You can make an approximation of one of these by removing the
    transformer from a wall wart (small wall plug mount supply) and unwind
    the low voltage winding. Then add a turn so of heavy wire in that
    space and pass your line current through that wire, using the original
    line voltage primary as the secondary.
    It will work to some extent as a very poor efficiency transformer core.
    No. All that is important is that the core easily pass magnetic
    field, be insulated from the windings, and less important, not
    circulate current around the core, under the wire. That last point is
    why good cores are made of lots of layers of metal tape or stacked
    laminations. These layers are insulated from each other, to keep the
    core from also acting like a short circuited turn.
    I suspect that this is reasonable since most of the energy put into
    the core by the primary is used up circulating current around the
    circumference of the bolt. Your .8 volts at the ends of 300 turns
    means that there is .8/300=2.7 millivolts generated in each turn.
    That doesn't sound like much, but think how much current the surface
    of the bolt must be carrying when you think of it as a sheet of metal,
    several inches wide, wrapped around the 1/4" rod. It must have a
    resistance well below a milliohm.

    If you are willing to wrap a coil by wrapping the wire around a dowel
    rod that is notched on the ends (like the end of an arrow) so you can
    pass the wire through a hole, you can do better by stacking up a bunch
    of washers (with a layer of tape between each for insulation) and
    wrapping your 30 gauge wire through the hole. Then pass your power
    lead through, also. But by the time you do all that, you could have
    made a better one from a voltage transformer with a split bobbin as I
    described, above. Or you could buy a commercial unit like the one I
    listed above, or one of the ones at the bottom of this page:
  4. Bordon

    Bordon Guest

    Is this open circuit voltage? Current transformers are usually loaded
    Yes I tried several resistors from 10R to about 360R.
    Its my understadning you must have a load on the secondary.
    Well really I tried a couple of LED's some of lower values as you mentioned.
    But obviously even for a small LED it was not enough.
    If all else fails I may try this, as AC/DC adaptors are cheap. I may get
    back to you on this. I have have one actually open on my desk right now.
    Its .25 A and 16.5v. However if you look at my diagram I think I will need
    to un solder the botom of the lamenated metal Pieces to get at the Primary.
    May be other tranformers dont have this problem. (I think its from an old
    modem or something)

    ,´--------- /|
    ,´-----------/ |
    /------------/ |
    +-------------+ |
    | | o-------o Lamenated Metal Pieces
    | +---------+ | |
    | |========o------------o Secondary (16.5v)
    | |=========| | |
    | |XXXXXXXXX| | |
    | |========o------------o Primary (120V)
    | |=========| | |
    |#+---------+#----------o Solder
    | | ' I guess where they put the
    +-------------+´ bottom part of Transformer
    in after they have put in
    winding asembly.
    (created by AACircuit v1.28.4 beta 13/12/04

    Interesting idea.

    I can get from my local Electronic store the a "Toroid" Dounut. They don't
    seem to have anything else eg Current transformers. Nearby anyway.

    Can I wrap one of these up. And would I get better response with this ? If
    So how many wraps would I use. The dounut is probably a 3/4 inch hole and
    about 1 3/4" across.

  5. Bordon

    Bordon Guest

    Woops reading this again I think what you mean is was there a load on the
    line 120v line. Yes I used a 2 Amp drill starting and stopping to draw
    current. The volt meter certainly showed that there was "action" on the
    secondary. So it works. It just isn't very much. Of course when you stop the
    drill the voltage drops to almost nothing on the secondary.
  6. Bordon wrote:
    I was thinking that you could pull the 16.5 volt winding out, one turn
    at a time, without taking the core apart.
    Most toroids are ferrite, which is not high enough permeability to
    work well at 60 Hz. Many are powdered iron (dispersed on insulating
    material) that are even lower permeability. If they have the specs
    for the core, you would want to get one that has a permeability of
    5,000 or 10,000. Most high frequency materials have a permeability
    between 10 and 1000. Most powdered iron cores have permeability below
    I guess it would work better than your U-bolt (the ferrite has a much
    higher resistance than iron, so it won't short out all your secondary
    energy), but it would be hard to get 300 turns in there plus the line
  7. Then you have built a very low efficiency current transformer.
  8. Jasen Betts

    Jasen Betts Guest

    use more wraps of magnet wire and houuse wire
    most LEDs will work at 2.5V or less
    it depends on the U bolt and how much current the AC mains circuit is passing
    Iron is better than mild steel (which you probably have) mild steel is
    better than high tensile, bigger is better.
    not really, a bigger problem is that solid metal transformer cores are
    inefficient, if you can replace your U bolt with a proper transformer core
    you'll get better results.
    that's about as good as you're likely to get. more turns of magnet wire will
    give more voltage if there's room for them. more turns of house wire will
    also help.

    one way to get more efficiency would be to take a "wall-wart" type plugpack
    powersupply apart, and take a chisel to the transformer secondary
    (the bit with the thick wires) and remove them, wind a few turns of your
    house main in their place and take the output from the primary
    (originally connected to the plug pins) -- something like that could
    produce upto 100V... using fewer turns of house wire will reduce the
    Try something that uses more power like a clothes iron, hairdryer, heat gun,
    or electric jug.
  9. Jasen Betts

    Jasen Betts Guest

    espeically the unemployed ones :)
    sometimes if you remove the aluminium (or steel) bracket the I sections are
    all together and come off easily allowing access to the windings.

    sometimes they are interleaved and that doesn't happen.

    possibly welded, I jabe a microwave transformer that's welded. it's probably
    not worth the bother trying to undo the welds (or solder joins) all that
    Iron is going to require a large soldering iron to unsolder.

    much easier is to (carefully) cut the secondary using a sharp knife or chisel
    as pull it out
    that would work, if you get a pre-wound toroid you could just
    make a few turns of mains wire through the centre, use the high voltage
    winding for output and ignore the low voltage winding.
    Toroids are typically more efficient than sheet-metal transformers.
    use as many turns as you can.

  10. Since the neutral wire has very little voltage with respect to ground,
    but carries the load current, it is the safest conductor to use with
    your transformer. Remember that one turn around the center leg of the
    E-I core is made by passing the wire down through one core hole and
    back up through the other one. It is a shame you didn't pull the low
    voltage winding out, a turn at a time (while counting turns). By the
    ratio of no load primary to secondary voltage, this would have told
    you about how many turns the original primary (now the current
    transformer secondary) has. But the core should be efficient enough
    that you should be able to connect an AC milliampere meter across it
    and, with a known primary current (from, say, the .83 ampere current
    of a 100 watt, 120 volt light bulb), measure the effective turns ratio
    of the CT.
  11. Bordon

    Bordon Guest

    Thanks Jasen,

    Didn't see your post till today because I was away. I have taken apart my
    Ac/Dc adaptor and as you and John suggested cut out the secondary. I used
    pointed wire cutters and just started cutting. I have not wrapped my hot
    line yet, as some other things have got in the way but I hope to try it out
    in the next day or so. I may also try salvaging the Ubolt with more wraps
    and see what happens. If you are still out there I have one other question.
    I'm assuming that I should wrap using the (white) neutral(AC) (not to be
    confused with the green ground) wire rather than the (black) hot.
    This is the way I have been doing it anyway.

  12. Bordon

    Bordon Guest

    Ouch, no I didn't count the wraps. ( I guess I'm not into music that kind of
    music. ;))

    Yes thanks for the confirmation. White I assumed would be the safest
    conductor to use .

    Wrapping, I suppose what you are doing is re-wrapping what you have taken
    out. Accept a lot less. I will try one wrap at a time and test it to see
    what I get with each wrap. I will also test it with different loads.
  13. Bordon wrote:
    That's right. You are replacing a winding that probably had at least
    a hundred turns with a winding of one to several turns. But it is
    still a winding.
  14. Jasen Betts

    Jasen Betts Guest

    neutral or hot it makes little difference electrically as the current I'm
    assuming you want to detect will be flowing in both of those conductors.

    using the neutral may result in a slightly safer design

    nevertheless for a little extra safety it's probably a good idea to earth
    the metal transformer core

  15. Gordon W

    Gordon W Guest

    If you are doing this as an interesting exercise you might like to visit a
    discussion on back in April this year titled 'current
    transformer winding wire help please'.

    You might also like to visit for more
    info. They also supply samples if you are interested in a professional


  16. Bordon

    Bordon Guest

    If you are doing this as an interesting exercise you might like to visit a
    Yes I will do that. Im a little depressed now because my last two
    experiments failed. The U bolt (although I may try adding more wire to it)
    and the transformer Idea. It didn't work either. I believe it was because
    the wire in the primary winding of the transformer must have been broken. It
    was a good transformer before I started, but once I went to all the trouble
    of removing the secondary, etc etc. it didn't seem to work as a current
    Transformer. I put an ohm meter on the ends of the transformer coil and got
    nothing mmm that is no continuity. The wire must have got broken some how.
    Well how the heck to you fix a break in a transformer ?? I tried various
    tests thinking that maybe it was bad soldering etc. but no luck. Soooo I
    have taken another transformer apart. But this one is smaller and it looks
    like taking the wires out will be real fun. It also has some diodes and a
    capacitor in it. Which I assume is to convert the power to DC and cushion
    things a bit.

    Thanks. I will persevere.
  17. Jasen Betts

    Jasen Betts Guest

    You can sometimes disassemble the laminations, unwind the core using a
    take-up spool driven by a hand drill held in a vice. solder the break,
    paint it with some insulating compound (eg nail polish) and re-wind it
    (driving the core spool with the drill this time) repairing any further
    breaks caused by snags during the re-winding. if your time is worth
    more than about $2 an hour don't bother unless you want the experience.

    I tried various
    a job like this really needs a vice without it it's too easy to stress the
    terminls or leads going to the primary while you're prodding and poking at
    the wire in the secondary, and that primary typically has very fine wire -
    and that smaller one will have even finer wire.

  18. Bordon

    Bordon Guest


    Im my case because we are just talking a cheap ac/dc adaptor I just put it
    aside and started on another ac adaptor. But thanks. Its all a learning
  19. Rich Grise

    Rich Grise Guest

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