# using step-down transformer as step-up

R

#### [email protected]

Jan 1, 1970
0
At first blush, it seems like you should be able to use a power
transformer as either step-down or step-up, simply by feeding the
input into either the primary or secondary. However, is this really
the case? For example, consider one of those pole transformers that
normally step down, say, 12kV to 220V. If the secondary is not
connected to anything, I would expect the primary current to be quite
small. However, if you were to feed 220V into the secondary with the
primary open, wouldn't quite a bit of current be drawn by the 220V
secondary? (I've never tried this, I'm running on intuition here, so
I would welcome any insight.)

A similar situation: one of those Weller soldering guns. It has a
transformer with many turns on the 120V primary, and a ONE-turn heavy
copper secondary going to the tip. Surely if you reverse that and
feed in 1V (or whatever the secondary voltage is) it will be almost a
direct short at 60Hz, even if the primary is unloaded?

Is my intuition right, or am I totally off base here?

If I am right, what is it that makes the transformer asymmetrical?
That is, different when used as step-up or step-down? How would a
step-up transformer that is meant to convert 220V to 12kV differ in
construction from the step-down pole transformer, and why?

Robert

P

#### Phil Allison

Jan 1, 1970
0
At first blush, it seems like you should be able to use a power
transformer as either step-down or step-up, simply by feeding the
input into either the primary or secondary. However, is this really
the case?

** It is - with some provisions.

For example, consider one of those pole transformers that
normally step down, say, 12kV to 220V. If the secondary is not
connected to anything, I would expect the primary current to be quite
small. However, if you were to feed 220V into the secondary with the
primary open, wouldn't quite a bit of current be drawn by the 220V
secondary? (I've never tried this, I'm running on intuition here, so
I would welcome any insight.)

** Enough current would have to flow in the secondary to magnetise the iron
core.

That level would be 12,000/220 times the normal level.

A similar situation: one of those Weller soldering guns. It has a
transformer with many turns on the 120V primary, and a ONE-turn heavy
copper secondary going to the tip. Surely if you reverse that and
feed in 1V (or whatever the secondary voltage is) it will be almost a
direct short at 60Hz, even if the primary is unloaded?

Is my intuition right, or am I totally off base here?

** Basically - you are wrong.

If I am right, what is it that makes the transformer asymmetrical?

** Basically, they are not.

Problems can arise with small AC supply transformers that have high
magnetising current levels and poor regulation factors.

These will still work OK in reverse, but at a somewhat lower voltages than
when used the normal way.

...... Phil

D

#### Don Klipstein

Jan 1, 1970
0
In <[email protected]m>,
At first blush, it seems like you should be able to use a power
transformer as either step-down or step-up, simply by feeding the
input into either the primary or secondary. However, is this really
the case? For example, consider one of those pole transformers that
normally step down, say, 12kV to 220V. If the secondary is not
connected to anything, I would expect the primary current to be quite
small. However, if you were to feed 220V into the secondary with the
primary open, wouldn't quite a bit of current be drawn by the 220V
secondary? (I've never tried this, I'm running on intuition here, so
I would welcome any insight.)

My wild guess is feed "normal secondary voltage" into what is normally
the secondary, with what is normally the primary being open, and the
voltage at what is normally the primary will be low-90's % of "nominal",
when the transformer is a bigger one of several to 10's of KVA. My
experience is that ones of a KVA or two achieve upper 80's % in that area.
A similar situation: one of those Weller soldering guns. It has a
transformer with many turns on the 120V primary, and a ONE-turn heavy
copper secondary going to the tip. Surely if you reverse that and
feed in 1V (or whatever the secondary voltage is) it will be almost a
direct short at 60Hz, even if the primary is unloaded?

I consider this a fairly inefficient transformer. I do not have one
handy to try. Though my guess is that running one of those in reverse
needs several amps to maybe around 10 amps to supply "magnetizing current"
from the low voltage side at "normal secondary voltage" (in addition to
soldering tip load current), to achieve output voltage from the line cord
maybe around 75-80 volts or so output voltage from the line cord. (Though
I would not be surprised with the line cord putting out to open circuit
or a voltmeter anywhere from 60 to 105 volts).
Is my intuition right, or am I totally off base here?

If I am right, what is it that makes the transformer asymmetrical?

Loss, which is probably at least somewhat evenly split into the two
directions.
That is, different when used as step-up or step-down? How would a
step-up transformer that is meant to convert 220V to 12kV differ in
construction from the step-down pole transformer, and why?

Due to losses, I expect 12KV-to-220V (more like 240V) to have turns
ratio a little less than 50 (12,000/240), probably more like 48 maybe 47.

Step-up from 240V to 12 KV means probably turns ratio of at least 53
even in 1-KVA capacity ballpark.

- Don Klipstein ([email protected])

R

#### [email protected]

Jan 1, 1970
0
For example, consider one of those pole transformers that
**  Enough current would have to flow in the secondary to magnetise theiron
core.  That level would be 12,000/220  times the normal level.
.....    Phil

Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller? Robert

P

#### Phil Allison

Jan 1, 1970
0
Phil Allison

** Enough current would have to flow in the secondary to magnetise the
iron
core. That level would be 12,000/220 times the normal level.

Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller?

** Your whole notion is false.

Transformers are essentially symmetrical devices.

....... Phil

B

#### [email protected]

Jan 1, 1970
0
Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller?  Robert

No. It means that the wire used on the 220V side would be thicker (to
carry the magnetising current without getting too hot) in a
transformer designed to be used to step up a 220V source to provide a
12kV output, as compared with a transformer designed to step down a
12kV source to provide a 220V output.

The turns raito for either transformer may well be slightly higher
than the nominal 54.5 to compensate for the voltage lost driving the
magnetising current through the winding resistance, but this ratio
should be the same for transformers driving the same output load in
either direction. In practice, it isn't always possible to wind a high
voltage winding with the optimal wire thicknesss - thin wire breaks
easily.

R

#### [email protected]

Jan 1, 1970
0
** Enough current would have to flow in the secondary to magnetise the
Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller?

** Your whole notion is false.
Transformers are essentially symmetrical devices. Phil

You ARE right. Transformers ARE essentially symmetrical. I think I'm
starting to see that was troubling me had nothing to do with symmetry
at all, but rather with the no-load current drawn by the winding used
as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my
220V wall socket, it'll probably blow the fuse even if nothing is
connected to the other side. But that's not asymmetry. It just means
that the transformer wasn't meant to be used like that. If it was, it
would have more turns on the 220V side (and a correspondingly larger
number of turns on the 12kV side to maintain the ratio) so that the
now greater inductance would draw less current while magnetizing the
core.

So it's not the transformer itself that is asymmetrical in behaviour,
but rather the requirements placed on the windings, depending on
whether you use them as primary or secondary. A primary winding
shouldn't draw much current when unloaded, and for a secondary, fewer
turns is probably an advantage since it gives lower impedance so the
voltage won't drop as much under load.

Does that make sense, or am I still wrong? Robert

R

#### [email protected]

Jan 1, 1970
0
** Enough current would have to flow in the secondary to magnetise the
Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller?

** Your whole notion is false.
Transformers are essentially symmetrical devices. Phil

You ARE right. Transformers ARE essentially symmetrical. I think I'm
starting to see that was troubling me had nothing to do with symmetry
at all, but rather with the no-load current drawn by the winding used
as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my
220V wall socket, it'll probably blow the fuse even if nothing is
connected to the other side. But that's not asymmetry. It just means
that the transformer wasn't meant to be used like that. If it was, it
would have more turns on the 220V side (and a correspondingly larger
number of turns on the 12kV side to maintain the ratio) so that the
now greater inductance would draw less current while magnetizing the
core.

So it's not the transformer itself that is asymmetrical in behaviour,
but rather the requirements placed on the windings, depending on
whether you use them as primary or secondary. A primary winding
shouldn't draw much current when unloaded, and for a secondary, fewer
turns is probably an advantage since it gives lower impedance so the
voltage won't drop as much under load.

Does that make sense, or am I still wrong? Robert

P

#### Phil Allison

Jan 1, 1970
0
Interesting....so does that mean that a transformer designed to step
220V up to 12kV would have more turns on the 220V side than a similar
step-down transformer, so that the current drawn by the 220V winding
in the absence of a load is smaller?

** Your whole notion is false.
Transformers are essentially symmetrical devices. Phil

You ARE right. Transformers ARE essentially symmetrical. I think I'm
starting to see that was troubling me had nothing to do with symmetry
at all, but rather with the no-load current drawn by the winding used
as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my
220V wall socket, it'll probably blow the fuse even if nothing is
connected to the other side. But that's not asymmetry. It just means
that the transformer wasn't meant to be used like that. If it was, it
would have more turns on the 220V side (and a correspondingly larger
number of turns on the 12kV side to maintain the ratio) so that the
now greater inductance would draw less current while magnetizing the
core.

So it's not the transformer itself that is asymmetrical in behaviour,
but rather the requirements placed on the windings, depending on
whether you use them as primary or secondary. A primary winding
shouldn't draw much current when unloaded, and for a secondary, fewer
turns is probably an advantage since it gives lower impedance so the
voltage won't drop as much under load.

Does that make sense, or am I still wrong?

** You are still wrong.

When I say transformers are essentially symmetrical - I mean they CAN be
used in both the normal and reverse directions.

Why not simply post a REAL question - ie one where YOU are not posing mad
theories and making up false examples.

Eh ????

...... Phil

R

#### [email protected]

Jan 1, 1970
0
Why not simply post a REAL question  - ie one where YOU are not posing mad
theories and making up false examples.

Eh   ????

.....    Phil

I'm sorry. I didn't mean to upset anyone. This is a real question to
me since I need a high-voltage step-up transformer for an RF amplifier
and was planning to use a power distribution transformer backwards,
but it's expensive so I wanted to make sure it will do what I want
before I buy one. Again, sorry I'm being a pain. I won't ask any
more dumb questions. Robert

P

#### Phil Allison

Jan 1, 1970
0
<[email protected]
Why not simply post a REAL question - ie one where YOU are not posing mad
theories and making up false examples.

Eh ????

I'm sorry. I didn't mean to upset anyone.

** Not upset, but you were going no-where with your absurd Q.

This is a real question to
me since I need a high-voltage step-up transformer for an RF amplifier
and was planning to use a power distribution transformer backwards,
but it's expensive so I wanted to make sure it will do what I want
before I buy one. Again, sorry I'm being a pain. I won't ask any
more dumb questions.

** If you had just posted that simple Q right off - with a link to the
long ago.

...... Phil

R

#### [email protected]

Jan 1, 1970
0
Trying again with a "simple" question
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=400028867104
(You have to scroll down past some irrelevant stuff. Then there are
pictures and a close-up of the nameplate.)
It's a 25kVA 4160/7200/12470V to 240V 60Hz step-down transformer.

What I'm wondering is, if I use this as a step-up transformer by
applying 240V to what is normally the secondary, what would be the no-
load current drawn by the 240V winding?

I suspect there isn't enough information here to determine that, but
perhaps someone who is familiar with transformers of this type would
have some idea?

Robert

R

#### Rich Grise

Jan 1, 1970
0
On Wed, 11 Feb 2009 18:24:46 -0800, renenkel wrote:
I'm sorry. I didn't mean to upset anyone. This is a real question to
me since I need a high-voltage step-up transformer for an RF amplifier
and was planning to use a power distribution transformer backwards, but
it's expensive so I wanted to make sure it will do what I want before I
buy one. Again, sorry I'm being a pain. I won't ask any more dumb
questions. Robert

That's been done before with "pole-pigs"; typically hams using 3300/240
trannies (local dist. lines were often 3300V back in those days). They
had no problem with excessive primary current - the magnetizing current
in the core is sufficient either way - consider a neon sign transformer.

Have Fun!
Rich

P

#### Phil Allison

Jan 1, 1970
0
Trying again with a "simple" question
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=400028867104
(You have to scroll down past some irrelevant stuff. Then there are
pictures and a close-up of the nameplate.)
It's a 25kVA 4160/7200/12470V to 240V 60Hz step-down transformer.

What I'm wondering is, if I use this as a step-up transformer by
applying 240V to what is normally the secondary, what would be the no-
load current drawn by the 240V winding?

I suspect there isn't enough information here to determine that, but
perhaps someone who is familiar with transformers of this type would
have some idea?

** Such large transformers have low * percentage * losses and the iron and
copper loss ( at full load) are about the same. The 240 volt winding is
rated a tad over 100 amps - so I reckon I mag might be 5 to 10 amps or so
on the 240 volt side.

But you will never be able to switch it on without using a big variac or a
well designed "sort start "circuit and you will never get more out than you
put in.

Clearly, you have some totally hair-brained, LETHALLY DANGEROUS and ILLEGAL
scheme in mind.

Kindly GO STRAIGHT TO HELL.

Bloody lunatic.

...... Phil

G

#### [email protected]

Jan 1, 1970
0
I'm sorry.  I didn't mean to upset anyone.  This is a real question to
me since I need a high-voltage step-up transformer for an RF amplifier
and was planning to use a power distribution transformer backwards,
but it's expensive so I wanted to make sure it will do what I want
before I buy one.  Again, sorry I'm being a pain.  I won't ask any
more dumb questions.  Robert

I've built a few low power RF amplifiers in the past. I "rolled my
own" impedance matching transformers for coupling between stages. I
don't think that transformers designed for 50- 60 Hz are going to wrok
well at RF frequencies. What frequency and power levels are you
working at?

George Herold

B

#### Baron

Jan 1, 1970
0
I've built a few low power RF amplifiers in the past. I "rolled my
own" impedance matching transformers for coupling between stages. I
don't think that transformers designed for 50- 60 Hz are going to wrok
well at RF frequencies. What frequency and power levels are you
working at?

George Herold

If he is talking about a PSU for a big (1KW) SSB linear amp I've used
microwave oven transformers to generate 3KV @ 1 amp. Its a low duty
cycle application. The transformers are free from the local dumpster !

W

#### whit3rd

Jan 1, 1970
0
A similar situation:  one of those Weller soldering guns.  It has a
transformer with many turns on the 120V primary, and a ONE-turn heavy
copper secondary going to the tip.  Surely if you reverse that and
feed in 1V (or whatever the secondary voltage is) it will be almost a
direct short at 60Hz, even if the primary is unloaded?

Yes, but only ALMOST a direct short; the secondary winds around
a magnetic core, and has, thus, some series inductance.
If one loads the primary, the effective impedance on the
secondary will go down. If you short the primary, it
acts like most of the series inductance vanishes, though
the primary has significant internal resistance which will
prevent the short circuit from being completely effective.

R

#### [email protected]

Jan 1, 1970
0
I've built a few low power RF amplifiers in the past.  I "rolled my
own" impedance matching transformers for coupling between stages.  I
don't think that transformers designed for 50- 60 Hz are going to work
well at RF frequencies.  What frequency and power levels are you
working at? George Herold

Actually, when speaking of running a step-down transformer backwards I
was thinking of the power supply transformer. I'm thinking of making
a high-powered induction heater with something like 100A at 240V input
to the transformer, and a 20kW water-cooled power triode as an
oscillator. Maybe my saying "RF" was a bit of an exaggeration; I was
thinking of something under 100kHz.

As for the "Darwin awards" (thanks, John , I guess I must have been
trying ever since I stuck my fingers into an energized light bulb
socket at the tender age of 6, but somehow I'm still around

As for Phil's lovely admonition to "go straight to hell", I hardly
think this is "illegal", or even "lethally dangerous" if handled
correctly. Units with far more power than I am proposing are used
routinely commercially. For example, here's a 150kW unit:
http://cgi.ebay.com/Inductoheat-150...06518207QQcmdZViewItemQQptZLH_DefaultDomain_0
I'm sure even that is small potatoes in some applications.

Also lurking in the back of my mind for some future day is a low-
frequency radio transmitter for, say, 8kHz. I believe frequencies
that low are unregulated, so there's no legal limit to the amount of
power one could use. (Please no flame wars if I'm wrong...just a
polite correction would be fine! The only problem is antenna
efficiency, but that's probably a topic for another thread....

Robert

R

#### [email protected]

Jan 1, 1970
0
If he is talking about a PSU for a big (1KW) SSB linear amp I've used
microwave oven transformers to generate 3KV @ 1 amp.  Its a low duty
cycle application.  The transformers are free from the local dumpster !
Best Regards:  Baron.

Thanks for the tip. I'm looking for something more powerful, but this
is a great idea for other projects. I see microwaves put outside on
the curb all the time, and never thought of cannibalizing them for
their transformers! I suppose the magnetron might be useful to some
folks as well, but perhaps it would be burnt out in a discarded
microwave. (Although, you'd be surprised how much perfectly
functioning stuff gets thrown out by consumers and industry alike. I
remember when I was a grad student, the amount of good stuff I
scavenged from the university's garbage bin was astounding! And later
when I worked at a big company too. Until they got wise and locked
the garbage. To make sure the perfectly good equipment actually DID

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