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Transformer drawing 1 amp

Discussion in 'Electronic Basics' started by [email protected], Mar 14, 2007.

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  1. Guest

    I have a transformer that is drawing about 1 amp when the secondary is
    not in use. Is this normal? Its a 15amp 12V CT transformer. The
    secondaries read 12 volts from the center like it should(its actually
    about 13V). It had what I guess what a thermistor(a rectangular box
    that as stuck inside the transformers windings) but I took it out
    because I thought it might be the problem.

    It hums like crazy when I connect power to it and everything seems to
    work fine with it except that its drawing 1 amp for no reason. I
    seriously doubt this is power loss in the transformer but I can't
    figure out whats wrong. The only thing I can think of is that a few
    windings might have fused together somewhere but surely this would
    cause a huge current draw?

    The transformer doesn't look like there is anything wrong with it(no
    charred spots) and it came from a car battery charger. I'm trying to
    turn it into a variable power supply (about 5 to 12V) but I just have
    no clue if I should chunk the transformer or not. I essentially have
    nothing connected to the transformer except a switch that works and
    the power cord that also works(they are not the issue here). I
    connected an amp meter between the switch and one of the transformer
    leads and its reading about .8A AC after start up(there is a surge at
    the start but it seems to settle about there).

    ------ SWITCH ------- ----- AMP meter ----- --- +

    ------ Neutral ------- ----------------------------------- +

    Anyone have any clues on whats going on or how I could farther
    diagnose the issue? Or is this natural for transformers to waste so
    much energy? (I thought they were pretty efficient and being that I
    have nothing connected on the secondary I would expect it not to waste
    any power except maybe losses in the core)

  2. Phil Allison

    Phil Allison Guest

    ** No.

    ** Yes it would.

    Dump it.

    ........ Phil
  3. chuck

    chuck Guest

    You didn't mention heat. Is the transformer as hot as a 100 watt light bulb?

  4. All transformers must draw some primary current, even when
    nothing is connected to their secondaries. This is because
    a transformer generates voltage across each turn in
    proportion to the rate of change of flux through the core.
    It takes some current to swing the core flux back and forth
    to get that rate of change. Your transformer has a full
    load rating of about 12*15= 180 VA, so with 120 volts across
    the primary, that load would require 180/120= 1.5A, in
    addition (sort of) to the magnetizing current I spoke of.

    But the magnetizing current is normally a small fraction of
    the full load current, so I suspect that something is wrong.
    Either you are applying more voltage than the winding is
    intended for, driving the flux swing into saturation, or
    there is a partial short, somewhere inside the transformer,
    which could take quite a while to smoke or produce an
    obvious high temperature on the outside, where you can feel
  5. default

    default Guest

    May be a shorted turn or two that would cause the symptoms. A single
    turn may only have a voltage potential of a fraction of a volt to the
    turns on either side - the voltage being so low it doesn't immediately
    blow fuses - but may be causing the hum.

    Like someone already suggested 1 amp 120 volts is 100 watts and in a
    short time you should feel it getting warm then hot.

    Or being a reactive load (the inductance of the winding) is causing
    some hinky reading with your meter.

    I used to work for a place that wound their own PS transformers - we
    measured them with an old fashioned moving vane meter for excitation
    current and a few milliamps, maybe as high as 10-20 ma in that power
    range would be normal - depending on how efficient the client wanted
    the supply to be.
  6. It may be designed with high leakage. This is a form of current regulation.
    A thermostat.

    What do you want to do with this transformer? It's probably only good for
    battery charging.
  7. This would be abnormally high for most transformers. However, one designed
    for a battery charger may be designed with a lot of reactance so it is
    current limited and inherently short circuit protected. If it does not get
    very hot after about 30 minutes, it is probably drawing mostly reactive
    current, and may be safe. For an ordinary power supply, it would be better
    to get a transformer designed for a well regulated output voltage.

  8. Guest

    Thanks guys,

    Could someone explain why and how a transformer limits its current?
    They all look the same to me. I know there is saturation of the core
    that limits stuff but I don't know any details about it. It seems that
    the only parameters that one can change(the main ones at least) are
    the winding ratio and total windings. (assuming standard transformer
    design) I suppose if you just have one winding on the secondary and
    100 on the primary its not going to function as well as if you have
    100 windings on the secondary and 10000 on the primary.

    In any case, I'm testing the transformer as we speak. Its been running
    for about 40 mins and now has a temperature of about 37C. For the
    first 20 mins its was quite cool and maybe rose about 5 degree's above
    ambient. My meter is telling me that its drawing 0.8A but another
    meter said it was only about 0.6A when I tested it. Not sure if the
    meters are bad, if it has something to do with the inductance, or I
    just made a mistake... or maybe the meters are not calibrated

    I had another transformer that is 25VCT @ 2A and it was drawing about
    0.1A without any load. I suppose if everything scales up linearly then
    the 0.8A is reasonable. Its already at 40C and drawing about 0.90A
    (it slowly has seemed to creep up from about 0.84A at the start to
    0.9A now) so is I draw another 1.5A from it then I'm sure it will get
    to hot.

    It seems to bee peaking out at about 42C(the current has dropped to
    about 0.84A) but I could be wrong. Its hot to the touch but not close
    to burning but if one presses pretty hard then it does get
    uncomfortable and feels like it might burn after a short time. I can
    only imagine that having a full load on it will make it impossible to
    use and will most likely burn it up? Also I'm not sure how well my
    thermometer is measuring the actual temperature. I do not smell any
    smoke or fumes of any kind either.

    What should I do? Chances are I will run the power supply for long
    durations and rarely will I draw more than a few amps and when I do it
    will be for short periods of time(in most circumstances). I just need
    something with a little more juice than my current ps for those rare
    occasions when I want to play with things like electromagnets and
    motors. Although I don't want to take any chance have the thing burn
    down the house either.

    If some transformers are designed in this way(to run "hot") could
    someone please explain why? I'm at a loss as to explaining the
    behavior of this transformer if its not broke but besides the extra
    current everything else seems, at least so far, to be ok. Although
    when that extra 15 amps goes through the secondary and extra 1.5A
    going through the primary probably spells disaster?

  9. Phil Allison

    Phil Allison Guest

    ** Resistance in the windings is by far the main limiting factor on output

    It is also by far the main cause of self heating.

    ** Saturation limits the max applied primary voltage and the lowest
    operating frequency.

    ** That tranny is probably OK.

    0.6 amps off load is very high but not fatally so.

    ** When you apply a load to the secondary, the primary magnetising current

    Some trannys, like those in a microwave oven, are never meant to be run off

    ** Bet that does not happen.


    You MUST use a "true rms" meter to test AC current transformer windings.

    Ordinary ( average responding ) meters will be way under the real number in
    most cases.

    ....... Phil
  10. John E.

    John E. Guest

    Although I don't want to take any chance have the thing burn
    How much is peace of mind worth? (New worry for the new century: it's no
    longer "Did I turn off the gas" as our grandparents used to ask each other in
    the movie theatre, but now "Did I turn off my power supply.")

    Buy a new transformer.
  11. When there is no secondary load, the inductance of th
    primary limits its current. That inductance drops
    precipitously, twice a cycle, if the core flux reaches
    saturation. This occurs if you apply excessive voltage to
    the primary.

    The transformer limits current to the secondary by the
    resistance of both primary and secondary windings (that
    waste some of the available voltage, so less is there to
    drive current through a given load resistance). But if
    there is a significant flux path around the primary that
    does not also pass through the secondary, then there is an
    additional inductive current limiting effect that acts like
    having an inductor in series with the secondary.

    Some transformers intended to survive large secondary
    overloads (like welders, neon sign, microwave oven and large
    battery chargers) provide this flux path by separating the
    primary and secondary coils and adding blocks of core
    material between them, with a small air gap.

    In this picture of a microwave oven transformer, the
    secondary has been removed (it wound through the holes where
    the coin lays) and you can see the two blocks of laminations
    that almost close the flux path around the primary winding.
    The critical spec that involves saturation is volts per
    turn, so for a given number of primary turns, the voltage
    you apply to the primary. Are you sure the winding you are
    connecting to the line is really one designed for that much
    voltage? If you have a variac you can use to turn down the
    line voltage, smoothly, you can use it to see if the high no
    load primary current abruptly goes away at some reduced
    voltage. That would prove that the current is not a result
    of a short (that would cause the current to drop in rough
    proportion to the applied voltage).
    And you are back to the volts per turn problem.
    If the current is a result of a few turns shorted, that
    current will fall as these turns get hot (till that hot spot
    causes more turns to short).

  12. John  Larkin

    John Larkin Guest

    Does it get hot after, say, an hour? If not, it's probably just a
    really cheap transformer but OK to use.

  13. The Phantom

    The Phantom Guest

    What are the dimensions of the transformer core? I happen to have a
    transformer rated 8 amps @ 24 volts, about the same rating as yours. It's wound
    on a 2" stack of EI-137 laminatinos:

    A really good way to detect shorted turns is to measure the unloaded loss of
    the transformer in question. This requires a wattmeter, which you probably
    don't have, but just to show some typical measurements, I slipped a single turn
    of 21 gauge wire around the center leg of my transformer, and also a single turn
    of 14 gauge wire.

    I then measured the current with 120 VAC applied, and with all secondaries
    open and no shorted turns. Then I successively shorted the single turn of 21
    gauge wire and the single turn of 14 gauge wire.

    The results were as follows:

    Primary current Wattmeter reading

    No shorts .187 A 10.5 watts

    21 ga shorted .280 A 25.0 watts

    14 ga shorted .65 A 67.0 watts

    With only 10.5 watts dissipated, the transformer should only get a little warm
    after an hour or so. With 67 watts dissipated, I would expect it to get hot.
    The description you give in another post sounds like it's hotter than it should
    be with no load.

    If it's about the same size as my exemplar, then the primary current of .6
    amps with no load, plus the temperature rise you are seeing, would seem to
    indicate a shorted turn.

    I also did a short circuit test by placing an ammeter across the secondary and
    bringing up the primary voltage slowly with a variac. With 8 amps (rated
    current) in the ammeter connected to the secondary, the wattmeter indicated 10
    watts dissipation. This means that at rated load, the copper loss in the
    transformer is 10 watts, and with a core loss of slightly less than 10.5 watts,
    the total loss (core + copper) is about 20 watts at full load, substantially
    less than the 67 watts shown above. If this transformer is similar to yours,
    and it were operated at full load, you would have more loss due to a shorted
    turn than due to the load.

    The insulation system is probably not designed for this much temperature rise,
    especially in the immediate vicinity of the shorted turn, The transformer would
    smell and more shorted turns would eventually result. :-( Without the
    protective overtemp cutout you removed, there would be danger of a fire.
  14. Guest

    With the variac I have the following

    20V 0.1 A
    40V 0.13 A
    60V 0.21 A
    80V 0.31 A
    100V 0.47 A
    120V 1.07 A
    125V 1.48 A
    130V 1.78 A

    This looks almost exponential although I can't tell as the first 5
    measurements are approximately linear and there is a huge jump when
    going from 100 to 120.

    What does this mean? There definitely seems to be something happening
    about 115V. (looks almost linear up to that point then linear again
    but with a much larger slope)

    I can't tell what the "core" is inside the bobbin but strangely there
    is a wood chip(looks like a wedge) that is sticking out on one side up
    above the bottom. It looks like someone wedged it in there but it was
    made like that(its obvious). Not sure if this could mean the core is
    changed or what. Although looking at the bobbin The two coils look
    like the are stacked on top of each other and are of the same width.

    It sounds like this transformer might be current limited like you guys
    were mentioning. If so, does this mean that I cannot use it for my
    purposes? The reason I want to use it is because its 15 amps and my
    current ps is only about 0.75A and every time I use it above 1A I get
    nervous. (it has a fan in the case though and seems to stay cool but I
    never run it long above 1A... sometimes though I run it at 2-3 amps
    for short periods of time( in seconds)). I don't actually need the
    full 15 amps out of the thing and ATM I only have a 4A bridge
    rectifier but I was plan on using that and if I ever have the need up
    it then I'll just replace the rectifier. Also I couldn't find any
    power mosfets larger than about 8A so I'm limited by that ATM too.

    I just don't see any reason to waste the transformer if it will work
    for my needs. Later on I want to try and build my own transformer when
    I get some more time. I've got a book on transformer theory and
    design but haven't got around to reading it yet. (trying to get this
    PS built so I can play with some motors).

    Does this current limiting have anything to do with resonance? If I
    were to keep increasing the voltage with the variac would I eventually
    see a resonance curve?

    Thanks for your help. I appreciate you taking the time out to explain
    things(and the other guys too. Everyone has been most helpful).

  15. Guest

    Its about 4x3 but a little smaller

    Its 3 4/5 x 3 1/5 x 2

    So maybe its just a cheap transformer? Seems to be about the same size
    as yours but rated at almost twice the current. Of course even if I
    used it only at 8A then it would still be great.
    How did you short the turns? Did you manually rub off the insulating
    layer and short them?
    Yeah. I'm not sure. It seems like it might have been designed to do
    that. I guess its just not really the best transformer for what I
    want but maybe it will work if its not shorted in any way.
    But if its current limited then this seems natural? Personally if it
    is current limited it seems like a huge waste of power but maybe its
    the easiest way. I'm thinking about adding a load to is and see what
    happens after an hour or so. Try and push and see if it smokes or
    anything(since its rated at 15 amps maybe I'll just push it to 10A
    since I'll probably never use it above that).
    I added a new 100C thermistor but maybe that was a waste of 1$ if this
    transformer is bad. I think I'll try and add some load to it and have
    it run for a while and see what happens. This should give me a good
    indication if its good or not. This being a car battery charger was
    surely designed to run for several hours straight? Although maybe the
    thermistor would cause it to turn off and cool down when it overheated
    so it wasn't really for continuous use? Is there any max temperature
    for transformers operating at full load?

    Thanks for taking the time out. Its been really helpful in that now I
    do have an idea, at least for some transformers, about there current
    draw and what happens when they are shorted. I'm not sure if it
    applies to transformers in general though but still is helpful(if I
    can remember the results ;).

  16. Guest

    Now that I think about it maybe the transformer is not 15 amps. I
    thought because the charger was 15 amps that the transformer should be
    but maybe it was peak? since its a battery charger maybe the peak
    current was many amps but the average sustained current was only maybe
    6-8 amps. (it seems like the same size as your transformer so maybe
    this is the case).

    In this case the 1.0A with no load seems awful large as the turns
    ratio is 10:1 and that means that its only drawing about 0.8A with
    full load. Although, again, maybe the transformer was designed this

  17. Guest

    On the case(of the charger, not the transformer) it says

    Input: 120VAC 50/60 Hz 2.8 Amps
    Output: 12VDC 15 Amps

    This is why I assumed it was 15 Amps.

    Now since the ratio is 10:1 that means that it would draw a max
    current of 1.5 Amps at full load on the primary due to the load. Since
    I'm drawing 1A without any load this is about 2.5A on the primary at
    full load. So it seems to work out. I guess this transformer is a
    current limiting one.

    Now if I understand a current limiting transformer it is made in such
    a way as to limit the max current drawn on the primary? It does this
    by sacrificing power? So at full load of 15A the primary will be
    pulling much more current than it should but if I tried to pull 20A it
    would not let me? Sorta like some sort of over current protection? A
    "normal" transformer would not get as hot but it seems to be ok for
    this one?

  18. Phil Allison

    Phil Allison Guest

    ** Smug stuborn, Groper Alert !!

    ** Read my post to you - you ASS

    ** Bollocks.

    ....... Phil
  19. chuck

    chuck Guest

    Phil Allison wrote:

    Only if there is core saturation or a non-linear load. Otherwise, the
    two meters ought to read identically.

  20. The Phantom

    The Phantom Guest

    Probably EI-125 lams:
    Notice that my transformer is 24 volts @ 8 amps, while yours is 12 volts
    at 15 amps. My voltage is higher and current is lower, but the product of
    the two is about the same as yours. This is why I say the ratings are
    similar. I notice in another post that you say the 15 amp rating is the
    rectified output. This is not the same as the AC current rating of the
    transformer, so the comparison is somewhat off.
    I used a piece of magnet wire and scraped off the insulation at the ends
    of the wire so I could short them together.
    Can you post a digital picture of it over on

    If it is indeed constructed of standard EI laminations, concentric wound,
    then is isn't current limited. I suspect that it is a "cheap" transformer
    that is designed with core flux density somewhat high to help lower the
    initial cost. This can cause the no-load current draw and temperature to
    be high. I'll respond to another of your posts where you measure the
    current vs. input voltage.

    Also, can you measure the resistance of the primary and secondary with
    your ohmmeter? When you do that, first short the ohmmeter leads together
    and measure the resistance of just the leads. Then subtract that value
    from the resistance you measure; this is especially important for the
    measurement of the secondary. Measure the secondary from the outside
    connections of the secondary, ignoring the center tap.
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