isolation transformer needed

[Rheilly Phoull]

"sbnjhfty" <fgsdfgrr@asfffwer.com> wrote in message
news:%llRm.94141$gg6.27160@newsfe25.iad...

I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.

So what do you want it for, to cut down on noise when you contact the "live"
when working on the PSU or to avoid shocks whilst working on the gear ??

Regards......... Rheilly P

 

[sbnjhfty]

Rheilly Phoull wrote:

"sbnjhfty" <fgsdfgrr@asfffwer.com> wrote in message
news:%llRm.94141$gg6.27160@newsfe25.iad...
I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.

So what do you want it for, to cut down on noise when you contact the "live"
when working on the PSU or to avoid shocks whilst working on the gear ??

Regards......... Rheilly P

To avoid shocks.

 

[Adrian C]

whit3rd wrote:

On Dec 1, 7:41 pm, sbnjhfty <fgsdf...@asfffwer.com> wrote:
I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?


Yes, they are. In terms of shock hazard, you can use a GFI to
the 1000W unit, and a small isolation transformer to run your
oscilloscope,
and get the same kind of result.

This must probably again be a voltage US/UK thing - or maybe me - but if
I was working on 240V live equipment I'd rather it was ALL floating
through an isolation transformer than rely on the complexities and
effectiveness of a GFI/RCD to prevent shocks at the bench.

--
Adrian C

 

[Adrian C]

sbnjhfty wrote:

Rheilly Phoull wrote:

So what do you want it for, to cut down on noise when you contact the
"live" when working on the PSU or to avoid shocks whilst working on
the gear ??


To avoid shocks.

In which country are you?

--
Adrian C

 

[sbnjhfty]

Adrian C wrote:

sbnjhfty wrote:
Rheilly Phoull wrote:

So what do you want it for, to cut down on noise when you contact the
"live" when working on the PSU or to avoid shocks whilst working on
the gear ??


To avoid shocks.

In which country are you?

USA

 

[bz]

mm <NOPSAMmm2005@bigfoot.com> wrote in
news:v3heh5l8roiu8ooio44fntl5o8g5hh7bcn@4ax.com:

On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd <whit3rd@gmail.com
wrote:

Yes, they are. In terms of shock hazard, you can use a GFI to
the 1000W unit, and a small isolation transformer to run your
oscilloscope,

Do they make plug-in GFI's? Like they make plug in surge supressors
and plug-in adapaters from 3 pin plugs to 2 slot receptacles, it
should be easy to make a plug-in, point of use GFI, but I havent' come
across one.

I have a 'short extension cord with a built in GFI'. I don't remember where
I bought it but you should be able to find such.
A google search for
extension cord with GFI
brings a bunch of hits.


--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

 

On Wed, 02 Dec 2009 19:35:30 +0000, Adrian C <email@here.invalid>
wrote:

PlainBill47@yahoo.com wrote:

Give a little thought as to how you are going to be using this. If
you want to isolate the input side of the power supply from the line
so you can look at the control ICs a relatively low capacity isolation
transformer will suffice. If you want to isolate 1000 watt power
supplies while testing at full load, you'd better have an isolation
transformer rated at something over 1000VA.

I would say something rated at 250 VA is the minimum you want. The
medical isolation transformers or the BK Precision TR110 would be
better.

PlainBill

Could perhaps the figure of 1000W be taken from what might be repaired
as a hobby interest - surround sound home theater amplifiers, where the
actual power draw from the mains is considerably less than that hyped
1000W - and if it's that item, ye wouldn't really (if you value your
ears) be testing at full load/volume anyway after fixing something that
perhaps failed after moderate use.
Well, the OP said 'Power supplies'. I have a bad habit of taking

people at their word. The last time I worked on an amplifier I noted
the power supply was an classic 'heavy iron' design, with the AC line
feeding a transformer. Still, I wouldn't be surprised to find a SMPS
in something rated at a pretend 1000 watts.

IMHO, it's a fool who repairs a power supply, then does not test it
with a dummy load BEFORE hooking it up to it's intended load. And
again, the OP has to decide if he wants an isolation transformer
capable of handling the full rated load. Personally, that is always
my recommendation. It avoids nasty surprises when one is in a hurry
to test and forgets to switch the power cord to an isolated source.

PlainBill

 

[whit3rd]

On Dec 2, 8:55 pm, mm <NOPSAMmm2...@bigfoot.com> wrote:

On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd <whit...@gmail.com
wrote:

Yes, they are.  In terms of shock hazard, you can use a GFI to
the 1000W unit, and a small isolation transformer to run your
oscilloscope,

Do they make plug-in GFI's?

Yes, of course; more important, one can get a GFI socket and
wire it into a handy box (or a string of boxes with multiple
plugs) and make a power distribution panel. There are
(expensive, alas) sockets that offer 'isolated ground' so that
the metal case(s) of the distribution panel can be grounded
while the special sockets can be floated. Thus, you can
wire sockets with ground-optional to the isolation transformer
and have only the distribution panel on the workbench (the
transformer can live on the floor or wherever).

 

[whit3rd]

On Dec 3, 6:20 am, Adrian C <em...@here.invalid> wrote:

whit3rd wrote:
...  In terms of shock hazard, you can use a GFI to
the 1000W unit, and a small isolation transformer to run your
oscilloscope,

This must probably again be a voltage US/UK thing - or maybe me - but if
I was working on 240V live equipment I'd rather it was ALL floating
through an isolation transformer than rely on the complexities and
effectiveness of a GFI/RCD to prevent shocks at the bench.

Working on live 120V or 240V, you can always touch two points
and get a shock. Either an isolation transformer, or a GFI
interrupter, prevents the shock if one of those points is GROUND.

I'm not sure why you would distrust a GFI, but they have a test
button. Use it, and be reassured.

The main plan, always, is not to touch the live wires.

 

[Dave Plowman (News)]

In article
<8122d63d-b1ef-41f7-bf49-24e4d16c165c@g12g2000yqa.googlegroups.com>,
whit3rd <whit3rd@gmail.com> wrote:

This must probably again be a voltage US/UK thing - or maybe me - but if
I was working on 240V live equipment I'd rather it was ALL floating
through an isolation transformer than rely on the complexities and
effectiveness of a GFI/RCD to prevent shocks at the bench.

Working on live 120V or 240V, you can always touch two points
and get a shock. Either an isolation transformer, or a GFI
interrupter, prevents the shock if one of those points is GROUND.

If one of those points is ground touching the other gives you a shock. If
neither is ground you can touch either one without a shock. The whole
principle of using an isolating transformer for safety. Makes for better
odds.

I'm not sure why you would distrust a GFI, but they have a test
button. Use it, and be reassured.

The main plan, always, is not to touch the live wires.

That is the best safety advice. But have a backup plan for if you do.

--
*A snooze button is a poor substitute for no alarm clock at all *

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[mm]

On Thu, 3 Dec 2009 13:57:06 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

On Dec 2, 8:55 pm, mm <NOPSAMmm2...@bigfoot.com> wrote:
On Wed, 2 Dec 2009 17:13:20 -0800 (PST), whit3rd <whit...@gmail.com
wrote:

Yes, they are.  In terms of shock hazard, you can use a GFI to
the 1000W unit, and a small isolation transformer to run your
oscilloscope,

Do they make plug-in GFI's?

Yes, of course; more important, one can get a GFI socket and
wire it into a handy box (or a string of boxes with multiple
plugs) and make a power distribution panel. There are
(expensive, alas) sockets that offer 'isolated ground' so that
the metal case(s) of the distribution panel can be grounded
while the special sockets can be floated. Thus, you can
wire sockets with ground-optional to the isolation transformer
and have only the distribution panel on the workbench (the
transformer can live on the floor or wherever).

Very good. Thanks to you and bz.

 

[GregS]

In article <50c415e195dave@davenoise.co.uk>, "Dave Plowman (News)" <dave@davenoise.co.uk> wrote:

In article
8122d63d-b1ef-41f7-bf49-24e4d16c165c@g12g2000yqa.googlegroups.com>,
whit3rd <whit3rd@gmail.com> wrote:
This must probably again be a voltage US/UK thing - or maybe me - but if
I was working on 240V live equipment I'd rather it was ALL floating
through an isolation transformer than rely on the complexities and
effectiveness of a GFI/RCD to prevent shocks at the bench.

Working on live 120V or 240V, you can always touch two points
and get a shock. Either an isolation transformer, or a GFI
interrupter, prevents the shock if one of those points is GROUND.

If one of those points is ground touching the other gives you a shock. If
neither is ground you can touch either one without a shock. The whole
principle of using an isolating transformer for safety. Makes for better
odds.

Its also necesary to make certain differential oscilloscope measurments if you don't
have a differential plug-in.

I also tend to use a battery scope for that instead.

greg

I'm not sure why you would distrust a GFI, but they have a test
button. Use it, and be reassured.

The main plan, always, is not to touch the live wires.

That is the best safety advice. But have a backup plan for if you do.

 

[GregS]

In article <%llRm.94141$gg6.27160@newsfe25.iad>, sbnjhfty <fgsdfgrr@asfffwer.com> wrote:

I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.

One thing you don't hear mentioned too much about is voltage.
We all know transformers are rated for a voltage with load.
The same thing can apply to an isolation transformer. i
have ordered custom wired transformers, and its NOT 1:1 with
no load. They increase the voltage to make up for the loss.
In some applications the voltage may be too high after this.
Good to use a variac along with an isolation transformer and monitor
voltage and current.

greg

 

[Phil Allison]

"David Lesher"

I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.


Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost.

** That is a very unwise suggestion:

AC supply transformers are NOT intended to be operated in reverse - cos the
magnetising current is intended to be carried by the supply side winding and
NOT the secondary.

A standard 500VA transformer operating from 120 VAC may well draw 1.5 amps
with no load - no problem since as the primary winding has only about 0.6
ohms of resistance and hence loses only 1.35 watts in heat.

The iron core losses will far exceed that.

However, if you try to make it work in reverse to deliver 120 volts at 500VA
from the primary - things get nasty.

First, the secondary will have to be fed with a ** higher voltage** than
the **off load** voltage by about 4% to cover voltage drop under load.

Naturally this increases the previously mentioned magnetising current level
by about 30%.

So now it is say 2 amps, referred to the primary.

2 amps at 120 volts = 240VA and that HAS to be *continuously supplied* by
the first tranny in the pair.

So, the result is that the pair of trannys can only deliver half the VA into
the load that one is capable of.


..... Phil

 

[David Lesher]

sbnjhfty <fgsdfgrr@asfffwer.com> writes:

I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.

Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost. Try all the usual
surplus suspects.

Of course the secondary power rating will be the limiting factor.


--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

[David Lesher]

"Phil Allison" <phil_a@tpg.com.au> writes:

Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost.

** That is a very unwise suggestion:

AC supply transformers are NOT intended to be operated in reverse - cos the
magnetising current is intended to be carried by the supply side winding and
NOT the secondary.

I've used this approach several times in the past, and never had
the catastrophe you imply. I recall doing it with a pair of plate
transformers [600V at 200-300 mA]; and later with some weird 60V@6A ones.

The idle currents were not egregious, and given the broad range of
"120 volt line" the loss unobtrusive. It was surely safer than working
on hot-chassis equipment without same.


--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

[Phil Allison]

"David Lesher"
"Phil Allison"

Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost.


** That is a very unwise suggestion:

AC supply transformers are NOT intended to be operated in reverse - cos
the
magnetising current is intended to be carried by the supply side winding
and
NOT the secondary.

I've used this approach several times in the past, and never had
the catastrophe you imply.

** Try reading my post again.

Only fuckwits snip posts out of site so they can write idiot
comments like you just did.

No "catastrophe" was predicted by me.

Anyone can claim to have done something wacky and got
away with it.

Sorry to say that is ** ABSOLUTELY NO BASIS **
for advising others in a public forum.
----------------------------------------------------------------

Some years back, I tried connecting a pair of new 240V / 6.3V, 15VA
transformers back to back to get an isolated 240 V output. The first tranny
in the pair ran hot with no load on the second.

A 15 watt load on the second dropped the voltage from 240V to 200V and the
first tranny then got very hot.

Testing revealed that the current drawn from the first tranny by the second
nearly equalled to its 15VA rating.

Useless.


..... Phil

 

[bz]

neet"Phil Allison" <phil_a@tpg.com.au> wrote in
news:7po31lFjqcU1@mid.individual.net:

"David Lesher"

I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.


Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost.


** That is a very unwise suggestion:

I am sorry to have to say this, but that statement is wrong.

AC supply transformers are NOT intended to be operated in reverse - cos
the magnetising current is intended to be carried by the supply side
winding and NOT the secondary.

The magnetic flux density depends on current AND the number of turns.

Less turns and higher current gives the same flux density.

This is why 'ampere-turns' are used in calculating flux density, not simply
'amps'.
http://mysite.du.edu/~jcalvert/tech/transfor.htm

The magnetic core doesn't care which winding induces the magnetic field.

A standard 500VA transformer operating from 120 VAC may well draw 1.5
amps with no load

At what phase? With no load, it is NOT in phase. It DOES cause IR losses in
the primary, however.

- no problem since as the primary winding has only
about 0.6 ohms of resistance and hence loses only 1.35 watts in heat.

1.5 amps at 120 volts = 180 W
1.5 amps at 120 volts at 89.57 degrees (or a power factor of 0.993) gives
1.35 watts.

Slightly over 1% loss. That is a bit high for modern power transformers
under no load, but a normal loss under max rated load.

The iron core losses will far exceed that.

Shouldn't. The iron core losses are PART of the total losses seen.

However, if you try to make it work in reverse to deliver 120 volts at
500VA from the primary - things get nasty.

No. You should only, at worst (approximately) double the losses (assuming
two identical transformers).

Let us assume that the secondary of T1 is 12 vac.
To deliver 500VA at 12 volts, the secondary is going need to see a 0.288
ohm load and will deliver 41.7 amps to the load. The DC resistance of the
winding will be much lower than the load resistance, on the order of 0.03
ohms for an inefficient transformer.

First, the secondary will have to be fed with a ** higher voltage**
than the **off load** voltage by about 4% to cover voltage drop under
load.

Naturally this increases the previously mentioned magnetising current
level by about 30%.

The identical but reversed 2nd transformer 'expects' that higher voltage
and exactly compensates for it.

So now it is say 2 amps, referred to the primary.

Wrong. If the output of the second transformer is carrying 1 amp, the
primary of the first transformer will carry 1 amp plus the iron and copper
losses of the two transformers. A poor efficiency is about 95% so with two
transformers, back to back, you might expect 10% losses resulting in 1.10
amps.

A low voltage, high current secondary is commonly wound with heavier wire
so it can stand more current and presents a much lower dc resistance.
When it is driven, as when the windings are reversed, it will run just
fine.

2 amps at 120 volts = 240VA and that HAS to be *continuously supplied*
by the first tranny in the pair.

Wrong. Only the current to supply the 'no load losses' needs to be supplied
'continuously'.

Under the 500VA load, the primary will present a 28.8 ohm load to the
110VAC line, drawing 4.2 amps of current while the back to back 12 volt
windings will be carrying 42 amps of current.

The no load losses will be quite low because the primary presents an
essentially pure inductive load rather than a resistive load to the ac
line. Only the core losses, on the order of .25 to .5% of the rating will
need to be supplied.

http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/

Under no load, the 12 volt windings would see a 'parasitic current' on the
order of 0.02 Amps and the 110 V primary would see a current on the order
of 0.1 Amp and present a 'resistive component' of the load of 115 ohms to
the supply lines.

So, the result is that the pair of trannys can only deliver half the VA
into the load that one is capable of.

The combo should be able to supply close to the rated VA of
_a_single_transformer.

You will have ~twice the losses and both transformers will reach the
temperature that one would have reached.

So, de-rate the pair of 500VA back to back transformers to 450VA to give
yourself plenty of safety margin and don't seal them into an airtight box
together, and you should be just fine.

.... Phil

Best Regards



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

 

[Jamie]

bz wrote:

neet"Phil Allison" <phil_a@tpg.com.au> wrote in
news:7po31lFjqcU1@mid.individual.net:


"David Lesher"


I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.


Buy 2 transformers with 120V primaries, and some voltage secondaries.
Hook them back to back.

The weirder the secondary voltage, the lower the cost.


** That is a very unwise suggestion:


I am sorry to have to say this, but that statement is wrong.


AC supply transformers are NOT intended to be operated in reverse - cos
the magnetising current is intended to be carried by the supply side
winding and NOT the secondary.


The magnetic flux density depends on current AND the number of turns.

Less turns and higher current gives the same flux density.

This is why 'ampere-turns' are used in calculating flux density, not simply
'amps'.
http://mysite.du.edu/~jcalvert/tech/transfor.htm

The magnetic core doesn't care which winding induces the magnetic field.


A standard 500VA transformer operating from 120 VAC may well draw 1.5
amps with no load


At what phase? With no load, it is NOT in phase. It DOES cause IR losses in
the primary, however.


- no problem since as the primary winding has only
about 0.6 ohms of resistance and hence loses only 1.35 watts in heat.


1.5 amps at 120 volts = 180 W
1.5 amps at 120 volts at 89.57 degrees (or a power factor of 0.993) gives
1.35 watts.

Slightly over 1% loss. That is a bit high for modern power transformers
under no load, but a normal loss under max rated load.


The iron core losses will far exceed that.


Shouldn't. The iron core losses are PART of the total losses seen.


However, if you try to make it work in reverse to deliver 120 volts at
500VA from the primary - things get nasty.


No. You should only, at worst (approximately) double the losses (assuming
two identical transformers).

Let us assume that the secondary of T1 is 12 vac.
To deliver 500VA at 12 volts, the secondary is going need to see a 0.288
ohm load and will deliver 41.7 amps to the load. The DC resistance of the
winding will be much lower than the load resistance, on the order of 0.03
ohms for an inefficient transformer.


First, the secondary will have to be fed with a ** higher voltage**
than the **off load** voltage by about 4% to cover voltage drop under
load.

Naturally this increases the previously mentioned magnetising current
level by about 30%.


The identical but reversed 2nd transformer 'expects' that higher voltage
and exactly compensates for it.


So now it is say 2 amps, referred to the primary.



Wrong. If the output of the second transformer is carrying 1 amp, the
primary of the first transformer will carry 1 amp plus the iron and copper
losses of the two transformers. A poor efficiency is about 95% so with two
transformers, back to back, you might expect 10% losses resulting in 1.10
amps.

A low voltage, high current secondary is commonly wound with heavier wire
so it can stand more current and presents a much lower dc resistance.
When it is driven, as when the windings are reversed, it will run just
fine.


2 amps at 120 volts = 240VA and that HAS to be *continuously supplied*
by the first tranny in the pair.



Wrong. Only the current to supply the 'no load losses' needs to be supplied
'continuously'.

Under the 500VA load, the primary will present a 28.8 ohm load to the
110VAC line, drawing 4.2 amps of current while the back to back 12 volt
windings will be carrying 42 amps of current.

The no load losses will be quite low because the primary presents an
essentially pure inductive load rather than a resistive load to the ac
line. Only the core losses, on the order of .25 to .5% of the rating will
need to be supplied.

http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/

Under no load, the 12 volt windings would see a 'parasitic current' on the
order of 0.02 Amps and the 110 V primary would see a current on the order
of 0.1 Amp and present a 'resistive component' of the load of 115 ohms to
the supply lines.


So, the result is that the pair of trannys can only deliver half the VA
into the load that one is capable of.



The combo should be able to supply close to the rated VA of
_a_single_transformer.

You will have ~twice the losses and both transformers will reach the
temperature that one would have reached.

So, de-rate the pair of 500VA back to back transformers to 450VA to give
yourself plenty of safety margin and don't seal them into an airtight box
together, and you should be just fine.


.... Phil


Best Regards



Nice vanity call you have there Mr. N5BZ..

For some reason I have a vision of seeing you on slow scan?

 

[Phil Allison]

"bz"

** I'll give this very confused radio ham just one try.

** That is a very unwise suggestion:

I am sorry to have to say this, but that statement is wrong.

** You are a brave man - and a very foolish one too.

AC supply transformers are NOT intended to be operated in reverse - cos
the magnetising current is intended to be carried by the supply side
winding and NOT the secondary.

The magnetic core doesn't care which winding induces the magnetic field.

** Not relevant.

The issue is the magnetising CURRENT !!

A standard 500VA transformer operating from 120 VAC may well draw 1.5
amps with no load - no problem since as the primary winding has only
about 0.6 ohms of resistance and hence loses only 1.35 watts in heat.

Slightly over 1% loss.

** Nonsense.

The power loss is mostly from the iron core when there is no load.

The previously mentioned 1.5 amps of magnetising current is inversely
proportional to the number of turns on the core.

The iron core losses will far exceed that.

Shouldn't.

** Fraid it does - pal.

However, if you try to make it work in reverse to deliver 120 volts at
500VA from the primary - things get nasty.

No.

** Fraid they do get nasty.

Even if nobody ever told you about it.

Let us assume that the secondary of T1 is 12 vac.
To deliver 500VA at 12 volts, the secondary is going need to see a 0.288
ohm load and will deliver 41.7 amps to the load. The DC resistance of the
winding will be much lower than the load resistance, on the order of 0.03
ohms for an inefficient transformer.

First, the secondary will have to be fed with a ** higher voltage**
than the **off load** voltage by about 4% to cover voltage drop under
load.

Naturally this increases the previously mentioned magnetising current
level by about 30%.

The identical but reversed 2nd transformer 'expects' that higher voltage
and exactly compensates for it.

** You have failed to see the issue of transformer "regulation", ie the
*off load* and *on load* secondary voltages are different - the voltage
always drops when load is applied. Ohms Law you know.

All transformers are wound so as to give the desired secondary voltage/s
when " on load" - ie the turns ratio is adjusted to compensate for the
regulation factor.

So now it is say 2 amps, referred to the primary.


Wrong.

** Fraid it is quite true - pal.

If the output of the second transformer is carrying 1 amp,

** The discussion is still about the no load situation.

You are miles away from any understanding of the issue.

2 amps at 120 volts = 240VA and that HAS to be *continuously supplied*
by the first tranny in the pair.


Wrong.

** Fraid it is correct - pal.

Only the current to supply the 'no load losses' needs to be supplied
'continuously'.

** That is exactly what I said.

Taking your example of a 12 volt secondary, the magnetising current drawn
when used in reverse is 10 times that when used the normal way. So, instead
of 1.5 or 2 amps of current - it is 15 or 20 amps.

Transformers are always rated in VA rather than watts - cos it is possible
to * fully load * a transformer with capacitance or inductance while drawing
no real power.

IOW - once the secondary *current rating* is reached, the game is up.

The no load losses will be quite low because the primary presents an
essentially pure inductive load rather than a resistive load to the ac
line. Only the core losses, on the order of .25 to .5% of the rating will
need to be supplied.

http://findarticles.com/p/articles/mi_m0BPR/is_10_21/ai_n6259812/

** How hysterically funny !!!!!!!!!!

The radio ham has quoted a page that discusses megawatt transformers the
size of houses.

Then he blithely assumes all transformers have the same characteristics as
these.

Wot a hoot.



...... Phil