Transformer drawing 1 amp

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 ----- --- +
 
Transformer
------ 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)

Thanks,
Bob

<Bob.Jones5400@gmail.com

>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.

** No.


> 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?


** Yes it would.

    Dump it.



.......  Phil 

Bob.Jones5400@gmail.com wrote:
> 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 ----- --- +
>  
> Transformer
> ------ 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)
> 
> Thanks,
> Bob
> 
You didn't mention heat. Is the transformer as hot as a 100 watt light bulb?

Chuck

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Bob.Jones5400@gmail.com wrote:
> 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?

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 
it.

On 14 Mar 2007 05:17:55 -0700, Bob.Jones5400@gmail.com wrote:

>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 ----- --- +
> 
>Transformer
>------ 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)
>
>Thanks,
>Bob

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.
-- 

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

<Bob.Jones5400@gmail.com> wrote in message 
news:1173874675.071958.105010@d57g2000hsg.googlegroups.com...

>I have a transformer that is drawing about 1 amp when the secondary is
> not in use. Is this normal?

No.

> The transformer doesn't look like there is anything wrong with it(no
> charred spots) and it came from a car battery charger.

It may be designed with high leakage. This is a form of current regulation.

> (a rectangular box that as stuck inside the transformers windings)

A thermostat.

What do you want to do with this transformer? It's probably only good for 
battery charging.

<Bob.Jones5400@gmail.com> wrote in message 
news:1173874675.071958.105010@d57g2000hsg.googlegroups.com...
>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 ----- --- +
>
> Transformer
> ------ 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)
>
> Thanks,
> Bob
>

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.

Paul 

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
properly.

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?

Thanks,
Bob

<Bob.Jones5400@gmail.com>
>
>
> Could someone explain why and how a transformer limits its current?


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

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


> 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.


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


> 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
> properly.


 **  That tranny is probably  OK.

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



> If some transformers are designed in this way(to run "hot") could
> someone please explain why?


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

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


> 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?


**  Bet that does not happen.

 BTW

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

> Although I don't want to take any chance have the thing burn
> down the house either.

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. 
-- 
John English