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    Elementary questions about electronics

    Modifying transformer


    I tore apart the battery transformer I mentioned here a while back. I'm
    going to remove a few turns on the primary to increase the voltage across
    the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
    so).

    I'm wondering if removing turns is a good idea? The core already saturates
    at about 110VAC and I would imagine that removing turns on the primary can
    only make things worse?  Although since it already saturates and wastes
    about 1A or 120W I'd imagine that since I'm not loading the secondary to
    anything near what its suppose to be that it won't be to bad? (its suppose
    to be rated for 15A but I'll probably draw about 4A max on rare occasions).

    Another issue was that the top part of the transformer was welded to the
    bottom part.  Now wouldn't this defeat the purpose of laminates?  Surely by
    welding them they would be electrically connected and therefore reduce the
    reason to use laminates?  When I cut the welds I was thinking that maybe it
    would be better not to weld it shut to remove any currents between the top
    and bottom halfs?  The magnetic flux should still concentrated in the core
    because the gap would be very small?

    I guess I'm going to play around with it and see what happens though.  Try
    without core first then piece it together.  Just want to get a few things
    clear before I wrap everything up.

    Thanks,
    JOn

    lol, now I see why the welded it... anyone know?

    -----------------------------------------------Reply-----------------------------------------------
    On Sun, 06 May 2007 22:46:24 GMT, "Jon Slaughter"

    <Jon_Slaugh@Hotmail.com> wrote:
    >I tore apart the battery transformer I mentioned here a while back. I'm
    >going to remove a few turns on the primary to increase the voltage across
    >the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
    >so).

    >I'm wondering if removing turns is a good idea? The core already saturates
    >at about 110VAC and I would imagine that removing turns on the primary can
    >only make things worse?  Although since it already saturates and wastes
    >about 1A or 120W I'd imagine that since I'm not loading the secondary to
    >anything near what its suppose to be that it won't be to bad? (its suppose
    >to be rated for 15A but I'll probably draw about 4A max on rare occasions).

    If it's already close to saturation, removing primary turns could be
    dramatic. 30/24 = 1.25, and few transformers are designed 25% away
    from saturation.

    Saturation isn't an absolute event, but losses climb radically as the
    core is driven harder. Smoking varnish is a reasonable expectation.

    John

    -----------------------------------------------Reply-----------------------------------------------

    "John Larkin" <jjlar@highNOTlandTHIStechnologyPART.com> wrote in message

    news:edos33p70ts989a3blt3ue5iil915e2og2@4ax.com...

    If I remember correctly it was only pulling 1A before without any secondary
    load. Now its about 2.5A and I've removed about 5 turns. That doesn't seem
    right but thats my readings. I think the extra current is because the core
    isn't welded so there is a great more deal of leakage. The voltage only went
    from 24.2 to 25.7VAC so I'd have to remove a lot more.

    I'll remove some more turn to get me to about 27VAC and then weld it shut
    again and then try and run it through some stress tests.  Unfortunately I
    can't easily add to the primary because of the huge wire it uses(I don't
    have any even close to it. I guess its about AWG 16-18 or something.

    I suppose if I could add to the secondary that I wouldn't have any of these
    issues? Would it even reduce the saturation if I did?  (I could always
    rewind the bobbins if I got the wire. I don't need the 15A the transformer
    is rated for but just about 8 or would be perfectly fine.

    Thanks,
    Jon

    -----------------------------------------------Reply-----------------------------------------------

    On Sun, 06 May 2007 23:47:26 GMT, "Jon Slaughter"

    It will be very nonlinear. I bet it's getting hotter, too.

     I think the extra current is because the core

    >isn't welded so there is a great more deal of leakage.

    Probably not; it's likely just saturation.

    > The voltage only went
    >from 24.2 to 25.7VAC so I'd have to remove a lot more.

    Yup.

    >I'll remove some more turn to get me to about 27VAC and then weld it shut
    >again and then try and run it through some stress tests.  Unfortunately I
    >can't easily add to the primary because of the huge wire it uses(I don't
    >have any even close to it. I guess its about AWG 16-18 or something.

    >I suppose if I could add to the secondary that I wouldn't have any of these
    >issues? Would it even reduce the saturation if I did?

    The primary determines saturation. If there's room for more secondary
    turns, maybe of finer wire, leave the original primary winding and
    work on the secondary.

    John

    -----------------------------------------------Reply-----------------------------------------------

    "Jon Slaughter" <Jon_Slaugh@Hotmail.com> wrote in message

    news:iOt%h.7195$2v1.3168@newssvr14.news.prodigy.net...

    It seems like a whole lot of bother to make a transformer do what it was
    not originally designed to do, and taking turns from the primary is the
    worst way possible. Once saturation occurs, the output voltage actually
    becomes somewhat regulated, so a large change in primary voltage produces
    much less change in the secondary. If you are not using a true-RMS meter,
    your readings will be false because of the increased distortion (clipping,
    or a flattened peak). Your current readings will also be erroneous because
    you will see even more distortion, but for current this will be sharp peaks
    corresponding to the flattened output voltage waveform.

    #16 or #18 wire is not huge, but of course I am a high current guy and I'm
    used to #4-0 welding cable and 6" x 1/2" bus bar. If you are willing to go
    to the trouble of unwelding a transformer and pulling out primary turns,
    you should be able to find an old automotive alternator or generator with
    wire heavy enough to use for a proper secondary. A motor winding shop might
    give you some old wire, or maybe an entire motor, for the price of its
    recycle value.

    Better yet, you should be able to find a more suitable transformer surplus
    or at a hamfest. The heavy items are often left behind. I've picked up
    (with some difficulty!) some big transformers and iron core chokes that
    were in the freebie piles and trash bins. Sometimes you have to pull them
    out of old tube equipment. If you can find a Variac or Powerstat, you
    basically have a toroidal core primary, and you can wind your own secondary
    for any voltage you want. I have a couple dozen of these with power rating
    of 500 VA to 2500 VA or more, and I've even repaired some that were burned,
    because all the windings are accessible.

    Paul

    -----------------------------------------------Reply-----------------------------------------------

    Well, maybe a bother to you but was fun for me. Learned a little bit about
    the practical side of transformers.

    Oh, the reason why I didn't want to mess with the secondary was that it is
    center tapped. I figured it would be much more complicated to mess with?
    I'd also have to worry about keep the taps even.  (Although I suppose it
    wouldn't be hard to add turns but since I didn't have any wire handy I just
    decided to mess with the primary.

    > #16 or #18 wire is not huge, but of course I am a high current guy and I'm
    > used to #4-0 welding cable and 6" x 1/2" bus bar. If you are willing to go
    > to the trouble of unwelding a transformer and pulling out primary turns,
    > you should be able to find an old automotive alternator or generator with
    > wire heavy enough to use for a proper secondary. A motor winding shop
    > might give you some old wire, or maybe an entire motor, for the price of
    > its recycle value.

    Well, I probably could have found something to use but wasn't interested in
    it so much.  I kinda ruined the transformer as now its pulling 4A but I am
    geting my 30V on the secondary.  Makes more noise than before but I didn't
    weld it all that well.  Only took a min to cut it open and another min to
    weld it closed. The hardest part was getting the bobbin out.

    > Better yet, you should be able to find a more suitable transformer surplus
    > or at a hamfest. The heavy items are often left behind. I've picked up
    > (with some difficulty!) some big transformers and iron core chokes that
    > were in the freebie piles and trash bins. Sometimes you have to pull them
    > out of old tube equipment. If you can find a Variac or Powerstat, you
    > basically have a toroidal core primary, and you can wind your own
    > secondary for any voltage you want. I have a couple dozen of these with
    > power rating of 500 VA to 2500 VA or more, and I've even repaired some
    > that were burned, because all the windings are accessible.

    Yeah, I'm going to get me a better quality transformer.  I was kinda
    experimenting with this one. I'm sure I could always rewind it if I wanted.
    I assume that I could reduce the saturation point by using more windings on
    the primary?

    The transformer died in a good cause though as atleast I have learned a
    great deal about them.  Wasn't a good transformer to start with so no big
    loss.

    Thanks,
    Jon

    -----------------------------------------------Reply-----------------------------------------------

    Oh, actually the secondary is about AWG 10 or 9(if there is a 9?). The
    primary is about 16. Not that it makes much of a difference.

    -----------------------------------------------Reply-----------------------------------------------

    "John Larkin" <jjlar@highNOTlandTHIStechnologyPART.com> wrote in message

    news:bmrs33pc8md7h488u65d725ja4dehbuoil@4ax.com...

    Now I have 30VAC secondary and 4A primary.  The transformer is pretty
    useless now unless I only plan on using it for a few mins or so(last time it
    took about an hour to heat up pretty good so now I would imagine that it
    would take a few mins)

    > I think the extra current is because the core
    >>isn't welded so there is a great more deal of leakage.

    > Probably not; it's likely just saturation.

    yeah, after I welded it there wasn't any significant decrease in
    saturation... just noise.

    Well, chances are this transformer sucked from the start? (from the other
    posts) It was already in saturation and I didn't want to mess with the
    secondary because it is CT'ed.  I didn't want ot mess with adding wire to it
    because theres not much room left.

    I suppose, if I'm really eager to mess with it, is I can just rewind the
    whole thing?

    If I use smaller secondary and primary windings I can probably keep it out
    of saturation(it was about 100 to 110VAC before it saturated at 120VAC)?
    Since I don't need the large secondary current I can use smaller wire on
    both... and since it won't saturate as much that means I can even decrease
    the size farther?

    I'm not sure if its the laminates that are the main issue or the turns?  I
    understand that there are better laminates but can this be offset by using
    larger turns or is there some upper bound that is reached?

    The way I see it is that I would need to rewind the primary using a smaller
    gauge and if I keep it out of saturation since its about AWG16 I might be
    able to go with AWG 18 or even 20? Of course I'd then have to worry about
    the secondary and its number of turns. If its about a 6:1 ratio then that
    means I can get about 6 amps out on the secondary for 1amp on the primary.

    The problems I forsee is that if I use to small of a gaugue to reduce
    saturation and have a large number of turns then I also have to have a large
    number of turns but larger gauge on the secondary.... Its possible that I
    might end up without the room on the bobbin for the secondary.  Although I
    guess I could always remove primary turns as long as it doesn't cause
    saturation.

    I guess I need to figure out what size wire I need on the secondary a given
    wire size on the primary at a certain max current?  Once I know this I can
    then estimate if the secondary will fit on the bobbin given the turns on the
    primary?

    Surely theres some software for this type of stuff?

    Thanks,
    Jon

    -----------------------------------------------Reply-----------------------------------------------

    On Mon, 07 May 2007 00:56:40 GMT, "Jon Slaughter"

    For a given core, it's primary turns that cause saturation; fewer
    turns makes higher flux density and heads for saturation. It sounds
    like this one was borderline to start with.

    >The way I see it is that I would need to rewind the primary using a smaller
    >gauge and if I keep it out of saturation since its about AWG16 I might be
    >able to go with AWG 18 or even 20? Of course I'd then have to worry about
    >the secondary and its number of turns. If its about a 6:1 ratio then that
    >means I can get about 6 amps out on the secondary for 1amp on the primary.

    Rewinding sounds like a huge hassle. There are lots of surplus
    transformers around.

    John

    -----------------------------------------------Reply-----------------------------------------------

    >>I'm not sure if its the laminates that are the main issue or the turns?  I
    >>understand that there are better laminates but can this be offset by using
    >>larger turns or is there some upper bound that is reached?

    > For a given core, it's primary turns that cause saturation; fewer
    > turns makes higher flux density and heads for saturation. It sounds
    > like this one was borderline to start with.

    yeah, but what I'm saying is that if I would have added turns to the primary
    then I could have reduce the saturation? Of course I would have decreased
    the secondary voltage too but...

    >>The way I see it is that I would need to rewind the primary using a
    >>smaller
    >>gauge and if I keep it out of saturation since its about AWG16 I might be
    >>able to go with AWG 18 or even 20? Of course I'd then have to worry about
    >>the secondary and its number of turns. If its about a 6:1 ratio then that
    >>means I can get about 6 amps out on the secondary for 1amp on the primary.

    > Rewinding sounds like a huge hassle. There are lots of surplus
    > transformers around.

    Well, I will try to look into that and see. In any case its a good learning
    experience and things always make more sense when I get my hands dirty.

    Thanks,
    Jon

    -----------------------------------------------Reply-----------------------------------------------

    On 2007-05-06, Jon Slaughter <Jon_Slaugh@Hotmail.com> wrote:

    > I tore apart the battery transformer I mentioned here a while back. I'm
    > going to remove a few turns on the primary to increase the voltage across
    > the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
    > so).

    > I'm wondering if removing turns is a good idea?

    good!  it's not.

    > The core already saturates at about 110VAC and I would imagine that
    > removing turns on the primary can only make things worse?

    yup.

    it might be better to re-wind the secondary with more turns
    (maybe use thinner wire)

    > Another issue was that the top part of the transformer was welded to the
    > bottom part.  Now wouldn't this defeat the purpose of laminates?  Surely by
    > welding them they would be electrically connected and therefore reduce the
    > reason to use laminates?  

    if the weld is thin (not right across the core) it doesn't form a shorted turn
    for eddy currents.

    --

    Bye.
       Jasen

    -----------------------------------------------Reply-----------------------------------------------

    On Sun, 06 May 2007 22:46:24 GMT, "Jon Slaughter"

    <Jon_Slaugh@Hotmail.com> wrote:
    >I tore apart the battery transformer I mentioned here a while back. I'm
    >going to remove a few turns on the primary to increase the voltage across
    >the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
    >so).

    >I'm wondering if removing turns is a good idea? The core already saturates
    >at about 110VAC and I would imagine that removing turns on the primary can
    >only make things worse?  Although since it already saturates and wastes
    >about 1A or 120W I'd imagine that since I'm not loading the secondary to
    >anything near what its suppose to be that it won't be to bad? (its suppose
    >to be rated for 15A but I'll probably draw about 4A max on rare occasions).

    If the core is indeed saturating, you'll only make it worse by
    removing turns.  It's anyone's guess how it will turn out - mine would
    be badly, but if it is only to be used occasionally for short periods
    it may work.  The transformer is rated at 15 amps at 24 out raising to
    30 will lower the output rating as well. 100 watts of waste is a fair
    amount of heat and cost if it is to run for protracted periods.

    Putting a small 4 amp 6 volt transformer phase wired to  boost the
    secondary is safer and probably easier.

    >Another issue was that the top part of the transformer was welded to the
    >bottom part.  Now wouldn't this defeat the purpose of laminates?  Surely by
    >welding them they would be electrically connected and therefore reduce the
    >reason to use laminates?  When I cut the welds I was thinking that maybe it
    >would be better not to weld it shut to remove any currents between the top
    >and bottom halfs?  The magnetic flux should still concentrated in the core
    >because the gap would be very small?

    Transformer manufacturers do take some short cuts and welds are one of
    them - you see it a lot in el-cheapo 12V battery charger transformers.
    microwave ovens, and other bulk produced consumer stuff.  Not as bad
    as it may seem at first because the weld usually has very shallow
    penetration and is on a part of the core where the field is weaker -
    but it will waste some power and I wouldn't try to re weld it. It is
    also hard to do unless you have a TIG setup and are very good and
    fast.

    >I guess I'm going to play around with it and see what happens though.  Try
    >without core first then piece it together.  Just want to get a few things
    >clear before I wrap everything up.

    >Thanks,
    >JOn

    You can add turns to the secondary if there's room.  
    --

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    -----------------------------------------------Reply-----------------------------------------------

    "Jon Slaughter" <Jon_Slaugh@Hotmail.com> wrote in message

    news:0FC%h.647$TE7.328@nlpi070.nbdc.sbc.com...

    As a learning experience what you are doing is fine, and maybe for a "rough
    and dirty" power supply it will be OK. I looked at your previous post in
    Mid-March (although the nym was BobJones, so maybe not you), the
    transformer was a 12 VCT at 15 A, and another was 25 VCT at 2 A. The
    purpose there was for a variable supply 5 V to 12 V.

    Anyway, much depends on what you intend to use the transformer for. A
    variable DC supply will be more efficient if you run the primary off of a
    Variac or Powerstat. At low outputs you will be below saturation and it
    will be very happy. You can also boost the 120 input to 140 and push the
    output at the expense of saturation.

    You could also use a phase modulated triac controller designed for
    inductive loads.

    If you just want to get higher DC voltage out, you can connect the output
    in various ways. A full wave bridge with 12 VCT will give you about 15-18
    VDC, while a full wave CT will give you half the voltage and twice the
    current. You can make a doubler circuit using two capacitors and two diodes
    to get 30-35 VDC. The regulation will be poor but such a circuit is also
    somewhat current-limiting, which may be good. You will need about 2000 uF
    per ampere.

    Have fun with your experiments.

    Paul

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